If the engine is reaching full rpm then the horsepower is adequate. With the boards up, if the speed is still lacking, it pretty much has to be a propellor problem or trans slipping since you say the bottom is clean and you are discounting weight as a factor. You might consider having a mechanic use a test tach to verify that yours is accurate, but that's grasping at straws. I have to admit that I would be tempted to offload everything I and a few strong friends could carry and then see what happens.
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40' Norman Cross trimaran