I've put an aluminum propeller on my '77 Thunderbird 18' tri-hull in place
of the original stainless steel one. Old prop is a Mercury 48-73982A4, 13
3/4" diameter, 21" pitch. New prop is a Michigan Wheel Vortex 992005, 14
1/4d, 21p. It's attached to a Merc 165 I/O. On the original prop, top speed
was 45MPH indicated and cruise was in the neighborhood of 25-30 indicated at
about 3200-3500 RPM.

Anybody want to predict what (if any) performance difference I'll see on
Saturday when I take it for a spin?