How big an outboard?

H

hotvrod

Cadet
Joined
May 21, 2010
Messages
9
I am restoring a 15' '62 Isham dual cockpit runabout and am wondering how big an outboard it will handle. I'm looking for an older outboard appropriate in style to the boat. I was thinking of around 50hp but i found an old 75hp evinrude that would compliment it. The transom is solid and the hull is partially glassed. I was thinking of rebuilding the transom anyway as it was painted and has one soft spot about the size of a golfball. My "restore" is basically replaceing decking and refinishing, and out fitting motor and helm controls. Any thoughts? Suggestions?
 
RotaryRacer

RotaryRacer

Lieutenant
Joined
Jul 18, 2004
Messages
1,361
Re: How big an outboard?

I think 75HP would be too much...although I have no idea what style hull your boat has.

Post some pictures so we can get a better idea what it may be able to handle power wise.

There is a pretty simple calculation that you can use to estimate what the hull can handle. I can't seem to find it right at the moment. I'll post back if I find it.
 
H

hotvrod

Cadet
Joined
May 21, 2010
Messages
9
Re: How big an outboard?

mohawkSister2.jpg
The hull design is identical to this though mine is plywood and glassed up to the red portion like this one. It's 15' at the waterline, 16' overall. I was thinking 50hp as my limit, though it would depend on engine weight more than hp.
3nb3ka3pe5Y45O15Z0a5f66c6628c5fb018d5.jpg
This is the motor I thought would compliment the boat but itsw 75hp.
 
H

hotvrod

Cadet
Joined
May 21, 2010
Messages
9
Re: How big an outboard?

It has a steel keel protector strip i will need to replace also, its getting pretty crumbly with the rust. I live in the northwest so I was thinking of replaceing the decking with cedar and the transom with a lam layer of okumne(?) 2x 3/4" marine ply with some aditional gusseting of redwood or cedar. (more for asthetics than strength. there aren't any in the original). I live on a freshwater lake but it might see some time in the sound.
 
H

hotvrod

Cadet
Joined
May 21, 2010
Messages
9
Re: How big an outboard?

We have several manufacturers of ply in the portland area so a more exotic decking may be an option.
 
CATransplant

CATransplant

Admiral
Joined
Feb 26, 2005
Messages
6,319
Re: How big an outboard?

hotvrod said:
View attachment 53897
The hull design is identical to this though mine is plywood and glassed up to the red portion like this one. It's 15' at the waterline, 16' overall. I was thinking 50hp as my limit, though it would depend on engine weight more than hp.
View attachment 53898
This is the motor I thought would compliment the boat but itsw 75hp.
Click to expand...

The outboard on the back of that photo is a 30 or 35 hp from the 50s. That's what I thought would be what the boat was designed for. There were no 75 hps in the 50s. The largest thing available was a 50hp V4, back in 1958.

So, I'd be willing to bet that your narrow 16' boat was designed for the 30-35 hp outboards of that day.

75 would be way too much. A Johnnyrude Big Twin would be just right.
 
You must log in or register to reply here.
Top