Using an old outboard on a new hull

[jeff_smith_0423]

Friends..

I'm pondering using my old Merc 850 on a new deep-v hull. I'm looking at the alumacraft fisherman 160cs. The alumacraft specs state a 40 hp max outboard motor. When i use the coast guard formula, i get closer to 80hp.

When rigging a setup like this would you trust the coast guard formulas? I really would like a new hull that's lighter and easier to tow than my 15' tri-hull. If my current rig can handle the power, is it possible that this one can't? I would really like to get a few more seasons of use out of the 85. It's a great running and good looking motor. I'll also be using a CMC tilt-trim unit if i go this route.

I believe this was a flywheel rated motor, so it was closer to 70-75 hp at the prop - plus, it's about 30 years old, so it's certainly not producing that much. I believe the dry weight on the motor is about 250 lbs.

So, what do you guys think?

 

[JB]

Re: Using an old outboard on a new hull

Putting 85HP, even flywheel rated, on a boat rated for 40HP MAX is a really, really bad idea, Jeff.

Aluminum boats don't have the rigidity of 'glass, so the Alumacraft guys know exactly what they are doing when they rate their boats.

Many years ago I put a Johnny 18 on a li'l aluminum semi-vee rated for 14. It took that Johnny one season to rip the tinny apart. Leaked at every seam.

This is more of an outboard topic, so I will move it to Non-Repair Outboard.

 

[Texasmark]

Re: Using an old outboard on a new hull

You are asking a multipronged question. Part of the answer should come from your insurance company; you won't like the results.

Friend had a round bottom 14' Alumacraft of the '60's vintage. Tough boat, not many rivets like Lone Star had and all. Mounted side console, remotes, and a 33 Johnny. Plenty of whoopie for that boat. The engine you refer to is way overkill.

I do frequently ponder the answer to the question about boat ratings prior to the mid '80's hp changeover in HP ratings and boat ratings. I guess, I would take the facts to the local authorities and insurance co. and seek their opinions. I would subsequently act upon them. Otherwise your arse is hanging out in the wind and anything could come by and whack it; and I'm sure you don't need that.

Mark

 

[Frank Acampora]

Re: Using an old outboard on a new hull

The Goast guard formulas (formulii?, formulae?) don't take into account the gauge of the aluminum or method of supporting the transom etc. And you may be interpreting incorrectly thus coming up with an invalid rating. The manufacturer knows the limits of the design; Most manufacturers are going to rate a boat for the max its design will allow, for advertising and pr.

So: in this case even though My motto is "too much horsepower is never enough." I would adhere to the manufacturers maximum horsepower rating.

As an addendum: I have a 14 foot fiberglass flat bottom that was probably rated for 35-40 HP. I have a 90 on it BUT--You don't want to know the work I put into it to handle the power AND I only use it once in a while to zip up and down the river for 2 hours. Then I go home and glue it for 8 hours. It is definitely NOT an everyday or every weekend boat.

 

[jeff_smith_0423]

Re: Using an old outboard on a new hull

Yeah - i thought it would be pushing it. I was just wondering if Alumacraft had a history of underrating.. They have several other 16 foot options that are closer in rating - just overkill on features gadgets. The bigger issue is that i need something smaller than 17 feet so it'll fit in the garage.

 

[achris]

Re: Using an old outboard on a new hull

...The bigger issue is that i need something smaller than 17 feet so it'll fit in the garage.

So... extend the garage... Geez man, why wasn't that so obvious!!!



Chris..............
(see thread on using a multimeter for a DVA adaptor for your existing meter)

 

[DJ]

Re: Using an old outboard on a new hull

Yeah - i thought it would be pushing it. I was just wondering if Alumacraft had a history of underrating.. They have several other 16 foot options that are closer in rating - just overkill on features gadgets. The bigger issue is that i need something smaller than 17 feet so it'll fit in the garage.

Have you thought about a swing-away or removable tongue, for the trailer? That's always been by solution.

 

[bassboy1]

Re: Using an old outboard on a new hull

Yeah - i thought it would be pushing it. I was just wondering if Alumacraft had a history of underrating.

There is a difference between a history of underrating, and a bit overboard. Underrating might be putting 115 percent of the rating on, like may possibly get done to mine in the future. But, what you are planning on doing is putting 213 percent of the rating on - a bit more than would be considered "reasonable."

 

[jay_merrill]

Re: Using an old outboard on a new hull

Overpowering aluminum boats around my part of the world seems to be a "national pastime." I see flat bottomed and semi-v jon boats here all the time, with motors much bigger than they are rated for. It seems that everyone wants to go 60 mph whether the hull is designed for such speeds or not.

As someone else mentioned, its not the Coast Guard or Marine Patrol that will be your problem, its your insurance company who will cause you major headaches. Whether your motor is old & tired, flywheel rated or whatever, they only thing they will look at if you get in an accident is the maufacturer's rating on your boat, and Mercury's horse power rating on your motor. You can bet that if you hit something or someone, as a result of your accident, "plaintiff's" lawyer won't be much interested in the CG formula or how your motor was rated either.

I think the points about excess stress on the boat are valid too. My experience in this type of construction is greater in regard to airplanes than it is to boats but this I can tell you - catastrophic failure of a rivited aluminum structure usually results from a couple of basic scenarios .. first, from metal fatigue caused by normal usage over many years, sometimes made worse by corrosion and, second, by repetitive overstressing of the structure over a shorter period of time. Throw a little corrosion into the second situation and you are pretty much begging for a not so pleasant "boating experience."

I can really understand wanting to keep that old Merc in use because, when they are right, the old motors are sweet - just like the Energizer Bunny, they just keep going and going! Depending on how you feel about a few things, maybe you should still get a new aluminum boat but just go for one that is a bit bigger.

 

[Solittle]

Re: Using an old outboard on a new hull

Plaintiff's lawayers love cases where the defendant has an engine in excess of the rated power.