Here is a photo of a Lund 14′ small hull that we run as high as a Mercury 200 on it. These are about the same weight as the 18 you are talking about, but the additional 2 feet is really important. This rig is at maximum with this motor on it.
I don’t remember what our Lund is rated for, but using the formula that’s been posted on here, I think I calculated it should be around 17-18hp., so I am not too far off. However like Frank said, in your case, that’s almost twice the rated power. We do have a 12′ Crestliner Commander 12 that is rated for 30hp, but with a Mk-25 on it, it is a real handful!
Steve