Going Bigger Than The Boat is Rated

[l008com]

This is mostly just a curiosity/hypothetical question, as I'll likely never have the money to do this, and if I did,I'd probably just buy a whole new boat anyway. BUT.

So my '85 thundercraft is rated for a 90HP motor.
But I imagine that engines today are probably much lighter right? So you could probably get a 115HP or even more powerful motor, that weighs the same or possibly less, than my current 88' 90HP Merc. Is there anything else this HP rating is based on, other than the weight of the motor? What kind of problems might you have doing this?

 

[ondarvr]

Re: Going Bigger Than The Boat is Rated

I won't comment on whether you should over power it or not, but the new motors are actually much heavier than the older ones, so even if you went with a current 90HP it would weigh more than what you have now.

 

[l008com]

Re: Going Bigger Than The Boat is Rated

Reeeeaaally hmm thats interesting. I would have guess that like everything else, things would be smaller and lighter. Mainly because they can probably get 90HP out of a much smaller, lower displacement engine than it took years back. GM gets 305hp out of a NA 3.6l v6 these days (for example, obviously not relevant, just sayin)

 

[tmcalavy]

Re: Going Bigger Than The Boat is Rated

The newer motors generally make more hp for their weight, are more efficient, yada, yada, yada...but they also are just as complicated, or more so, than car/truck engines. They are quieter, too, and engineered so that it takes an engineer to find/diagnose/fix any problems that pop up. Ain't technology great??

 

[Silvertip]

Re: Going Bigger Than The Boat is Rated

Weight of the engine is a small part of the equation regarding HP ratings. If the boat is rated for 90 HP then it should not be fitted with a motor more than than regardless what it weighs. Besides, do the words lawsuit, attorney, incident, liability, insurance, negligence, accident and others like it mean anything to you. The fact that you even asked the question means this is something you would do in a heartbeat if a bigger engine came along. Have an accident involving personal injury and I will almost guarantee that the injured party sees an attorney and they immediately go after you for having an overpowered boat. Not only that but the insurance company would not stand by you either because you obviously lied to them telling them you have a 90 HP motor when you really have a larger motor. See where this all leads?

 

[Old & In the way]

Re: Going Bigger Than The Boat is Rated

BTW

I had a '92 Arima where the HP rating was actually increased and Arima created a new certified Coast Guard plaque for my boat.

I ended up putting a new 90 HP Yami on the boat, but in my research I found that the unproven (IMHO) Evenrude 2 stroke, fuel efficient, like the 115 HP had a very attractive HP to weight ratio.

Just my
$ .02

Ed

 

[tommykeith]

Re: Going Bigger Than The Boat is Rated

I had an old Starcraft (late 60s)rated for 45 hp and had a newer (80's) 55 hp. Called Starcraft and talked to one of their engineers. He said in the 70's hp rating changed from being measured at the power head to being measured at the prop so that the weight difference could be overwhelmed by the hp increase. End result, he sent me a new ID plate rating that boat for 55 hp.

Unfortunately, while trailering it one day, a car came across the yellow line and tore off the transom and she sits at the bottom of a lake for divers to dive to.

So check with the manufacturer.

 

[jakebrake]

Re: Going Bigger Than The Boat is Rated

sidin' with silvertip...add the word "drowned". the only thing between you, and a particularly nasty death is a very oscure law of physics, so stack the odds in your favor.

a transom is horsepower rated for a reason, and i don't feel like seeing you on you tube.

 

[l008com]

Re: Going Bigger Than The Boat is Rated

I had an old Starcraft (late 60s)rated for 45 hp and had a newer (80's) 55 hp. Called Starcraft and talked to one of their engineers. He said in the 70's hp rating changed from being measured at the power head to being measured at the prop so that the weight difference could be overwhelmed by the hp increase. End result, he sent me a new ID plate rating that boat for 55 hp.

Unfortunately, while trailering it one day, a car came across the yellow line and tore off the transom and she sits at the bottom of a lake for divers to dive to.

So check with the manufacturer.

So does that mean that once your boat was damaged, you intentionally just sank it?

 

[Frank Acampora]

Re: Going Bigger Than The Boat is Rated

Horsepower ratings are derived from a number of factors: Length of boat hull, width at widest point, tramsom width and height, freeboard height, type of hull--deep vee or flat or round bottom, closed deck or open, and steering. Weight of the engine plays a small part of the rating. It is related to the load carrying capacity of the hull. So, your hull is rated for 90 HP based on the size and design. A similar sized hull as yours MAY be rated for say 115 or maybe 85--depends.

I don't remember the manufacturer, but it was an offshore, multiple outboard hull. The manufacturer reinforced the transom with aluminum angle to get a 50 HP increased rating Coast Guard approved on the same hull.

That said: Too much horsepower is never enough. You should hang at least a 125 or 150 on the back of that boat. Then take it out and scare the bejeesus out of everyone on the water. LOL

Actually, increasing horsepower on a given hull is a diminishing returns proposition. You never get the speed you expect and the same horsepower, lets use 150 for example, on your boat would probably give the same or better top speed more safely on a hull rated for it.