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Re: question
By:Paul G. Jacobson
Date: 6/25/2001, 3:59 pm
In Response To: Re: question (Ben Staley)

: I was thinking about weight per linear foot on a kayak, a foot of boat in the
: center weighs a lot more than a foot at the ends....fatter. Do you know if
: you get the same performance advantages by adding the length at the ends? ..

One way or another the length is always added to one end or the other :) What you are changing is the outline of the hull. If you space each form a bit more than called for ( say 13 inches apart instead of 12) then with 16 forms you'll add about 15 inches, and the lines will not bbee to much different. The angle of entry will become a bit pointier, and the beam will stay the same.

If you shift half the boat forward, keep the spacing between frames, and add another (duplicate)middle frame you keep the original entry angle and the original beam, but the center of the boat flattens. Some people "fix" this by redrawing the boat plans to make a new, wider, center frame. This keeps the lines of the boat, but makes the beam bigger.

If you just space out the frames at the ends then you get a very pointy entry angle, with little increase in bouyancy. This might be done for a racing boat design.

You can do some rough math to get an idea of how much weight you'll add to the boat. Start by trying to figure out howmany square feet you have curently, divide the weight by that to get an approximate weight per square foot, and then figure out how many square feet of boat you'll be adding.
I estimate things by figuring straight line shapes. If the boat is 18 feet long and 2 feet wide the bottom is roughly 18 square feet. The deck will be about the same.

If the boat is about 1 foot tall, then each side is similar in shape to a band 1 foot wide and 19 feet long You have a left side and a right side, so double that. Overall you get 74 square feet as an estimate. actualy it is less because of some rounded corners and curves, but this is a number I can work with.

If the boat, without cockpit coaming, hatches or seat, weighs 37 pounds then each square foot of materials, with glass and resin, weighs about 1/2 pound. If the boat weighs 50 pounds then each square foot weighs about 2/3 of a pound.

Now figure out how much you are adding to the boat. If your addition is strictly at the ends, figure you are adding a section 1 foot long by the cirircumference of the boat at that point. If the hull is only 6 inches wide, and the deck is only 6 inches wide, but the boat is still 1 foot tall, you are adding 2 sides, the hull width and the deck width to get 3 square feet. That will add 1.5 to 2 pounds to the boat's weight. On the other hand, if you leave the frame spacing alone at the ends and add middle sections, for each foot of additional length, assuming a 1 foot height and a beam of 2 feet you get, two sides plus hull width plus deck width equaling 6 square feet. This adds 3 to 4.5 pounds to the weight of the boat.
The adding at the widest part of the boat gives you additional displacement of 2 square feet times the depth of the boat at the waterline (let's figure 4 inches) that would be 2 square feet times 1/3 foot or a total of 2/3 cubic feet of displacement at the normal water line. Figuring a rough 60 pounds of water per cubic feet, you get 40 pounds of flotation, minimum. since you can push the boat deeper into the water you can get probably twice that as a maximum. If the old maximum was 350 pounds it would now be 430 pounds. On the other hand, the first case, where the additional effort was spent in making a pointier front gives you an added square footage of hull of just 1/2 squre foot. that is jsut 1/4 of these calcualtions, so figure you'ld get 10 pounds additional flotation at the waterline, and 20 pounds max.

Hope this helps.

: . . .
: What if I left the 4 forms in the middle alone and spaced the end ones out 2
: inches further? Would I end up with a longer boat with all of the
: advantages but without as much additional weight? Of course the cockpit
: area would remain the same and the large foot challenged among us would
: still need to raise the deck a little.

: Just thinking, no plans for a G-18 yet :)

Messages In This Thread

Guillemot "L" Launch *Pic*
Dave Seales -- 6/24/2001, 11:40 pm
Re: Guillemot "L" Launch
Roger Nuffer -- 6/25/2001, 3:39 pm
Great looking boat/design! *NM*
Bob Deutsch -- 6/25/2001, 2:48 pm
Re: Guillemot "L" Launch
Ben Staley -- 6/25/2001, 10:33 am
Re: Great Job! It's a fun boat *NM*
Shawn Baker -- 6/25/2001, 10:26 am
Nice Job
David Hanson -- 6/25/2001, 9:49 am
Re: Nice Job
Dave Seales -- 6/25/2001, 12:17 pm
Re: Nice Job
David Hanson -- 6/25/2001, 3:41 pm
Re: Nice Job
LeeG -- 6/28/2001, 8:30 pm
how transparent was the dynel
mike allen ---> -- 6/25/2001, 5:32 pm
Fairly clear *Pic*
David Hanson -- 6/26/2001, 9:36 am
Re: how transparent was the dynel
David Hanson -- 6/25/2001, 5:44 pm
Re: Vacuum-Bagging Dynel
Shawn Baker -- 6/25/2001, 4:06 pm
An idea for you
Brian Nystrom -- 6/25/2001, 5:22 pm
Re: An idea for you
Bruce Schultz -- 6/26/2001, 9:51 am
Re: Vacuum-Bagging Dynel
David Hanson -- 6/25/2001, 4:23 pm
Re: Vacuum-Bagging Dynel
Shawn Baker -- 6/25/2001, 4:31 pm
Re: Weights
Shawn Baker -- 6/25/2001, 12:36 pm
Re: question
Ben Staley -- 6/25/2001, 1:03 pm
Re: Save weight with materials/methods, not size
Shawn Baker -- 6/25/2001, 4:29 pm
Re: Save weight with materials/methods, not size
Ben Staley -- 6/25/2001, 5:00 pm
Re: Save weight with materials/methods, not size
Tom -- 6/26/2001, 1:41 pm
Re: Save weight with materials/methods, not size
LeeG -- 6/26/2001, 2:40 pm
Re: Save weight with materials/methods, not size
Tom -- 6/27/2001, 9:43 am
Re: Save resistance with optimum hull design
Shawn Baker -- 6/27/2001, 11:23 am
a problem w/ matching resin/glass wt?
mike allen ---> -- 6/25/2001, 6:15 pm
Re: question
Shawn Baker -- 6/25/2001, 4:02 pm
Re: question
Paul G. Jacobson -- 6/25/2001, 3:59 pm
Re: Weights
Dave Seales -- 6/25/2001, 12:52 pm