-----Original Message----- From: John Winters <735769_at_ican.net> To: PaddleWise (E-mail) <PaddleWise_at_lists.intelenet.net> Date: Saturday, May 22, 1999 7:21 AM Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form" >. >>From a wave making standpoint in smooth water it does not matter which form >you use so long as the longitudinal center of buoyancy (LCB) lies within >the appropriate range (roughly 48 to 55% aft of forward waterline ending. >This surprises most people who look upon the waterline shape as cutting >through the water and the water being pushed to the side. The water >actually dives under the boat and travels roughly along the buttock lines. Why do you think it appears to do that? Are these hulls pushing hull speed and the trough that follows the bow wave is actually what is "diving under". It sure seems to me that the net effect is that the (incompressible) water had to be displaced to the side and upward first and then is falling back to below grade as the hull is passing through. >If you can get hold of Taylor's "Speed and Power of Ships" you can see >some fascinating drawings of the water flow around a wide variety of hull >forms. I don't have a copy but confirmed this with some other texts. Were any of the tests pictured done at very low speeds? I have trouble believing that the flow along the hulls surface will follow the buttock lines at low speeds. > >This problem with smooth versus rough water led to the debacle with the >English America's Cup challenger Sceptre. They tested only in smooth water >an the results looked great. Unfortunately they had to race in rough water >where the finer American boat that had more resistance in smooth water won. > >The location of the LCB does make a difference aft where the fuller stern >provides virtual lengthening and fills in the boundary layer aft. Like >anything, you can carry this too far and cause separation drag aft (see >above). > >Another difference has to due with performance in rough water. Finer ends >with flared topsides have less energy loss than fish form boats and also >tend to pitch less. I forgot to mention that one. I'll have to remember to add it to my list. I'd like to learn more about this before I make that claim though. Do you have a source that explains why there is less energy loss. Is it just the loss due to pitching more or is it also the greater pressure exerted on the blunter ends by the waves? > >One of the more interesting aspects of this has to do with "how" a boat has >its form. For instance, some boats have swede form waterlines but the LCB >still lies forward of amidships. The opposite can occur as well. Many >people look at the waterline and say "fish form" when in reality the boat >has a Swede form hull. Some of the Inuit Greeenlandic boats have this shape >I.E. fine waterlines forward coupled with a full underbody forward. All >very confusing but fun if you enjoy that kind of thing. Most people aren't looking at the waterline at all but at the seam line (top view) when deciding if a boat is Swedeform or fishform and that can be even more misleading than just looking at the waterline. For instance, our Escape looked way more Swedeform than it actually was because there was considerable flair behind the cockpit (to get more secondary stability and storage room) but at the same time the flare was reduced markedly to near vertical sides just in front of the cockpit (in order to make it narrow in the paddling area). It then flared out again further forward into a ship like bow. It looked radically Swedeform but it was only moderately so. Matt Broze www.marinerkayaks.com > >*************************************************************************** >PaddleWise Paddling Mailing List >Submissions: paddlewise_at_lists.intelenet.net >Subscriptions: paddlewise-request_at_lists.intelenet.net >Website: http://www.gasp-seakayak.net/paddlewise/ >*************************************************************************** *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
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