Boat max horsepower rating?

[StuBone278]

Okay, so is the max horsepower rating based off the torque of the outboard on the transom, the weight of the motor, or both? I'm just wondering because these small new 4-stroke outboards are usually just as heavy as a 2-stroke of greater horsepower.


eg, My boat is rated for max of 20hp. I have a 20hp 4-stroke on it that weighs as much as a 25hp 2-stroke. It makes a slight creak sound when I go to get it on plane. I don't remember it doing that a while back. I checked all the transom and it seems to be in perfect condition. Thanks.

 

[tashasdaddy]

Re: Boat max horsepower rating?

that is one thing you have to be careful with, when it comes to older boats. the rating was based on the constuction of the boat. to handle speed and the weight of the motor.

 

[CATransplant]

Re: Boat max horsepower rating?

What kind of boat do you have? If it's a typical aluminum fishing boat, it's possible to reinforce the transom, using new braces from the transom to the gunwale of the boat. I've seen some pretty nice setups done that way.

If your transom's creaking, it sound a little overloaded with that heavy 20 hp 4-stroke. Perhaps a little strengthening is in order.

 

[stubblejumper]

Re: Boat max horsepower rating?

Be aware that outboard motors used to be rated at the powerhead,but now they are rated at the prop.As such,the newer outboards actually produce more power than an older engines with the same rating.This could be a factor when installing a newer outboard on an older boat.

 

[Frank Acampora]

Re: Boat max horsepower rating?

Boats are rated for horsepower using a Coast Guard formula that factors Transom Height, Transom width, Total hull width, hull material, and hull style--ie. Round bottom, flat bottom, deep vee.

Typically a 20 inch transom can handle more hp than a 15, and a deep vee takes the highest hp with round bottom and flat bottom in that order.

obviously, barring inadequate construction, the larger hull is rated for higher horsepower.

Even though newer engines are rated at the prop, it is not going to make a radical difference. Remember, the Coast Guard formula has a safety factor in it. However, if your boat is old and you hear noises from the transom with the heavier and higher horsepower engine, it is telling you that it needs reinforcement.

Me? I prefer to use at least twice the rated power--LOL

I actually do have a 14 foot flat bottom hull designed in the 1960s to handle about 35 horsepower. I rebuilt and strengthened it to carry a 90. ---Lartsa fun--faster than some jet-skis.

 

[StuBone278]

Re: Boat max horsepower rating?

Alright, thanks a lot yall. My boat is a 2007 WeldBuilt 1436 so I would imagine they got the factoring in right that 4-strokes are becoming more popular. The creaking is in no way significant, either, I was just kind of curious.