Disclaimer: I am not a boat builder
, but I am an Engineer
. Take my comments with a pinch of salt
and large amount of Tequila
For the sake of the discussion, lets name your fibreglass supports looking left to right, A, B, C & D.
In the drawing that you have shown, blocks A & D will not distribute any of the vertical load, only horizontal loading. If you place the compression post in between supports B & C, you will need to spec a plate with much thickness to resist bending. It is best to place a continuous support under the post, something like a square section (sort of like a stringer) of fibreglass or ss section. This will distribute the loading a little better than what is shown.
The plate which is bolted at the top & bottom, is not 'structural' as such, it serves to first, obviously keep the post in place, and secondly to distribute any horizontal loading that may occur. Other than that, you can keep it nicely polished!
I can see what the Surveyor
has tried to achieve, but with the wrong solution. Essentially you can say that any point loading that is placed on a beam, will distribute at 45deg in all directions in the horizontal plane (think of a cone spreading out from under the post dia) After that, the load is not spread too well, and becomes ineffectual.
I hope this has helped a little, or helped to understand the problem.