How Moving Water Affects Your SpeedCoach
Peter King
Says that moving water has no effect on a speedcoach. Whilst I
understand the theory, I don't believe this.
Paddling upstream at Molesey, I average 2:20-2:25, when I turn and
head downstream, my average is 2:10-2:15 for steady state UT2. My
heartrate is the same both ways about 165.
Can someone enlighten me?
Pete
Luke Shepherd
> Says that moving water has no effect on a speedcoach. Whilst I
> understand the theory, I don't believe this.
have you considered wind resistance in this situation: when going with the
flow, your "airspeed" will be higher when there is little or no wind, and
against the flow it will be lower and thus drag from the wind will also be
lower all other factors being constant
might just make the difference
luke
Rob Collings
water speed doesn't. And shouldn't. Maybe there is a wind that you don't
notice too much?
Rob.
Ewoud Dronkert
> Paddling upstream at Molesey, I average 2:20-2:25, when I turn and
> head downstream, my average is 2:10-2:15 for steady state UT2.
Think about it; the impeller is in the water, not in any contact with the
land. So all it really does is measure speed relative to the water.
I can't think of any reasonable explanation for your observation. Maybe you
are happy to go home?
Carl Douglas
Peter -
Moving water should have no effect, but:
1. having the bank go past slower, & knowing we have longer to go
against the stream, may psychologically affect how we perform on the
upstream leg
2. there could have been a wind resistance effect (upstream at Molesey
is to the west) which you might not have taken into account?
3. but, most importantly, it is harder to row upstream on a relatively
narrow river because the flow shear situation makes sculling more
difficult.
To briefly expand on the last point:
I presume you were not in the middle of the river going upstream? So,
if there was much stream, then the blade nearest the bank was in water
flowing downstream significantly slower than the blade nearer the mid-
stream. That meant that, unless you worked harder on the mid-stream
blade, you'd drift into the main stream.
I am sure that it is more physically demanding to maintain a given speed
under these asymmetric circumstances (you actually will do more work)
than in the reverse situation where you return in mid-stream (before
collecting that errant quad scull, that is!) with both blades in stream
flowing at equal velocity.
To underline the relative difficulty of sculling upstream on a narrowish
river, I'd note that the velocity profile across a flowing river is such
that there is a lot of shear (change in flow speed as you move across
the river) near the bank but little shear near the centre. So the with-
stream sculler's blades are in water with velocity similar to that in
which the boat runs, while the against-stream sculler has a large
difference in water flow velocity between both blades & between each
blade & the boat.
Cheers -
Carl
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JZ, UK
Email: ca...@carldouglas.co.uk Tel: +44(0)1784-456344 Fax: -466550
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
Steven Maynard-Moody
is a difference in head wind, I find it easier to keep a lower split going
upstream than down. Often going down stream the boat against the bank is
going much faster (I cover the same ground in less time) but my splits are
higher.
Oh, to be young and do steady state rows at HR 165 bpm!
Steven M-M
"Peter King" <peter...@uk.pwcglobal.com> wrote in message
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Henry Braun
`head downstream, my average is 2:10-2:15 for steady state UT2. My
`heartrate is the same both ways about 165.
`
`Can someone enlighten me?
Upstream near the bank there's a very large difference in the speed of the
stream past your two blades, which means that one of them at least isn't
going to be moving the boat efficiently if you pull them through at the
same speed. I couldn't tell you which, though.
Downstream in the middle there's much less difference in the stream across
your boat, so your blades can both work equally, and more of your effort
goes into moving the shell not the water.
Richard Packer
Except that the river at Molesey's not tidal, so you shouldn't be in
the middle going downstream. Upstream and downstream you should be on
the right, so the bowside (starboard) blade will always tend to be in
the slower water nearer the bank, with the strokeside (port) blade in
quicker water nearer the centre of the river.
Richard
Henry Braun
`Except that the river at Molesey's not tidal, so you shouldn't be in
`the middle going downstream. Upstream and downstream you should be on
`the right, so the bowside (starboard) blade will always tend to be in
`the slower water nearer the bank, with the strokeside (port) blade in
`quicker water nearer the centre of the river.
Drat. Good explanation, wrong problem. Have you thought about moving to
Putney?
On the very wiggly river above Oxford this summer---so with only the
tiniest current flowing---it was noticeably less work to scull downstream
than up, because upstream even a small amount of current pushes one's bows
towards the outside bank on every corner, while downstream it does the
steering for you.
Anu Dudhia
faster downstream.
http://www.atm.ox.ac.uk/rowing/stream.html
marc gwadz
"downhill"? I mean this literally. That, rowing upstream one can also be
forcing the boat to gain altitude, while heading downstream you lose altitude,
and therefore gain speed relative to the current.
I think this notion is not as silly as it may sound. A more pronounced effect
in whitewater kayaking clearly demonstrates that the it is not merely the
speed of the stream that determines boat speed through the water, but also the
angle of the water surface relative to the boat. kayakers will often "surf" a
wave by riding down the face of a glassy wave and get the forces of the
stream and gravity to balance out. it is fairly easy to maintain ones
position, often even without paddling, in a current that would otherwise wash
you quickly downstream, unless you were to paddle all-out.
perhaps a more familiar situation for rowers would be rowing in large wakes or
long wavelength tidal waves, in which the boat alternates surging and drooping
in speed depending on which side of the wave you are on.
rowing up a flat river the effects may be a lot more subtle. but perhaps the
inches or feet one gains in altitude during a leg of a row make a difference,
not only in current speed but in storing potential energy.
Anu Dudhia
Actually, thinking about it a bit more, perhaps it does explain why an
impeller mounted under the hull *would* indicate a faster velocity
downstream (bigger vertical shear), although it's all a bit hand-wavy.
Marc Monplaisir
due
to tide, and the difference of 5-10 seconds sounds reasonable at
steady-state based on unscientific observations.
But I also agree with the other comments that factors such as
wind and how currents flow in the channel make a much greater
difference, and
that "all things being equal" comparisons seldom apply in actual rowing
conditions.
Since I row on a tidal estuary off of San Francisco Bay, this is a
subject of interest.
Here's a two-bit theory. All of you hydrodynamicists, feel free to poke
holes.
The boat does not move at a constant speed relative to the water, but
rather has an acceleration/deceleration curve tied to the stroke.
I find it hard to believe that the curve moving against the tide would
be exactly
the same as moving with the tide. Even if the maximum speed reached
relative to the water
during the stroke were the same, wouldn't the peaks be a little sharper,
and the
valleys a little flatter, going against the tide, therefore giving the
SpeedCoach a different
reading? Would this effect also be more exaggerated at lower cadences?
Two scenarios as an analogy:
1. I'm in a car driving 50 MPH with a 10 MPH tailwind, relative speed =
40 MPH.
2. I'm in a car driving 30 MPH into a 10 MPH headwind, relative speed =
40 MPH.
If I suddenly take my foot off of the pedal, won't the car in #2 lose a
greater % of its speed faster, since it has less momentum? Or perhaps
my physics are flawed....
I guess another question would be, has anyone found that the
SpeedCoach measurements ARE in fact consistent regardless of water flow?
Thanks,
Marc
--
Marc Monplaisir
707-765-4653 home office
707-885-0108 fax
Marc Monplaisir
Thanks,
Marc Monplaisir
Rob Collings
>
<snip>
> I guess another question would be, has anyone found that the
> SpeedCoach measurements ARE in fact consistent regardless of water flow?
>
> Thanks,
> Marc
I have. We are only on a small river (non-tidal) and I seem to pull the
same splits in each direction on the 2 days each year that there is
little wind. I've definately notice the shear effects that are being
discussed - or maybe I just can't steer...
Rob.
DanDaRower
If your oars are on the water they'll slow you down more against the
current, expesially when it rough
Jim Dwyer
shear comes into play either. The boat is only a few centimeters in the
water so the shear should be neglible unless there is a very strong wind
blowing against the current. Rowing close to shore will affect steering due
to a difference in the current on either side of the boat.
No current would be like rowing in a huge swimming pool that is not moving
relative to the ground. Rowig upstream would be like pulling the swimming
pool and all of the water it contains in on direction while you row in the
other. Your speed coach will read the same in both cases and also if you
row with the current. Wind will change your splits.
Jim
"Peter King" <peter...@uk.pwcglobal.com> wrote in message
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CaptStash
"Marc Monplaisir" <mar...@yahoo.com> wrote in message
news:3BE989A5...@yahoo.com...
>
> Here's a two-bit theory. All of you hydrodynamicists, feel free to poke
> holes.
>
> The boat does not move at a constant speed relative to the water, but
> rather has an acceleration/deceleration curve tied to the stroke.
> I find it hard to believe that the curve moving against the tide would
> be exactly the same as moving with the tide. Even if the maximum speed
> reached relative to the water during the stroke were the same, wouldn't
> the peaks be a little sharper, and the valleys a little flatter, going
> against the tide, therefore giving the
> SpeedCoach a different reading? Would this effect also be more
> exaggerated at lower cadences?
>
> Two scenarios as an analogy:
> 1. I'm in a car driving 50 MPH with a 10 MPH tailwind, relative speed =
> 40 MPH.
> 2. I'm in a car driving 30 MPH into a 10 MPH headwind, relative speed =
> 40 MPH.
>
> If I suddenly take my foot off of the pedal, won't the car in #2 lose a
> greater % of its speed faster, since it has less momentum? Or perhaps
> my physics are flawed....
>
>
On a very still day (no wind):
1. You are in a car driving 50 mph over the roadway. The roadway itself is
moving in the same direction as you (see Heinlein "The Roads Must Roll" ) at
10 mph. Speed over the ground = 60 mph. Relative wind = 60 mph from dead
ahead.
2. You are in a car driving 50 mph over the roadway. The roadway itself is
moving in the opposite direction as you at 10 mph. Speed over the ground =
40 mph. Relative wind = 40 mph from dead ahead.
The car in scenario No. 1 will require more horsepower than that in No.2 for
a given speed due to the apparent wind. In other words you theoretically
need to row harder in a following current for the same speed through the
water as rowing upstream. To better understqnd this take it to the extreme.
Imagine rowing into a current so strong it matches your speed through the
water. You will stand still relaitve to the ground and the air, thus
encountering no air resistance.
Sounds like there is something else also going on that you are not taking
into account; i.e., prevailing wind or other hydrodynamic effect.
CaptStash....