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Saint
12-13-2000, 07:51 PM
What is a good Prismatic Coefficient to shoot for in a 17" Sea Kayak
(Swedeform, 250 lb displacement, hard chine). I am designing in Carlson's
Chine Hull Designer and have played for a while with different hulls and got
my value down to .49. I did a bit of reading in Dave Gerr's book and he
states/implies that an ideal is .54. Does that stand true for the long and
narrow extreme of kayaks as well as other displacement boats. Am I trying
to get as low as possible? I understand what the Prismatic Coefficient is,
but I don't know what is I should try to get it to say.

Todd Bradshaw
12-13-2000, 10:36 PM
I looked through the specs of some well tested, proven sea kayak designs and the P.C. figures varied from .50 to .54. The midship coefficients for the same boats were in the .75 to .80 range. The designer claims that his intent was to produce boats with minimal resistance at normal cruising speed. For the kayaks that I own, that seems to be about 3.5 - 4.5 knots on the G.P.S.

I do own one longer boat (18' 11") that, on paper, is probably faster. In real life though, the speed at which it really starts to glide nicely and show it's stuff (about 5.5 knots) takes more energy than most people (myself included) would care to put out for very long. It's also less tolerant of choppy water, where it seems to bog down and feels heavy and slow. I can testify to the premise that sometimes, in the real world of water, rather than paper, and at the speed that one normally paddles, the "slower" boat can be faster.

Saint
12-15-2000, 01:27 PM
It is not that hard to find the specs on differnt boats, but it is harder to really know what they mean. I guess I'll just have to build it and paddle it, then I know what .54 feels like. <grin>

Todd Bradshaw
12-15-2000, 02:49 PM
If you look at a lot of sea kayaks, you'll find that there are often radical differences in shape from one to the next - Swede-form, fish-form, hard chine, soft chine, round bottom, V-bottom, etc. Some do certain things better than others, but a lot of them paddle very well, in general.

The boat that I usually paddle is Swede-form, round bottomed in front, heavily V-bottomed in the back, soft-chined except for about 6' in the middle where it is hard-chined and has a ton of flare built into the ends. My wife's boat is fish-form, shallow arch with a hint of V all the way down the hull and moderate flare on both ends. Both cruise at about the same speed, are reasonably sure-footed in rough water and we can switch boats without it feeling particularly strange.

When I discovered that my 19.5" waterline beam kayak was more stable than my 21.5" waterline beam boat, I pretty much stopped paying attention to specs. Since we usually paddle at least 4 miles in a typical outing, I pay a lot more attention to the design of the seat, which is one place that a lot of designers could spend a little more time.

I'd say "Go for it!" You might become the next great kayak designer.

BrianCunningham
12-16-2000, 10:29 AM
Take a look at the design section of the Kayak Forum
http://www.kayakforum.com/cgi-bin/Design/index.cgi

Do a search on Prismatic Coefficient
There's been quite a few posts on the subject there.

George Roberts
12-16-2000, 07:57 PM
Ideal for what? Speed? Handling?

In point of fact, all of those numbers are so much garbage for kayaks and most other boats.

Take two boats, one good and one bad for the same use, they can have all the same numbers.

You design the boat for the use and these numbers fall out. That does not mean that they have any meaning.

Saint
12-18-2000, 10:04 AM
In response to George... "Honest, I knew better than that"...<grin> By 'ideal' I was meaning most efficient at a normal all day cruising pace. (I guess my telepathical typing didn't come through.) I wouldn't doubt that what you say about the numbers may be true, but they at least give a start to defining the shape. I have been playing with Chine Hulls app and have been very surprized at how much I can change the shape of the hull and still have pretty much the same numbers for the righting moment. Not sure if I just don't understand what the numbers mean or how the numbers are derived.

fair&fair
01-12-2001, 07:01 PM
Those numbers are far from garbage. All different types of hulls will perform best if certain parameters are met. For example, if you wanted to design a planing power boat with a displacement of 30,000 lbs or so, a speed/length ratio of 3 or higher is imperative. Along with that number, a prismatic around .7 is also desirable. LCB, LCF and all of the other numbers have would have their own values that would best suit this design critera. If the numbers were garbage then yacht desgin would be a meaningless endeavor. The fact is, you would not want to design a kayak with a prismatic of .7. The ends would be way too ful for this tyoe of boat and it would infact perform like crap.