What determines HP rating on hulls?

[62Scout]

I'm curious as to how HP ratings are determined for hulls, and if there's a SAFE way to increase that at home? At this point, I'm looking at this purely from an academic standpoint, and just curious for the knowledge. At some point in the future, I may look into building a boat from scratch at home, though that won't be happening any time soon.

I understand that engine weight, and by default, hull balance, along with torque loading on the transom plays into this. I'm also assuming that there's a significant amount of engineering that goes into the final determination, and that the simple answer is "buy a boat rated for more HP". Part of this is the thought that if I ever found myself needing to repower a boat, and I found a screaming deal on something with more HP than my hull is currently rated for, what would I do? Though if I'm thinking if I ever got to the point where I wanted to put an engine with a serious amount of power in a boat, I'd be looking at I/O setups anyways.

 

[tpenfield]

The USCG has some formulas and guidelines for the boat manufacturers to follow.

Beyond that there here may be some manufacturer’s discretion.

 

[Scott Danforth]

, and if there's a SAFE way to increase that at home?



I'd be looking at I/O setups anyways.

boat with outboards are rated per international maritime law (USCG formulas), and there is no way to increase that rating without certified testing (some manufacturers make same hull for multiple brands, and one brand was tested, making that brands version of x hull capable of higher hp)

boats with I/O's are not rated. there is nothing stopping you from putting a 2000hp blown aluminum 632 monster motor in a 14' boat with an I/O

 

[62Scout]

Ahh, just found the USCG Boat Builder's Handbook. Looks like that will be some good weekend reading material

I know that the inboards and I/O boats aren't officially rated, as opposed to outboards, and I know that inboards load the hull differently than outboards do as well. I know that I wouldn't be able to legally increase the rating, much the same as changing the axle and suspension in my SRW F350 to a DRW setup didn't legally increase the payload to that of a DRW truck.

It was a thought that occurred to me as we were discussing boats here at work, and how I'd likely never be able to afford something like a nice vintage Chris Craft wooden boat, but I may be able to build a nice boat myself one day, which then transferred into thoughts of "how do I determine how much weight/HP that hull would handle??".

 

[Lightwin 3]

Number of people = vessel length (ft.) x vessel width (ft.) ÷ 15

 

[Keyboardman]

I have an old 75 Johnson on my old trihull that's only rated for 70. Wonder if the local law would bust me if I ever got checked!

 

[5150abf]

Just be sure to weigh the possible end result, your transom can't hold the extra power which you will find out at WOT going really fast.

Transoms generally don't sort of fail.

You can make certain your trans is in the best shape it can be, if the boat is 10-15 years you will want to look at anyhow.

From a liability stand point I don't think anyone is going to tell you to over power your boat.

 

[cassabby]

I have owned my 1988 Crestliner Nordic V1500 since new. It came with a 1988 Evinrude 60HP outboard. In 1999 I upgraded the outboard to a Force by Mercury I-4 1.7L 120HP. This is above the HP rating for this boat but the boat was built well, the motor was on the lighter side for it's HP and the boat runs extremely well. I have tuned in the mount height to the 3rd hole from top and run a 3 blade SS Lazer II 22p prop. I do not pull skiers just lake fish for Walleye and Pike and stay out of the top rpm's unless there is some kind of contest to get back to camp first. No issues in 30 years. Very happy with this survivor.

 

[Scott Danforth]

I have an old 75 Johnson on my old trihull that's only rated for 70. Wonder if the local law would bust me if I ever got checked!

I have owned my 1988 Crestliner Nordic V1500 since new. It came with a 1988 Evinrude 60HP outboard. In 1999 I upgraded the outboard to a Force by Mercury I-4 1.7L 120HP. This is above the HP rating for this boat but the boat was built well, the motor was on the lighter side for it's HP and the boat runs extremely well. I have tuned in the mount height to the 3rd hole from top and run a 3 blade SS Lazer II 22p prop. I do not pull skiers just lake fish for Walleye and Pike and stay out of the top rpm's unless there is some kind of contest to get back to camp first. No issues in 30 years. Very happy with this survivor.

the PO is talking I/O which are not governed by the rules that OB's are when it comes to horse power

Just be sure to weigh the possible end result, your transom can't hold the extra power which you will find out at WOT going really fast.

Transoms generally don't sort of fail.

You can make certain your trans is in the best shape it can be, if the boat is 10-15 years you will want to look at anyhow.

From a liability stand point I don't think anyone is going to tell you to over power your boat.

first, its an I/O and the USCG formula(s) for HP do not apply. Hull weight carrying capacities and people capacities still do apply. the forces on an I/O are on both the transom and stingers which is tied together by the hull. the typical 2" thick transom with 3/4" stringers is the same design if you have 100hp or 1000hp.

an outboard loads only the transom (ignoring any splashwell) with a horrendous moment coupled only to the transom at the clamp area. outboards area literally trying to pry the transom off the boat from the top. Add the weight of the outboard adding to this moment and you have the reason why OB hulls are rated. where an I/O still has a moment, however it is down at the X-dimension which is near the bottom of the transom and a thrust vector. the moment arm on an I/O is so much shorter than an outboard. hull reaction loading for an I/O boat transom for a given hp is about 1/3 that of the loading of an outboard. weight of the I/O motor is on the hull, not the transom

I seriously doubt the PO plans on anything more than maybe a 3.0 to a 4.3 swap or even a 5.7 swap or even a supercharged 6.2 stroker. hull design, x-dimension and pad design determine stability which would be more of a concern than HP capacity. HP output will determine which drive will survive. adding power to an I/O boat is not a structural issue unless the hull structure is rotten (then the hull needs to be restored anyway).

 

[high&dry]

For three years my hull was called the Nantucket 190 and was part of the value line from Boston Whaler that included packaging of trailer, engine and boats and certain options. Then with the introduction of the then new Montauk 190 in 2007 looming on the horizon the Nantucket 190 was rechristened the Outrage 190, the price went up but it included some new option packages and extra details (all of which I have added to mine).

So, why tell this, the maximum horsepower and engine weight went from 520 pounds and 150 horsepower to 550 pounds(?) and 200 horsepower and aside from that, there is not one single structural difference and I have confirmed that with the engineer at BW. Now, 150 horses is plenty to push my boat to near 50 MPH with the original prop with just me and my wife (little people) and no load. So, 200 horses would pin the ears back big time. And apparently all it took was a snap of the fingers. So to some extent, many of these ratings are very conservative and can be expanded whenever the manufacturer decides it is a goof thing. Like to justify a pricing increase from $33,000ish to $65,000ish.

In typcial northern Gulf chop I am doing good to stay on top at 25 MPH, not sure what I would do with 50 more horsepower. But, I guess it might be funner once and a while. It would be a solid 50 MPH boat for sure.

H&D

 

[QBhoy]

Something in between naval architecture and common sense and hull ability.
I always worry when I see older 80’s and early 90’s boats with an original recommended OB size rating referring to a 2-Stroke of the era....then someone decides it will be fine with a newer equivelant 4 stroke and a third or a half the weight on top. Also thoughts around the transom integrity after all those years.