Max hp listing (rating)

[nathanhooper]

This is not technically about a boat motor, more so the boat, but it does involve motor issues, so I am not sure where to post this. But here is a question I hope someone can solve for me.

I have a short shaft motor I want to put on a boat. It is a 50hp one and I have not been able to find a boat to put it on. Well I think I found one, it has the high transom sides that then come down to a low point where the motor mounts. It currently has a 35hp on it, so I know its rated for that much, I would assume.

The problem is I cannot find any documentation on it. It does have the model number, but no max hp rating. According to the model, it can accept up to a 80hp, but if you add one little letter to the end of the model number then it is only rated for 40hp, that is according to the records here at iboats.

So my question is, if it is indeed only rated for a 40, how would I know that for sure? And, if the transom is strong, would putting the 50hp on there cause some major issues right off the bat, or is it something that would be bad in the long run.

For instance, could I put the 50hp on there, and if it floats all right and everything like that, is it safe to use? I guess the bigger question is how do they come up with the max hp listing to begin with? I hope this makes sense, and you understand my question. thanks

 

[nathanhooper]

Re: Max hp listing

Re: Max hp listing

I guess a question to ask to clarify my other question is it a matter of the boat sinking because of weight of too large a motor, or safety of running too fast, or the transom ripping off because of the weight?

Hope that helps.

 

[roscoe]

Re: Max hp listing

Re: Max hp listing

The transom ripping off because of the thrust, but more likely, it is a a handling and control issue.

 

[BuzzStPoint]

Re: Max hp listing (rating)

A few things will play into this.

If the transom is solid, and you're going over by 10hp. It may not matter. How old is the motor? If it's old, is that 50 horse even pushing out 50 anymore?

My max rating of my boat is 45, I'm putting a 55 horse on it. From what I'm as long as you dont run over the max weight of the boat, it should be fine..

But, If I go by capacity plate, it says 45, if I go by the coast guard formula, it's 50 horse.

 

[nathanhooper]

Re: Max hp listing (rating)

Not sure on the transom yet. I have not purchased the boat yet, because I do not want to get something that will not fit the motor. I will check that out. it looks as thought it is indeed solid, and it is an open aft so I could easily "beef" up the transom if needed.

The motor is old, not sure what it puts out anymore, but that is a good point you bring up. I guess at that point it would just be a weight issue. I have it narrowed down, as far as max hp rating is concerned. If it is indeed the model he is saying it is, then the max hp is 80, if not, then the only other models that would come close are rated at 40hp max. Either way i think that I would be safe. But as always, if someone has true experience with this and wants to comment I would be very appreciative.

 

[BuzzStPoint]

Re: Max hp listing (rating)

Here you can compute the max via Coast Guard rules as well.

Compute a factor by multiplying
the boat length in feet by the maximum
transom width in feet excluding
handles and other similar fittings, attachments,
and extensions. If the boat
does not have a full transom, the transom
width is the broadest beam in the
aftermost quarter length of the boat

(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".

 

[roscoe]

Re: Max hp listing (rating)

My max rating of my boat is 45, I'm putting a 55 horse on it. From what I'm as long as you dont run over the max weight of the boat, it should be fine..

Buzz, you are in WI, land of the regulated.

You are illegal when powering your boat beyond the rated hp.
See page 21 of the WI boating regs, "Prohibited Methods of Operation."
Concidered "willful violation" under the law, meaning you are automatically guilty.

Just thought I'd let you know.

Happy boating.

 

[ClassyGlassy]

Re: Max hp listing (rating)

you also need to look at whether or not the hp rating is "actual" hp or hp rated at the powerhead!

My '76 Merc 150hp is rated at the powerhead but through gear reduction through the prop I'm somewhere around 120-130hp. Whereas my father's '94 Merc 150hp is rated at the prop and delivers 150 "actual" hp........

Your boat is rated for "actual" hp...........

 

[roscoe]

Re: Max hp listing (rating)

you also need to look at whether or not the hp rating is "actual" hp or hp rated at the powerhead!

My '76 Merc 150hp is rated at the powerhead but through gear reduction through the prop I'm somewhere around 120-130hp. Whereas my father's '94 Merc 150hp is rated at the prop and delivers 150 "actual" hp........

Your boat is rated for "actual" hp...........

Try explaining that to the conservation warden and see where it gets you.

 

[109jb]

Re: Max hp listing (rating)

A few things will play into this.

If the transom is solid, and you're going over by 10hp. It may not matter. How old is the motor? If it's old, is that 50 horse even pushing out 50 anymore?

My max rating of my boat is 45, I'm putting a 55 horse on it. From what I'm as long as you dont run over the max weight of the boat, it should be fine..

But, If I go by capacity plate, it says 45, if I go by the coast guard formula, it's 50 horse.

1 - if a manufacturer rating exists, it trumps the C.G. formula.

2 - your 55 horse motor will be over BOTH the manufacturer rating and the C.G. rating.

3 - The formula you posted L x W x 2 - 90 = hp is for boats with a transom depth of 20 inches or more. We don't know what the transom depth is on the boat in question but we do know it had a short shaft motor on it.

4 - I'm assuming we are talking about an older boat. We don't know what limited the manufacturer rating. Could have been horsepower, handling, or engine weight or a combination of these. If weight was a factor, then you must also consider if the a replacement engine is comparable in weight to the older outboards. A new 4-stroke may be too heavy.

5 - Even if powering above the manufacturer rating works, it is in my opinion a bad idea for 3 reasons. First, in many states it is illegal. You argue that it is the prop hp the boat is rated for. I live in Illinois and lived in Wisconsin earlier, and the regulations for both those states say that you can't power over what the capacity plate says. Arguing that a motor that has a big 55 on the side of the hood is actually only a 40 or whatever ain't gonna cut it. Second is liability. If something unfortunate happens then you are opening yourself up for big trouble. If you don't mind I don't car but I'm not gonna open myself up for that. Third is kind of related to the second one. If I overpower my boat then my insurance company could deny any claim. Is it likely they would?? I don't know, but again I am not going to take the chance and give them any kind of out.

 

[ClassyGlassy]

Re: Max hp listing (rating)

Try explaining that to the conservation warden and see where it gets you.

It was the Colorado State Parks person that explained it to me when I started building my boat.......my boat was a stern drive boat rated for 140hp that it came with and I now have a sturdier transom with an older 150hp motor!

I would contact your local State Parks person to explain YOUR state's regulations..........