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What's the recommended prop size for 1991 yamaha 70 2 stroke on a 1989 15 SS?

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Any suggestion would really help. Size and pitch recommendations?

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We need some additional information to help you:

1) How high is your engine mounted (see here: http://www.whalercentral.com/articles...cle_id=106)?
2) What is the gear ratio of your engine?
3) What is the WOT operating range of your engine?
4) What prop do you currently have installed (brand, material, diameter, and pitch)?
5) What is your current WOT rpm and speed?

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I have a 13 1/2x 17 Ballistic prop on my 70 Yamaha 2-stroke and have been very happy with it. The boat is a !964
16'6". Ballistic is made by Michigan wheel.

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I would go with a Yamaha K series painted stainless steel prop in 17" pitch.

Engine mounted 3 holes up.

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The engine is all the way down. Runiing 13x19 aluminum now. Wot is now 5800 rpm. Recommended is 5500 to 6000. Hitting 39 mph .

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Your gear ratio should be 2.33 and the WOT 5000-6000: http://www.yamaha-motor.com/assets/se...5_1390.pdf. Confirm this is consistent with what your user manual says.

Using the Mercury prop slip calculator (http://www.mercuryracing.com/prop-sli...alculator/) I calculate a slip of 13% which is high. You can use this app to get a general idea of what prop pitch is appropriate.

Raise the engine 3 holes up and then lake test again and report back with updated data. If the engine aerates while riding onto the plane let us know that. Also be careful about over-reving the engine. The rpm should rise by about 150-200rpm per hole up (roughly) and the prop slip should decrease.

I will defer to Phil on the ultimate prop recommendation as he is the "prop guru".