Hi Mick
The missing piece of info I did not mention is that I also progressively reduced the Cp with each scale down. So I did not answer the question properly (mea culpa). Both the prismatic as well as the block cf's stay the same for any proportional scaling. Reducing length by 90% and keeping beam and draft constant reduces displacement by 90%. Any two dims reduced by 90% will reduce the displacement by (0.9^2) and so on. The block coefficient would thus never go to1 unless the whole yak fills the lxwxd block. Sorry about the confusion.

Scaling all the station dims by 95% but maintaining the 'original' draft reduces the displacement proportional to the kayak design's sinkage. For instance, the Cape Ann Storm displacement is reduced to ~93% not the 86% which would be the case otherwise.
As far as the maximum capacity goes, I was speaking figuratively. This measure is definitely important but I consider it meaningless in the absence of other information. One may ask, what does it really mean? Is it OK to load the kayak to the max and expect it to behave well or will one extra pound just sink it. The question to ask is: what is the sinkage, righting/heeling moments, maneuverability at max capacity. Prismatic coefficient matters when the kayak is loaded around the designed displacement because you can expect it to perform but when you get to the max it really doesn't play a role in speed as much as in seaworthiness.

The volumetric coefficients are illustrated in the link below.

Vaclav