Dinghy Engine Max HP?

[Beersmith]

I am almost ready to splash my newly built Spindrift 10ft Nesting Dinghy. The website says the max HP for the boat is 2-3 hp. What exactly is the limiting factor on the engine size? We beefed up the transom to maybe handle a bigger engine, but is the limiting factor weight/floatation?

If that is the case, if I found a 4 HP motor that weighed the same as a 3 HP (in theory), the 4 HP motor would be fine? Or is that much power an issue of some sort?

[Horror Hotel]

Can never have to much power. If it's the right weight a few extra HP won't bum you out. On the odd occasion you have 8 jerry jugs you'll still zip right along.

[nimblemotors]

HP is related to the amount of stress the boat is designed to handle.
4hp vs 3hp is not going to make a difference.

[Beersmith]

Originally Posted by nimblemotors

HP is related to the amount of stress the boat is designed to handle.
4hp vs 3hp is not going to make a difference.

how much of a jump in HP would make a difference though? Where would I cross the line in terms of HP?

[Cheechako]

That's an absurdly low rating for a 10 ft boat. I would have thought about 10 HP or even more. Is there something maybe about the nesting feature where it's bolted/attached (?) together that makes it at risk to separate in a big wave going fast? Obviously they dont want you planing.... anyway, bottom line I wouldnt hesitate to put the lightweight 4 hp on there if you are just putting along.

[nimblemotors]

Originally Posted by Beersmith

how much of a jump in HP would make a difference though? Where would I cross the line in terms of HP?

You'd have to ask the designer of the boat, but double the HP is well, twice as much as you should. With these tiny outboards, the actual HP probably varies quite a bit.

[Cheechako]

Originally Posted by Beersmith

how much of a jump in HP would make a difference though? Where would I cross the line in terms of HP?

My guess is the real limiter is not the HP, but speed. If the 10 ft boat planes, and crosses a 3 ft wave landing in the trough... the stresses want to break it in half. If not laning the stresses are much lower. Just a guess.

[Beersmith]

thanks a bunch for the answers

[Cheechako]

10-6 RIB rated at 15hp
10' Livingston rated at 10hp
I found this:
"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".

[SkiprJohn]

Originally Posted by Cheechako

10-6 RIB rated at 15hp
10' Livingston rated at 10hp
I found this:
"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".

Very helpful! Thanks.

[ranger58sb]

Originally Posted by Cheechako

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

And I think that's the basis for the USCG-compliant sticker often (always, these days?) found on the transom of small boats... which generally state both HP and weight limit for outboards, as well as passenger limits, relative to that specific boat.

-Chris

[Beersmith]

Originally Posted by ranger42c

And I think that's the basis for the USCG-compliant sticker often (always, these days?) found on the transom of small boats... which generally state both HP and weight limit for outboards, as well as passenger limits, relative to that specific boat.

-Chris

does this mean that max HP is a legal requirement too? If the scale says I should have a 3 hp, but I put a 4 hp on there, could I be ticketed or fined???

[HappyMdRSailor]

Originally Posted by Beersmith

I am almost ready to splash my newly built Spindrift 10ft Nesting Dinghy. The website says the max HP for the boat is 2-3 hp. What exactly is the limiting factor on the engine size? We beefed up the transom to maybe handle a bigger engine, but is the limiting factor weight/floatation?

If that is the case, if I found a 4 HP motor that weighed the same as a 3 HP (in theory), the 4 HP motor would be fine? Or is that much power an issue of some sort?

I have a 4 hp Seagull you can have..... IF...
You promise to take video and post it every time you need it to start...

Originally Posted by Beersmith

does this mean that max HP is a legal requirement too? If the scale says I should have a 3 hp, but I put a 4 hp on there, could I be ticketed or fined???

Absodamnlutley-tootenley....

If you have a MFG placard and the engine is larger... You most certainly can expect an overzealous ticket book holder to hand you a page from it...

(edit-in practice though... from 3 to 4 you could really make a pretty persuasive argument... 15??? NOW yer pushin' it !)

[fjwiley1]

Beersmith....I doubt you would get "fined"...the issue of exceeding the "recommended" HP rating by Manufacturer/designer would arise if you became involved with some law suit......
Like others have stated, 1 hp difference would not likely matter......have Fun with your new dink....

[Beersmith]

thanks for the tips everyone