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 Post subject: Bottom wave effect
PostPosted: August 28th, 2006, 5:41 pm 
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Joined: June 20th, 2001, 7:00 pm
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While cruising on my latest escapade I noticed that in some shallow water the boat seemed to be slow. (this wasnt the first time I have noticed it but I gave it a couple of hours thought this time)

I do know that there is a wave generated by the hull that travels downwards.

What is the relationship between hull speed and water depth? Does the length of the boat have any effect?

Please be gentle John. While I enjoy math it sometimes takes a little time for me to catch onto complicated relationships.


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PostPosted: August 28th, 2006, 6:02 pm 
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Joined: July 16th, 2006, 8:59 pm
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Location: Now in Sudbury
I too have noticed this paddling up a swiftly moving stream. It is especially true when I can only get a couple of inches of paddle into the water. Coincidence? I think not!


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PostPosted: August 28th, 2006, 6:40 pm 
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Hi Kim,
Until John jumps in you may find your answer HERE
Glad you asked the question as I never did know why this was happening.
GG


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PostPosted: August 29th, 2006, 6:52 am 
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Shallow water effects are the result of waves created by the boat and the wave interaction with the bottom. It is common to believe there is a wave generated downward from the hull (as Kim mentioned) but this is not the case. Not sure where the idea originated. All waves created by the canoe are surface waves. There is, however, an interaction between the hull and bottom which is the Bernoulli effect where the water velocity increases in the constricted space between the hull and bottom causing a drop in pressure resulting in the boat sinking slightly lower in the water. This is the law of conservation of energy at work. This, however, is not what causes the resistance increase that paddlers notice.

Wave speed in deep water is a function of wave length but, in shallow water, (wave length longer than the water depth) depth governs wave speed and, consequently wave length. When a fast-moving hull encounters shallow water, the waves created by the hull grow steeper and higher and require more energy to maintain for a given speed. This is the increase in resistance paddlers feel when paddling abruptly from deep to shallow water. The formula for the critical depth is V = the square root of g x D , where D = the water depth, V = the velocity and g = the acceleration due to gravity. The increase in resistance as a boat enters shallow water can reach 150% according to the Italian scientist Rota who researched shallow water effects many years ago. With greater speed the wave changes to one lying roughly normal to the direction of travel with all the wave making energy concentrated in a single wave traveling at the same speed as the boat (often called a wave of translation). With even more power, the wave pattern changes to one of convex diagonal waves (when looking at them from above) rather than the common concave "V" created in deep water. At this point resistance begins to fall and can reach levels slightly below that in deep water. The difference is dramatic enough that marathon paddlers believe their boats jump over their bow waves and begin traveling downhill. In reality the wave-making phenomenon has altered and the production of energy-draining transverse waves ceases to be replaced by a diagonal wave system that drains away less energy. This lacks the drama of boats leaping over their bow waves but it avoids the difficulty of having to explain how the boat travels down the front of a wave it has yet to create.

Most of the time we can feel the boat slow down in shallow water but if you can get going fast enough while in deep water and then hit the shallow water at high speed you might be able to feel the speed increase. Hard to maintain it though.

In the literature about the fur trade the traders reported a "suction" in some rivers. This was the Bernouli effect at work (as well as wave resistance) and not usction but rather a reduction in pressure.

I once delivered a 54' power boat across the Inland waterway in Florida. In some places the boat would actually roll over on its side if we were going too fast due to the proximity to the bottom and the shape of the hull (Probably too much deadrise) It was a scary experience the first time it happened.

Hope that is helpful and not to techie.

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Cheers,

John Winters


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PostPosted: August 29th, 2006, 10:35 am 
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Thanks John, that was just right.

Our "friend" Bernouilli is certainly active therefore and anyone canoeing is likely to encounter him. I never thought of the BE under the canoe.

I especially curse him on windy days trying to paddle between constrictions of land, such as islands.


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