Question about wave making resistance.

Discussion in 'Boat Design' started by river runner, Sep 25, 2011.

  1. river runner
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    river runner baker

    If you were paddlig a canoe accross a lake, you'd want it to be Swede form bacause of wave making resistance. I was wondering what effect current has, if any. If you were traveling upstream at zero knots in a four knot current, would there be the same wave making resistance as traveling at four knots on a lake? And if you were traveling downstream at eight knots in a four knot current, again, would there be the same wave making resistance as traveling at four knots on a lake?
     
  2. philSweet
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    philSweet Senior Member

    The short answer is yes, same wave resistance, all else being equal, which it never is, which is why lake and river canoes tend to have different shapes and features. Hope that helps. :D
     
  3. messabout
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    messabout Senior Member

    Please explain why it needs to be swedeform because of wave making. Is that a better shape than fishform or symmetric parabolic waterlines in terms of wavemaking?
     
  4. DMacPherson
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    DMacPherson Senior Member

    Not to put too fine a point on it, but the wave-making resistance is the same only if you omit shallow water (channel) effects. For example, while you may be running downstream at 4 kts with a 4 kt current, you still are running 8 kts against the channel, versus zero kts the other way - and the "blockage" effects on wave-making drag will be different between the two directions. You can visualize this by changing reference. Keep the vessel still and move the channel bottom. You will have fluid shearing from the channel in one direction and none in the other. The magnitude of the differences would depend on the channel and vessel, but with reasonable canoes and waterways I would suspect that it could be significant, certainly measurable.
     
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  5. rxcomposite
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    rxcomposite Senior Member

    I would say yes because the movement is relative. In a tank test, a model is not making any wave while at zero speed and an airfoil in a wind tunnel is not making any lift with no wind velocity. But move any of the medium and you get result, IE, move the boat in the tank or move the air in the tunnel.
     
  6. gonzo
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    gonzo Senior Member

    It depends on the depth. A shallow river or an obstruction will create turbulence and standing waves which change the dynamics.
     
  7. jehardiman
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    jehardiman Senior Member

    One needs to be very careful here. Effectively, the resistance would be equivalent as long as the relative speed through the water is the same. There are some Renyolds and energy effect differences but those are down in the weeds. And the water depth issues are also dependent on relative velocity.

    So for 0 speed over ground against a 4 knot current, relative speed 4 knots; same as 8 knots over ground with a 4 knot current or 4 knots in still water. In an absolute sense there is a slight difference in resistance due to celerity propagation effects, but most likely immeasurable in the real world.
     
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  8. river runner
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    river runner baker

    Hmmm. I guess the root of my question is, if I wanted to paddle a canoe (or anything else) upstream, would I want Swede form? And I guess the new question is, would that change with water depth? If the water was really shallow, would a swede form hull stop being more efficient?
    Regarding messabouts question, all racing canoes, kayaks, shells, balast keel sailboats etc. are Swede form. Swede form has been proven to me more efficient in a displacement hull. Add enough horsepower and everything changes.
     
  9. philSweet
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    philSweet Senior Member

    Shallow water tends to increase the proportion of wave resistance to skin resistance at a given speed. So the question becomes- what governs the swede/fishfrom transition, skin friction or wave making (aero considerations also play a part in the real world, if not in the test tank world.), and I don't think we know the answer to this yet. If you've got enough water to paddle properly, the shallow water effect is quite small for a 16' canoe at four knots, say the wave pattern on the hull would be equivalent to a deep water wave pattern on the same hull travelling about 5% faster. But the skin friction would be less, so the crossover would be only a couple percent speed difference at equal total hull resistance. The preceding based on 1 meter depth. Sorry for the mixed units. None of the issues you have raised are likely to affect the hull choice for a canoe at four knots. River turbulence, windage/weathervaning, and maneuverability are more important.
     
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  10. messabout
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    messabout Senior Member

    River Runner; I disagree that "all" the racing kayaks and such use the swedeform layout. Some very quick human powered boats and extremely fast multi and monohull sailboats do not use that planform at all. Take a good look at a modern K1 or K2 or an International 10 meter canoe, a Tornado, even a Hobie 16.

    I am most inclined to accept Leo's parabolic planform equation as the probable optimum for skinny boats.
     
  11. Easy Rider
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    Easy Rider Senior Member

    I always look at it this way. If I had a symmetrical hull in motion a bit below hull speed which end of the boat would produce the greatest resistance? I feel quite sure the fwd end does and that tells me the fwd end needs to be optimized. By changing to a swedform hull w lower PC and entry angle fwd the resistance is reduced if going fast enough to make wave making the dominant element of resistance. So I agree that swedeform hulls are probably faster. However at slower speeds where skin friction is the dominant resistance I suspect that the fish form may be faster or rather requiring less power. And at some point in between the symmetrical form wins. Since kayaks and canoes usually travel much slower than hull speed generally I suspect the fish form is a better choice as far as resistance is concerned. Just say'in.
    Easy Rider
     
  12. DCockey
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    DCockey Senior Member

    Speed depends on the paddler and the boat. Some paddlers like to paddle close to "hull speed", at least for short bursts.
     
  13. river runner
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    river runner baker

    I have never seen a K-1 or K-2 that was not swede form. If anyone can find a modern racing kayak that is fish form, please post it here, I'd like to see it.
    I tend to agree with Easy Rider, that fish form may be more efficient at slow speed, but swede form is more and more efficient the closer to hull speed you get.
    All racing canoes and kayaks I have seen are indeed swede form and not fish form. Swede form wins the races, so I would have to conclude that the designers of these boats know what they are doing.
     

  14. portacruise
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    portacruise Senior Member

    Human pedaled or paddled hydrofoils hold some short distance records, but I don't know if these are considered swede after liftoff.

    P.
     
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