Motor weight on boat

[EHardgrave]

We recently replaced a johnson 60hp 2 stroke with a 70hp 4 Stroke. This motor weighs in at approx 350Lbs. We haven't put it in the water yet. This is on 1991 175 SeaNymph that says it is rated for up to 90Hp (2 stroke I assume). Would there be any problems with the weight of this motor on this boat?

 

[Jilly - 5]

Re: Motor weight on boat

HP rating has less to do with weight on the transom, as it does with torque moment on the transom from the prop force. Prop is anywhere from 2 to 2 1/2 feet below transom mount, creating high torque on the transom lip. Use good sized wooden backing plate (say 4" x 1" x 15" - teak, mahagony, etc) inside boat to distribute engine rotating moment on transom.

 

[dajohnson53]

Re: Motor weight on boat

What did your old motor weigh? What is the weight of a current 90 hp 2 stroke (which is what you assume the HP capacity to be based on). How does your new 70's wt. compare to a 90 2 stroke?<br /><br />As the other guy said, the HP rating is based on the forces imparted by the engine, and also the weight. My personal opinion (worth what you're paying for it) is that the engine torque force would be approximately the same for a 90 HP 2 or 4 stroke, therefore a 70 HP 4 stroke would be unlikely to exceed that. Therefore, if you know the weights involved for comparison, you might feel comfortable (or not). Better yet, why not contact the boat manufacturer?

 

[EHardgrave]

Re: Motor weight on boat

I assume the torque of a 4 stroke wouldn't be any more than a 2. Since the boat is rated up to 90hp I think all of that is ok. I just didn't know if the back of the boat would be buoyant enough for the weight of the bigger motor.<br />Good ideas, thanks. I will gather that info and compare. I think Lowe bought out Sea Nymph so I don't know if the info there is readily available. I will see.

 

[CaptainPuget]

Re: Motor weight on boat

You’ve added about 100 lbs to the stern. The boat has plenty of buoyancy to float an extra 100 lbs but you probably find that the handling is affected and not in a positive way. The only way to find out is to put it in the water and test it. There are methods (tabs, foils etc.) to increase the stern lift to compensate for the extra weight if you find that you need it.

 

[JB]

Re: Motor weight on boat

Your boat will have no problem with the 4 stroke 70. Most 2 stroke 90s weigh in the vicinity of 300 lb.<br /><br />Hook her up and go boating.<br /><br />Good luck.

 

[beniam]

Re: Motor weight on boat

WY-Johnson<br />Done that. Went from a lighter 2 stroke to a heavier 4 stroke on a 16 alum. boat. Moved fuel forwards to compensate for additional 100 lbs.Works fine. Maybe your passengers can sit forward. Be happy, enjoy life, boat lots. Batavier

 

[dajohnson53]

Re: Motor weight on boat

WY: post what you find re: weight of old engine vs. new, and what you found when you water tested it. I'm curious. Thanks

 

[roscoe]

Re: Motor weight on boat

Why does everyone assume that boats are rated differently for a 2 stroke vs a 4 stroke? Or that boat hp ratings were all based on you having a 2 stroke motor? I didn't see any reference to different hp ratings based on the type of motor was being used the last time I looked at a rating plate.<br /><br />4 stroke motors have been around for many many years. <br />I'm sure the boat manufacturers knew that 4 strokes were out there, as they were being sold at the same dealerships that sold the boats.<br /><br />Homelite Bearcats in the 60's, Hondas around 1970, some much earlier.<br /><br />Your boat comes with a hp AND a weight rating. Use them both to rig your boat and you should be fine.<br /><br />If it can't take it, your transom probably wasn't in as good of condition as you thought.

 

[roscoe]

Re: Motor weight on boat

1995 J60 248-255#<br />1995 J90 301#<br />2003 J90 335#<br />2003 J70 4stroke 359#<br />2003 J90 4stroke 421#