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Posts: 33
| Just bought a new 2008 tuffy osprey side counsel with 115hp optimax. Have a steel 4 blade prop on looking to change to alum the prop is a 23 pitch runs 43mph at 5000rpms. Any suggestions would be helpful to change it to alum. thanks |
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Posts: 3480
Location: Elk River, Minnesota | Hiya,
Seeing as though the max RPM for your motor is 5750, the switch from the 23 pitch 4 blade to something different in my humble opinion is a good idea. My gut says your motor is slow out of the hole, but handles well.
In most cases if you make a switch from steel to aluminum in the same blade count, pitch and diameter, you will be looking at a gain of roughly 200 to maybe 300 RPM.
If you go to a 3 blade prop in the same pitch, you would probably gain around another 200 RPMs as well. So...a 23 pitch aluminum should get you around 5400 to 5500.
To get to the 5700 range, I would say drop 2 more inches in pitch to a 21 and you should be close if not a little over the max RPM. This is around the optimum RPM range when you are lightly loaded and by yourself with a full tank of gas. By setting up your boat in this manner, you can load down a bit without going outside of the WOT operating range of the motor.
If you are looking for aluminum, I would suggest a solas propeller which will be a little less expensive than an OEM prop. The squeeze cast design they use means the blades will flex less, but they contain a decent amount of cup to help with handling at higher motor heights. I would say the Amita 3 would be a good start. Another good prop in aluminum is the Mercury black max. This might give you a little lower RPM as it has bigger "ears" so to speak than the amita.
One thing to always consider, though is that any prop you choose is a compromise in some area, so one prop might work well, and another not so well. Testing is your best bet, but finding a demo prop program (blue props) sometimes is a hard thing to come by.
Hopefully this gives you a start...
Steve |
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Posts: 633
Location: Madison, WI | I have the same boat with a 115 merc 4-stroke.(2007) I have an aluminum 4 blade prop on it right now and my boat runs 38-39 full speed hammer down at 5000 to 5100 rpms? i would be happy to get to 43mph, should i drop pitch? |
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Posts: 3480
Location: Elk River, Minnesota | Hiya,
In a nutshell...yes.
A 4 blade prop will not give optimum speed as they are designed for better handling and in most cases big boats in big water. 3 blades will be faster (less drag due to one less blade contacting water) and with the right prop, the motor can be raised a bit off the transom without losing bite (most notably with highly raked and highly cupped steel props)
sounds like the same scenario as the first post...
Steve |
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Posts: 32889
Location: Rhinelander, Wisconsin | It's probably not a 23 pitch, a 115 can't turn that big a wheel, especially on that boat. I already answered this via email.
The boat should have a 19 pitch prop if you choose a 3 blade aluminum. |
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Posts: 33
| you are right it is not a 23 pitch must of had a brain fart when typing it, is only a 21 pitch |
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props