Great 8 steering at the Charles

[Jim Dwyer]

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit=Refresh&event_leg_id=23686&school_id=6691&sculler_id=

Defective boat and they still won the race!

[Steve S]

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit...
Defective boat and they still won the race!

The boat wasn't defective, rather the forces on the carbon rudder/skeg
exceeded the design load ( + factor of safety) and it snapped off in a
practice turn 15 min before the start. Quickasaflash, they pulled in
to BU boathouse and jury-rigged a rudder from a smaller boat,
variously remembered as a pair or a 4+ rudder, that turned out to be
good enough for all the turns but Weeks and Eliot, both of which were
sweeping turns to port.

At Weeks the cox jammed her left hand in while steering with her
right, and the bowman dropped out for 4 strokes. Video here

At Eliot the cox did the same thing but the bowman dragged his oar.
You can see the Eliot turn here. http://nesports.tv/ (click on "Day
2 (Races 38-54)" and then scroll until it shows about 31:30 on the
right hand side of the time bar).

[Steve S]

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit...
Defective boat and they still won the race!

The boat wasn't defective, rather the forces on the carbon rudder/
skeg
exceeded the design load ( + factor of safety) and it snapped off in
a
practice turn 15 min before the start. Quickasaflash, they pulled in
to BU boathouse and jury-rigged a rudder from a smaller boat,
variously remembered as a pair or a 4+ rudder, that turned out to be
good enough for all the turns but Weeks and Eliot, both of which were
sweeping turns to port.

At Weeks the cox jammed her left hand in while steering with her
right, and the bowman dropped out for 4 strokes.  Video here
http://youtu.be/ykkrdW1FYLM

At Eliot the cox did the same thing but the bowman dragged his oar.
You can see the Eliot turn here.   http://nesports.tv/  (click on
"Day 
2 (Races 38-54)" and then scroll until it shows about 31:30 on
the 
right hand side of the time bar).

To clear up a possible quibble ahead of time, I should add that the
stroke, Iztok Cop, rows bow-side (starboard), so the boat was set up
such that the bowman, Tim Maeyens, was rowing stroke-side (port) and
so when Maeyens dropped out or dragged, the full starbord power would
swing the boat to port.

[sully]

Post by Steve S

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit...
Defective boat and they still won the race!

The boat wasn't defective, rather the forces on the carbon rudder/
skeg
exceeded the design load ( + factor of safety) and it snapped off in

I don't get this. How is this not defective?

It's a rowing shell being rowed. Failure of a rudder and/or skeg in
the act of being rowed and in the absence of some sort of abuse or
collision is a defect.

Design or materials, doesn't matter!!

[Jim Dwyer]

I heard that they had to replace 3 or 4 rudders over the weekend because
they kept breaking. Is that true?

Post by Steve S

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit...
Defective boat and they still won the race!

The boat wasn't defective, rather the forces on the carbon rudder/
skeg
exceeded the design load ( + factor of safety) and it snapped off in

I don't get this. How is this not defective?

It's a rowing shell being rowed. Failure of a rudder and/or skeg in
the act of being rowed and in the absence of some sort of abuse or
collision is a defect.

Design or materials, doesn't matter!!

[Carl Douglas]

Post by Jim Dwyer
I heard that they had to replace 3 or 4 rudders over the weekend because
they kept breaking. Is that true?

Post by Steve S

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit...
Defective boat and they still won the race!

The boat wasn't defective, rather the forces on the carbon rudder/
skeg
exceeded the design load ( + factor of safety) and it snapped off in

I don't get this. How is this not defective?
It's a rowing shell being rowed. Failure of a rudder and/or skeg in
the act of being rowed and in the absence of some sort of abuse or
collision is a defect.
Design or materials, doesn't matter!!

I'm with Sully on this. How does a typical eights rudder ever get to be
overloaded?

Jim, have you any idea what sort of rudders these were?

Cheers -
Carl

[Steven M-M]

A good recap on Rowing News cite:
http://www.rowingnews.com/index.php?option=com_content&view=article&id=533:theheroofthegreat8&catid=47:headline&Itemid=125

I agree equipment shouldn't break under normal racing demands; this
was an robust test, however, with a powerful crew and demanding
course.

Steven M-M

[Steve S]

Post by Jim Dwyer
I heard that they had to replace 3 or 4 rudders over the weekend because
they kept breaking.  Is that true?

Post by Steve S

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit...
Defective boat and they still won the race!

The boat wasn't defective, rather the forces on the carbon rudder/
skeg
exceeded the design load ( + factor of safety) and it snapped off in

I don't get this.  How is this not defective?
It's a rowing shell being rowed.   Failure of a rudder and/or skeg in
the act of being rowed and in the absence of some sort of abuse or
collision is a defect.
Design or materials, doesn't matter!!

I'm with Sully on this.  How does a typical eights rudder ever get to be
overloaded?
Jim, have you any idea what sort of rudders these were?
Cheers -
Carl
--
Carl Douglas Racing Shells        -
     Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write:   Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find:    http://tinyurl.com/2tqujf
URLs:  www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)

Whether the boat was defective is a matter of semantics, I guess,
though I tend to think that "defective" means that the boat, or, this
case, an appendage, was not up to spec. My point was the design, not
the boat, was defective.

I suspect the designers did not forsee this performance situation:
1) a crew with quite astonishing power. The average 2k erg in that
boat was less than 5:46, according to Warren Anderson, the USA
participant, who sat 6.
2) as sharp turns as one can make, at full speed, with that power.

I'm pretty sure these are the details. In the course of practice, they
bent two standard rudders. These were metal. They installed a carbon
one race morning. It snapped when they simulated one of the big turns.
They installed a smaller-than-spec metal one, borrowed from a pair or
a four, and raced with that. It had a smaller lifting surface and the
same size post. It neither snapped nor bent. But it was too small to
suffice for Weeks or Eliot. So the cox and the bowman improvised, and
the crew went on to a brilliant victory.

[Henry Law]

Post by Steve S
I'm pretty sure these are the details. In the course of practice, they
bent two standard rudders. These were metal. They installed a carbon
one race morning. It snapped when they simulated one of the big turns.

I know from reading this group that intuition, when applied to
engineering or particularly to fluid dynamics, is often dead wrong.
Nevertheless my intuition - and a certain amount of subsequent research
on the web into rudder forces - tells me that this cannot be the case;
or at least that the cause of damage to the rudder was not just the
force of the turn.

Let's say the rudder is 100mmx50mm (that seemed about right, sitting in
my front room): 0.005m^2. 2000m in 5 min would be 40m/sec (surely too
high). I think I see that to a very rough approximation the force on a
rudder is around speed x cross sectional area, given in Kg force when
dimensions are in meters. So 40*0.005 = 200g force. Can a rudder not
sustain that amount of load? And I think this calculation is for a
flat plate normal to the flow of water, i.e. stalled, which no rudder
ever should be with a good cox aboard.

Carl will be along in a moment I expect, to tell us what the real
calculations are, and to show me that I'm out by an order of magnitude,
or something.

[Richard du P]

Post by Henry Law

Post by Steve S
I'm pretty sure these are the details. In the course of practice, they
bent two standard rudders. These were metal. They installed a carbon
one race morning. It snapped when they simulated one of the big turns.

I know from reading this group that intuition, when applied to
engineering or particularly to fluid dynamics, is often dead wrong.
Nevertheless my intuition - and a certain amount of subsequent research
on the web into rudder forces - tells me that this cannot be the case;
or at least that the cause of damage to the rudder was not just the
force of the turn.
Let's say the rudder is 100mmx50mm (that seemed about right, sitting in
my front room): 0.005m^2.  2000m in 5 min would be 40m/sec (surely too
high).  I think I see that to a very rough approximation the force on a
rudder is around speed x cross sectional area, given in Kg force when
dimensions are in meters.  So 40*0.005 = 200g force.  Can a rudder not
sustain that amount of load?   And I think this calculation is for a
flat plate normal to the flow of water, i.e. stalled, which no rudder
ever should be with a good cox aboard.
Carl will be along in a moment I expect, to tell us what the real
calculations are, and to show me that I'm out by an order of magnitude,
or something.
--
Henry Law            Manchester, England

Not for me to get in front of Carl on the sums - but if the cox
totally lost faith in the rudder without even trying it, and simply
stuck a rudder-sized part of her hand in the Charles, would it feel a
force of less than half a pound?
[Henry knows I'm a lot older than him, and I still think in
Fahrenheit]

Richard du P

[Pete]

Post by Henry Law
Let's say the rudder is 100mmx50mm (that seemed about right, sitting in
my front room): 0.005m^2.  2000m in 5 min would be 40m/sec (surely too
high).  I think I see that to a very rough approximation the force on a
rudder is around speed x cross sectional area, given in Kg force when
dimensions are in meters.  So 40*0.005 = 200g force.  Can a rudder not
sustain that amount of load?   And I think this calculation is for a
flat plate normal to the flow of water, i.e. stalled, which no rudder
ever should be with a good cox aboard.

Your calculation isn't in the right units, therefore clearly wrong.
Apart from anything else, it thinks that the resistance is the same in
air and water. Also, your minutes seem to contain 10 seconds...

Force on a rudder can be pretty high; it's got to be, in order to move
most of a ton around. That said, it shouldn't break rudders whoever
you put in the boat.

Pete

[Carl Douglas]

Post by Pete

Post by Henry Law
Let's say the rudder is 100mmx50mm (that seemed about right, sitting in
my front room): 0.005m^2. 2000m in 5 min would be 40m/sec (surely too
high). I think I see that to a very rough approximation the force on a
rudder is around speed x cross sectional area, given in Kg force when
dimensions are in meters. So 40*0.005 = 200g force. Can a rudder not
sustain that amount of load? And I think this calculation is for a
flat plate normal to the flow of water, i.e. stalled, which no rudder
ever should be with a good cox aboard.

Your calculation isn't in the right units, therefore clearly wrong.
Apart from anything else, it thinks that the resistance is the same in
air and water. Also, your minutes seem to contain 10 seconds...
Force on a rudder can be pretty high; it's got to be, in order to move
most of a ton around. That said, it shouldn't break rudders whoever
you put in the boat.
Pete

The forces on rudders are not at all high, so do I hope that all the
hi-falutin' tosh that is spinning off this latest urban myth can be
promptly laid to rest.

The fact that an eight might weigh nearly a ton has little bearing on
the rather small forces needed to swing its stern. A decent rudder
operating within ~2m of the stern of a 16m boat will generate a large
turning moment, even though it is lightly loaded.

Eights fitted with conventional rudders never turn rapidly because those
rudders are so hopelessly inefficient, & their rudders remain very
lightly loaded, whatever the steering input. When the design is mere
crap, the turning force is crap & so, too, is the hydrodynamic force on
said crappy rudder.

Now let's re-run Henry's calcs:
Rudder face area = 0.1 x 0.05 = 0.005 m^2
Boat velocity = 2000/(5 x 60) = 6.7 m/s
(& that's more of an extreme sprint speed than head race cruise!)
Density of water = 1 tonne/m^3

Assume:
1. rudder perpendicular to uninterrupted water flow (an alignment which
no rudder should even remotely approach)
2. drag coeff, Cd, = 1
Force on rudder = area x velocity^2 x density x Cd/(2 x g)
where g is acceleration due to gravity, = 9.81 m/s^2

So:
Force on rudder = 0.005 x 6.7^2 x 1 x 1/18.8
= 0.012 tonnes force
= 12 kgf or 26lbf

[Actually this is far too high, since a typical eights rudder operates
well within the slowed & disturbed flow of the hull's boundary layer. A
more realistic velocity would be around 5m/s, giving a force on a
fully-stalled rudder plate of about 6.6kgf or 14.6 lbf]

Now consider the loads on the rudder pin:

Assume:
1. centre of load acts at middle of rudder plate, e.g. at 50mm from end
of pin.
2. take that length as the lever arm for the application of the force to
the rudder pin

So:
Pin bending moment = 0.012 x 0.05
(BM) = 0.00085 tonne force.m
= 6.2 Newton.m

Assume
1. pin is of 6mm diameter type 316 stainless steel
2. 0.2% proof stress of that steel is ~200 MPa (= 2 x 10^8 Newton/m^2)
(don't even want pin to deform, but ultimate tensile stress is 2.5
x higher)

Extreme fibre stress in a 6mm diameter rod under bending load is:
S = BM x R/(pi x R^4/4)
Bar radius, R = 3mm = 0.003 m
S = 8.3 x 0.003/(3.142 x 0.003^4/4)
= 2.9 x 10^8 Newton/m^s

Now that _is_ an interesting result (if my maths is correct)! Just
supposing the rudder was turned at 90 degrees to the fullest flow while
at top speed, there could be enough pressure on it to _slightly_ bend a
6mm pin. However, the pressure would still only be about 50% of that
required to completely bend it flat. And, as soon as the pin starts to
bend, the load reduces.

One could suppose that if, a) cox were so unwise as to apply 100% rudder
at full speed (but why would she want to subject her crew to a sudden 12
kilogram drag penalty?) & b) the quality of welding was poor, then it is
possible to imagine a rudder breaking off.

This does not allow anyone to attribute these multiple equipment
failures to the strength of the crew. That crew was marginally faster
than those it beat &, while that margin was enough to enable it to win
convincingly despite the lack of adequate steering, it was insufficient
to drastically increase rudder loadings.

Now let's consider how a rudder, any rudder, works & how it is supposed
to be used.

The rudder's function is to generate controllable side-forces, acting at
right angles to the direction of the boat. Those side forces, which
pull the stern sideways causing the boat to change direction, should be
generated through induced hydrodynamic lift, whereby the placing of a
"foil" at a (small) angle to a flow generates substantial (lift) forces
on the foil perpendicular to that flow while incurring almost no fluid
drag. They should achieve this without significantly increasing fluid
drag, & they do not work, as some think, by creating drag.

As with aircraft wings, efficient rudder has are of so-called aerofoil
shape - long (top to bottom) & of approximately teardrop cross-section.
Such shapes induce flow to follow their surfaces cleanly, even when
under high loads, without "separation" (tearing away).

Furthermore, the rudder should act in undisturbed flow! The water
layers flowing close to the hull of a rowing shell are highly disturbed
& being dragged along as a result of fluid friction. So it is axiomatic
that any rudder or any part thereof that is set close to the skin, &
which thus lies within this disturbed "boundary layer", will be
relatively ineffective & inefficient.

Finally, a rudder should be part of joined up thinking on boat steering,
not an afterthought. By that token, if you have a large fin, then
tacking a little bit of metal just behind it, & inside the boundary
layer, & asking it to act as a rudder, is rather pathetic: while the
fin does its level (but often rather poor) best to keep the boat running
straight, the poor little tab of metal you call a rudder is struggling
to deflect enough flow to produce some sort of turning effect. No
wonder that convention fin-&-rudder systems are, shall we say, a touch
slow to act & uncertain in performance?

In short, typical rowing rudders are rather ridiculous. Yet crews
happily race each other carrying equally hindering systems, oblivious of
the drag penalties they thus incur. Those drag penalties come from
several sources:
1. the common plate-fin/appended-rudder devices have high drag because
flow conforms poorly to flat plates with sharp leading edges, even when
they aren't a bit bent. Non-conforming flows separate from the fin
surfaces & spin energy off into vortices - all loss & drag.
2. when cox tries to use the silly little rudder, it fights the fin. The
steering result is just the crumbs left over from that fight - the rest
is lost energy that slows the boat.
3. because conventional systems are uncertain, the boat is constantly
changing direction under wind loads & varying blade loadings, even if
cox is not trying to correct the last over-correction. So the boat
follows a slightly but significantly meandering course (watch boat
courses on a filmed multilane event). Every meander sets the boat
slightly sideways to the direction in which it is moving. Shells incur
the least drag when going the way they're pointing, & drag rises rapidly
when they're even slightly out of line.

There is _one_ truly effective & race-proven steering system - AeRowFin.
We invented it. We designed it. And we make it (see our website).
It is an undoubted race-winning tool. It costs less than one new oar.
It works precisely, & it truly empowers the cox. It doesn't break off
when you steer with it, even when you make an urgent steering correction
to avoid collision ;) .

More recently, AeRowFin has been ripped off (badly) by Empacher. You
could say, as some do, that imitation is the sincerest form of flattery.
Personally speaking, I think it stinks. Incredibly, just a few months
ago Empacher got very rattled by my occasionally pointing out what they
had done. They sent me an email in which they had gall to threaten to
sue me if I continued to tell the truth on this matter. Quite stunning
cheek, you might think. In explaining my disdain for this thuggish
threat I also reminded Empacher how pissed off they were when they
complained to FISA, at a boatbuilders' meeting in St Catherines '99,
about the Chinese ripping off "their" hull shapes. I was very
sympathetic with them at the time. I'm a lot less sympathetic now.

Cheers -
Carl

[Charles Carroll]

Carl,

Didn't you used to sell AeRowFin rudders to Empacher? It seems to me that I
remember Empacher's offering an AeRowFin rudder as an option when ordering
one of their boats. Or is my memory just playing tricks on me?

If so, is there any significant difference in the two rudders? Or did
Empacher simply study and then just brazenly rip off your design?

Am I naïve to think that the burden of demonstrating the difference between
Empacher's new rudder and your AeRowFin rudder belongs to Empacher?

Maybe I have just never grown up, but I don't understand how big companies
can conduct themselves in such an unconscionable manner.

There is a film about just this subject, "A Flash of Genius." I don't know
if you can get the film in the UK, but if you can I highly recommend it. I
think that it might prove very instructive to anyone who is contemplating
selling something that they have invented.

I am especially fond of the dictionary scene where Robert Kearns puts an
engineer on the witness stand and has him read from Charles Dickens. Same
words we all use, same twenty-six letters of the English Alphabet, but not
everyone puts them together in the same way! It is a wonderful scene.

Cordially,

Charles

[Carl Douglas]

Post by Charles Carroll
Carl,
Didn't you used to sell AeRowFin rudders to Empacher? It seems to me
that I remember Empacher's offering an AeRowFin rudder as an option when
ordering one of their boats. Or is my memory just playing tricks on me?
If so, is there any significant difference in the two rudders? Or did
Empacher simply study and then just brazenly rip off your design?

Nothing significant. They say they've changed the profile - by
introducing a semi-elliptical tip profile. They claim, in writing to
me, that they did this to make it different from AeRowFin - which admits
that they chose to imitate AeRowFin.

Being a gentleman, I did not to remind them that this change makes their
rip-off resemble the wing profile of a famous WW-II British fighter
plane..... More to the point, they've failed to realise that this
change does not enhance performance over our profile & does impair the
steering output. In other words, they made that change for
non-functional reasons.

Post by Charles Carroll
Am I naïve to think that the burden of demonstrating the difference
between Empacher's new rudder and your AeRowFin rudder belongs to Empacher?

Does the plagiarist stop to worry about demonstrating differences from
the original? Generally they brazenly claim it as their own "advanced"
design. This damages the reputation of the original. As when an
Empacher rip-off malfunctioned in Beijing.

Post by Charles Carroll
Maybe I have just never grown up, but I don't understand how big
companies can conduct themselves in such an unconscionable manner.

Commerce is afflicted by the presumption that might is right. Many
businesses are many highly ethical but, sadly, not all. And in some
parts of the world no intellectual property is safe.

Post by Charles Carroll
There is a film about just this subject, "A Flash of Genius." I don't
know if you can get the film in the UK, but if you can I highly
recommend it. I think that it might prove very instructive to anyone who
is contemplating selling something that they have invented.
I am especially fond of the dictionary scene where Robert Kearns puts an
engineer on the witness stand and has him read from Charles Dickens.
Same words we all use, same twenty-six letters of the English Alphabet,
but not everyone puts them together in the same way! It is a wonderful
scene.

To an engineer or scientist, words in a technical context have precise
meanings. Some others treat words as meaning whatever they wish them to
mean.

The corrosive effect of intellectual property theft is that it
discourages innovation. The concept of intellectual property is
meanwhile debased by its extension to cover all manner of distinctly
un-intellectual garbage & by the high cost of its defence. Deep pockets
make abuse easy - it reduces to the bully vs victim game. Meanwhile the
state, which will do nothing to protect the small guy, cheerfully spends
our taxes to protect the systematic overpricing of, e.g., big name
clothing brands by seizing cheaper copies. I don't condone copying, but
one has to wonder about priorities when a state works only for those who
maintain grossly inflated prices through heavy brand image promotion.

So I very much admire the way in which Mr. Dyson, for example, risked
all to fight & defeat the much larger firms that had thought they could
with impunity steal the intellectual property around which his vacuum
cleaners were designed.

Carl

[Carl Douglas]

Post by Carl Douglas

Post by Charles Carroll
Carl,
Didn't you used to sell AeRowFin rudders to Empacher? It seems to me
that I remember Empacher's offering an AeRowFin rudder as an option
when ordering one of their boats. Or is my memory just playing tricks
on me?
If so, is there any significant difference in the two rudders? Or did
Empacher simply study and then just brazenly rip off your design?

Nothing significant. They say they've changed the profile - by
introducing a semi-elliptical tip profile. They claim, in writing to
me, that they did this to make it different from AeRowFin - which admits
that they chose to imitate AeRowFin.
Being a gentleman, I did not to remind them that this change makes their
rip-off resemble the wing profile of a famous WW-II British fighter
plane..... More to the point, they've failed to realise that this
change does not enhance performance over our profile & does impair the
steering output. In other words, they made that change for
non-functional reasons.

Post by Charles Carroll
Am I naïve to think that the burden of demonstrating the difference
between Empacher's new rudder and your AeRowFin rudder belongs to Empacher?

Does the plagiarist stop to worry about demonstrating differences from
the original? Generally they brazenly claim it as their own "advanced"
design. This damages the reputation of the original. As when an
Empacher rip-off malfunctioned in Beijing.

Post by Charles Carroll
Maybe I have just never grown up, but I don't understand how big
companies can conduct themselves in such an unconscionable manner.

Commerce is afflicted by the presumption that might is right. Many
businesses are many highly ethical but, sadly, not all. And in some
parts of the world no intellectual property is safe.

Post by Charles Carroll
There is a film about just this subject, "A Flash of Genius." I don't
know if you can get the film in the UK, but if you can I highly
recommend it. I think that it might prove very instructive to anyone
who is contemplating selling something that they have invented.
I am especially fond of the dictionary scene where Robert Kearns puts
an engineer on the witness stand and has him read from Charles
Dickens. Same words we all use, same twenty-six letters of the English
Alphabet, but not everyone puts them together in the same way! It is a
wonderful scene.

To an engineer or scientist, words in a technical context have precise
meanings. Some others treat words as meaning whatever they wish them to
mean.
The corrosive effect of intellectual property theft is that it
discourages innovation. The concept of intellectual property is
meanwhile debased by its extension to cover all manner of distinctly
un-intellectual garbage & by the high cost of its defence. Deep pockets
make abuse easy - it reduces to the bully vs victim game. Meanwhile the
state, which will do nothing to protect the small guy, cheerfully spends
our taxes to protect the systematic overpricing of, e.g., big name
clothing brands by seizing cheaper copies. I don't condone copying, but
one has to wonder about priorities when a state works only for those who
maintain grossly inflated prices through heavy brand image promotion.
So I very much admire the way in which Mr. Dyson, for example, risked
all to fight & defeat the much larger firms that had thought they could
with impunity steal the intellectual property around which his vacuum
cleaners were designed.
Carl

And here's an interesting slant on "might is right" commercial practices:
http://www.guardian.co.uk/lifeandstyle/2009/nov/05/seamstress-takes-on-chanel

Carl

[Carl Douglas]

Carl Douglas wrote:>
<snip>

Post by Carl Douglas
Pin bending moment = 0.012 x 0.05
(BM) = 0.00085 tonne force.m
= 6.2 Newton.m

Seems I made some slight errors while reviewing numbers, which no one
saw or, more likely, cared to flag:

Bending moment would be 0.0006 tonne force.m = 5.9 Newton.m

Post by Carl Douglas
Assume
1. pin is of 6mm diameter type 316 stainless steel
2. 0.2% proof stress of that steel is ~200 MPa (= 2 x 10^8 Newton/m^2)
(don't even want pin to deform, but ultimate tensile stress is 2.5 x
higher)
S = BM x R/(pi x R^4/4)
Bar radius, R = 3mm = 0.003 m
S = 8.3 x 0.003/(3.142 x 0.003^4/4)
= 2.9 x 10^8 Newton/m^s

which should have been:
S = 5.9 x 0.003/(3.142 x 0.003^4/4)
= 2.8 x 10^8 Newton/m^2

Insignificant changes, fortunately.

Carl

[Mike De Petris]

Post by Carl Douglas

Post by Carl Douglas
Pin bending moment = 0.012 x 0.05
     (BM)          = 0.00085 tonne force.m
                   = 6.2 Newton.m

Seems I made some slight errors while reviewing numbers, which no one
Bending moment would be 0.0006 tonne force.m = 5.9 Newton.m

I think you introduced the error intentionally to check if we were
following :-)

Post by Carl Douglas
     S                = 5.9 x 0.003/(3.142 x 0.003^4/4)
                      = 2.8 x 10^8 Newton/m^2
Insignificant changes, fortunately.

this is why we didn't comment O:-)

anyway, thanks for the numbers Carl, I have a better picture now, I am
curiously following the thread, as I've been a cox for few years when
young, and still have a lot of those feelings about putting a hand
into water, front and stern/loaders, manouvering at high and low speed
with different kind of steering systems, lot of experince with very
old-style rudders anyway, as it happend more then 20 years ago.

[Pete]

Post by Carl Douglas

Post by Pete

Post by Henry Law
Let's say the rudder is 100mmx50mm (that seemed about right, sitting in
my front room): 0.005m^2.  2000m in 5 min would be 40m/sec (surely too
high).  I think I see that to a very rough approximation the force on a
rudder is around speed x cross sectional area, given in Kg force when
dimensions are in meters.  So 40*0.005 = 200g force.  Can a rudder not
sustain that amount of load?   And I think this calculation is for a
flat plate normal to the flow of water, i.e. stalled, which no rudder
ever should be with a good cox aboard.

Your calculation isn't in the right units, therefore clearly wrong.
Apart from anything else, it thinks that the resistance is the same in
air and water. Also, your minutes seem to contain 10 seconds...
Force on a rudder can be pretty high; it's got to be, in order to move
most of a ton around. That said, it shouldn't break rudders whoever
you put in the boat.
Pete

The forces on rudders are not at all high, so do I hope that all the
hi-falutin' tosh that is spinning off this latest urban myth can be
promptly laid to rest.
The fact that an eight might weigh nearly a ton has little bearing on
the rather small forces needed to swing its stern.  A decent rudder
operating within ~2m of the stern of a 16m boat will generate a large
turning moment, even though it is lightly loaded.

Rough numbers: an eight's moment of inertia is probably at least 4,000
kg m^2 (assuming turning about the centre, which may not be all that
realistic). The moment provided by the rudder is about 8 times the
(sideways) force on the rudder. So, the angular acceleration is about
1/500 times the force on the rudder.

If the force is 2 N (Henry's guess) then even if there is no
resistance to turning, it will take about 25 seconds for the eight to
turn through 90 degrees. If there is resistance to turning, (which,
realistically, there is quite a bit of) then, much longer. So, a 2 N
sideways force wouldn't be much use.

If the force is instead 120N (your calc, and I know that wasn't
sideways force, but for illustration...) then that changes to about
3.5 seconds and leaves the boat spinning rapidly. Which is perhaps not
very surprising, given there's a factor of 60 between the two force
numbers. Whether you call this a 'high force' is of course up to
debate; put it this way: if your outstretched hand is required to
provide a 120N force for a prolonged period of time, you will not find
it easy, especially if you're a small cox.

A realistic sideways force is somewhere between the two values -
memory suggests 40N in a hard turn, but memory may well be wrong.

As to breaking rudders, your calculation explains nicely why a well-
made rudder won't break even if the cox manages to badly stall it at
full sprint. What it's missing (perhaps because your rudders don't fit
this description) is the possibility that the cheap plastic rudder
plate might break itself off the metal attachment, or break the (much
thinner) metal rod that goes up the rudder plate.

Pete

[Carl Douglas]

Post by Pete

Post by Carl Douglas

Post by Pete

Post by Henry Law
Let's say the rudder is 100mmx50mm (that seemed about right, sitting in
my front room): 0.005m^2. 2000m in 5 min would be 40m/sec (surely too
high). I think I see that to a very rough approximation the force on a
rudder is around speed x cross sectional area, given in Kg force when
dimensions are in meters. So 40*0.005 = 200g force. Can a rudder not
sustain that amount of load? And I think this calculation is for a
flat plate normal to the flow of water, i.e. stalled, which no rudder
ever should be with a good cox aboard.

Your calculation isn't in the right units, therefore clearly wrong.
Apart from anything else, it thinks that the resistance is the same in
air and water. Also, your minutes seem to contain 10 seconds...
Force on a rudder can be pretty high; it's got to be, in order to move
most of a ton around. That said, it shouldn't break rudders whoever
you put in the boat.
Pete

The forces on rudders are not at all high, so do I hope that all the
hi-falutin' tosh that is spinning off this latest urban myth can be
promptly laid to rest.
The fact that an eight might weigh nearly a ton has little bearing on
the rather small forces needed to swing its stern. A decent rudder
operating within ~2m of the stern of a 16m boat will generate a large
turning moment, even though it is lightly loaded.

Rough numbers: an eight's moment of inertia is probably at least 4,000
kg m^2 (assuming turning about the centre, which may not be all that
realistic). The moment provided by the rudder is about 8 times the
(sideways) force on the rudder. So, the angular acceleration is about
1/500 times the force on the rudder.
If the force is 2 N (Henry's guess) then even if there is no
resistance to turning, it will take about 25 seconds for the eight to
turn through 90 degrees. If there is resistance to turning, (which,
realistically, there is quite a bit of) then, much longer. So, a 2 N
sideways force wouldn't be much use.
If the force is instead 120N (your calc, and I know that wasn't
sideways force, but for illustration...) then that changes to about
3.5 seconds and leaves the boat spinning rapidly. Which is perhaps not
very surprising, given there's a factor of 60 between the two force
numbers. Whether you call this a 'high force' is of course up to
debate; put it this way: if your outstretched hand is required to
provide a 120N force for a prolonged period of time, you will not find
it easy, especially if you're a small cox.
A realistic sideways force is somewhere between the two values -
memory suggests 40N in a hard turn, but memory may well be wrong.
As to breaking rudders, your calculation explains nicely why a well-
made rudder won't break even if the cox manages to badly stall it at
full sprint. What it's missing (perhaps because your rudders don't fit
this description) is the possibility that the cheap plastic rudder
plate might break itself off the metal attachment, or break the (much
thinner) metal rod that goes up the rudder plate.
Pete

Thanks for those interesting comments, Pete.

It might be worth making a point on how eights turn:
When the rudder is applied, this moves the stern out to one side. This
puts the boat at an angle to its current direction of motion, which
markedly increases its drag.

It also accentuates the inherent, if counter-intuitive, tendency of
boats both long & short to try to swing their sterns ahead of their
bows, for reasons detailed in the next paragraph.

For any boat the skin friction dragging on a unit area of wetted hull
surface is greatest in the bows and falls off progressively as you move
towards the stern. This means the centre of drag is likely to be
somewhat, & sometimes a lot, ahead of the centre of mass. Get the
centre of mass even a little bit out of line with the direction of
motion & this generates a modest but rapidly growing couple (rotational
moment) which is trying to swing the stern out further. Anyone who's
paddled a rudderless kayak knows all about this effect - you're
constantly correcting its course as it tries to put stern before bow.
It's also one reason why projectiles which aren't spin-stabilised
usually have tail fins.

The hull's drag forces can be further shifted forwards by the sharper
leading & under edges of the bow engaging positively with the water as
the bow starts to be pushed sideways at the water as the stern swings out.

These 2 effects help to turn the boat, once the rudder or some other
influence has started the turning process. They also explain why it can
be so hard to keep a shell on a true course with an inadequate rudder.

Cheers -
Carl

[Kit]

Post by Carl Douglas

Post by Pete

Post by Carl Douglas

Post by Pete

Post by Henry Law
Let's say the rudder is 100mmx50mm (that seemed about right, sitting in
my front room): 0.005m^2.  2000m in 5 min would be 40m/sec (surely too
high).  I think I see that to a very rough approximation the force on a
rudder is around speed x cross sectional area, given in Kg force when
dimensions are in meters.  So 40*0.005 = 200g force.  Can a rudder not
sustain that amount of load?   And I think this calculation is for a
flat plate normal to the flow of water, i.e. stalled, which no rudder
ever should be with a good cox aboard.

Your calculation isn't in the right units, therefore clearly wrong.
Apart from anything else, it thinks that the resistance is the same in
air and water. Also, your minutes seem to contain 10 seconds...
Force on a rudder can be pretty high; it's got to be, in order to move
most of a ton around. That said, it shouldn't break rudders whoever
you put in the boat.
Pete

The forces on rudders are not at all high, so do I hope that all the
hi-falutin' tosh that is spinning off this latest urban myth can be
promptly laid to rest.
The fact that an eight might weigh nearly a ton has little bearing on
the rather small forces needed to swing its stern.  A decent rudder
operating within ~2m of the stern of a 16m boat will generate a large
turning moment, even though it is lightly loaded.

Rough numbers: an eight's moment of inertia is probably at least 4,000
kg m^2 (assuming turning about the centre, which may not be all that
realistic). The moment provided by the rudder is about 8 times the
(sideways) force on the rudder. So, the angular acceleration is about
1/500 times the force on the rudder.
If the force is 2 N (Henry's guess) then even if there is no
resistance to turning, it will take about 25 seconds for the eight to
turn through 90 degrees. If there is resistance to turning, (which,
realistically, there is quite a bit of) then, much longer. So, a 2 N
sideways force wouldn't be much use.
If the force is instead 120N (your calc, and I know that wasn't
sideways force, but for illustration...) then that changes to about
3.5 seconds and leaves the boat spinning rapidly. Which is perhaps not
very surprising, given there's a factor of 60 between the two force
numbers. Whether you call this a 'high force' is of course up to
debate; put it this way: if your outstretched hand is required to
provide a 120N force for a prolonged period of time, you will not find
it easy, especially if you're a small cox.
A realistic sideways force is somewhere between the two values -
memory suggests 40N in a hard turn, but memory may well be wrong.
As to breaking rudders, your calculation explains nicely why a well-
made rudder won't break even if the cox manages to badly stall it at
full sprint. What it's missing (perhaps because your rudders don't fit
this description) is the possibility that the cheap plastic rudder
plate might break itself off the metal attachment, or break the (much
thinner) metal rod that goes up the rudder plate.
Pete

Thanks for those interesting comments, Pete.
When the rudder is applied, this moves the stern out to one side.  This
puts the boat at an angle to its current direction of motion, which
markedly increases its drag.
It also accentuates the inherent, if counter-intuitive, tendency of
boats both long & short to try to swing their sterns ahead of their
bows, for reasons detailed in the next paragraph.
For any boat the skin friction dragging on a unit area of wetted hull
surface is greatest in the bows and falls off progressively as you move
towards the stern.  This means the centre of drag is likely to be
somewhat, & sometimes a lot, ahead of the centre of mass.  Get the
centre of mass even a little bit out of line with the direction of
motion & this generates a modest but rapidly growing couple (rotational
moment) which is trying to swing the stern out further.  Anyone who's
paddled a rudderless kayak knows all about this effect - you're
constantly correcting its course as it tries to put stern before bow.
It's also one reason why projectiles which aren't spin-stabilised
usually have tail fins.
The hull's drag forces can be further shifted forwards by the sharper
leading & under edges of the bow engaging positively with the water as
the bow starts to be pushed sideways at the water as the stern swings out.
These 2 effects help to turn the boat, once the rudder or some other
influence has started the turning process.  They also explain why it can
be so hard to keep a shell on a true course with an inadequate rudder.
Cheers -
Carl
--
Carl Douglas Racing Shells        -
     Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write:   Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find:    http://tinyurl.com/2tqujf
URLs:  www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)

Barely 8am and I have learnt something new, interesting and useful
today already.
Regards
Kit

[Mike De Petris]

Post by Kit

Post by Carl Douglas
These 2 effects help to turn the boat, once the rudder or some other
influence has started the turning process.  They also explain why it can
be so hard to keep a shell on a true course with an inadequate rudder.

Barely 8am and I have learnt something new, interesting and useful
today already.
Regards
Kit

And so much interesting for COASTAL rowing too!!
Coastal rowing boats have flat stern and 'V' shaped bows, tailwaves
sometimes make it impossible to keep the boats from spinning.

[fatsculler]

Post by Mike De Petris

Post by Kit

Post by Carl Douglas
These 2 effects help to turn the boat, once the rudder or some other
influence has started the turning process.  They also explain why it can
be so hard to keep a shell on a true course with an inadequate rudder.

Barely 8am and I have learnt something new, interesting and useful
today already.
Regards
Kit

And so much interesting for COASTAL rowing too!!
Coastal rowing boats have flat stern and 'V' shaped bows, tailwaves
sometimes make it impossible to keep the boats from spinning.

All this engineering maths is well over my head, but it got me
thinking that a rudder is so small because a racing 8 is not expected
to make significant turns whilst at maximum speed. Most 8's (and 4's
and pairs for that matter) are designed to compete on straight courses
and therefore only need to make relatively small adjustments whilst
racing. A question: how many crews at the Charles will have changed
their rudder to something bigger (if that indeed would make any
difference) and how many expected to be able to race with rudders
designed for 2K multi-lane courses?

[Carl Douglas]

This post might be inappropriate. Click to display it.

[d***@aea.be]

Post by Carl Douglas
FWIW, if you do have to steer with a wet hand, then much better than
holding it out at arm's length would be to hold the flattened hand
vertically from a vertical fore-arm, with fingers tightly against each
other, immersed down to the wrist & no more, & steering by simply
rotating a few degrees either way.  The hand held in that way makes a
passable imitation of an aquatic mammal's flipper, which does a good job
of resembling a well-shaped (hydro)foil.

I wondered when we would get to the science of handsteering :-)

I had in mind extending your arm out as far back as you can
comfortably manage, bending the wrist until the hand is parallel to
the boat, thn dipping it in the water with the fingers trailing. Then
by cupping the fingers slightly you could create a passable simulation
of a foil.

No, wait a minute, that would surely force your hand and therefore
your arm inwards towards the hull, dislocating your shoulder.

OK, scrub that, back to Carl's Plan A.

Dave H

[David Biddulph]

Post by fatsculler

Post by Mike De Petris

Post by Kit

Post by Carl Douglas
These 2 effects help to turn the boat, once the rudder or some
other influence has started the turning process. They also explain
why it can be so hard to keep a shell on a true course with an
inadequate rudder.

Barely 8am and I have learnt something new, interesting and useful
today already.
Regards
Kit

And so much interesting for COASTAL rowing too!!
Coastal rowing boats have flat stern and 'V' shaped bows, tailwaves
sometimes make it impossible to keep the boats from spinning.

All this engineering maths is well over my head, but it got me
thinking that a rudder is so small because a racing 8 is not expected
to make significant turns whilst at maximum speed. Most 8's (and 4's
and pairs for that matter) are designed to compete on straight courses
and therefore only need to make relatively small adjustments whilst
racing. A question: how many crews at the Charles will have changed
their rudder to something bigger (if that indeed would make any
difference) and how many expected to be able to race with rudders
designed for 2K multi-lane courses?

I do remember when my club bought our Karlisch 8 in 1977 it was supplied
with 2 rudders, so in principle we had a choice of putting on the larger one
if we felt we needed it on bendy courses. When I was coxing it I usually
reckoned that I could cope with the smaller one, particularly if the crew
were competent (which they usually were), but at least we had the choice
available to us. We also had a choice of fins, one very deep but one
shallower one which I suppose might have been wiser if there had been vast
amounts of driftwood around or if we were rowing over very shallow
sandbanks. I remember one head race on a very bendy course when I was
somewhat disappointed at my steering during the race, then when we came
ashore I found that the fin (the deep one) was missing, presumably having
been broken off as we went afloat in a shallow bit of river.

[Jim Dwyer]

Post by Carl Douglas
Jim, have you any idea what sort of rudders these were?
Cheers -

Carl:

I don't think that it was a CD rudder. They rowed a Filippi boat.

[Alistair]

This story sounds pretty cool, respect to the coxswain for clear
thinking (who was she?).

It's my opinion that the fastest way to get round a really tight bend in
an eight (or any boat) is to be skidding for the minimum amount of time,
and therefore anything that approximates to a car doing a handbrake-turn
is better than 40s of just relying on the hydrodynamic forces of the
rudder.

(Furthermore, it's difficult to tell when a rudder stops being effective
and starts being a brake, and the temptation on a tight bend is to steer
as much as possible which will usually ensure that the rudder has stalled.)

Based on nothing more than gut-instinct, I wouldn't have dropped out
two, I would have dropped out stroke and had him chuck an anchor, I
can't explain why though.

BUT before all the tight-bend coxes (think Bumps, Durham etc.) see this
video and start jamming their arms in the water going round their tight
bends, I think that's just daft and not likely to do anything but slow
the boat down. I've stuck my arm in the water myself once or twice when
coxing and the first thing you notice is the ENORMOUS pressure you have
on your arm, you'd have to be a goliath I reckon to be able to keep your
arm 3/4 immersed in the water with a fast eight.

But I just can't see it providing much turning force on the boat. Or, in
fact, really anything more than a negligible amount. I'd be happy to be
shown to be wrong, but it seems to me that sticking in a brake (one's
arm) so close to the centre of the boat is going to produce very little
torque at all.

It's still a great story.

AJP

Post by Steve S

https://www.sportgraphics.com/events/3946/photo_browser/page/4?commit...
Defective boat and they still won the race!

The boat wasn't defective, rather the forces on the carbon rudder/
skeg
exceeded the design load ( + factor of safety) and it snapped off in
a
practice turn 15 min before the start. Quickasaflash, they pulled in
to BU boathouse and jury-rigged a rudder from a smaller boat,
variously remembered as a pair or a 4+ rudder, that turned out to be
good enough for all the turns but Weeks and Eliot, both of which were
sweeping turns to port.
At Weeks the cox jammed her left hand in while steering with her
right, and the bowman dropped out for 4 strokes. Video here
http://youtu.be/ykkrdW1FYLM
At Eliot the cox did the same thing but the bowman dragged his oar.
You can see the Eliot turn here. http://nesports.tv/ (click on
"Day 
2 (Races 38-54)" and then scroll until it shows about 31:30 on
the 
right hand side of the time bar).
To clear up a possible quibble ahead of time, I should add that the
stroke, Iztok Cop, rows bow-side (starboard), so the boat was set up
such that the bowman, Tim Maeyens, was rowing stroke-side (port) and
so when Maeyens dropped out or dragged, the full starbord power would
swing the boat to port.

[Tinus]

Is it actually allowed for the cox to put his or her hands in the
water? The reason why I ask this is because this practice would,
besides speed reducing steering, also allow the cox to increase the
speed by means of paddling with the hands. The great eight should have
been disqualified!