Wake Riding


by russ_watters
Tags: riding, wake
russ_watters
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#1
Aug31-06, 08:09 PM
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A friend of mine emailed me this question:
Wake riding: Can it be explained in plain English whether a lead boat
with 20 people paddling (average weight of the boat is 185) can be
slowed down by a boat following, with the same weight distribution, that
is riding the lead boat's wake? This is an ongoing debate among some of
the people I paddle with on the National Dragon Boat Team.
At first glance, I expected it to be easy, but I'm really not sure what the answer is!

Drafting in a car works because the rear car pushes a high pressure area in front of it, partially filling the low pressure area behind the lead car, reducing the drag for both cars. But do you have to be near directly behind for it to work? I'm thinking you do.

Canada Geese (and military aircraft) fly in v formation more for wingtip vortex reduction, iirc.

Boats on the surface work more like supersonic flow (which is what water tables are for), with little to no forward propagation of the pressure waves above a fairly low speed. So that makes me think that unless the boats are close enough for the trailing boat to push the little "pile" of water in front of it close to the lead boat, there will be no effect on the lead boat.

So a boat could ride the trough between waves or surf the bow wave, reducing its own drag while not affecting the lead boat at all. The biggest complication I can think of, though, is the effect of the propulsion. Power boats push water behind them, creating a high pressure zone, so if you sit behind a power boat, you'll be pushed backwards. A dragon boat is paddled, so you'd need to ride the wake far enough away that there is no adverse effect on your boat (or your boat's paddles). Either way, the trailing boat will not help the lead boat unless the trailing boat is so close horizontally that they are both riding each other's wakes.

I can't think of any situation anywhere where a trailer would actually slow down the leader.

Comments?
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Bystander
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Aug31-06, 08:31 PM
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That's gotta depend a lot on hull forms --- there might be a possibility of following interference for "cruiser" sterns, but don't go betting a whole lot of money on it.
Danger
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Aug31-06, 11:14 PM
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I can't see the rear boat having any effect on the lead one. While the accoustic effects will travel at the speed of sound in water, they wouldn't have much impact. Surface waves are far slower, and don't generally outpace the craft that's creating them. Also, they dissipate rapidly.

russ_watters
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Sep1-06, 05:47 AM
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Wake Riding


Right, the vast majority of the drag on a boat comes from surface waves because to outrun your own wake, your boat has to phyically lift itsef up onto its bow wave. And I don't think it is possible to alter the shape of that wave from behind.
Clausius2
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Sep1-06, 02:28 PM
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To be honest, I don't know the answer. It is true that the wake of the leading boat gets distorted because of the trailing boat. The question is: does the distortion of the wake of the leading boat have influence in its drag?. I'm pretty sure it has, but I don't know how to show it analytically. Russ, you are thinking as of pure supersonic flow, this time applying instead of the Mach cone, the Froud cone. Your thoughts are justified in the sense that the Shallow Water equations are hyperbolic in supercritical flow (Froude#>1). But pay attention because I am not sure that "all" perturbations are propagated downstream and so the wake plays little role. The energy contained by the kinematic motion of the waves of the wake is in fact the defect of energy caused by the drag. With that I mean that the vast drag energy is dissipated in the decayment of those waves in the trailing edge.

To sum up, I think there may be some ellipticity caused by internal gravity waves. With ellipticity I mean that the perturbations in the wake are also propagated upstream. Also, it seems you are thinking only in what happens just on the surface, whereas there is motion of fluid beneath the boats. In fact, the motion of deeper fluid is very important in ship dynamics, and the protuberance in the underwater nose of some large ships is put there for reducing the drag. The underwater dynamics of a ship generate internal gravity waves.

Sorry for not being too consistent here.
Danger
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Sep2-06, 12:07 AM
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Clausius; as always, it's a pleasure to read your posts... especially in areas like this where fluid dynamics are involved. In this case, however, I'm not sure that your analysis matches the original question. Russ was asking about sculls or similar rowed boats. I can't see that gravitational interactions would have much effect. We're not dealing with jet boats or aircraft carriers or anything else exotic. It's probably closer to the interactions among adjacent swimmers.
On the other hand, I'm drunk right now and you're the expert. You might have understood the OP better than I did. (Somehow, though, I think that's not the case. )
Clausius2
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Sep2-06, 12:21 AM
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Yeah, I think I'm gonna shut up here and wait for reading a little bit more about the topic. I brought some potential observations to the board though...
Danger
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Sep2-06, 12:47 AM
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Don't you dare shut up! If you do, I'll have nothing to read and have to go to bed.

Wait a sec... W is down there... I'm up here... W is female... I'm drunk... and horn...

Never mind. Shut up, Claudius.
Clausius2
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Sep2-06, 01:38 AM
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Quote Quote by Danger
Don't you dare shut up! If you do, I'll have nothing to read and have to go to bed.

Wait a sec... W is down there... I'm up here... W is female... I'm drunk... and horn...

Never mind. Shut up, Claudius.
Oh my gosh, even if all this is off-topic, I think it's funny.

Go to bed, and tomorrow morning burn the alcohol doing a bit of excercise.
Astronuc
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Sep2-06, 07:31 AM
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The rear boat riding on a wake would have little or no effect on the preceding boat. The wake is a consequence in the medium and simple reflects the momentun transferred from the lead boat to the water.

Only if the second boat could follow close enough to the lead boat to create an increase in pressure behind the lead boat, or cause a bow wave which would have to 'push' the lead boat, would the rear boat have an effect on the lead boat. I suspect a bow wave (from a rear boat) might actually retard the lead boat, since it would tend to drop the bow of the lead boat thus increasing the loss of momentum from the lead boat.

Canada Geese (and military aircraft) fly in v formation more for wingtip vortex reduction, iirc.
i believe that trailing geese or aircraft take advantage of the upwelling of the vortices created by the lead goose/aircraft, which means the trailers get a little extra lift, thus reducing energy requirements.

ENERGY SAVINGS IN FORMATION FLIGHT OF PINK-FOOTED GEESE pdf file.

Precision and Dynamics of Positioning by Canada Geese Flying in Formation
http://jeb.biologists.org/cgi/conten...ract/128/1/445
ray b
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#11
Sep2-06, 09:41 AM
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true trailing boat [bow behind the stern of the leader] I see no effect
on the lead boat and gain for the trailing boat
BUT a trailing boat with overlap [bow ahead of leader stern]
then you get a much more complex wave interaction
esp if the trailing bow is close abeam and ahead of the mid point of the leader

a tank test could get drag sweet spots data for wave interfearance paterns
Danger
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#12
Sep3-06, 10:04 AM
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Clausius, my apologies. I just realized now that I mis-spelled your name in that previous post, and it's too late to edit it. Blame the beer.


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