130SPORTINCT wrote:[I assume that the location] of the painted waterline should be a fair point of reference [for where the boat's static trim should be altered] to get back to ultimately.
That MIGHT be true if the boat was originally rigged with the heavy four-stroke-power-cycle engine, AND if there has been ingress of a very large amount of water and concentrated at the stern which in some unknown manner has now become absorbed into the foam in the interior of the Unibond hull.
But that seems like a lot of things had to happen. A much simpler explanation is the boat came with a lighter engine when the original bottom paint was applied.
Or even simpler, the original bottom paint was applied incorrectly, and the boat has always had a static trim that puts the waterline higher than the anti-fouling paint.
Or, best of all, find someone else with a c.2006 130 SPORT that is rigged with the heavy Mercury 40-HP FOURSTROKE engine, and get a measurement from that owner about how much freeboard remains at the stern with the engine tilted up as shown in your Figure 1, in the same type of brackish water, at the same water temperature.
Note that the density of water varies with salinity and temperature, so very cold, very salty seawater is sufficiently denser than tropical fresh water that the volume of the weight of the water displaced by a hull for a given total hull weight varies considerably. Commercial ships have a Plimsoll line reflects that change in water density for various conditions.
Aerated water is also effectively less dense, which is why there was a great danger to commerical ships when Russia's North Sea natural gas pipeline failed and began venting enormous amounts of natural gas into the North Sea. A large ship that was heavily loaded could have been at risk of foundering if it sailed into those aerated waters.