Mileage from an outboard - SailNet Community
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post #1 of 11 Old 04-02-2010 Thread Starter
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Mileage from an outboard

This may be a quirky question, leading to even quirkier answers, but here goes:

How many miles (or hours even) can you expect to travel on a river or Atlantic ICW given a 3000lb displacement fin keel sailboat under 8 hp 4 stroke Tohatsu outboard power only, not considering current?

Thinking about an extended trip this summer and wondering about gas tanks.

Thanks,

Fred Nelson
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post #2 of 11 Old 04-02-2010
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Depends a lot on how hard you push the boat. Backing off even a little from the boat's hull speed will greatly reduce the fuel consumption. You don't give much info about the boat. If I had to guess, I would say about 1/2 a gallon per hour at around 5 knots, so figure a gallon every 10 miles.
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post #3 of 11 Old 04-02-2010
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I have a 3000 # boat but my outboard is only 3.5 hp w/ a 0.3 gal tank. I can get 25 minutes in a headwind (slowly), and dead calm I can motor for 45 minutes around 5 knots.

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post #4 of 11 Old 04-02-2010
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Well

I would think you would move close to hull speed at 1/2+a bit throttle as my 4Hp dos that on the J24 with not much speed after 5 knots no matter what

When we use to take trips with the boat we allways had two 6 gallon tanks which will allow a full day of motoring under allmost any conditions

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post #5 of 11 Old 04-03-2010
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Too many varibables, but:

8 (4-str)
5,500 RPM
0.85 Gallons per hour

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post #6 of 11 Old 04-03-2010
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Quote:
Originally Posted by TohatsuGuru View Post
Too many varibables, but:

8 (4-str)
5,500 RPM
0.85 Gallons per hour
That's full throttle though, which would be necessary to push my 7000lb boat, but not one weighing less than half that.

I would estimate 1/3 to 1/2 gallon per hour, 10-13mpg or so


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post #7 of 11 Old 04-03-2010
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I have a 5000 pound fin keeler, a Haida 26, with a Honda 8hp 4 stroke. It has a 21 foot waterline, and a calculated hull speed of around 6 knots. I normally keep the RPMS a little low, and go 4 knots, just starting to push a noticeable wake. This keeps me going at less than a gallon per hour.

I also motored this same boat with a Nissan 5hp 2 stroke at around 3 knots and less than half throttle, this didn't even put out a noticeable wake, and was painfully slow, but we used barely any gas at all, probably less than a half gallon per hour.

Wave making resistance - Wikipedia, the free encyclopedia

Your waterline length is going to be an important factor in determining how efficiently she will go through the water. A longer waterline means a higher hull speed.

Also, a high thrust prop will waste less gas than the stock prop.

I made the mistake of running my outboard at very low rpm for around 8 hours, and fouled the plugs, so keep that in mind, occasionally you should clear out the carbon by revving her up.
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post #8 of 11 Old 04-03-2010
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One thing no one has mentioned yet is the current. It depends on how well you time your passage to maximize the advantage of current... This can make a huge difference.

If your boat normally motors at 4.5 knots...and you're going against a 2 knot current, you're now doing only 2.5 knots, or 56% of your normally cruising speed... likewise, if you time it so that you're going with the current, you're now doing 6.5 knots or 144% of your normal cruising speed.

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post #9 of 11 Old 04-05-2010
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Diesel has 16-18 hp per hr per gallon, and gas is 12 to 14 hp per hr per gallon.
I run a 2GM20 at 2800-2900 rpm at 40% prop slip for maximum efficiency.
Say 6 knots SOG at 78% of full power ( 80%) is the most efficient for diesels, I get over 15hrs cruising range at 6 knots= ~100 miles.


I did my calcs by using a gps and monitoring fuel usage. You would have to do the same with your setup be be sure of actual performance.
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post #10 of 11 Old 04-05-2010
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Catalina 25

I have a Catalina 25, weighs about 6000 lbs cruise loaded. I have a Tohatsu 9.8 4 stroke. I usually motor sail at about 1/4 throttle, at 5 knots. I get 10 nautical miles per gallon. On a cruise I use a guesstimate of 15 nmpg because I try to sail as much as possible and during that time the motor is off (of course).

If I push the boat up to 6 knots the mpg goes WAY down.
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