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Kayak and Canoe Design Bulletin Board
Re: Design Water Line
Posted By: Kris Buttermore In Response To: Re: Design Water Line *LINK* (RiverRaider)
Date: Monday, 29 January 2007, at 10:24 a.m.
In general, people that create these hypotheticals (within somewhat standard available kayak length ranges) will arrive - through roundabout methods, twisted math, and generally poor science - at the same rather obvious conclusions:
1. Length adds friction drag (long thin tube has more surface area than a sphere of same volume).
2. Longer hulls create longer wave system allow less wave making drag at higher speeds (to anyone that understand "Hull Speed" is not a limit and is simply about wave length gets it automatically, everyone else needs to read up or leave these sort of discussions and go paddle more).
3. Attaining the speeds to take advantage of the longer hulls wake making advantage depends on the paddler (uses/fitness/ability).
4. With sea kayaks the transition point where wave drag becomes the larger factor is often around 4-4.5 knots. Above that, length matters, below that it does not benefit as wave drag isn't the main factor. Wetted surface/skin friction is - but at less than 4 knots the differences are generally too small to care about. They're all easy, so there's not really much of a penalty either.
5. None of this matters to 90% of paddlers - who do more like 2-3 knots for a few miles, with lots of stop and go fishing/socializing/whatever and call it paddling.
For people that regularly cruise at 4-4.5 knots or above over longer distances it matters very much. For everyone else - WHO CARES? Ironically, "everyone else" usually includes the people who engage in debates about the efficiency of ANY hull under 17'! *L* At 3 knots it's true a 14' kayak might have less drag - but a 17' kayak is so easy to paddle at 3 knots who cares! It's a useless advantage, arguable on paper only. There are valid reasons to use shorter kayaks - but this sort of efficiency just isn't one of them.
I'm no racer, but I do paddle over 4 knots regularly (Less that 4.3 average over distance I feel I'm slacking), and so have a definite preference for longer narrower hulls. A little longer than I can really take advantage of in fact. In this I may have some small drag penalty under 4 knots - but at lower speeds the effort level is low and differences minor. I'll gladly take that (don't notice it anyway) to keep easier fast cruise, higher sprint potential (safety), and a boat that encourage me to get fitter/faster rather than penalizing me when I keep hitting the wall. At my normal speeds, I'm no where near the wall - at my sprint speeds I am - but it's still not the hull's wall - it's my own.
People need to be more concerned with their own fitness and what they want to do with a kayak before worrying about numbers. Since most kayaks fall within the zone most paddlers operate in most of the time - it make more sense to focus on the users needs and abilities - because outside of that context it's all moot.
Any hypothetical will not be realistic. It's very rare to find shorter kayaks with same beam as longer ones, and even if you did you'd sink it lower. At same beam a shorter hull is also less stable (smaller waterplane area).
Total displacement (no matter what the shape, as you still have to move a certain amount of water) and paddler (the one doing the moving) matter more than any of this by a huge margin.
Kris
: Gentlemen,
: Capt. Jimbo is evidently going to continue to blow smoke up your collective
: butts as long as he can, so allow me to intervene.: The motivation for Jimbo's initial question was specious, he knows the
: answer.. he's been haunting a few of the other design boards as well,
: looking for ammunition to fuel the debate that has raged over at KFS for
: waaaaay to long now. (link Included): Instead of just asking a straight question.. and telling you why he's asking,
: he's gonna play dumb and jerk everyone around?: Jimbo's not wrong per se, but he's employing the Straw Man for certain.
: The original post over at KFS dealt with hypotheticals, it was meant to start
: a discusion about (obviuosly) boat length. I wanted to look at just how
: much difference there was in resistance as you moved up through the common
: sizes of rec. boat's, and I wanted to keep some things constant, (I was
: specificaly interested in Cp for example) so the boats were all modled
: after one another, and the Cp remained constant, which i thought was a
: nifty way to illustrate how numbers changed: if the paddlers weights had been constant, then I would have had different Cp
: specs.. not my point at all.: I said from the outset of that post that the displacement numbers would not
: remain constant. I mad ethe mistake, however, of posting the input paddler
: weights (before feeding it through the program) istead of the output
: weights ... Jimbo was off and running.. and here we are: If I was wrong i have no problem with that.. I do have issue with people who
: are less than straightforward however!: Sorry for the interruption, sorry KFS's dirty laundry made it out in public.
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