Reinforcement of panel cut-outs

I'm not an engineer, but work with them frequently and understand how production boats are built.

Cutouts like those are generally irrelevant to the structural integrity for a few reasons.

The typical flat(ish) panel anywhere on the boat is normally over built. A flat panel can easily meet the stress loads required, but flex too much for a boat owner to be comfortable with. This means these areas are over built to satisfy the owner that its perceived strength is adequate. CSM is frequently used to bulk it up because it's cheap and quick, plus additional strength isn't required. Sometimes flotation foam is sprayed on the backside to increase the perceived strength.

You can make swiss cheese out of them without too much concern.

Mounting pumps or other stressed fittings or brackets changes it a bit, but holes aren't a big problem.

Cracks may indicate a problem, but cracks more often develop from poor methods used when cutting the hole itself.

Hole saws, jig saws, drills and other tools when used incorrectly tend to create tiny fractures in the gel coat and or laminate. These micro fractures can become larger from vibration or stress. You can have a very solid structure, but cracks may develop over time.

Decks and liners do see some stress, but are frequently over built so the parts are easier to build, de-mold, assemble and not flex when people are walking around or contacting the surface in some other way.

 

he isn't fixing anything; he wants to add something

noone made any argument that he has a problem with the existing setup

he asked how to calculate the load; the answer is not don't bother

 

Glassing under that would be difficult to do a clean job. I suggest you pull the inspection hatch, pump bellows and electric connection. make a pattern and transfer it onto marine grade plywood. 6mm or 9mm with or without glass cloth, skippers choice. Definitely epoxy sealed with 2 or 3 coats. Following best practice bond it to the inside and Bob's yer uncle. Extra glass under the pad eyes.