Maximum Outboard Horsepower Capacity (Guide and how-to)

MeanMouth · Post by **MeanMouth** » 15 Feb 2010, 05:41

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".

Post by **Brine** » 15 Feb 2010, 15:13

Nice....

We should make this a sticky

lswoody · Post by **lswoody** » 01 Mar 2010, 20:46

Brine wrote:Nice....

We should make this a sticky

I agree. Thanks!!!!!

https://www.youtube.com/user/ranchero50 · Post by **Ranchero50** » 24 Sep 2010, 02:51

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

Outdoorsman · Post by **Outdoorsman** » 24 Sep 2010, 11:19

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

https://files.dnr.state.mn.us/education ... ilders.pdf

Outdoorsman

MOBowhunter · Post by **MOBowhunter** » 21 Nov 2010, 19:09

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

Flat Bottom · Post by **Flat Bottom** » 09 Feb 2011, 21:58

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

FishyItch · Post by **FishyItch** » 09 Feb 2011, 22:46

Brine wrote:Nice....

We should make this a sticky

+1 more

KICKEDBACK · Post by **KICKEDBACK** » 05 Mar 2011, 17:17

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.

Decatur · Post by **Decatur** » 05 Mar 2011, 18:23

Thanks!

Industry · Post by **Industry** » 02 Apr 2011, 08:09

KICKEDBACK wrote: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 · Post by **mtnwkr** » 03 Apr 2011, 03:30

Industry wrote:
KICKEDBACK wrote: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.

TinBoatToo · Post by **TinBoatToo** » 02 May 2011, 19:47

Thanks!

LonLB · Post by **LonLB** » 02 May 2011, 20:46

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.

jdsgrog · Post by **jdsgrog** » 25 May 2011, 10:34

MOBowhunter wrote: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).