what throttle setting gives best fuel economy?

[rbigg]

I've had several boats with big single and twin o/bs in the past. I remember most as having a "sweet spot", which was just down from wide open throttle (WOT). Many of you are probably familiar with a sweet spot. Cars and trucks have been known to have them. Seems like going to WOT can eke out a little more speed, but things are straining, coming back down a little seems to let everything harmonize and get in the groove, and run more smoothly.<br /><br />The question I have is: assuming a boat with o/b has a sweet spot, is my fuel consumption more efficient at the sweet spot than wide open? Sounds like it would be -- a little bit---as you come back on the throttle to get to the sweet spot. Or, is it that there's no real reason to run at the sweet spot, other than it "feels right", and good?<br /><br />Just curious

 

[ZmOz]

Re: what throttle setting gives best fuel economy?

You will get less gas mileage at WOT. The place where you get the best mileage depends on the motor; however, it is usually around 3-4000 RPMs.

 

[Solittle]

Re: what throttle setting gives best fuel economy?

OMC shows it as a curve with the rated WOT at the top of the curve. The most efficient is around %90 of rated. So if your rated rpm is 5500 the most effieient would be 4950.

 

[JasonJ]

Re: what throttle setting gives best fuel economy?

I get my best economy at about 3600 rpms, anything beyond that is just pure speed and consumption (max rpm is 5800, I can tickle 6000 if I want). I can hear the sweet spot on mine, the sound is very noticebly different when it fall below the sweet spot, and a screaming gnashing banshee above the sweet spot.

 

[airman]

Re: what throttle setting gives best fuel economy?

Go to www.boattest.com They have figures for just about any boat you can find. Typically the best mileage happens at a little over 1/2 speed (not 1/2 rpm) but it's never at WOT.

 

[MajBach]

Re: what throttle setting gives best fuel economy?

When i was shopping for an outboard a few years back, i did a lot of reading. Comapring many charts of speed vs. g.p.h. on a variety of 4-stroke motors and boats, it appeared that best range was typically just a little higher than planing speed (not lowest planing speed). TO clarify this though, it was the speed you get after leaving the trottle set at just enough to get you on the plane. After getting over the hump, the boat speeds up of course. This was typically around 3500 rpm.<br /><br />BTW, the difference was signifcant. Of course, best mileage was always at trolling speed, providing you're not travelling against a good current.

 

[CaptJB]

Re: what throttle setting gives best fuel economy?

I have a 25 foot walkaround boat equiped with twin 175 Mercs and the boat has a twinscan floscan fuel meter. It is very intersting to watch fuel consumption versus RPMs and MPH. I find my best economical running is around 3800 -4000 rpms on each engine. I am doing about 22-23 mph and each engine is drinking 8 gph. This speed is just slightly past planing. At 4000 RPMs, I have 1200 RPMs in reserve but this last 1200 rpms cost another 8 gph for each engine. When I am running at 3800-4000 RPMs, I can move up 100-300 RPM for about an additional 1 gph. Those last few 100 RPMs cost a lot of fuel. My experience backs up what airman stated, about 1/2 max speed but up on plane is the the most economical to run at.<br />Final note, the floscan or any other fuel flow meter is a valuable piece of equipment to have on a boat that has the capability of really drinking the fuel !

 

[18rabbit]

Re: what throttle setting gives best fuel economy?

Somewhere in the specs for my engine (Cat diesel) there is a chart that shows anticipated fuel consumption to ‘load’ rations for given rpms. In some applications, a higher rpm affords more economy, in others, lower rpm as the best economy. I think it depends on the ‘work’ that is expected of the engine, i.e. if alternators or hydraulic pumps driven off the engine kick in, a change in rpms is needed to maintain the best economy. Unless there is a constant, static load on the engine, there may not be a single preferred rpm.<br /><br />Anyway, that’s what I’ve gotten out of the Cat engine manual. I imagine it would be the same for all engines.

 

[Capn Mike]

Re: what throttle setting gives best fuel economy?

A fuel flow meter surprises you. I have a 23' Striper w/200 Johnson. Before I got my fuel flow meter, I figured I knew where the sweet spot was...and yes, I could "hear & feel" it. It was definitly @ 4400-4500 rpm, just about 800 rpms above planning speed.<br />Then I got the meter and charted it. Mine is fed into my Navman GPS and gives me fuel flow and GPH, allowing me to figure MPG. Hmmmm. <br />Sure enough, I get maximum MPG @ 4500 rpm. Then it drops off. But I as I increase the throttle AND TRIM UP THE MOTOR, I get almost the same MPG (only 1 MPG less) @ full throttle, motor trimmed up as high as possible. I can't run with the motor trimmed high in a heavy chop or swell....but the previous posts are right: it opens your eyes to all sorts of possibilities. I also believe it paid for itself the first season. <br />Can't say that about too many other boat options!

 

[mattttt25]

Re: what throttle setting gives best fuel economy?

airman gave you the best advice. you can probably find close to your setup there. another good source of 10-15 tests a month is boating magazine.<br /><br />from what i have read, most good sized outboards provide the best fuel economy at the 3500 rpm mark, give or take a few hundred rpms.

 

[kd6nem]

Re: what throttle setting gives best fuel economy?

I'm hearing a lot more talk about engines than hulls here. I agree with what I'm hearing, but let me throw in yet one more opinion: Prop pitch and efficiency, hull design, biological load (barnacles & marine growth), trim, load weight and distribution all play a role in this. So might how big the engine is. Too big or too small might be less efficient. Most efficient RPM for the motor might just be one thing but most efficient hull speed might be when the engine is not at its favorite speed in some cases. We'd like to see the two balanced, and hopefully in most cases they pretty much are. Fuel flow meter and GPS do tell the bottom line best. Charts and graphs are typical scenarios under test conditions. How about the real conditions I'm in right now?<br /><br />Of course we're only talking about planing hulls here. Displacement hulls are a whole different discussion, though most of the same principles apply. I agree the RPM range is a great ballpark starting place, and is in fact plenty good for my purposes. But remember engine RPM alone doesn't tell the whole story if you are really trying to watch fuel economy. If you can afford a big powerful boat get the fuel computer for sure! The rest of us will merrily power along at our favorite speed and enjoy every minute of it.

 

[mattttt25]

Re: what throttle setting gives best fuel economy?

bearcat- that's what airman and i were saying- the best way to guestimate your most efficient rpm is to compare your setup with those that were professionaly tested. i've seen many test results of 18'-22' center consoles running on 135-175 hp outboards, and most have a sweet spot of 3500 rpms. most likely means my 20' with 150 hp engine running at 3500 rpms is close.<br /><br />you're advice was good. all of those factors do play in the equation.