Re: 18hp on 16ft fiberglass Quachita ????
there's a reason it came with a 50. replacing it with a motor that is too small, just because you have it, might not be a wise choice in the long run, when you find you aren't satisfied with it being so limited. You may wear out the 18 if you have to run it wide open against a relatively big load all the time. and there can be a safety issue.
A guy near me keeps a sailboat on a mooring ball; he lives on the water a short distance away. He got one of those "free" generic old 16' bowriders and stuck a 9.5 on it, so he has less invested than a new dinghy/tender would cost, and more size. But his rig isn't good for much of anything else.
So to answer your question, we'd need to know your intended use. For general use, I'd opine it's too small. And I'm not one to seek top speed or favor overpowering boats; we used to get along fine with a lot less HP than you typically see today.
