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Old 13-07-2018, 10:50   #1
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Dinghy Max HP/Weight question

So...If a dinghy is rated for, lets say as an example, a maximum 9.9 HP motor and a maximum motor weight of say 90 lbs. What exactly are these ratings based on?

What if you have a motor that exceeds the HP rating but is under the weight rating as you would see with a 2 stroke vs a 4 stroke?

Speaking with Defender they stated that exceeding either of the maximum ratings would void the warranty on a new inflateable.....

Any input?
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Old 13-07-2018, 11:39   #2
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Re: Dinghy Max HP/Weight question

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Originally Posted by Mazu View Post
So...If a dinghy is rated for, lets say as an example, a maximum 9.9 HP motor and a maximum motor weight of say 90 lbs. What exactly are these ratings based on?

What if you have a motor that exceeds the HP rating but is under the weight rating as you would see with a 2 stroke vs a 4 stroke?

Speaking with Defender they stated that exceeding either of the maximum ratings would void the warranty on a new inflateable.....

Any input?
I tend to agree with Defender.

The transoms are rated for wieght (especially when hanging in davits), but also maximum thrust. A larger horsepower motor will put more strain on the transom (and other parts).

Stick with what the manufacture recommends.
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Old 13-07-2018, 12:17   #3
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Dinghy Max HP/Weight question

Iíd say the weight rating may be a structural issue.
However the max HP is almost always derived from a formula of beam and length I think, it is only very rarely actually tested for stability, itís a USCG formula I believe.
I would have no issue with exceeding max HP, but then I donít use full throttle except for getting on plane. Just as a point, at full throttle 5800 RPM our dinghy with both of us in it or making 20 kts, back off 500 RPM to 5300, and itís making 19 kts. So why run full throttle on this one?
Itís rated for 20 Hp and has a 20 Hp motor, that has a 6 Hp sticker on it

Hp formula and test method, testing is to my understanding very rarely done, and is almost always only done if a manufacturer is seeking a higher HP limit than the formula gives them.

https://www.law.cornell.edu/cfr/text/33/183.53
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Old 13-07-2018, 12:18   #4
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Re: Dinghy Max HP/Weight question

The ratings are a safe working load for weight and hp. They are separate to each other. The boat is designed to that rating. Insurance and culpability in an accident will also be issues.

You will probably get away with small increases but maybe not.
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Old 14-07-2018, 03:28   #5
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Re: Dinghy Max HP/Weight question

Rough Rules of Thumb

Capacity Calculation:
Number of (150 Lb) people = vessel length (ft.) x vessel width (ft.) ų 15 (or 18)

Gross weight capacity: Number of people (from above) x 150 (or 180) = capacity

Maximum Horsepower Calculation: Boat length x boat width = boat square footage
Then:
35 square feet or less = 3HP
36 - 39 sq ftt = 5.5HP
40 - 42 sq ft = 7.5HP
43 - 45 ft = 10HP
46 - 52 sq ft = 15HP


See ➥ http://newboatbuilders.com/store/Cap...raftSample.pdf

And ➥ https://www.uscgboating.org/regulati...AFELOADING.pdf
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Old 14-07-2018, 09:06   #6
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Re: Dinghy Max HP/Weight question

"Speaking with Defender they stated that exceeding either of the maximum ratings would void the warranty on a new inflateable....."

Easy to be solved - don't by inflatables, they usually cause troubles trough all the year.
And: Damn who needs a dink with 10 HP usually 3,5 are far enough or at max 6 Hp if you got wave breaking.
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Old 14-07-2018, 17:51   #7
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Re: Dinghy Max HP/Weight question

[QUOTE=GordMay;2673232]Rough Rules of Thumb

Capacity Calculation:
Number of (150 Lb) people = vessel length (ft.) x vessel width (ft.) ų 15 (or 18)

Gross weight capacity: Number of people (from above) x 150 (or 180) = capacity

Maximum Horsepower Calculation: Boat length x boat width = boat square footage
Then:
35 square feet or less = 3HP
36 - 39 sq ftt = 5.5HP
40 - 42 sq ft = 7.5HP
43 - 45 ft = 10HP
46 - 52 sq ft = 15HP


See ➥ http://newboatbuilders.com/store/Cap...raftSample.pdf

And ➥ [URL]https://www.uscgboating.org/regulations/assets/builders-handbook


So 1 square ft is the transition point. This is where I get confused. If you move up only 1 square ft, you increase HP. And itís not like you move up 1 hp, you move up several HP.......
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Old 14-07-2018, 18:01   #8
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Re: Dinghy Max HP/Weight question

Quote:
Originally Posted by moseriw View Post
"Speaking with Defender they stated that exceeding either of the maximum ratings would void the warranty on a new inflateable....."

Easy to be solved - don't by inflatables, they usually cause troubles trough all the year.
And: Damn who needs a dink with 10 HP usually 3,5 are far enough or at max 6 Hp if you got wave breaking.
Lost a 9.8ft Achillies between here and Puerto Rico. So I have the 15HP motor but would like to get something lighter that will handle the 15 but at roughly the same length they are rated at max 9.9 HP, some less. Unless I go with another rigid hull I am looking at a different motor. The 10HP panel floor in hypalon at 9.8í would knock off about 40lbs compared to my old double hull RIB.
I believe the ratings are based on 4 stroke. My 15 2 stroke weighs 88lbs. A 4 stroke 9.9 Mercury is 93 lbs.

The inflatable works for me as I dive and lift a 50 lb dog on and off the dink. I use a folding lift to lower the motor on and off the rail. The only hard dink I saw that looks stable is the walker bay....with the optional inflatable tubes....
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Old 14-07-2018, 18:28   #9
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Dinghy Max HP/Weight question

I know I should not say this, but if your concerned, put a 9.9 sticker on it.
My Suzuki for instance, Iím pretty sure the 9.9, 15 and 20 Hp motors are the same physical motor, difference is in tuning.
Iíll use the power to get on plane without struggling, then back off.
Every car I have ever had will very easily exceed the speed limit, but I donít. I donít see the problem myself.

Two or four stroke doesnít play into it, itís weight and HP, those are two different limits, your not supposed to exceed either. Of course itís usually easier to get over weight in a four stroke, but not always.
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