Motor Swap

[elh79]

I have an old 15 foot fiberglass tri-hull that I have completely stripped and put a new floor in. I have painted it and use it to bass fish and duck hunt. It has a 1975 Johnson 50 on it that runs really good. The boat is rated for max 85HP. I have the opportunity to buy an early 80's johnson 70HP for $400. My buddy guarantees this motor and he is a pretty trust-worthy friend. My question is....is this worth the money and trouble. Will I see a noticeable difference in boat performance. I don't mind changing props if I have to. I definitely would like to get more holeshot, and more top-end speed. Is this possible moving up 20HP? Thanks for the help.

 

[Dhadley]

Re: Motor Swap

It's a direct swap except for the prop. Go for it! If you dont like it, swap it back. I'd bet you'll love it.<br /><br />Set it up for 5800 with an average load. Actually your 50 should be the same.

 

[elh79]

Re: Motor Swap

Pound for pound what do you think the speed increase will be? I know there are no guarantees...but I would just like to know a rough estimate.

 

[Dhadley]

Re: Motor Swap

That all depends on how the 50 is set up and how the 70 will be set up. If the 50 is set up at -- say 5500 rpm and you set up the 70 at 5800-6000 the difference will be tremendous.<br /><br />If both motors are set up properly and running as they should youre still going to see a big difference. The difference in torque alone is great enough to provide that. Plus the 70 swings a larger diameter prop which will carry the boat much better.

 

[WillyBWright]

Re: Motor Swap

Going from a twin to a triple ... you'll notice a significant increase in speed, better holeshot, increased fuel consumption. Not as much of a difference as if you hung a V-4, but still plenty significant. +5mph wouldn't suprise me at all.<br /><br />Ballpark guess...<br />Twin ... lower 30s<br />Triple ... upper 30s<br />V-4 ... lower 40s.

 

[tashasdaddy]

Re: Motor Swap

go for it. the cost won't be that much, after you sell the 50.

 

[ob15]

Re: Motor Swap

I would definitely go with the 70. Just make sure it is in good shape & you get the setup correct. Set it up the way Dhadley mentioned.<br /><br />One thing also, you will be able to cruise at the same speed as the 50 but at a lower rpm. This may reduce fuel consumption or be the same as the 50.