: If a planset is scaled up or down, is there any reliable way to calculate the
: change in the load the boat can carry? In other words, if the manufacturer
: states that the kayak can handle 300 lbs, and I build it to 95% scale,
: should I expect the result to handle 285 lbs (95% of 300)?

: I'm talking about scaling the whole thing, not just changing the length
: between stations. Although, that is another question: how does lengthening
: or shortening a design while keeping the beam the same affect things?

: Thanks for any comments / help,
: Bill Graham

Scaling kayaks up or down (length for example) doesn't have a linear relationship to either the displacement or the wetted surface.
For small reduction in scale, the relationship is roughly as follows:

reduction in LWL (waterline length)
95% reduces displacement(lb)to 89.4% and wetted surface(ft^2)to 94%
92% -------------------------83% ------------------------- 88%
88% -------------------------75% --------------------------82.5%

These ballpark figures come from proportional longitudinal scaling of kayak lengths (not just bringing the two middle stations together for instance) while keeping the draft and BWL same (well, +- 1/4"). You can use these figures (within reason) to extrapolate the changes in displacement. Small changes in draft have enormous effect on the displacement as well.
I must say however, that manual resizing will be more than painful. You might also want to discuss this with the designer to see how they feel about the alterations.

I hope this helps
Vaclav Stejskal

Also,the thing to look at is the designed displacement and not the maximum capacity which is pretty much meaningless. At 300lb load you might as well be riding a sinking log.