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Lyman
07-27-2000, 11:54 AM
I've got a 16 foot classic (5.0L) that came with a 1.62:1 drive. It runs an honest 62mph on slight chop with me alone in the boat. I'm turning just under 5000rpm and occasionally bump into the rev limiter. The prop is a 21p Vengeance. Question: Will switching to a non-plus 21p Mirage lower my RPM's slightly? Should I expect a few more MPH? Vengeance is a 13.75" diameter, Mirage is 14.25". This speed may be as fast as this hull can go as it starts to chine walk. I've got a deal on a new in the box Mirage and need to act fast.

[This message has been edited by Lyman (edited 07-27-2000).]

Forrest
07-27-2000, 02:07 PM
See if they will let you try it first. You just never know about arm chair, "do you think if . . ." approach to prop selection.

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Forrest

GEOO
07-27-2000, 02:58 PM
Lyman, I like small diameter props on small boats. I think the larger diameter prop will cause more chine walk and torque steer. GEOO

GEOO
07-27-2000, 03:10 PM
Lyman, I did some prop calculations. Your numbers do not make sense. Something is off (rpm, pitch, gear, or speed). A 1.62 ratio @ 5000 rpm with a 21 pitch will give a theoretical speed of 61mph, (no slip). Most Donzi's run between 10-15% slip. 10% = 55mph, 15%=52 mph. Before you spend money on a new prop find out were the error is. The prop my have been repitched, your tach may be off? GEOO

AVickers
07-27-2000, 05:26 PM
I come up w/ the same numbers as George. And with the slip, I agree that you're going about 55...if your prop really is a 21p. Did you come by this speed value w/ GPS? or Radar? That's really the only way.

As for diameter, I've heard that at high RPMs, props begin to use only the outside ends of the blades and everything inside a certain diameter becomes less and less important. It would seem reasonable that if you had more at the outside of the arc on a given prop, you'd have more "bite" at higher Rs.

Don't know this for sure, but I had both 14" and 15" aluminum props with the same measured pitch for my old 18". With the 14" on, as I approached the highest RPM my engine would run, it seemed to be blowing out. Almost like the Rs and the speed would climb together to a point, and then the Rs would go up the last little bit, but the speed wouldn't.

With the 15", the top-end Rs wouldn't change, but it felt like it was getting a good "bite" all the way to the top. Indicated Rs were the same for both props, but the indicated speed was a bit higher (1 or 2 MPH) with the 15". Plus it just felt better at WOT...

Both had been reworked (several times), so there's no way of knowing if other factors were coming into play (rake, progression, blade shape or cup), but the 15" did seem to perform better. (They looked to be the same prop -- only one had bigger blades.)

One thing about both props though, is that they suffered bad cavitation burns on the rear face at the center of the blade about an inch-and-a-half outside of where the blade attaches to the hub -- evenly on all three blades. Lots of energy going up in steam, I suppose...

[This message has been edited by avickers (edited 07-27-2000).]