reviving old seacraft (propeller question)

[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?

[eggsuckindog]

Not sure on Dia but its a 19 and no I haven't added any cup, to do that would bea 1/3 of a new one. 5600 rpm

[Fr. Frank]

Dixon, a couple more questions:

Do you have a negative trim limiter, such as a rod extending from one side of the transom mount to the other, which limits how far down the engine will trim?

With the motor at neutral trim (cavitation plate exactly parallel to keel line), how high above the keel is the bottom of the cavitation plate?

I'm trying to figure out if you're cavitating or ventilating, or both. The fix is different depending on the cause of the air on the blades, because the cause affects where/how the air enters onto the blade, and produces the most vacuum.

If you're cavitating (or mostly cavitating), the fix is to change the angle of attack and departure angles on the blades by greater progressive rake, and/or increasing diameter of the prop, and/or increasing cupping, and/or increasing blade tip size.

If you're ventilating (or mostly ventilating), the fix is to increase the ability of the thrust side of the prop to hold water while surfacing, which is accomplished with increased cupping, and/or smaller blade tips with lots of cupping and reduced blade flex.

I'd like to actually see your boat out of the water in person, and have several props to try at once. I can bring my 15.25" x 16.5" PowerTech. You probably have the 14.5x19 SS Black Max, which is a mid-range propeller.

I can get a couple of different props to try from AAA Outboard, in different style and pitch. Just gotta set up a date and meeting point that works for both of us, if you're interested. And no, you don't have to buy any props. It's just nice to know what works best.

Maybe I can bring them to Long Point, unless you want to do this earlier?

[eggsuckindog]

It will trim negative a bit I think, I only get cavatation when I try to trim the bow for maximum top end. The prop just doesn't pull the bow up much then looses grip, its not bad except for attaining top end which I know could improve. I will probably be able to find a couple Mirage's at Longpoint to try maybe, as they a readily available on the used market, mine is just fine vs $450 for a new one.