Prop Size...Suggestion

[Fr. Frank]

Assuming average high-performance SS propeller slip of 12%-15% at WOT, with, say, a 22"P Trophy prop by Quicksilver, you're looking a a top speed at 5000 rpms of around 54-55 mph. That assumes, of course, that you have the power to turn that prop at that rpm with that weight.

If you have to drop to a 20"P prop, your WOT speed will be 48-50 mph, assuming the 12%-15% slip.

Prop slip tends to increase with lower speed, so at the given rpms figure a cruise speed with the same 22"P prop of 32-35 mph, and with a 20"P prop of 31-33 mph.

Things like drive angle, height of cavitation plate above the kelson, hull smoothness, amount of rocker or hook in the hull, etcetera, all affect the efficiency of the prop.

A slip of under 10% at WOT is possible, but rarely achieved without mods like nosecones, carefully adjusting drive height and trim angle.

I was able to get a slip ratio of between 8% and 9% when I had my 2.0L Merc XR4 V6 on my Seafari, with slick racing paint designed for sailboat hulls, a Bob's nosecone and low water pickup, motor mounted pretty high, and a very expensive, custom-made, one-of-a-kind propeller made for me by PowerTech.

If you did achieve a slip ratio of 8% at WOT, then your top speed at 5000 rpms would be 58 mph with the 22"P prop, and 52 mph with the 20"P prop.