Boat Building Forum

Find advice on all aspects of building your own kayak, canoe or any lightweight boats

Re: length vs speed.
By:Nick Schade - Guillemot Kayaks
Date: 5/8/2016, 10:51 am
In Response To: Re: length vs speed. (Etienne Muller)

In your typical wind waves found in short fetch bodies of water, length doesn't provide that much advantage down wind. I keep up with my surf ski friend quite well in my 14' kayak on occasionally breaking waves going 7 to 10 mph. The waves are defining how fast you go. The 14' kayak will sprint to 8 or 9 mph for a few stroke if need be, but the waves will usually provide a lift before there is a need.

On faster waves and swell it can be harder get enough speed, while holding course. This is when the rudder of a surf ski and its higher sprint speed start to be of benefit.

Due to being shorter and typically less muscular, smaller paddlers are less able to apply turning torque to the kayak, so over long designs can be difficult to handle in the wind. The tendency to broach while surfing is harder to overcome.

There are a number of factors that are hard to balance when designing for a smaller paddler. Lets say you have a kayak designed for a 200# paddler that you would like to adapt for a 100# paddler. This is 1/2 (50%) the intended weight. A simple scaling of all dimensions would require scaling by the cube root of 1/2 or about 80% of original.

So now your displacement is correct, but since the length is 80% of the original the amount of torque needed to turn the boat is reduced by about 80%, not 50% and wetted surface area is at about 63% original. So the tracking is stiffer, but the drag is less.

With stability, the righting torque will be reduced by the 4th root of 1/2, but since the paddler's weight is 1/2 the original with half the displacement, it is only the righting arm which is reduce by 80%, so the kayak would feel 80% as stable for the smaller paddler than the original kayak would feel to a 200# paddler.

All this means that I don't scale a design by more than about +/- 10% and for a small paddler the boat will still be longer than ideal and less stable. A new design specifically for a small paddler will probably be proportionally shorter and wider to achieve the analogous performance.

Messages In This Thread

S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/5/2016, 3:59 pm
Re: S&G: Looking for feedback from skinnies.
Mark N -- 5/5/2016, 5:44 pm
Nice lawn *NM*
Jaybabina -- 5/5/2016, 6:13 pm
Re: Better Picture of my lawn
Mark N -- 5/6/2016, 3:31 pm
Re: Better Picture of my lawn
Dan Caouette (CSCWC) -- 5/7/2016, 9:53 am
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/5/2016, 6:29 pm
Re: S&G: Looking for feedback from skinnies.
Justin -- 5/6/2016, 4:33 am
Re: S&G: Looking for feedback from skinnies.
Dan Caouette (CSCWC) -- 5/6/2016, 7:40 am
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/7/2016, 11:08 am
Re: S&G: Looking for feedback from skinnies.
Mike Bielski -- 5/6/2016, 7:54 am
Re: S&G: Looking for feedback from skinnies.
Bob Johns -- 5/6/2016, 10:29 pm
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/7/2016, 5:52 am
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/7/2016, 11:15 am
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/7/2016, 11:59 am
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/7/2016, 6:58 pm
Re: length vs speed.
Nick Schade - Guillemot Kayaks -- 5/7/2016, 8:32 pm
Re: length vs speed.
anewhouse -- 5/7/2016, 9:24 pm
Re: length vs speed.
Alex Ferguson -- 5/7/2016, 9:36 pm
Re: length vs speed.
anewhouse -- 5/7/2016, 9:52 pm
Re: length vs speed.
Nick Schade - Guillemot Kayaks -- 5/7/2016, 10:17 pm
Re: length vs speed.
Etienne Muller -- 5/8/2016, 4:16 am
Re: length vs speed.
Nick Schade - Guillemot Kayaks -- 5/8/2016, 10:51 am
Re: length vs speed.
Etienne Muller -- 5/8/2016, 11:59 am
Re: length vs speed.
Bill Hamm -- 5/8/2016, 8:32 am
Re: length vs speed.
Roy Martin -- 5/8/2016, 10:41 am
Re: Fine Ends
Nick Schade - Guillemot Kayaks -- 5/8/2016, 11:28 am
Re: S&G: Looking for feedback from skinnies.
Jaybabina -- 5/8/2016, 11:11 am
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/8/2016, 12:16 pm
Re: S&G: Looking for feedback from skinnies.
Nick Schade - Guillemot Kayaks -- 5/8/2016, 3:00 pm
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/8/2016, 6:24 pm
Re: S&G: Looking for feedback from skinnies.
jaybabina -- 5/8/2016, 6:52 pm
Re: S&G: Looking for feedback from skinnies.
Bill Hamm -- 5/8/2016, 9:19 pm
Re: S&G: Looking for feedback from skinnies.
bsoderholm -- 5/9/2016, 9:40 pm
Re: S&G: Looking for feedback from skinnies.
Howard Spira -- 5/9/2016, 10:24 pm
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/10/2016, 3:35 am
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/10/2016, 5:25 am
Re: S&G: Looking for feedback from skinnies.
Nick Schade - Guillemot Kayaks -- 5/10/2016, 10:37 am
Re: S&G: Looking for feedback from skinnies.
Etienne Muller -- 5/10/2016, 4:46 am
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/10/2016, 5:51 am
Re: S&G: Looking for feedback from skinnies.
Bill Hamm -- 5/10/2016, 6:46 am
Re: S&G: Looking for feedback from skinnies.
JohnAbercrombie -- 5/10/2016, 11:22 am
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/10/2016, 9:09 pm
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/10/2016, 9:33 pm
Re: S&G: Looking for feedback from skinnies.
Nick Schade - Guillemot Kayaks -- 5/11/2016, 9:07 am
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/11/2016, 6:28 pm
Re: S&G: Looking for feedback from skinnies.
Paul D -- 5/11/2016, 11:43 pm
Re: S&G: Looking for feedback from skinnies.
JohnAbercrombie -- 5/11/2016, 12:50 am
Re: S&G: Looking for feedback from skinnies.
Alex Ferguson -- 5/11/2016, 1:01 am
Re: S&G: Looking for feedback from skinnies.
Justin -- 5/10/2016, 7:38 am
Re: S&G: Looking for feedback from skinnies.
JohnAbercrombie -- 5/10/2016, 11:31 am
Re: S&G: Looking for feedback from skinnies.
JohnAbercrombie -- 5/10/2016, 1:48 pm
Re: S&G: Looking for feedback from skinnies. *PIC*
Etienne Muller -- 5/10/2016, 5:47 am
Re: S&G: CirrusLT *PIC*
Etienne Muller -- 1/3/2017, 10:33 am