reviving old seacraft (propeller question)

[Fr. Frank]

Dixon, have you measured the angle of thrust on your motor at operating trim angle? If you cannot trim at all without ventilating your prop, that means you're probably at a negative trim angle (thrust angle aims slightly downward), and if so, you're running too high out of the water. You need a positive thrust angle of at least 4-5 degrees, or you're literally driving the bow down into the water, creating a lot more drag. You want to get the stern up, then you want to lift the bow for best speed. If you run with your motor trimmed all the way down, put a carpenter's protractor on it and measure the angle to the cavitation plate. If you don't have access to a big protractor, just stick a long level or good straightedge along the keel line of the boat. If the cavitation plate is exactly parallel with the keel line, or the front of the cavitation plate is higher than the rear, you need to adjust your mounting height upward. You need about 10* of negative trim (trim "in") to get on plane, then you should run at about 4*-10* of positive trim (trim "out" or "up") at full throttle.

[CaptLloyd]

If the cavitation plate is exactly parallel with the keel line, or the front of the cavitation plate is higher than the rear, you need to adjust your mounting height upward.

Fr. Frank,

Could you verify that for this situation, the motor needs to be mounted higher?

Thanks,

Lloyd

[Fr. Frank]

If the cavitation plate is exactly parallel with the keel line, or the front of the cavitation plate is higher than the rear, you need to adjust your mounting height upward.

Fr. Frank,
Could you verify that for this situation, the motor needs to be mounted higher?

OOPS!! I make a boo-boo. Sorry, guys.

Assuming a transom mounted engine, if the situation is just as described, where the motor ventilates too easily when trimmed, then no, the motor needs to be mounted LOWER.

LOWER LOWER LOWER

This should increase performance AND efficiency. It's about the angle of attack. The idea is to be able to fine-tune the angle of attack without introducing air to the back edges of the propeller blades. How much lower to mount is the unknown. I'd go one bolt hole to start.

[eggsuckindog]

Frank- Its all the way down on the transom now but its not that bad, I just can't get much more than a few degree's positive, just need a prop with a longer blade/rake and cup. Very similar to my 23 when we put the V8 on, the SST wheel would cavatate on the flats but a big long blade Protester was the ticket and you couldn't blow it out if you tried - which I did I think a Mirage + or Tempest will do fine, need a 19

[Fr. Frank]

I agree on the Mirage+, it's a great prop. But you may need a 17 or 18 to get max RPMS while heavily loaded. What kind of prop are you running now?

[eggsuckindog]

Standard Merc SS wheel with round sort of Mickey Mouse ear looking blades, probably great for most applications, gets the boat up very smooth and good at cruise.

[Fr. Frank]

I understand you're running a Black Max prop. What diameter and pitch? Have you ever had the cup increased? What are your max rpms now, fully loaded at max gross weight?