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Discussion Starter #1
Actually, how many miles per gallon do you get from a 9.9 horsepower inflatable? Assume you are up on plane.

This would be useful to know if, for instance, you were taking your RIB from one island to the next to get groceries or whatever.
 

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The per hour rate sound dead on for motors...

I cannot imagine anything more that 2 gal and hour and that is probably with a strong headwind and seas, this is what google came up with, Outboard Fuel Consumption - World Cruising and Sailing Forums
But milage in a displacement hull is very speed sensitive, and on a planing boat it is going to be very weight sensitive. For example, my little 4hp will plane my sport boat and me at a very respectable pace, but when a second person is added - displacement mode and less than 1/3 the speed. Your boat loaded with provisions and 2 people may not plane with 9.9HP, and if it does, it will take all of it.

The consumption figure of 0.1 gph per HP is, of course, wide open.

Load her up and find out!
 

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Discussion Starter #4 (Edited)

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Discussion Starter #5
Another reason to ask this question. If you hit a shipping container, how far could your dinghy carry you, with X gallons of gas?
 

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That is exactly what the link said I posted but who knows if that is right, 10% and .1 per HP are the same thing.
 

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Discussion Starter #7
Sorry guys, I misread it. 0.1gph PER HORSEPOWER. That seems right.

So If I'm up on plane and using 8 hp of my engine's capability, and I'm doing 20 knots with 5 gallons of gas.... I can go 5*20/0.8 = 125 nm on a tank. That's really something.

I wonder what ocean conditions will allow a dinghy to go up on plane.
 

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Check Mr. Wuffles message for the link.

Where's that from. That sounds way too low.
But I do think it is close. I know my 18 hp would burn just less than 2 per hour and my 9.9 hps burn about 1 per hour.

By the way, the 18 hp was on a Stiletto cat. At 18 hp I got 11 knots and ~ 5.7 mpg. At ~7 hp I got 6.5 knots and 8.0 mpg. That is not a displacement hull, either.
 

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My 4 hp dinghy motor burns about 1/4 gallon of gas per hour. Consumption goes up if I take it on a plane (though I rarely do, it's not that much fun), may be double that.
 

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Discussion Starter #10
Thanks for the posts. I'm wondering now if anyone has used their dinghy as a lifeboat and has any experience with this.
 

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Bene, if you hit the container you probably want your crew in the dinghy as well. Then you can forget 20 knots.
I did some testing:
Flat as it gets (lagoon in St martin),
Tohatsu 9.8 HP 2 stroke in reasonably good condition,
Carib RIB dinghy, reasonably smooth bottom (Sunsail size)
Two people, no gear, or provisions (except one small camera, one gps, one bottle of water and one towel)
We made 13 knots (almost 14 with heads down) on full throttle.
Add a third person and you may not plane (man + woman + child = plane, three man = not plane)

Just outside the lagoon (coastal wave conditions) we made about 10 to 11 knots, but after we have changed the direction we hardly made 10.

3 or more people and some provisions: count 4 to 6 knots max.

I also did get into dinghy about 6 miles NE of Anegada BVI.
The conditions were easy trade wind sailing from a Bene 423.
It looked completely different from the dinghy.
I did put on a life weest (to keep my wife happy), but I was glad I had it on just moments later.
Those waves are preventing you to come to plane or you can get killed. You can only plane in one direction: broad reaching with the waves.
Downwind: you are too fast for the waves, upwind - forget it.
You must be alert - once I missed a freak wave from the side and almost flipped the dinghy. Thanks god I was sitting very low and was holding hard.
In moments I have lost the visual contact with the sailboat and that is one very scary feeling. And it was a nice January trade wind day.

Let me see if I find a photo.
 

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In Theory...

An in tune two stroke outboard burns about .6 to .8 pounds of fuel per horsepower per hour. On a plane, a 9.9hp outboard is probably using about 60% of its horsepower at 3/4's throttle or call it 5.94 hp. With this, the fuel burn is roughly 4.2 lbs/per hour. Gasoline with a 50:1 fuel;oil ratio weighs in at roughly 7lb per gallon so the burn rate is roughly .6 gal per hour.

Likely as not, one runs at less than full plane most of the time so I personally figure .75 gallon per hour. I.e. a 6 gallon tank last a bit over 8 hours assuming it is run bone dry. Figure 6-7 hours to be safe.

FWIW...

s/v HyLyte
 

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Discussion Starter #13
More great information. +1 Tom
 

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It's generally accepted that gasoline engines burn 1 gal/ hour for every 10 HP. That is equal, of course, to the already discussed .1 gal/ HP.

Diesels burn 1 gal for every 20 HP that is generated.

Open that little 9.9 up and let 'er rip and you'll burn about 1 gal/hour.
 
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