It's not grossly under-powered, but ..
Rule of thumb is 2hp per ton, your boat being 4 tons (8000 pounds displacement) your 15 should be fine - and again BUT -
Your 15 is really less than 10 at normal operating RPM's and and then you have to subtract the transmission loss and if you are lucky you are delivering 8hp to the propeller.
When said propeller is not fully efficient you have a problem.
BTW, the "15" isn't ever real - just like the Universal 25 is really 21hp, the Y15 is really 12hp.