Need you infinite wisdom

Hello folks,

I have a technical question for you folks. I have a 1968 Larsen runabout, rated for a max motor of 55hp. (According to the plate). I have a 1976 70hp Johnson I was thinking about mounting on the boat. I know that typically, in this situation, a 70hp is too much motor. However...I was told by a mechanic, that with the changes in horsepower from 1968 to 1976, that technically, in 1968, a 55hp was the equivalent of a 70hp in 1976. Is this true? Am I crazy for thinking about putting this motor on this boat? If it is true about a 1968 55hp motor being the equivalent to a 1976 70hp motor? Is there documentation of this, so I can be legal? I know the Coast Guard would frown on a 70hp motor mounted on a boat rated for 55hp. However, if I can prove the above, I may be able to move forward with this motor. (No pun intended).

Thank you very much for any help you may be able to provide!

Drew Doman
Portland Or.

Re: Need you infinite wisdom

Hey Drew:

If there is a plate on your boat that states the max size of engine on your boat, then that is what the CG s going to go by. Not any story from a mechanic.

On the other hand, you could replace the numbers on the engine so that it says it's a 55.

BTW - whenever I have been stopped and checked by CG or other water police, they haven't checked the engine to boat size. They have only cared about number of people and life jackets, bailing bucket, signaling devices, throwing line, and oars.

Randy

Re: Need you infinite wisdom

You need to be more concerned about hungary lawayers than the CG.

Re: Need you infinite wisdom

I would be thinking about the 32 year old transom.

Re: Need you infinite wisdom

I just cut up a late 60's Larson Trihull, it was rated at 55hp too, after cutting that boat apart for disposal, I'd say that a 55hp is about all I'd want to hang on it. That boat was pretty much an open dual console type, bowrider design trihull with a very light duty looking transom.
I would say that motors got stronger in later years, the hp to weight ration got better, so maybe weight wise you may be close, but I have both a 50HP 1969 motor and a 1978 55hp motor here and there's not much difference in weight, but there's a huge difference in weight compared to the 70hp 3 cylinder Johnson on my Starcraft. I can hoss the 55 hp on an off by myself, but no way would I attempt that with the 70hp, I'd guess that motor has to be in the 250lb weight range, while the 55 is at least 50 lb lighter being only a twin cylinder with a much lighter lower unit and midsection. If your 70hp also has power tilt and trim, the weight difference is even greater.

If your looking to upgrade power, I'd look to the largest hp rating in the same engine weight class or family at best, and even at that there could be liability issues, but I like others have mentioned above have never had anyone check my motor vs the boats rating. I think with a boat that old, the weight alone would worry me more than the hp. Adding that much more weight to an old transom could be trouble.

Re: Need you infinite wisdom

Thanks for the input!