You have to be careful when doing a repair to advanced composite structure that you use materials and methods that duplicate as best as possible the function of each layer.

The cloth used to build a high performance catamaran is definitely not the same cheap $hit cloth that they sell at the local boatyard or auto parts store. Those junk open weave cloths are prefectly fine for repairing stuff built with a chopper gun, but they will result in a repair that is very resin-rich and brittle. Try to find out what cloth style the boat was built with and use that or something close. Failing that, use a good quality aerospace/industrial grade like 7781 or 1581 cloth. Do pay attention to fiber orientation and alternate it between layers(0-90, 45-45).

Likewise for the core material; don't install a substitute core that is like 10X harder and stronger that the original foam core. You will just make a stress concentration point that will probably crack later. A very dry mixture of resin and plastic microballoons should do nicely. Avoid chopped strand in the core substitute as it will make it too strong.

Many sailors avoid boats with repairs since most repairs have a bad track record on longevity and they typically cause the boat to be overweight. This is because the above guidelines are often not followed. In aviation, we return busted advanced composite parts to original form strength and weight all the time, so it can be done and actually does not take any more work, just a little knowledge.

Also use a piece of thin(.024") 2024-t3 aluminum for the back up plate for the repair. Since it is so thin, you will not have to widen the hole to get it inside the boat like you might for a balsa back up plate. It's important to use a structural grade like 2024-t3 for the backup plate, too. The non-hardenable architectural (junk) grade they sell at the local home centers are too soft and weak to be useful as a backup plate. The difference here is dramatic, not subtle. You can then pull pretty hard on the wires attached to the plate and force it into good contact with the curved inside of the hull without the backup plate breaking or the wires pulling through like it might with balsa.

Other than these little details, Scooby's tutorial is spot
on, so just follow that while observing the above and your boat can be the equal of new in strength, form and weight.

Jimbo