Wolfer has it correct.
I think you mean PCP.
The amount of weight it takes to sink ANY BOAT 1" has NOTHING to do with displacement. NOTHING. Say it three times.
LOL, no, I got it.
"lbs. per inch immersion" is all about waterplane or the boat's "footprint" in the water.
I think "Archimedes" when I think that. We can even measure the weight of boobs that way
In any case, the more "spread" the waterplane is the more it's immersed, or the faster the plane grows in area, the more it will resist immersion (simply because the volume grows proportionally faster). I know you know that, but, I thought I'd attempt to explain it with my own words.
Two 40'er of grossly different displs can have very similar water plane areas so they will have very similar lbs. per inch immersion.
Yes, but when adding or subtracting lbs, the change of shape of the waterplane matters (again, think vertical cylinder vs. inverted cone shape).
No sailing vessel short of a Mac 26 or a radical trapeze dinghy is designed to be a " truly planing boat". If they were they would look like power boats that ARE designed to be truly planing boats.
I don't disagree with regards to keel boats not being truly planing boats, but I disagree that if they were, they would look like planing powerboats. They wouldn't, as the propulsion of a powerboat (center of effort for want of a better term), is below the waterline, and at the back. A sailboat, or kite boat or whatever, doesn't have the propulsion down there.