-----Original Message----- From: Matt Broze <mkayaks_at_oz.net> To: 'Paddlewise' <paddlewise_at_lists.intelenet.net> Date: May 28, 1999 2:28 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" Was going to add this to the other post but it looked a bit cumbersome. (SNIP) >Why I said the water might be going back and forth (as well as up and down >and forward and back) is that if the hull is flared outward (as most that >don't look like a plow are) the hull would be in the way of pure up and down >motion of the individual water molecules. It would force them to the side as >the crest passes through them and they would fall back next to the hull with >the next trough. To see this more clearly (my way) imagine a long flared >boat with many crests along its side at the same time. Sorry, Can't see it. This might help explain why I can't see it. Those who have paddled whitewater know that the waves in a chute stand still while the water moves past the bottom. In the immediate area of the boat (by immediate I mean all the area affected by the boats passage until the waves leave the hull to form the wave train) the water acts as if it were flowing over the bottom (upside down as it were). Waves at the point of creation by a moving solid object or in flowing water do not act like free waves in open water. The rotational motion comes after the wave creation not integral to it. (SNIP) > >Perhaps we have a different definition of hull speed. I see it as the speed >at which massive increases in power create little additional speed due to >the limitations on the speed of a wave. If the speed a wave is able to >travel is reduced then by my definition the speed that a hull is able to >easily move is also reduced accordingly for the same reasons that "hull >speed" exists in the first place. William Froude invented the term "hull speed" to describe the approximate point where the bow and stern waves coincide (approximately Froude Number 0.40). For many boats (most sea kayaks included ) the "hull speed" does not represent a point where "massive" increases in power are required for greater speed. In some cases resistance may increase rapidly (kind of a relative term) at hull speed (typically for hulls with displacement length ratios in excess of 100) but in others one hardly notices it. Sprint boat paddlers can't even tell when they passed it on the way to Froude number 0.60 or more. Like the sound barrier, "hull speed" is not a barrier. How much power one needs to exceed "hull speed" depends upon the size of the waves the boat creates. >What you just described sure sound to me to me to be very similar to whate >happens to a planing boat. No, but it may look that way. If we placed a tripod on the boat with a target over the CG and a level on shore and then tracked the boat through the shallows we would see that the CG need not rise to enter the region of increased speed and reduced resistance. Planing involves the rise of the CG where the displaced water is reduced. This need not happen with the shallow water phenomenon. This does not mean it is impossible to plane in shallow water. Ernie Tuck did some neat studies of what he called surf skimmers and properly shaped hulls can and do plane in shallow water. So far I have not seen any proof that anyone could get a kayak or canoe to plane (see below on the definition). (SNIP) >I'm not so sure that the fast canoe doesn't plane some since it is in a >regime that may make exceeding the speed of the wave possible. May depend on >our definition of planing. I like to use Savitsky's definition since he seems to have lots of credentials in this respect. He defines planing as the point at which wave making resistance begins to decrease due to the vertical rise of the CG resulting from dynamic forces resulting from the vessel's own power. The beauty of this definition derives from its ability to differentiate between planing, shallow water effects and surfing where the forces and phenomenon differ. Obviously we can use any definition of planing we want but it would be nice if we followed the lead of those who have done the most research on the topic. (SNIP) >Another question: Is there anything happening in this shallow water regime >that is akin to "ground effect" (the reason Pelicans can glide for so long >with their wingtips just above the waters surface)? I don't know about ground effects but in shallow water some boats and ships do weird things. I once delivered a 54' trawler across the Inland waterway in Florida. At just a few knots the damned thing would roll over on its side and round up usually onto a mud bank. Ran aground several times when we did not react quickly enough. The owner really freaked out. >I have no idea what you are trying to say here but I am very familiar with >the Bernoulli effect thank you. I'm wondering if you are making it the cause >of wavemaking when I see it as the effect. No, I am not making a cause for it being a cause of wavemaking. Just wondered if you had failed to consider its effects in the discussed context. >>John said: >>No one climbs their bow wave. It is a bit like lifting yourself by your >>bootstraps. > >We can try to climb it but we will just waste a lot of energy in the attempt >because we lack the horsepower. That is what outboard motors are for. (see >last post for my take on this) No one climbs it even with all the horsepower in the world. Sorry to repeat myself but as the dynamic forces increase the CG lifts vertically and the bow wave diminishes in size. We don't climb it when we plane it just goes away. >> >John responded: >>The water need not go down (or upwards) if it increases its velocity. As >>Nick and I have pointed out the water eventually does rise but it is pushed >>out of the way by the water passing under the boat (verified by tank >>testing and mathematical methods), > >When I throw a rock in the water it displaces some of the water upwards and >then gravity brings it back down but it overshoots equilibrium. Are you >telling me that the trough along side of a boat moving along at near "hull >speed" is caused by the Bernoulli effect rather than normal wave >propagation? No. (SNIP) >in order to get from front to back Seems to me to be the other way around. The orbital motion of >the wave (that has already been created and would be there regardless of >Bernoulli) could be the cause of the Bernoulli effect on the hull. See earlier discussion on waves and wave making of the hull. >Check out the picture on page 279 of "Aero-Hydrodynamics of Sailing". It is >enough to make me wonder what is happening there. There are possible >confoundings due to the keel and I don't know the speed length ratio it was >taken at but it seems to support my theory. I will check it out when I can get the book (we live a long ways from a library large enough to have this kind of thing) but before assuming it supports your theory you might also want to check out Taylor, Inui, and others whose theories seem to disagree with yours. I wish I had some theories of my own on this topic but I just did not get the chance to spend enough time working with this kind of thing. As a result I have had to rely upon those people we commonly call "experts" who have done research and have published their findings for peer review. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:32:59 PDT