motor rating

[broncosguy711]

how is it decided when a boat is rate for what power of engine? and why would a 16, 17 or 18 foot boat be rated for a bigger engine over a 19 foot boat?

thanks

Broncs

[BobT]

I'm guessing here but I would imagine it has to do with a number of variables such as

1. How robust the transon is

2. How much water is displaced by the boat

3. Boat width, height, length, weight together probably determine how high the boat floats on the water. Don't want to be without some clearance.

6. Boat length to width ratio. I imagine as you get closer to 1:1, handling becomes more irratic therefore hp rating is reduced.

Bob

[delcecchi]

Not nearly that complicated. I googled and got

How is the maximum rated power determined?

In response to a mandate from Congress contained in the Federal Safe Boating Act of 1972 (FSBA), the United States Coast Guard issued regulations covering small outboard boats and the maximum rated horsepower for them. The regulations bind the manufacturer to provide a capacity plate that shows the maximum rated horsepower for each hull.

The applicable regulations can be found in 33CFR183.53 My interpretation of those regulations is that the manufacturer must state the horsepower as no more than that computed under the formula, except, if the manufacturer conducts the performance testing procedure, the results of that testing can be used (with some qualifiers). However, the manufacturer is apparently free to state a lower rating if desired, and sometimes does. Note that the regulations only require ratings for monohull boats under 20-feet in length, but Boston Whaler has historically provided a rating plate for its boats longer than 20-feet as well.

The formula provided in the regulations reduces to:

(2 X L X W) -90 = rated horsepower

Where:

L=boat length

W=transom width; if the boat does not have a full transom,

the transom width is the broadest beam in the

aftermost quarter length of the boat.

The rated horsepower may be rounded up to the nearest "5".

[Valv]

Delcecchi, I think your formula is wrong, if I try it with a 17ft boat that's what I get:

2 x 17= 34

x 92" (width) = 3,128

3,128 - 90 = 3,038 HP

I think that's a little too much for a 17ft

It will come out OK with "divide by 90" (instead of -90) which makes 33.7hp, but still too low for a 17ft Lund Explorer 1700

[hcrider]

We might need an example of how it works out. Valv, you've switched units in your equation. You used feet for the length and inches for the width. I suspect that may result in a significant problem for the overall equation.

[Valv]

Quote:

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We might need an example of how it works out. Valv, you've switched units in your equation. You used feet for the length and inches for the width. I suspect that may result in a significant problem for the overall equation.

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If I transfer everything in inches it makes it even worse....

Let's do the math then.

[stmichael]

I transferred the 92" inches into Ft (7.7 rounded up from 7.666666666666666666666666666666666666666666666666666666666666666666666666666666666666)sorry button stuck

2x17(ft)=34

x7.7(ft)=261.8

-90=171.8 / 175 h.p.

"rounding up to the nearest 5"

[Valv]

That's too much HP.

I think weight has to do soemthing toom but not sure.

Funny part, a bass boat of same dimensions has a more HP capacity than others.

Also rating from some years ago are lower than current.

[upnorth]

There has to be a bit more to it than 2xlxw type of thing. Just look at what they put for HP on some of the lund pro v boats. And then look at a boat of not quite so high end and it is 50 to 100 horse less. There has to be more to it than that.

Kinda piqued my curiosity so I did some digging and found a pdf that explains it fairly well. Here is the link.

http://www.uscg.mil/d8/mso/louisville/WebStuff/comdtpubp16761_3b.pdf

Steering type, transom height and a few other factors come into play.

[stmichael]

maybe the materials/structural integrity of the transom? Cheaper the materials = lower rating?

[Wade Joseph]

The hp rating is based on what the boat will maintian level floatation with. This means if you swamp the boat, it must maintian level floatation with the motor its rated for. As long as you follow the USCG label plate (the yellow one) and do not exceed the weight and hp limits, the boat will stay afloat even if full of water.

The problem that is looming (and some older boat owners already have run into this when repowering) is how to distinguish between a 4 stroke that weighs as much as 200 lbs more than the comparable sized 2 stroke such as an e-tech. I know some guys who have repowered offshore boats that had twin 150 merc two strokes with 150 hp 4 strokes and wonder why they sat 6 inches lower in the stern. It was the added weight of the 4 stroke motors.

Actual HP has nothing to do with the rating. when the system was started, all the engine mfg's motors all weighed roughly the sdame for a given hp.

[Valv]

I don't think there's any boat that stays afloat even full of water, with the exception of some Boston Whalers.

The flotation foam will slow down the sinking process and keep boat afloat longer, but it will still go under.

[broncosguy711]

Wow, I thought this/there would be a easier answer.

[stmichael]

Isn't it amazing how the simplest issues turn out to be almost as complicated as relationships. (I was going to say women, but, I thought about it and decided it would make a good afterthought.) I hope my wife does not read this thread...

[BobT]

I hope not too but what about the other who knows how many women that are reading this? You might just have opened a hornet's nest.

Bob

[delcecchi]

For console steering and a 20 inch transom, that reference gives 2*l*w-90 where length and width are in feet. For a 17 foot by 7 foot boat that would be 150 HP. 17*7 is 119. 119*2 =238. 238-90 is 148 round up to 150. My 1775 ProVee is rated at 150. Some probably depends on where you measure the width. At the transom or somewhere else.

[Wade Joseph]

Valv..I am not gonna argue or bicker, but they should in fact stay afloat if they are not overloaded beyond the posted weight limit of persons motor and gear. Thats why the numbers are posted on the plate.

[upnorth]

I work with a guy who swamped his Alumacraft and it still floated enough to get to shore.

[Valv]

Quote:

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Valv..I am not gonna argue or bicker, but they should in fact stay afloat if they are not overloaded beyond the posted weight limit of persons motor and gear. Thats why the numbers are posted on the plate.

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Well then why they sink ? I don't want ot argue neither but they are not made to float if swamped or filled with water.

There are many instances where boats sink at dock due to failure of bilge pump, it gets filled with rainwater or a leak, and they end up on the bottom of lake/sea.

Flotation foam is not to float boat, it's to help mantaining it afloat for longer time, but will eventually sink, unless boat is capsized and some air is trapped inside.

[BobT]

I don't think I can agree with you on this. The boat will either sink or float. Adding foam will not make it hang for a while then sink. Once submerged, if it is going to sink it will begin to do so immediately and not after some period of time.

Do you have some information from the boating design specifications to back up the claim that foatation is for the purpose of slowing the sink rate?

Bob

[Fishin4Life]

Post deleted by Fishin4Life

[ewirz]

OK, so here's a question....

I have an Alumacraft Tourny Pro 175 tiller. The equation gives 80hp. (which is what the plate says). The counsel version comes out to 175h.p. Nowadays, with large tiller motors, does that mean I can put 100, 125, 150h,p tiller on my boat safely?? (I'm not actually going to do it, just hypothetical)

ERW

[BobT]

Quote:

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The hp rating is based on what the boat will maintian level floatation with. This means if you swamp the boat, it must maintian level floatation with the motor its rated for.

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Here's something that seems to dispel this theory. I have noticed that boats with consoles are usually rated higher than boats with tiller design. Here, the balance of the load inside must play a role. With a tiller, a certain part of the load is always located near the rear. With a console, that same constant is located more nearly centered. Seems to be pointing back to handling and load distribution.

Bob

[Wade Joseph]

Your exactly right Bob.Although its not a "theory". "LEVEL floatation" cannot be achieved if you have a 250 lb man AND a 250 lb motor at the back of the boat...but put the man in the front 1/3rd and then it will, or lighten the stern load with a smaller motor to balance things out. It doesn't hold much water in reality now days with motors of several HP ratings being made on the same frame/powerhead. ie..75/90/115 hp evenrude (the 75 ficht I have is the same as a 90 or 115).

As I also said, the "system" as designed has flaws now days because of the weight difference between say a 150 e-tech and a 150 honda/yamaha 4 stroke. Depending on the motor..this difference can be over 100 lbs.

[delcecchi]

go to the coast guard page and read up on it. Basically the answer is no. Not and meet the regs.

[upnorth]

If you read the coast guard link I posted earlier. Steering console boats are rated higher than tiller boats. Not sure why but that is how they rate them probably has to do with where the operator sits in the boat and how things will balance.