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Just bought a 17 ft Tashmoo Boston Whaler and want to fit my 70 HP Mercury. I was told my engine would be ok as the length of the engine shaft is 20 inches and the transom was 22 inches deep. Prop is coming out of the water after 20 MPH. Positive Trim all the way in? Looks like it's cavitating? Do I need a 4 blade propellor or can I install a engine bracket? Is my 70 horse too small for a 17 ft Tashmoo.

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This doesn't sound right to me.

How did you determine your engine is a 20 inch shaft?
Do you have the serial and model number of the engine so members here can see what you might have?

Is the engine mounted all the way down on the transom?

A 70 hp is plenty and should push the boat about 30 to 32 mph...

Definitely do not use a external bracket to lower the engine. If your engine is does not have a 20 inch shaft, I would either buy a used extension kit or sell the engine and buy a motor with a 20 inch shaft.

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Thank you very much for your quick response. Local Canadian Mercury Dealer said 3 cylinder 70 horse 2 stroke is good for a 22 inch transom as you have 3 inches to play with from transom mount to water intake. Do you think that I may need a 4 blade prop with a different pitch. I will have to get the engine serial number on the weekend. Thank you very much.

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If they sold you a 15 inch shaft, this will NOT work...

Before anyone here can recommend anything, we need to verify that this is in fact a 20 inch shaft motor.

I am guessing that this is a 15 inch shaft motor and that is why you are blowing the prop out of the water.

All 17' Boston Whaler Classic hulls use a 20 inch shaft motor.
Nauset, Tashmoo, Montauk, etc... These models all have the same basic hull. Just the interiors are different.

More info is needed.
Year of motor.
Model and Serial number of motor. (this should tell us what length shaft but not positive)
Existing prop.. diameter and pitch

Here is a 1978 Montauk with a 20 inch shaft main motor. This is the same basic hull that you have with a couple slight changes.
http://users.sisqtel.net/jkriz/Montau...ntauk.html

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Problem solved with engine, the shaft is only 15 inches, we need a long shaft. One marina told us to cut the transom, but that is not happening. Other Marina said, fit the bracket but don't think so.. We are going to buy a longshaft. Boat is out of the water and husband is going to restore her.

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My guess in my post above was correct. Your engine is a short shaft and will not work on the 16/17' models.

Glad you got it figured out.

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Thank you very much for your input. Did you see the pictures that we posted?