Once more about the NK impeller - calibration

[AnnaM]

My club just purchased a brand new Filippi Italia/S which I think is
the F15 mould (70kg boat). I am the happy individual racing in it.
Ufortunately the NK impeller can not be installed at 5m from the bow in
this boat due to its measurements. My club is located at sea front and
there's no place to go to where one could row an exact distance and
check the calibration for the speedcoach impeller. Has anyone got a
calibrated speedcoach where the pick-up is located at about 4.70m from
the bow? I would be interested in the calibration value. I will
meanwhile try to locate someone with a GPS device...

Thanks!
Anna M
Helsinki, Finland

[Ewoud Dronkert]

Post by AnnaM
meanwhile try to locate someone with a GPS device...

Much more difficult than it is for them to locate themselves.

[Steven M-M]

Post by AnnaM
My club just purchased a brand new Filippi Italia/S which I think is
the F15 mould (70kg boat). I am the happy individual racing in it.
Ufortunately the NK impeller can not be installed at 5m from the bow in
this boat due to its measurements. My club is located at sea front and
there's no place to go to where one could row an exact distance and
check the calibration for the speedcoach impeller. Has anyone got a
calibrated speedcoach where the pick-up is located at about 4.70m from
the bow? I would be interested in the calibration value. I will
meanwhile try to locate someone with a GPS device...
Thanks!
Anna M
Helsinki, Finland

Anna,

Congrats on your Filippi. Anyway, I don't think there is a direct
relationship between the location of the impeller and calibration. The
NK folks are very helpful and I would contact them directly with your
question. I think the "5m from bow" location has to do with keeping the
little propeller outside the turbulent boundary layer of water that
forms near the hull. That layer gets fatter toward the stern. I've
tried to calibrate mine as we have a marked course on the river where I
row, but it still seems like an approximation rather than an exact
measurement. If you contact NK, I would be curious to learn their
answer.

Steven M-M

[Jay L]

Post by AnnaM
My club just purchased a brand new Filippi Italia/S which I think is
the F15 mould (70kg boat). I am the happy individual racing in it.
Ufortunately the NK impeller can not be installed at 5m from the bow in
this boat due to its measurements. My club is located at sea front and
there's no place to go to where one could row an exact distance and
check the calibration for the speedcoach impeller. Has anyone got a
calibrated speedcoach where the pick-up is located at about 4.70m from
the bow? I would be interested in the calibration value. I will
meanwhile try to locate someone with a GPS device...
Thanks!
Anna M
Helsinki, Finland

Anna,

For what it's worth my brother's boat (very similar shape to the
Filippi) also had the 5m issue, as far as I recall he set it to 0.96 but
that would only be guessing, I know that it over reads a bit.
Incidentally we had the same issue on a 2x/- which was just about
exactly on 5m also used 0.96 for that as far as I recall, haven't used
the boat in a while so I can't remember if it was out.

Regards

Jay

[Carl Douglas]

Post by AnnaM
My club just purchased a brand new Filippi Italia/S which I think is
the F15 mould (70kg boat). I am the happy individual racing in it.
Ufortunately the NK impeller can not be installed at 5m from the bow in
this boat due to its measurements. My club is located at sea front and
there's no place to go to where one could row an exact distance and
check the calibration for the speedcoach impeller. Has anyone got a
calibrated speedcoach where the pick-up is located at about 4.70m from
the bow? I would be interested in the calibration value. I will
meanwhile try to locate someone with a GPS device...

First, the matter of location: 5 metres is not a magical figure. On a
faster boat the equivalent distance would be nearer to the bows, in a
slower boat nearer to the stern. Are you even as fast as a fast man in
a 1x, let alone an eight?

Next, the matter of accuracy: In a race you'll know if you're fast
enough by whether you win. In training what matters is not how fast you
are but whether you are improving, how rapidly & is that the best you
can do. So concentrate on that & use the device to be the best you can,
because there is never a time when you would not wish to go even faster.

BTW, do you intend to race with spinner & mount in place? Such a
stick-on object has a significant level of drag which must slow you down
& could cost you a race (or we'd cover the boat in the things!). I'd
guess at about 1 to 2 watts of drag at race pace. That might be 0.5 to
1% of your net propulsive work rate (rowing has around 65% propulsive
efficiency), so you might expect to lose 0.15 to 0.3% of boat speed, or
3 to 6m/2k

Cheers -
Carl

[Steven M-M]

Carl,

According to NK, "Laboratory tests were performed on the hull mount
impeller to determine the drag effects on a rowing shell. For single
sculls, the drag from the hull mount impeller is approximately 0.1% of
the total boat drag, and proportionately smaller for larger boats.
Depending on the size and speed of the shell, the added time
corresponding to this drag is at MOST 0.2 seconds over a 2000 meter
race."

If you used this claim-one that I can't evaluate-would you come
up with the same 3-6m loss in a 2k race?

I do wish NK had developed a reliable, no-drag "clip in/clip out"
bracket for the impeller. Using tape to attach it to the hull so that
you can remove it is a poor solution. I do find the SpeedCoach a great
training aide and use it for longer races. But I leave the red box off
my shell for 1k or shorter races; getting the drag from the impeller
without any info re stroke rate and speed.

Steven M-M

[J Flory]

Steven, you can slip the impeller and its shaft off, leaving only the
bracket on the hull. Probably reduces the drag somewhat. But a
two-piece bracket would be a great improvement.

BTW, it's a good idea to mark the bracket location once you know the
pickup is working well. That way if the bracket is removed for a race
or knocked off by debris it's easy to replace it. A black Sharpie
permanent marker pen can be used to make a small dot on the hull at the
center of both ends of the mounting flange (unless the hull is
black...).

[Jim Dwyer]

Car top your boat to a marked course to calibrate the SpeedCoach. That is
what I do although I only have to drive abot 20 minutes to get there. The
other alternative might be to take a GPS unit with you in the boat and row a
straight course 1000m long and compare.

Jim

Post by AnnaM
My club just purchased a brand new Filippi Italia/S which I think is
the F15 mould (70kg boat). I am the happy individual racing in it.
Ufortunately the NK impeller can not be installed at 5m from the bow in
this boat due to its measurements. My club is located at sea front and
there's no place to go to where one could row an exact distance and
check the calibration for the speedcoach impeller. Has anyone got a
calibrated speedcoach where the pick-up is located at about 4.70m from
the bow? I would be interested in the calibration value. I will
meanwhile try to locate someone with a GPS device...
Thanks!
Anna M
Helsinki, Finland

[Steven M-M]

Post by Jim Dwyer
Car top your boat to a marked course to calibrate the SpeedCoach. That is
what I do although I only have to drive abot 20 minutes to get there. The
other alternative might be to take a GPS unit with you in the boat and row a
straight course 1000m long and compare.
Jim

Post by AnnaM
My club just purchased a brand new Filippi Italia/S which I think is
the F15 mould (70kg boat). I am the happy individual racing in it.
Ufortunately the NK impeller can not be installed at 5m from the bow in
this boat due to its measurements. My club is located at sea front and
there's no place to go to where one could row an exact distance and
check the calibration for the speedcoach impeller. Has anyone got a
calibrated speedcoach where the pick-up is located at about 4.70m from
the bow? I would be interested in the calibration value. I will
meanwhile try to locate someone with a GPS device...
Thanks!
Anna M
Helsinki, Finland

The good folks at NK said there is a 2-part impellor of the sort I was
suggesting above:
http://www.speedtech.com/asp/product.asp?recorprod=1&product=115&cat=5&ph=&keywords=&recor=&SearchFor=&PT_ID

The NK folks doubt that taking the little fin off would have much
effect on drag as the base that stays on the hull may cause as much
drag as their fin. But something to check out.

Steven M-M

[s***@in2rowing.com]

I hate to spam too bad - but we just released a new amplifier with GPS
built in. www.in2rowing.com that may work a bit better for you. It's
basically a CoxBox melded with a SpeedCoach then injected with some
additional features.

-Nathan

Post by AnnaM
My club just purchased a brand new Filippi Italia/S which I think is
the F15 mould (70kg boat). I am the happy individual racing in it.
Ufortunately the NK impeller can not be installed at 5m from the bow in
this boat due to its measurements. My club is located at sea front and
there's no place to go to where one could row an exact distance and
check the calibration for the speedcoach impeller. Has anyone got a
calibrated speedcoach where the pick-up is located at about 4.70m from
the bow? I would be interested in the calibration value. I will
meanwhile try to locate someone with a GPS device...
Thanks!
Anna M
Helsinki, Finland

[mpruscoe]

Post by s***@in2rowing.com
I hate to spam too bad - but we just released a new amplifier with GPS
built in. www.in2rowing.com that may work a bit better for you. It's
basically a CoxBox melded with a SpeedCoach then injected with some
additional features.

Not in a scull unfortunately. Not much use for moving water either.

[oarsman]

If you are rowing on salt water, that will have more an effect on
measured speed than impeller position. Water density plays a major
factor in the measured speed, so calibration on one body of water at
one temperature will not hold other conditions. If you want a accurate
measurement, you will need a gps in the boat.

[Carl Douglas]

Post by oarsman
If you are rowing on salt water, that will have more an effect on
measured speed than impeller position. Water density plays a major
factor in the measured speed, so calibration on one body of water at
one temperature will not hold other conditions. If you want a accurate
measurement, you will need a gps in the boat.

Rotational friction on the NK bearings is relatively low (that
rotational friction is _not_ the same thing as the overall drag of the
stick-on installation). In that case the rate of rotation of the
spinner should depend only on the stream velocity & the spiral pitch of
the spinner, whatever the fluid. Put another way, whether in air, water
(fresh or salt) or mercury, the device should give the same reading at
the same stream velocity.

That's why you should tape the spinner when car-topping the boat because
car velocities might be almost 10x those in water, without water's
cooling & lubricating qualities. I'd also be careful about sculling in
mercury.......

Cheers -
Carl

[sue t]

If you must scull in mercury, be sure you don't flip ...

I'd also be careful about sculling in mercury.......
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

[oarsman]

"Put another way, whether in air, water
(fresh or salt) or mercury, the device should give the same reading at
the same stream velocity."

This would say that the speed coach would register my walk down to the
dock, which is does not. Explaination?

[p***@hotmail.com]

Post by oarsman
"Put another way, whether in air, water
(fresh or salt) or mercury, the device should give the same reading at
the same stream velocity."
This would say that the speed coach would register my walk down to the
dock, which is does not. Explaination?

friction will cause the impeller to stay still until a sufficiently
large torque is exerted. Air at that speed doesn't. Go faster and it
would, though. When a sufficiently large torque is exerted, the
impeller will begin to spin and accelerate until the torque on the
impeller decreases to equal the friction (this happens very very
quickly); while the terminal speed _does_ depend on the density of the
fluid, it's only as a very small (exponentially small in density and
terminal speed) gap down from the terminal speed you'd expect if the
fluid were very dense.

in real terms, Carl is right except when the impeller moves slowly in a
fluid of low density, such as walking to the dock.

Pete

[AnnaM]

Thanks to all of you respondents, this thread became a rather
interesting discussion instead of "at 4.70m mine is calibrated at
1.064"-type thing. Great. I have organised to borrow a GPS device from
a colleague at work next week; now it remains to put the impeller in
place!

bestest
AnnaM

[Carl Douglas]

Post by p***@hotmail.com

Post by oarsman
"Put another way, whether in air, water
(fresh or salt) or mercury, the device should give the same reading at
the same stream velocity."
This would say that the speed coach would register my walk down to the
dock, which is does not. Explaination?

friction will cause the impeller to stay still until a sufficiently
large torque is exerted. Air at that speed doesn't. Go faster and it
would, though. When a sufficiently large torque is exerted, the
impeller will begin to spin and accelerate until the torque on the
impeller decreases to equal the friction (this happens very very
quickly); while the terminal speed _does_ depend on the density of the
fluid, it's only as a very small (exponentially small in density and
terminal speed) gap down from the terminal speed you'd expect if the
fluid were very dense.
in real terms, Carl is right except when the impeller moves slowly in a
fluid of low density, such as walking to the dock.
Pete

Yup. It's called stiction. To move against solid-solid friction
requires a certain critical force to be exceeded - the same slip-stick
process by which the rosin-coated violin bow hairs keeps the string
vibrating.

The torque available at any given fluid velocity is proportional to that
fluid's density. Air has 1/800 the density of water, so at low speeds
in air the available torque may not be enough to initiate motion.

Cheers -
Carl