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Fuel consumption estimate

Old 09-27-2020, 01:38 PM
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Default Fuel consumption estimate

Is this estimate realistic? About to buy my first boat

14 foot aluminum boat, no current, no waves

15 hp Johnson engine.

20 mph cruising / 1.5 Gallon per hour = ~13.4 mpg
Old 09-27-2020, 01:43 PM
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I used to have a 16 ft lake style aluminum boat with a 40 hp evinrude 2 stroke engine. I couldn't believe how much fuel that thing used. I used to take it offshore in Ca (15 miles or so) so I'd put 30 or 40 miles on it. I brought 2, 5 gallon and one 10 gallon fuel tank with me for those trips. I hope your engine doesn't use that much fuel.
Old 09-28-2020, 11:59 AM
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Originally Posted by saltwaters View Post
I used to have a 16 ft lake style aluminum boat with a 40 hp evinrude 2 stroke engine. I couldn't believe how much fuel that thing used. I used to take it offshore in Ca (15 miles or so) so I'd put 30 or 40 miles on it. I brought 2, 5 gallon and one 10 gallon fuel tank with me for those trips. I hope your engine doesn't use that much fuel.
I don't know what to believe in anymore. I guess i'm just gonna have to buy it and find out for myself I guess.

I've been watching speed tests of small boats with outboards on youtube and here's the "data":

40hp Yamaha: 33 mph / 4 Gallons per hour = ~ 8.25 MPG

10hp Mercury on a 16' boat: 19 mph / 1 Gallon per hour = ~ 19 MPG (!)

But then I read about horror stories like yours:

Correct me if I'm wrong but here's what I gather from your data: 35 miles / 15 gallons = ~ 2.3 MPG

That's appalling.

Of course it all depends on the state of the engine, timing, reed valve? etc...

I also checked the method of dividing the horsepower by 10 to get the consumption at wot and the g/kwh figure looked good.
Old 09-28-2020, 12:05 PM
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16’ alumacraft with a 60” bottom. Carb merc 25hp. 20-24 mph averaged 10mpg the 3 years I had it using distance tracked on gps/fuel burn.
Old 09-28-2020, 12:25 PM
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Keep in mind. NMP/G economy can often vary quite a bit between best-economy cruise rpm and WOT . Backing off just a little can save a lot of fuel/NM even for a 15 hp ....if the boat will achieve decent planing attitude at under WOT RPM/.
Old 09-28-2020, 12:28 PM
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Gas engines make about 10-12hp for every gph they burn. Two strokes a bit worse, four strokes a bit better. This only applies to mid-upper power settings. At low power or near idle the burn rate per hp is much worse. Diesels are 16-20hp per gph.

To the OP's question, with a 15hp at cruise power (not full power) expect 1-1.5 gph.
Old 09-28-2020, 12:38 PM
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2 stoke or 4 stroke? Makes a huge difference. A current gen 4 stoke 15 HP should be under 1 GPH


20 MPH would probably not be WOT if no wind or waves to deal with, and the boat isn't overly loaded.
Old 09-28-2020, 12:40 PM
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Originally Posted by christuna View Post
I don't know what to believe in anymore. I guess i'm just gonna have to buy it and find out for myself I guess.

I've been watching speed tests of small boats with outboards on youtube and here's the "data":

40hp Yamaha: 33 mph / 4 Gallons per hour = ~ 8.25 MPG

10hp Mercury on a 16' boat: 19 mph / 1 Gallon per hour = ~ 19 MPG (!)

But then I read about horror stories like yours:

Correct me if I'm wrong but here's what I gather from your data: 35 miles / 15 gallons = ~ 2.3 MPG

That's appalling.

Of course it all depends on the state of the engine, timing, reed valve? etc...

I also checked the method of dividing the horsepower by 10 to get the consumption at wot and the g/kwh figure looked good.
modern 4 stroke engine will get much better fuel economy. Old 2 strokes (mine was a early 1970's model) won't get good economy. If the engine is a 2 stroke, the best thing you could do is sell the engine it has and get a brand new one.
Old 09-28-2020, 12:48 PM
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Originally Posted by christuna View Post
Is this estimate realistic? About to buy my first boat

14 foot aluminum boat, no current, no waves

15 hp Johnson engine.

20 mph cruising / 1.5 Gallon per hour = ~13.4 mpg

Mod-V ?
Deep V ?
flat bottom?
Weight?

My 25hp would get almost twice the fuel mileage on a flat bottom compared to a v-hull. Different hulls have more or less drag. So without knowing hull, impossible for anyone to give you a correct answer. I also highly doubt they ever made a Johnson that got 13.4mpg on any hull.
Old 09-28-2020, 12:56 PM
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Originally Posted by Jeepman View Post
Mod-V ?
Deep V ?
flat bottom?
Weight?

My 25hp would get almost twice the fuel mileage on a flat bottom compared to a v-hull. Different hulls have more or less drag. So without knowing hull, impossible for anyone to give you a correct answer. I also highly doubt they ever made a Johnson that got 13.4mpg on any hull.
the one I was looking at was a V hull I believe

Thanks for your comment.

I'm still looking around so I don't know what I will end up buying.

I will have to do more research on outboard makes/types/spare parts/economy
Old 09-28-2020, 01:19 PM
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At WOT (full throttle) you are going to burn 1.5ish GPH, take a GPS or your phone with a GPS app that will give you MPH and do the math on a test ride. That's the best we can tell you.

If you want to be conservative assume 1.8 GPH.

If you can plane and cruise below WOT you would get better economy.
Old 09-28-2020, 02:48 PM
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I suggest that you don't worry so much about miles per gallon.

Decide that you'd be ok with 6 miles per gallon and be happy that you get over 10.

60 miles is a LONG trip in a motorized rowboat
Old 09-28-2020, 04:18 PM
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Your gph sounds ok, but 20mph from 15hp on a 14ft boat sounds ambitious - 10hp (2 stroke) on a 10ft boat gives me 19mph which equates to about 19mpg - even though it’s a two stroke, it drinks by the thimble full.
BTW, you could offer me a brand new four stroke to replace that little engine (Tohatsu) and I wouldn’t take it.
Old 09-28-2020, 05:19 PM
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I had a 14 foot early aluminum boat that sprayed you pretty good in chop. and moved to a light 16' that bounced in the chop. The 16' was lighter, faster, and planed well at 2/3 throttle, and got much better range on a tank. The hull was more important than the motor.

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