Merc 200hp on a 1974 Tsunami 20' - Propping Performace

[Fr. Frank]

You can run 14" and larger diameter props. On my previous 20' Seafari I ran a PowerTech 3B prop that was 15.25" diameter with 16.5" pitch on an 2.4L Merc, achieving .08 slip at WOT. (originally 150 hp, but reman'd including balancing and polishing to produce >200 hp on the dyno, although I had changed the lower unit to a 4.75" Merc 1:62 lower from a 3.0L 225).

It isn't just diameter and pitch:

• total rake,
• rake progression,
• total cupping,
• number of blades,
• blade cup %,
• blade length to base ratio,
• total blade base circumference (possible to have over 100%, i.e.; overlap for extra stern lift)
• blade-tip shape and angle

ALL of this makes a difference. Different designs have different slip ratio on different hulls, as well.

.25 slip ratio at cruise is about average. Slip below .2 at cruise is great. Slip below .1 is unheard of except at max performance numbers. Most high performance boat builders shoot for a .25 - .20 slip ratio at cruise, and .15-.20 slip at 90% power.

Propeller slip is unused/unavailable horsepower and torque that you're still burning the fuel to produce.