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Can I put a 25hp 4stroke on an old fishing boat

S

swimrunbiker

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Hello, I have a 14ft sea nymph fishing boat from the 80's. The plate states that the max size engine I can put on would be 25HP. Back then, a 25 motor wasn't available in a 4 strokes, and today's 4 stroke would be much heavier. Would you suggest that I not put a Mercury 25hp 4 stroke on a boat like this one? Or would it not be an issue to put this heavy motor on it? thx
 
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sea nymph was sold a couple times...it was a Lowe boat and lowe is now part of the Brunwick boat group...you may give them a call and luck up and get an oldtimer familer with that boat....
what do you have on the boat now?you may just add the difference with something in the back of the boat and see how it sits in the water...if its a short transom boat with minimal flotation i would not do it...
the real danger in this situation is when you are going with the waves and lose power...then the waves coming aft will swamp you almost instantly...thats what you do not want to be exposed to..
 
I think the way HP ratings are calculated has changed since the '80's - from at the flywheel to at the prop. So, today's 20 HP (maybe lighter than today's 25 HP?) might give you close to the same performance.
 
if i remember correctly the old 25 evinrudes weighed 77 pounds without the electric start.. dont know about the new 4 stoke but the dealer should know...
 
ok..looked it up....25 evinrude 4 stroke is either 166 or 181 pounds depending on model...dont see a short shaft model...both are 20 inches...
 
A short shaft 25XD (back in the 80's) tipped the scales at 114 pounds.

The new 20 horse @ 115 pounds would be a good substitute (I don't like 4 strokes but this is a pretty sweet little motor - they also build the 15 horse now on the same platform).

The rating system from brake to shaft horsepower was not a "specific" date, but the major mfg's mostly changed over in the mid/late 70's so your 1980's boat would have most probably been rated for SHP (today's standard).

The actual weight of the motor is generally not the issue, it's the torque created where the motor is essentially trying to rip the transom off the boat.

The prop is trying to (crawl) under the boat which in turn "torques" (think crowbar) the top of the motor (powerhead) backwards. Since the bracket holds the motor it ends up creating thrust to move the boat forwards.

So if the loaded/rigged boat weighs say 1000 pounds, that is what is truly stressing the transom, not the 100 or even 150 pounds simply hangin' on the back - the 500 plus pounds of thrust (from a 20 horse) from the motor pushing on the transom to get the boat moving is easily 3 times the weight of even a heavy modern 4 stroke within your horse rating.

Don't get hung up on the weight of the motor. If the motor is significantly heavier than what it was designed for it will create issues, but not for the transom - it will change the attitude of the hull (make it bow high) which can lead to stability issues or proposing, but a couple hundred pounds on the transom won't "hurt" anything structurally...
 
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agree...swamping the boat is the only concern....and that would occur when the boat is stopped suddenly for any reason...
 
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