115 hp johnson too much?

[jeremy_nash]

I have a 1988 16' thundercraft regency boat. it is rated at 90 hp max. I had until today a 78 johnson w/ a 13 3/4X13" solas 4 blade prop on it. acceleration is decent, but top end is lacking. so I bought a trashed boat with a good running 1980 115 johnson. with it being a newer hull with an older motor and due to the way motors were rated in the early 80's I think I will be ok, of course I will be using the power wisely. boat is mostly used for skiing, tubing, and wakeboarding. with the occasional just take it out to go swimming and cruise around on

 

[jeremy_nash]

Re: 115 hp johnson too much?

fiberglass, deep vee hull

 

[mikeasb]

Re: 115 hp johnson too much?

I have a 1975 16' Starcraft deep V with a 115 Merc Inline Six. Runs like a scalded dog. Handles just fine and I have no problems with it. Run a 19p 3 blade Alum prop. The data plate is long gone so I have no idea what the boat was rated for. When I bought it it had a Merc 950 (95 hp) on it and it ran good with that (about 35 mph w/gps) but the hole shot was a little weak. Now it jumps up and onto plane in just a couple of secs, and tops out at about 41 gps.

 

[86 century]

Re: 115 hp johnson too much?

I would say you will be ok as long as your boat is solid. The engines weigh the same so thats not an problem however there may be liability concerns. And you should never over power a boat.

Now that I am done with the disclaimer. My '80 18' starcraft ss is rated for 150hp she has a modified 200hp merc. It is a lot of fun but I find myself constantly checking the transom for signs of stress.
Good luck

 

[wire2]

Re: 115 hp johnson too much?

I believe the practical reason for stating a horsepower limit on a boat is to avoid placing too much weight on the transom with a huge o/b. Having more HP than the stated limit does not necessarily make it dangerous. It's how the driver handles it.
Cars are not limited, a new Caddy CTS-V has 556 hp. Is it dangerous? No. It can be driven in a dangerous manner. So can an old Chevy Vega.

 

[CheapboatKev]

Re: 115 hp johnson too much?

I believe the practical reason for stating a horsepower limit on a boat is to avoid placing too much weight on the transom with a huge o/b. Having more HP than the stated limit does not necessarily make it dangerous. It's how the driver handles it.
Cars are not limited, a new Caddy CTS-V has 556 hp. Is it dangerous? No. It can be driven in a dangerous manner. So can an old Chevy Vega.

Id agree with the "practical" statement, however there is also the fact that certain hulls are simply not designed for speeds above "X" .

All sorts of unpredictable things happen, chine walking, sliding...all sorts of spooky stuff

 

[tx1961whaler]

Re: 115 hp johnson too much?

The actual weight of the outboard is of minimal concern. You could probably put 400 static lbs. on the transom without damage. It is the torque produced by the propeller thrust at the end of the motor leg (lever) that is the issue and what the ratings are all about. The lever (leg) is trying twist the transom off of the boat when it is under power. More HP means more force.
Going from a 90 to a 115 might give a 5 mph bump on the top end.

 

[JB]

Re: 115 hp johnson too much?

Second guessing the engineers who rate a boat for HP and then trying to rationalize overpowering by nit picking engine weight and other features is a bad, bad idea.

If you put over 90HP on your boat you are OVERPOWERING and VIOLATING MANUFACTURER'S SPECS. No yabuts mean anything.

If you are unhappy with your boats performance with a legal engine learn about and optimize the propping and rigging of your boat. Fine tuned propping and rigging are worth 25-35% of engine power.

 

[cr2k]

Re: 115 hp johnson too much?

If the motor falls out of the Caddy you just come to a stop on the road. If the motor falls off the boat (bolted to the transom) you sink.

Not quite the same. Not even close.

 

[5150abf]

Re: 115 hp johnson too much?

Wire2, the Cadillac was engineered to have that much power, the boat not so much, add in that it is 22 years old and certainly not as strong as it was new and you are asking for trouble.

Look at it this way Jeremy, if you were rated to hold 90lbs over your head and I put 115 on you what would happen?

I would imagine you could hold it for awhile but eventually it is coming down on your head, the boat is the same, it will take the bigger engine for awhile then fail, almost always on throttle at speed.

tx1961 got it, it has little or nothing to do with the weight of the engine, it is the torque it puts on the transom, think of the engine as a big lever, it is attached at the top and force is applied at the bottom, it is designed to hold 90 not 115.

 

[scoutabout]

Re: 115 hp johnson too much?

+3 on the last 3 posts. Those are wise words.

Just to be a bore I'll reiterate, dead weight sitting on the back of your boat is one criteria. Power output is of far greater consequence.

Is going from a 90 to a 115 an extreme example? Nope. We've seen people try to justify lots worse here. But you may have just put yourself in the "instantly liable / not insurable" category for any mishaps with that boat that you probably don't want to wear.

 

[Solittle]

Re: 115 hp johnson too much?

Lawyers love guys who overpower their boats. They will go for everything you own.

 

[Bob_VT]

Re: 115 hp johnson too much?

Lawyers love guys who overpower their boats. They will go for everything you own.

and anything you might have planned to own in the future could be interfered with too!

 

[Cannondale]

Re: 115 hp johnson too much?

In almost every case, I agree with Bob, et al. But there are those rare circumstances that go against the conventional wisdom.....like my baby boat.

I've got a 1990 Quantum 16' bass boat. In '90, the motor that came packaged was a 50hp Force and the hull capacity plate reflected this with a max recommended 50hp.

But a year later, when Force got a 70hp added to its lineup of motors, Quantum immediately upgraded the exact same boat to 70hp with no other changes to the hull, except increasing the max hp on the hull capacity plate.

So, sometimes it's only what's available that dictates the max hp rating.