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Old 10-08-2011, 06:22 AM
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Default best speed for fuel mileage

If i have 1 gal of gas can I go farther ideling or on plane?
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Old 10-08-2011, 06:26 AM
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Wind, sea conditions? In heavy wind idling might have you going backwards. Idling up river might have you speaking spanish in a week or so.
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Old 10-08-2011, 06:32 AM
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idling is better if no wind or wind at back, but very slowwwww
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Old 10-08-2011, 06:47 AM
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Mph is heavily dependent on the motor, boat, prop, and conditions. Depending on the current I'll see anywhere from 4.25-7 mph at slow idle. Best cruise is 3000-3300 rpm at 4-5 mpg. At 4000-4500 rpm I'm down to 3.5 mpg then it goes back up over 4 mpg at wot.
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Old 10-08-2011, 06:58 AM
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As nate points out, different motors, different optimal speeds.

Generally speaking, if you have ZERO wind, ZERO waves, ZERO current, you will get better mileage at idle, just in gear. Lets say idle is 800 rpm. Anything over 1000, and you start losing mileage. The most efficient speed above 1000 rpm, is "JUST" on plane. Say you plane at 17 mph, you want to go 18 mph to get the best fuel.
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Old 10-08-2011, 07:00 AM
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Default best mileage

I want my propulsion to use an engine running at efficient rpms, with a flat power band that allows over propping.

Weismann's shop is right here in Costa Mesa: http://www.weismann.net/
I'll definitely be paying him a visit. They build a bullet proof semi - automatic transmission, used for world championship offshore boats.

I first thought about automatic, people seem to prefer the two speed power glide. I want to avoid the loss associated w/ automatic. Fact - manual shift gets better mileage than automatic. Weismann's trannys can be remote shifted, and they definitely will be a wealth of knowledge.

Ask a veteran about how river dwellers would basically tie a small block to a stick, with the props haft attached straight to it. They would lift the prop out of the water for neutral.

I want a horizontal prop. Outdrives definitely transfer power better than inline.

I could either see if I could afford one of his retired outdrives, or maybe transmission only, using my synchronous belt outdrive idea. There are plenty of Muncies sitting around in old chevelles, but that would take some adapting. A clutch would still be involved. The final gear on weismann's drive becomes a direct input, without taking loss from the rest of the transmission.

This sounds very interesting. I can run 200 amp alternators from the main, and ditch the genset idea. I can still power dc motors on the outer tab drives. I can hold more fuel. I can cam the main for peak torque at 2000 rpm, and just grunt on through.

Anyone like this idea?

my previous thread brought me to this conclusion: http://www.thehulltruth.com/boating-...ting-more.html

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Old 10-08-2011, 07:02 AM
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Quote:
Originally Posted by Chiefsurfer View Post
As nate points out, different motors, different optimal speeds.

Generally speaking, if you have ZERO wind, ZERO waves, ZERO current, you will get better mileage at idle, just in gear. Lets say idle is 800 rpm. Anything over 1000, and you start losing mileage. The most efficient speed above 1000 rpm, is "JUST" on plane. Say you plane at 17 mph, you want to go 18 mph to get the best fuel.
Putting around at idle won't make your spark plugs too happy.
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Old 10-08-2011, 07:07 AM
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Quote:
Originally Posted by nate. View Post
Mph is heavily dependent on the motor, boat, prop, and conditions. Depending on the current I'll see anywhere from 4.25-7 mph at slow idle. Best cruise is 3000-3300 rpm at 4-5 mpg. At 4000-4500 rpm I'm down to 3.5 mpg then it goes back up over 4 mpg at wot.



Look at some of the data:
http://www.yamaha-motor.com/outboard...px?ls=outboard
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Old 10-08-2011, 07:18 AM
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Not to derail but to look at the question in a different way:

If your out at sea, (on a bay, lake, river) trolled more then you thought, went out further then you wanted chasing fish, whatever, therefore used more gas then planned.
You have an OH $H!T!!! moment when you realize how much fuel you have and concerned that you may not make it "all the way" back and want to get as close to land/port as possible, just in case.

The distance out is irrelevant in this hypothetical situation.

What RPM's will you run?
Will you adjust any weight (gear) in the boat?
Ask the passengers to move forward, mid ship, back, some of each?


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Old 10-08-2011, 07:38 AM
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Quote:
Originally Posted by Top Knot View Post
If i have 1 gal of gas can I go farther ideling or on plane?
Opens the door for a whole lotta "Ghee Whiz" answers.
Our 37' hull, SeaHunter, Trip 300 Yamaha's:
1000 RPM'S, 7.5 MPH, 4.2 GPH, 1.8 MPG
1500 RPM'S, 9.8 MPH, 6.9 GPH, 1.4 MPG
3500 RPM'S, 37.3 MPH, 23.5 GPH, 1.6 MPG

So, I'll go a longer distance, faster, and achieve more MPG @ 3500 RPM'S than 1500 RPM'S, and pretty close to 1000 RPM'S, idle speed.

Our 18' Flats boat:
1000 RPM'S, 4.2 MPH, .8 GPH, 5.3 MPG
1500 RPM'S, 5.6 MPH, .9 GPH, 6.2 MPG
3500 RPM'S, 23.8 MPH, 4.9 GPH, 4.9 MPG

I know, it can drive one crazy.
A lot of fun though doing fuel burn vs. speed, vs. economy and distance at what fuel burn.
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Old 10-08-2011, 07:45 AM
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Depends on motors. If a carb'd 2-stroke you are usually most efficient on plane at low to moderate cruise speed. If a DFI 2-stroke or 4-stroke, usually most efficient at idle speed in-gear (these motors can be 2 to 3 times as efficient at this speed than on plane).
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Old 10-08-2011, 09:16 AM
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Practically speaking, if you're trying to get as far (or close to home) as possible, keep in mind the 'loss' (or wasted fuel) that comes from the 'S' turns you'll be making at slow idle. Most Auto-Pilots won't work <~5kts, so going 'straight' will trump a small increase in mpg... I'd bet under real world conditions, finding your most efficient (on plane) cruising speed will be your best option...
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Old 10-08-2011, 09:24 AM
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Quote:
Originally Posted by Top Knot View Post
If i have 1 gal of gas can I go farther ideling or on plane?
Some boats need a gallon of gas just to get up on plane!
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Old 10-08-2011, 09:29 AM
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^You most certainly have to consider a LOT when answering this question. Getting on plane burns a lot of gas. Also, if you have some nice 3 ft rollers, you could be gassing WAAAAY up when going up the swell, and letting up on another. If you throttle down to where you fall off plane, you burn a lot more getting back up.

That said, if I was in a very tight spot on fuel, wanted to compare fuel with time, and possabilities of getting stranded, I would also find the optimal fuel burn rpm, once on plane.
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Old 10-08-2011, 10:13 AM
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Quote:
Originally Posted by Chiefsurfer View Post
^You most certainly have to consider a LOT when answering this question. Getting on plane burns a lot of gas. Also, if you have some nice 3 ft rollers, you could be gassing WAAAAY up when going up the swell, and letting up on another. If you throttle down to where you fall off plane, you burn a lot more getting back up.

That said, if I was in a very tight spot on fuel, wanted to compare fuel with time, and possabilities of getting stranded, I would also find the optimal fuel burn rpm, once on plane.
This makes the most sense for a long run.



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Old 10-08-2011, 02:12 PM
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Depends on many factors. 1 Gal on idle on a 4 stroke 2 hp vs 1 gal on a 4 stroke 350hp. If I had to make assumptions I bet at idle or on plane you would see more distance out of the 2hp. Again depending on the hull the motor was affixed, atmospheric conditions, tide etc.
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Old 10-08-2011, 02:14 PM
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or to answer the question of the thread title "best speed for fuel mileage"

Zero. If the motor is not running it is 100% efficient. Just drift.
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Old 10-08-2011, 03:30 PM
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No easy answer to this question. Simple solution, but a good deal of work, is to plot speed against fuel burn for your boat which will show most efficient speed/rpm combination for you. Flowscan (or equivalent) is a big help in this situation.
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Old 10-08-2011, 03:47 PM
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I'll bet 7/10 it's right about hull speed. The performance bulletins are on 2 motors. I shut down 1, run about 8mph with a waterline length of about 30' on my 32'er. On a single motor I get about 2.8mpg same speed on both I'm at 1.8. Best mileage at cruise is 1.6mpg.
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Old 10-08-2011, 07:19 PM
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I think you are generally going to get better gas mileage in displacement state. The trade off of course is speed.

I tend to range from 5 to 10 mpg at 5 mph SOG (around 1100 rpm). On plane, I never do better than 2.7 mpg but can get that from 25 (say 3500 rpm) to about 33 mph (4600 rpm).....I can go to wot without degrading below about 2.4 mpg but my ETEC is eating xd100 oil at an amazing clip...not just gas.

My worst mileage is when I am oomphing onto plane but not quite. I can do as bad as say 1.0 mpg at 2400 rpms for example.

I put engine trim, rpm, SOG and mpg on the HDS for findng the best spots. THere are no fixed answers, especially as wind and currents are factored in.
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