The theoretical speed of a vessel in knots through water can be generally calculated as 1.34 x SqRt of the length of the water line. Therefore, if a 17 foot (length on deck) kayak has a water line length of 16 feet, then the theoretical maximum speed is 5.36 knots. This assumes a displacement hull condition, that is that the vessel is not planing on top of the water, and that there is enough forward thrust available to achieve "maximum hull speed". Here's a very simple explanation of why this is so. As the kayak moves through the water it creates a bow wave. The faster it goes, the higher the bow wave. The higher the bow wave, the longer the wave length (the distance from one wave top to the next wave top. Behind that bow wave peak is a trough. When the vessel achieves a speed such that the trough of the bow wave occurs at or near the stern of the vessel drag increases dramatically. I know that others with a better grounding in the theory will probably disagree with this interpretation of the effect, but it has always seemed to me that when the stern sinks into the trough of the bow wave, the vessel is, in effect, constantly going up hill. The only way to overcome this effect is to increase forward thrust sufficiently to cause the vessel to plane. That's why a 30 foot sailboat is able to achieve hull speed with a modest 15 to 25 horsepower engine. And, why a ski boat of half that size is equipped with a monster v-8 engine. Now others can jump in with the exactly correct information. Incidentally, according to the formula above, your mile long kayak would be capable of achieving a speed of 97.4 knots given sufficient thrust. I leave it to you to determine the correct paddle length and the size and physical condition of the paddler. Steve King PaddleWise wrote: > > PaddleWise Sunday, May 13 2001 Volume 01 : Number 1509 ------------- > > Date: Sat, 12 May 2001 12:53:05 -0700 > From: "Evan Dallas" <Evan_Dallas_at_notes.ntrs.com> > Subject: [Paddlewise] Kayak speed vs length > > I've often heard (and intuitively accept) that all other factors being equal, a > longer kayak will be faster than a shorter one. I've got a friend who disagrees > with this, but I've had a hard time explaining why this would be true. I would > speculate (fancy word for "guess") the following possible reasons: > > 1. Paddling on the ocean, there are generally little (or big, for that matter) > waves and chop that push against the kayak from different angles. The longer > the kayak, the more these "local" disturbances would tend to cancel out and thus > interfere less with the boat's movement through the water. If this is true, > then would this advantage disappear when paddling in very smooth, flat water? > Or would the longer kayak continue to be faster? > > 2. A longer kayak has more floatation than a shorter boat (again assuming other > factors are equal), so it would float a little higher in the water, thus > reducing drag. I assume that most of the friction comes from the movement of > the bow cutting through the water and that the additional length contributes > very little additional drag. (I expect I'm going out on a limb on this one, > having once gazed at Matt's speadsheet on this subject (phew!)) I know enough to > know it's ultimately more complicated than that). > > My friend countered my comments by saying if that were true, then paddling a > boat a mile long would be faster than a 17 footer. Obviously a REALLY long boat > like that would weigh a lot and thus have a lot of inertia to overcome getting > it moving, but once up to speed -- would it be faster than the 17 footer? Or > perhaps another way to ask it: would it require less paddle effort to maintain > a given speed than a 17 footer?? What if it were so loaded with weight that it > floated the same height in the water as the shorter kayak -- would it still be > faster? > > Lastly, is there a simplified formula for estimating the speed, given a certain > beam and length (holding other factors constant)? > > Evan Dallas > Woodinville, WA. > *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I always get a little nervous when the talk of "speed" comes up regarding kayaks. Quite honestly I think hull speed is nowhere as important as efficiency. I want to know how much effort is it going to take me to maintain a constant speed, whether it's the maximum or not. I truly believe that most sea kayaks today are going to have close enough maximum hull speeds, that MOST people will not notice a difference. Obviously I'm not talking about comparing a Greenlander Pro with a Wilderness Systems Pungo. Whenever people ask me about speeds I always counter with, they'll both go plenty fast, but which one would you rather keep at this speed for a long period of time? It all comes down to effiiciency as far as I'm concerned. Happy paddling. Mark Mastalski At 09:12 AM 5/14/01 , Stephen King wrote: >Obviously a REALLY long boat > > like that would weigh a lot and thus have a lot of inertia to overcome > getting > > it moving, but once up to speed -- would it be faster than the 17 > footer? Or > > perhaps another way to ask it: would it require less paddle effort to > maintain > > a given speed than a 17 footer?? What if it were so loaded with weight > that it > > floated the same height in the water as the shorter kayak -- would it > still be > > faster? *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Hi all, The hull speed formula is an interseting one to determine the maximum speed for displacement hulls. But keep in mind it is only an approximation. It is a good one for not to slender hulls with a pronounces bow and stern, like a lot of sailing yachts. It is a much less good approximation for slender hulls. Hulls with a length to beam ratio (at waterline) above 8 can go faster in displacement mode than calculated by the formula. Another point is that the for-aft distribution of displacement (prismatic coefficient) changes the hull speed of the boat. Boats with very fine ends have a lower hull speed that boats with the same water line length and fuller ends. This is becuase the fuller ends increase the distance between the bow and stern wave. So just mounting two feet of razor blade at the bow and stern of your boat increases the water line length, but not the hull speed. It does affect handling, however.... And like mentioned by others, at speeds below hull speed the skin friction is more important that just the water line length. For people that are interested in this stuff I can recomend Marchaij's "The aero and hydrodynamics of sailing". It is a kind of bible for the design of sailing craft and contains a lot of interesting information on hull design. Greetings, Merijn Wijnen ****************************** Merijn Wijnen Vinkenhofje 8 5613 CN Eindhoven The Netherlands Tel.: 040-2939991 (job: 040-2650539) Fax: same as tel., call before sending or try twice E-mail: Home: merijn_at_music.demon.nl Job: m.wijnen_at_ind.tno.nl Web-site: http://www.music.demon.nl *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Yes, but is it a EUROPEAN kayak, or an AFRICAN kayak? .ashton, who had to. On Mon, 14 May 2001, Stephen King wrote: > Date: Mon, 14 May 2001 09:12:46 -0500 > From: Stephen King <steveking2000_at_home.com> > To: PaddleWise_at_paddlewise.net > > The theoretical speed of a vessel in knots through water can be > generally calculated as 1.34 x SqRt of the length of the water line. > Therefore, if a 17 foot (length on deck) kayak has a water line length > of 16 feet, then the theoretical maximum speed is 5.36 knots. This [snipped] *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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