Maximum Outboard Horsepower Capacity (Guide and how-to)

MeanMouth

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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".
 

Ranchero50

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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
 

MOBowhunter

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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
 

KICKEDBACK

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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. :?:
 

Industry

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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.
 

mtnwkr

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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.

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

LonLB

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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:
 

jdsgrog

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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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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!
 

TCHunter

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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?
 

momof2inmn

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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!
 

Lennyg3

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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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