I have an 18 ft. fiberglass Chris Craft runabout, with a 140
mercruiser. I run it at about 3-3200 rpm.

I could go up to 4000 but it doesn't prove to be much benefit. just
more wear & tear, and sucks more gas. I had a fin on my old lower end
and it helped plane quicker, but cut the over all speed. When I
installed the new lower end, I left the hydrofoil fin off, and it took
jsut moments longer to plane out, but seems like the top end was helped
dramatically, and the boat seemed to run smoother, probably because of
less drag.

Glad to hear that your boat runs fast and smooth now that you have
removed the horizontal fin in the lower-unit of the out-drive. The
outboard in my boat doesn't have anything like that, and it gets on
plane just as well. I guess I don't need to worry about this.

Jay Chan