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I have a 1972 Boston Whaler Tashmoo 16'-7" boat. It has a 1989 Yamaha 90 hp outboard. I bought the boat with the current prop and it needs a new one. It currently has a 13" x 17" on it now.
I went to the dealer and asked what the recommended size was and I didn't have the prop or know the size at the time and they wouldn't look it up but did tell me it should be taching out over 5000 RPM. Knowing how the boat was I wouldn't be surprised to have the wrong prop on it to begin with. The boat is little slow out of the hole with a skier and tachs out between 4200 and 4400 RPM. I am wondering if I should try a 13" x 17" or if that would be too little pitch. miwheel.com says 13" x 17", but it doesn't take in account the heavy Whaler hull.
I posted this on thehulltruth.com and they gave me some good advice and also referred me here. Some of the things that came up was condition of the bottom which is smooth, compression should be good but I will check it again when I go back to the boat, it is left on Cape Cod and I am one hour away. One one thing that I do not know is if the foam is saturated in the hull and I don't know how to check for that. The boat gets onto plane fast and easily and it looks OK in the water. I will try and get some pictures next time I go to the boat in the water.
Your dealer is correct that that motor should turn between 5000-5500 RPM at WOT.
The Yamaha two stroke 90 uses a 2.33:1 gear ratio and usually pushed a 16'-7" Whaler to 40 MPH and to do that usually need a 19" pitch propeller. If you are only using a 17" and only getting 4200-4400 RPM, something is TERRIBLY WRONG.
Please verify the gear ratio of your 1989 Yamaha 90. Your owner's manual will have this information. Perhaps my off-the-top-of-my-head recollection is not good this morning.
OK, I looked it up, The 1989 Yamaha 90 uses a 2:1 gear ratio. A 17" pitch prop should be just about right for this motor on a 16'-7" Whaler and should allow the motor to reach 5500 RPM at 40 MPH.
Obviously there is something amiss.
We need to know the top speed of this boat to help verify if the tachometer is reading correctly.
Sorry I haven't been able to get to a computer for a few days with work and all. My mistake, it has a 19 pitch prop on it now, I was thinking of buying a 17pitch if I get a new one. I am going down the cape this weekend, I will take my handheld GPS and get the top speed with the current prop and go from there I guess.
