I think if you put and A cat mast on an F18, and only put an A cat sail on it, no jib, no spin, and only one trap wire with only one sailor on it, it would be fine. You may not go very fast, as the F18 is a 'Dock' compared to a light A cat hull, and the A cat sailplan doesn't have the horse power of the F18 sailplan, but I don't see where the mast is going to care what it's attached to, at the bottom. If you keep the same amount of stay tension on the mast when you step it on the F18, as it had on the A cat, how would the mast even know what it was attached to?