70 hp jhonson 15.5 bassboat. What prop?

[Ghost1958]

Repowering a 15.5 ft 82 model astroglass bass boat with a 78 model 70 HP Johnson outboard.
The motor is coming off a freshwater house boat so I'm wondering will I need a different prop?
If so which one?
Thanks in advance.

 

[jimmbo]

Without knowing how the engine is mounted on the transom, the weight of the boat and load, all anyone can do is give a guess based on their experience. Since you never mentioned what prop was used on the old boat, or what rpm it attained it is not really relevant
If I were rigging it with the engine at the lowest transom height, I would start with a 17 in pitch and see where the rpms are. Your engine has a WOT rpm range of 4500 - 5500. That engine does perform best when propped around 5300 - 5500 rpm

 

[NYBo]

If the motor was properly propped for the houseboat, you will undoubtedly benefit from one with more pitch. But how much is kind of a wild card at this point without the info noted in jimmbo 's post above.

 

[David Young]

I agree with jimmbo. My 1977 15 foot tri-hull with a 70 hp motor does 5,500 RPM with a 17 inch pitch propeller

 

[Ghost1958]

Thanks guys.
Called the boat mechsnic I got the motor from today . Mounting the motor, controls adjusting, carb rebuild and guaranteed properly running motor is all part of the deal.
He said houseboat but it was actually 21 ft pontoon.
I mentioned the prop and he said it would likely need reproped for my boat that he has a bunch of good props so he will change to what it needs for nothing since he will be servicing the motor and lower anyway.

So I guess that's out of the way.
Hoping this motor turns out better than the one that came on the boat.