Outboard fuel economy

[haddles]

When I was a teenager I had some pretty wild cars, all of them v8's that I thrashed mercilessly. The thing is, no matter how hard I drove them, they didn't chew anywhere near as much fuel as my 4hp 4 stroke on my 3m (10ft) aluminium car topper dinghy. (on a distance/fuel basis). This motor will plane the boat (just) after which I can back off and use around 2/3 throttle. <br />I don't even think a F350 towing a horse trailer would even use as much fuel.<br />Is the resistance a boat has to overcome even on the plane so great that they are so inefficient? It always seems to amaze me. My 40hp Yamaha 2 stroke at high throttle uses roughly the same fuel as a Kenworth towing a double trailer.

 

[rodbolt]

Re: Outboard fuel economy

yep<br /> if ya dont believe the load effect at 20 MPH throw both out of gear and see how far each will coast. at 40 MPH the average boat will fall off plane and be about dead in the water in maybe 25-50 ft or less. at 40 MPH on a flat surface you may coast 1/4 mile or so by just tossing the truck tranny in neutral.<br /> thats why marine powerplants have to run at or near max output all the time.<br /> and the fuel economy on them is rather steep

 

[haddles]

Re: Outboard fuel economy

So if I put the 4hp on a kayak it theoretically should be quite economical as these types of hulls seem to coast after paddling has ceased.

 

[lark2004]

Re: Outboard fuel economy

to a point this would be true. <br />But the motor will only push a kayak effiently until it reachs it's hull speed. <br /><br />After that, you run into a whole range of pyhsics problems to do with the dynamics of the hull desighn and the way it passes through the water and way that the water pressure blah blah blah.....

 

[rodbolt]

Re: Outboard fuel economy

what lark says. once a hull passes its displacement speed fuel economy is left to the sailboaters

 

[QC]

Re: Outboard fuel economy

Yeah, what Lark and rodbolt says. Check any test data on a planing hull and the best economy is at Idle (usually near "hull speed"). Not true with rolling stuff, they like around 50 MPH. Just totally different animals . . .<br /><br />rodbolt, I like the "coast" test. Great way to explain some of the dynamics . . .

 

[dafox99]

Re: Outboard fuel economy

Agreed! What a great and simple way to get the point across. I'll remember the "see how far each will coast" example. Thanks, rodbolt.<br /><br />I'm no expert, but the way I've always thought of it is, once past the hull speed, the boat is creating waves, and pushing/moving water (wave drag, or residual resistance). Water weighs a lot (certainly a lot more than air, which is what cars are trying to move), and it takes a lot of power to push it to create those waves ...

 

[fishingdan]

Re: Outboard fuel economy

Comparing rowing a boat or peddling a paddle boat as fast you can to riding a bicycle. Which requires less effort for a given speed?

 

[steelespike]

Re: Outboard fuel economy

There are boats capable of highway economy ratings but at displacement hull speeds, 4 to 6 mph.In "Wooden Boat" there was an article on a 16 ft boat with a little deisel engine that delivered something like 28 mpg at displacement speeds.And it ran on vegitable oil.Your little 10 footer is probably about the worse size for a good MPG rating. Long relatively narrow hulls with a gentle run aft to keep the transom out of the water give the best displacement performance.

 

[Scaaty]

Re: Outboard fuel economy

Stand in the water at about belly button level, and swing your arm up though the air..<br /> Now...<br /> Same thing only through the water....

 

[dafox99]

Re: Outboard fuel economy

Another good one!<br /><br />If you happen to know the hp at the prop, and how fast your boat is at WOT, you can see the force required to push the boat (at that speed). You can take hp X 375 / speed (in mph) to get the force required to push the boat at that speed. In my case, I have 40 hp and run right at 30 mph, so, there is about 500 lbs of force pushing on the transom to keep the boat at that speed. This ignores hp losses from prop/water. Also .. is my '64 40 hp rated at the prop or the powerhead? <br /><br />Found an interesting formula that is supposed to calculate your top speed if you know your gross weight, hull type and hp.<br /> <br /> http://web.continuouswave.com/ubb/Forum4/HTML/000225.html <br /><br />According to this, my 14ft aluminum runabout with me, the motor and gear weighs 1440 lbs. not sure that is accurate.

 

[Bradster941]

Re: Outboard fuel economy

In other words, water is 25 times more dense then air, thus force needed for movement for your boat is greater than for your truck, (resistance of the medium you are moving through).

 

[phantoms]

Re: Outboard fuel economy

Don't forget air altogether, The boat still has to move air just like a car, in addition to water.

 

[rodbolt]

Re: Outboard fuel economy

what Phantoms said is valid as well. <br /> if you dont believe it watch a large center console drop 700 RPM with the addition of a hard top and curtains, then try to convince the customer he now will spend another 1000 dollars on another set of props.

 

[mjbrueck]

Re: Outboard fuel economy

You may want to check to make sure your float bowl isn't stuck. With a 4 hp, you should be using less than 1/2 a gallon per hour. If your tinnie will plane (more than 10 mph), you should be getting over 20 mpg. Still not small car mileage, but better than a hard-driven V8. On my 9.9 4-stroke, I get 15-20 mpg while on plane and close to 30 mpg at displacement speeds (gps verified).

 

[rbruce63]

Re: Outboard fuel economy

You could install a Doel-fin in your guzzler small horsepower motor. It has helped mine reach planing speed at lower throttle opening and now I can back off the throttle 1/3 turn to save some gasoline. Also, it has improved the ride on tough waters!<br /><br />With my current set up of a Chrysler 20 HP and a Boston Whaler Sport 11' I am spending as much as my 4.3 liter V6 GMC Jimmy close to 16 miles to the gallon. Before it was a little more than 2 gallons per hour at wide open throttle.