Posts: 3508
Location: Elk River, Minnesota | Hiya,
Sled was right...I'd chime in on this one...
couple of things with the Ranger 690 series of boats...the in-floor tank is more forward than the following models of 619, 620 series, thus the reason your 21 pitch aluminum cannot get anything more when you trim...it just does not have the lifting capabilities to get the boat out of the water. My bet is you have side spray that is up by the console seat as you are at full speed.. I would suspect that is why you cannot get any more RPMs out of it
With this particular boat, a good 4 blade like that Rev 4 mercury makes is a great option. Only problem is I don't believe that will fit your motor as the spline count and hub design will be different.
OMC/Bombardier have a couple of good props to try, but they are steel, which will get the entire boat out of the water more. The Renegade series of 4 blade props has both a bass style, but also has an off shore style designed for heavier boats. The offshore design is the one I would be looking at as it is not quite as aggressive as the bass model. It would allow for higher RPM's and, you would also be able to raise the motor up a few notches to get even more out of it as well. My gut instinct with you running aluminum is your motor is all the way down on the transom, or is one hole up. Definitely not the most efficient position for the motor.
If you choose to stay in an aluminum prop, a good long look at the Solas Amita series of 4 blade props would be your best bet for a prop that through its design and casting process will be thinner than a stock aluminum, flex less than a stock aluminum, yet has the breaking characteristics of aluminum to lessen shock on your lower unit. But...and this is a really big but, is if you knock a rock at speed with aluminum, the aluminum isn't going to save you...the prop shaft will bend due to the fin impact, even though the prop fin will break. If memory serves me correctly, that motor should be running about 5500 at full throttle when trimmed, and I'm betting a 21 pitch is about right. Much of that depends on the gear ratio, and that number I am unsure of. I would think it is right around 2:1, but could be around 1.85:1. If it were the smaller ratio, then even trying a 19 pitch might help to get RPMs up.
Being you are in the twin cities, I would check with Nelson marine by white bear lake. I believe they may still have a demo prop program (blue props) where they will allow you to take a prop out and run it to get the best prop for your motor.
If you are still having some issues with things, it would be very easy to make a run down here and we could do some testing on the river, being I am in hastings.
So...in a nutshell, a good 4 blade is your best bet. Renegade Offshore for steel, or Amita 4 in aluminum. The steel props should have a good amount of cupping and moderate rake, and most likely will be vented as well. If the Mercury REV4 has an exhangeable hub system in it that would allow you to run it on your motor, that would be a dandy prop to try. A few different guys like "Sled" on here run that prop with very good results.
Steve
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