70 hp jhonson 15.5 bassboat. What prop?

Ghost1958

Seaman Apprentice
Joined
Apr 7, 2017
Messages
31
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

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
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

Admiral
Joined
Oct 23, 2008
Messages
7,107
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

Chief Petty Officer
Joined
Jul 12, 2015
Messages
485
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

Seaman Apprentice
Joined
Apr 7, 2017
Messages
31
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.
 
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