# Maximum Outboard Horsepower Capacity (Guide and how-to)



## MeanMouth

Hopefully this guide will help you calculate the maximum amount of H.P. your boat can be insured for.
- This is to only be used as a guide.
- You can round your final HP number to the closest multiple of 5. (31 HP would round to 30 HP. 33 HP would round to 35 HP)


*Step One - Compute your Factor* 

A. Length of Boat x Transom Width (measure in feet: 14, 15.5,16, etc).



*Step Two - Compare your factor to H.P. rating*

A. 0-35 = 3 hp
B. 36-39 = 5 hp
C. 40-42 = 7.5 hp
D. 43-45 = 10 hp
E. 46-52 = 15 hp



*Step Three - If your factor is over 52 (Choose only A or B) *

*A. With Remote Steering (steering wheel) and at least 20" transom height*

Maximum HP = (Your factor x 2) - 90

Example: 19' 6" long , 7 feet wide = (19.5 x 7 x 2) - 90 = 183 (Round up in multiples of 5. This boat's max is 185 hp).


*B. With Tiller Steering (hand or extension) or less than 20" transom height (Choose only 1 or 2)*

*1. Flat bottom, or hard chined boats (hull has hard angles)*

Max HP = (Your factor x 0.5) - 15

*2. Soft chine, rounded hulls* 

Max HP = (Your factor x 0.8) - 25


*Conclusion*

I hope this has helped you all who are looking for max hp ratings. If your boat is insured, make sure to check with your insurance company for any restrictions.

This guide was found at NewBoatBuilders, which was ultimately used from "Title 33-Code of Federal Regulations-Subpart 183.53".


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## Brine

Nice....

We should make this a sticky


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## lswoody

Brine said:


> Nice....
> 
> We should make this a sticky




I agree. Thanks!!!!!


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## Ranchero50

I think you need to clarify it a little bit. 

On my 1448MV rated at 25hp tiller with a 15" transom. If I use the transom width of 48" on the bottom with tiller steer it shows I can only run a 12hp motor, if I use the @6' top the factor shows 27hp. 

Seems lind of ironic that I'm able to run an 85hp inboard jet with a low center of gravity that handles like it's on rails.

What are the sources for this capacity limit formula? I don't remember reading it in the US Coast Guard's boat builders handbook. They do go into specifics on handling and power capacity limits that way.

Jamie


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## Outdoorsman

Above info and more can be found here....

https://files.dnr.state.mn.us/education_safety/safety/boatwater/backyardboatbuilders.pdf

Outdoorsman


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## MOBowhunter

Is the transom measured at the top or bottom? It makes a difference between max rating of 40 H.P to a rating of 70 H.P. Please advise. Thank you, Matt


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## Flat Bottom

I think that's fairly close when checking my boats specs...


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## FishyItch

Brine said:


> Nice....
> 
> We should make this a sticky



+1 more


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## KICKEDBACK

Very interesting info. I have a 1979 Mirro-Craft "resorter"(14) and the hull I.D. tag says max hp is 25. if you look at the exact same model on Mirro-Craft's web the max rating is now 15hp. I wonder if 25hp in 79' is less powerfull than todays 25hp? Just curious about it. Anyone know the answer. :?:


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## Decatur

Thanks!


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## Industry

KICKEDBACK said:


> Very interesting info. I have a 1979 Mirro-Craft "resorter"(14) and the hull I.D. tag says max hp is 25. if you look at the exact same model on Mirro-Craft's web the max rating is now 15hp. I wonder if 25hp in 79' is less powerfull than todays 25hp? Just curious about it. Anyone know the answer. :?:


I was told that outboards prior to the early 80's were rated on hp at the crank, and were later rated for hp at the prop shaft. Maybe that is why they changed the boats rating.


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## mtnwkr

Industry said:


> KICKEDBACK said:
> 
> 
> 
> Very interesting info. I have a 1979 Mirro-Craft "resorter"(14) and the hull I.D. tag says max hp is 25. if you look at the exact same model on Mirro-Craft's web the max rating is now 15hp. I wonder if 25hp in 79' is less powerfull than todays 25hp? Just curious about it. Anyone know the answer. :?:
> 
> 
> 
> I was told that outboards prior to the early 80's were rated on hp at the crank, and were later rated for hp at the prop shaft. Maybe that is why they changed the boats rating.
Click to expand...


Also new boats are rated with 4strokes in mind which are much heavier.


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## TinBoatToo

Thanks!


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## LonLB

I'm not sure about how accurate that is. My max HP should be 130hp according to that.

I would be afraid to run full throttle with 130hp. :shock:


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## jdsgrog

MOBowhunter said:


> Is the transom measured at the top or bottom? It makes a difference between max rating of 40 H.P to a rating of 70 H.P. Please advise. Thank you, Matt



The transom is measured at the "widest point" on the transom. Doesn't matter if it's top middle or bottm. This formula is a general formula for those who have homemade boats or have boats without USCG capacity tags, especially on the old boats that didn't require them at the time. I'm not certain if this is an absolute "industry" standard used by boat manufacturers, but maybe someone in the industry can chime in. Bottom line, if you have a USCG tag, the formula is a moot point.

One point of correction, you need to round up to the next multiple of 5 hp. Like for my boat, the actual rating based on the formula is 33hp. But I would round up to 35hp which my boat is actually rated for based on the USCG tag.

In terms of hp rating, I believe pre-mid eighties hp was measured at the shaft and thereafter was measured at the prop. There was about a 15% loss from shaft to prop, so for older outboards, their actual rating would be about 15% less than modern outboards (e.g. an older 25hp is about equivalent to a 21-22hp by modern standards). But hp ratings can be a little misleading vs actual performance since actual performance is actually based on a lot of other factors (prop pitch, gear ratio, motor weight to hp ratio, etc).


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## mechanicworkman

OK, I got a 1436Jon Flat bottom

Across the top its 51" or about 4.25 Feet

Across the bottome its 36" 3 Feet

I have 15HP Mercury

Can someone help me figure this out cause i am still a bit confused! I may have figured out it but dont knwo if i even did it right? 

Can anyone help!


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## TCHunter

Very interesting thread. I image that 2 strokes are lighter than 4 strokes. I have a 1962 Lonestar that I am restoring. The tag says max is 24hp. So how much did a 1962 24 hp 2 stroke weigh?


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## roblj65

Thanks! 
This is great as I was wondering what the max HP was that I could put on my 1962 Starcraft


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## momof2inmn

Wondering if anyone can help answer a question my dad has. He has a 1979 14' aluminum Deep and Wide Lund and he can't read the motor rating number on the boat because of all the wear (he thinks it may say 25 or 35hp). He's had a very old Johnson 15 hp on it for many years and now wants to put a newer 25 hp on.

Can anyone tell me what the motor rating for this boat might be? Also, from some of the posts in this thread it sounds like the rating may be different that what the boat would have said anyway because it is pre-1980.

Thanks for any help you can give. We go to the lake in three weeks and are cutting it close on motor replacement!


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## Lennyg3

How does a max hp rating work with a jet? If the boat is rated for max 150hp, does that mean a 150 power head or can I run a 200/140 jet? Is the max hp more of a weight issue or an overpowered issue?


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## Canoeman

Lennyg3 said:


> How does a max hp rating work with a jet? If the boat is rated for max 150hp, does that mean a 150 power head or can I run a 200/140 jet? Is the max hp more of a weight issue or an overpowered issue?



Depends on state laws and insurance and regulations. I know some states go buy the power-head and others by the foot/prop. MN goes by what is stamped on the motor.

Downside is if you overpower your boat and then say you hit and hurt someone or someone gets hurt in your boat you may be liable because you exceeded your manufacturer hp rating. 

If you want to be safe contact your hull manufacturer and have them re-rate it for over what your putting on, if they will work with you. If they wont then it is a gamble.


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## Lennyg3

Canoeman said:


> Lennyg3 said:
> 
> 
> 
> How does a max hp rating work with a jet? If the boat is rated for max 150hp, does that mean a 150 power head or can I run a 200/140 jet? Is the max hp more of a weight issue or an overpowered issue?
> 
> 
> 
> 
> Depends on state laws and insurance and regulations. I know some states go buy the power-head and others by the foot/prop. MN goes by what is stamped on the motor.
> 
> Downside is if you overpower your boat and then say you hit and hurt someone or someone gets hurt in your boat you may be liable because you exceeded your manufacturer hp rating.
> 
> If you want to be safe contact your hull manufacturer and have them re-rate it for over what your putting on, if they will work with you. If they wont then it is a gamble.
Click to expand...



Well, I got a hold of PA fish and Game, and apparently they go by HP at the foot. The plate rating is 130hp. Now what is the factor for powerhead/jet? some places say 70%, and some say 66%. Being as though they don't make a factory Jet above 150/105 that I know of, which method would I use for say a 175 or 200? I know the general consensus is that a 200 is a 140 at foot, which would put me over, but using the .66 multiple gives me [email protected] foot, and I would think legal.


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## JohnT

Ok , I did the math a coupe of times , My boat is a Lowe Bass 170 . It's rated for a 70 hp and when I do the math it come's out to a 115 hp how is this so . Please help !!! Don't get me wrong I would love to throw a 90 hp or a 115 hp on there for some go fast fun ,for sure .


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## LA_Gas_Man

I have a 1988 16' starcraft all welded jon boat, 70" beam, 66" (5.5') wide transom, steering, 20" transom, that came with a 1989 evinrude 40hp. The sticker says its rated for a 35hp. When I first read this post I calculated my numbers and came up with 90 hp. Here are my numbers;

16' long x 5.5' wide = 88
88 x 2 = 176
176 - 90 = 86 rounds up to 90 hp

The 40hp evinrude that was on the boat pushed it ok, but I knew it could definitely handle more hp. After going over these calculations several times, I found a real good deal on a 1986 85hp suzuki.
Dropped it on the boat and it runs real good. Handles great. With the boat loaded w/3 people, tackle, & 6 gallons fuel I got a steady 38 mph on gps.


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## Frogman Ladue

...odd, should I have a cousol in my boat, it would be rated for -20hp acording to this formula. Tiller would put it at 18hp, factory tag says 30. And, that's doing the math a few times.


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## Lowe 2070 CC

Seems like there is something being lost in the translation with this formula. For example my own situation is:

- Lowe 2070 CC: 20' long, 8' beam, 1,110 pound hull weight, rated for a 115.

- (20 x 8 x 2) - 90 = 230.

So should I really be rated for a 225?? I don't think so (I'm running 50mph now with a 2 stroke 115). My dad runs a 225 on his 22' Ranger glass boat (it pushes 70+ mph). 

My old 16' Xpress, factory rated for 60hp, (16 x 6 x 2) - 90 = 102. No way I'd try to run a 90 on that boat.

There has to be something else factored in there that you're missing.


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## nick4203

Lennyg3 said:


> How does a max hp rating work with a jet? If the boat is rated for max 150hp, does that mean a 150 power head or can I run a 200/140 jet? Is the max hp more of a weight issue or an overpowered issue?




200/140 jet? why are jets always stated like this whats it stand for?


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## jeremy0998

Jets are always listed like that because there is hp loss when going from a prop to a jet. In the instance of a 200/140 jet, if you were to take off the jet boot and put on a prop, the engine would be rated at 200 hp. 

In PA, the restrictions on your plate only pertain to the hp at the propulsion. Like for my boat, which is a 1760, is rated for a 90 hp motor. So, I could put a 115/80 jet on the back but wouldn't because of the weight factor. You would be suprised how much weight is added to the motor as the hp listing goes up.


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## wired

There are other difference with the 150/105 mercury motors that just the bolted on jet pumps. The 105 motors have considerably higher ports than standard 150's. much lower compression and are equipped with 5600 RPM rev limiters. I have a fresh rebuilt 2.0 liter 1996 150/105 that I changed the lower to a standard prop lower. Cranking compression was 95 lbs. I milled enough material off the heads to drop the heads CC to 28 CC from 38 CC and still the compression is only up to 110 lbs. The ports are so high its very difficult to build high compression. On the other hand with the high ports it will push a 23 pitch prop to 65 MPH at 6000 RPM.


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## BloodStone

*What about an older aluminum 14.5' L x 5' W Meyers semi-v boat (front looks like a snub nose bullet, totally flat bottom), originally designed for a tiller, middle bench seat removed, side console put in, reinforced sides (braces) with additional flooring & interior transom wood replaced with new 3/4" red oak with a 1982 25 Evinrude on back? And no, I can no longer read the tag. * 8-[


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## Country Dave

_


Check out Federal Boating Safety Act of 1971 (FBSA), 


This has proven to be a hot topic for as long as I can remember and we don’t have any reason to believe that’s going to change anytime soon. It doesn’t seem to matter which side of the fence you’re on concerning this issue, what is clear is that people seem to be passionate about their opinion ”Whatever that is” and they are certainly initialed to that. 

While I’m not trying to change anyone’s opinion what I am trying to do is make clear the facts and legalities of repowering any boat “above the manufacturing max HP rating” Through extensive research including numerous phone call to different governing bodies such as “local marine patrols” “Sheriff’s offices” and the “US Coast Guard” these are my findings. 
Simply put “it is not illegal/against the law to repower above the manufacturing max HP rating” It’s not a criminal offence in the state of Florida. 

This may be different in your state and with a little research you can easily find out. It’s certainly plausible that your state may have restrictions regarding this however; I really don’t see it as something they target or seem to be very critical about from people I’ve talked to in other states. 

Let’s take a minute and look at what we do know about how a manufacture comes up with a max HP rating in the first place. There is a formula that is somewhat standard but it leaves much to the imagination and is open for interpretation. Length, with, transom thickness and height along with the hull materials all factor in. The one thing that is crystal clear is they are always going to error on the side of caution. 

Safety first and being reasonable human beings we don’t disagree with that, however there is much more here to consider. What’s not so clear is, how the formula was conceived and constructed, again keeping in mind we don’t have any idea what their standards were. Was it 30% under estimated braking strength of the transom, or was it 10%? Was it 30% under a calculated do not exceed speed for the hull, or 10%? We just don’t know what the baseline criteria were for the said formula and that leaves us with more questions than answers. 

From a manufactures standpoint yes safety first but believe you me they are looking out for number one and who can blame them. You can bet liability plays a huge factor when considering what they say should be the max HP motor you should but on a given hull. Any smart business will try and limit their liability as much as possible. So with that being said it’s at least reasonable to believe that there is a pretty good cushion/margin of error that favors a lower HP rating than what the hull could really handle. 

In addition if you have a new boat that’s under the manufactures warranty and you over power it “ according to the max rated HP” there is a good chance the will void your warranty. How about insurance companies and your personal liability? I’ve had many boats and I’ve only insured one of them and that’s because I financed it and they wouldn’t to the deal without it. I’m just not a big fan of insurance, maybe you are but it’s a scam as far as I see it. 
They will be more than happy to collect your premiums yet God forbid you have to make a claim and now you are the bad guy and it’s like pulling teeth to try and get anything done. 

So before we get any further off track what do the insurance companies say about repowering above the max rated HP? Well it’s pretty diverse, no real standard form what I’ve been able to find out. It really all depends on what company you talk to. Some don’t seem to have an issue with it whiles others do and some just want a higher premium. Check it out first if you want to insure your boat and have a higher HP motor than recommended. Personally I wouldn’t tell them chit about my motor unless they asked me very specific questions and always read your policy closely. They will try anything to scam you if they can. LOL 

Ok last but certainly not least what about your personal liability if you’re running a boat with a motor that has more HP than the hull is rated for? Let me say this. It’s a numbers game. Have you ever been in a boating accident? Have you ever been in a boating accident were there were injuries? Have you ever been sued over a boating accident? Yeah me ether. However some have and some will be but the chances are not likely from the data that I’ve been able to compile. The bottom line is, if you are involved in a boating accident that was a result of your negligence you may very well get sued and the layers couldn’t give a rats azz how many ponies your outboard has. 
If the accident was a result of your negligent it doesn’t matter. Let me say it this way. If you get into a boat accident and it’s your fault you could be paddling it and it doesn’t change anything. 

So let’s summarize. 

#1 It’s illegal to put a motor on your hull that exceeds the manufactures max HP rating.” Not true in my state, maybe it is in yours. It’s not a criminal offence however you may be fined. 

#2 You can’t get your boat insured or they will automatically drop you and or deny a claim based on engine HP alone. “Not true” I found that there are insurance companies that will insure you although you will pay a higher premium. 

#3 You automatically assume responsibility and liability if you are involved in an accident or incident if your motor exceeds the manufactures max rated HP even if you were not negligent. “Not true” there is nothing automatic in criminal or civil law. _


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## Motv18

To ease some confusion don't forget to x by the modifier. .5 or .8 for hard or soft chine 

230 x .5 = 115 bringing it close to the uscg tag.


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## Y_J

Thanks a bunch. This actually gave me a starting place. Wooot


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## onthewater102

federal regs. I can find only govern marking the max HP on the tag on the boat - it provides nothing to do with installing a motor in excess of that amount:

§ 183.53 Horsepower capacity.

https://law.justia.com/cfr/title33/33-2.0.1.8.44.html#33:2.0.1.8.44.3.217.1


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## thill

You know, as I read this thread, there is one huge factor that really must be taken into consideration, particularly in an aluminum boat:

Construction.

Transom construction, hull thickness, number of ribs, strakes, bunks, supports, and even the bottom contour. All of these have to factor into how much HP you put on a boat.

For instance, a 16' x 60" rowboat, made with very thin construction, 3/4" partial transom plywood, no strakes, and very few supports, may LOOK like it can easily handle a 50 HP motor, but the reality is, it will start breaking apart in short order if you start skipping across the lake in it.

Case and point, there are a lot of guys around here that will try to "soup up" their small aluminum boats by pulling the USCG plate off and adding a big motor. Sure, they will go fast, but when they go to sell it after a summer or two, you will see a dented transom, and cracks in the hull and supports. NOT the boat the innocent victim who buys it thinks it is.

Just something to think about...

-TH


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## onthewater102

I did eventually find it in the CT regs, no surprise there, if the nanny state can meddle it will...


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## earl60446

Yah, manufacturer rates mine for 60hp, this formula says 102 hp. It is plenty fast for me with the 48hp I got on it.
Tim


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## rednecknproud21

This helps alot. Plenty of good information on this site.


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## DaleH

> ... fairly close when checking my boats specs...


*Way off* ... as in 40% HIGH :shock:  ... when assessing my 16' Starcraft specs.


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## ppine

Or look at the plate riveted to the hull with hp rating.


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## RiverRoost

Thanks. Good information.


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## gnappi

Using my 2017 Lowe 1040 the calculations come out as shown if I measure the transom width as in the graphic below. The calculations either come out conservative or over but, failing not having factory specs this may help some with older boats or those where specs are not available.

Note: The 48" measurement was taken inside the boat.


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