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Old 09-28-2012, 08:34 AM   #1
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"Efficiency"

Folks are rightly concerned with fuel burn , and attempt to make guesstimates based on a Mfg. hp and prop graph.

Sadly this is a futile exercise as so much information (efficiency at various loads and RPM) is lacking.

I have always suggested to use a Fuel Map tho these are really hard to get from the engine folks.

Most folks have never even seen a fuel map, so here is a BMEP,map , about as close as you can get..

This is for a car engine , but it can be seen that there are areas to operate that are more efficient to operate than others.

For a long range cruiser , selecting power , there is no better info, IF you can obtain it.

http://ecomodder.com/wiki/index.php/...hyperbolae.png

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Old 09-28-2012, 11:09 AM   #2
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Well Fred, I respectfully disagree.

Most diesel manufacturers supply four curves for their engines: 1) hp vs rpm at wot, 2) horsepower vs rpm based on a 2.5 or 2.7 exponent formula, 3) torque vs rpm at wot and 4)fuel consumption vs rpm which corresponds to the prop hp curve. Some also give fuel consumption at wot vs rpm but that is mostly irrelevant.

Sure the horsepower curve is based on a pure formula, but the fuel consumption curve that they supply that corresponds with that horsepower curve is derived from just the sort of fuel map you talk about.

Yes, not every trawler will follow the 2.5 or 2.7 exponent curve but it will be close enough to make a pretty decent determination of expected fuel burn at different rpms, assuming the boat is propped to reach the rated hp at wot.

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Old 09-29-2012, 06:44 AM   #3
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1) hp vs rpm at wot,

Useless for deciding where to set cruise rpm and loading

2) horsepower vs rpm based on a 2.5 or 2.7 exponent formula,

Of no help if selecting a cruise regime other than WOT prop selected by Mfg.

3) torque vs rpm at wot

Great for sport fish that live at WOT

4)fuel consumption vs rpm which corresponds to the prop hp curve.

Again a cruiser will be loading the engine for max range , rather than max speed , so the prop chosen will NOT be the prop a mfg would chose to endure light loading at less than WOT.

Remember the Mfg is interested in no warentee calls , doesn't give a darn about your fuel use at cruise.
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Old 09-29-2012, 12:21 PM   #4
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This may be a little off from your technical discussion but using the fuel burn curves on my Cummins 6BT 5.9 liter engine yields results that are pretty close to real world fuel burn in the 2300 rpm range. The chart shows a burn of 9.5 gal per engine at 2300 rpm or 19 gal per hour total. I get an average of 18 knots at that rpm and that's very close to my real world average of 1 mpg.

At engine rpm between 1500 and 1900 the charts may not be accurate because the boat is above hull speed but not yet on plane. I suspect fuel burn in this range would be above the fuel burn at 2300 rpm.

I however operate the boat at only 2 speeds. 18 knots at 2300 rpm or 6.5- 7 knots on one engine at 1300 rpm. Operating it at any other speed makes no sense from a fuel economy standpoint. This is of course unique to my boat due to the planning characteristics of the hull and the fact that I get only a 1.5 knot increase in speed at 1300 rpm using two engines vs one.
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Old 09-29-2012, 02:59 PM   #5
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If only repropping and keeping the same engine(s), the major efficiency contribution would be from the bigger prop(s). Moving the engine to another operating curve wouldn't yield much, except for the possible case of turbocharged engines running well off boost. Anyway, it's easy enough to get the best range speed for the existing drivetrain using prop charts and a simple speed-power polar obtained in calm conditions. I'd think a good prop shop could calculate the power required to achieve that speed for various cruise prop sizes. Then back into the prop charts for a fuel burn estimate. Yes, it's ballpark, but it would probably tell you that it's not worth the effort unless circling the globe or maybe doing the Loop...

I've seen similar results as Timjet for running a twin on one. Good solution if the transmission and autopilot can handle it. Again, most benefit for turbo engine running well off boost. Still want to do the pipe wrench test...lock shaft to investigate drag difference between free wheeling and stopped prop.
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Old 09-29-2012, 03:44 PM   #6
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Quote:
Originally Posted by skidgear View Post

I've seen similar results as Timjet for running a twin on one. Good solution if the transmission and autopilot can handle it. Again, most benefit for turbo engine running well off boost. Still want to do the pipe wrench test...lock shaft to investigate drag difference between free wheeling and stopped prop.
Very good point! Make sure especially the tranny is lubricated and the shafts too. In my case I called the manufacturer and confirmed the tranny does not need the engine for lubrication. I have stuffing boxes. Once trimmed the autopilot does not know one is shut down.

I would not lock the shaft for safety reasons and if I had to for lubrication reasons would not shut one down.
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Old 09-30-2012, 08:14 AM   #7
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The big hassle of course is the only way a boat can be really efficient is if that IS the design criteria from the start.

The items like a CPP or a 2 speed transmission are easy to build in from the start , but not a realistic expense for an older boat.

For the old boat , probably the most results for the effort is keeping the prop clean, second would be a clean hull, third UNLOAD! some of the "stuff" that builds up over the years.
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Old 09-30-2012, 11:04 AM   #8
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Very good point! Make sure especially the tranny is lubricated and the shafts too. In my case I called the manufacturer and confirmed the tranny does not need the engine for lubrication. I have stuffing boxes. Once trimmed the autopilot does not know one is shut down.

I would not lock the shaft for safety reasons and if I had to for lubrication reasons would not shut one down.

I wouldn't lock the shaft in normal operations either, but I'm curious about that drag differential number.

The Twin Disk MG502 does depend on the engine running for trans oil pump pressure. I've read that TD is OK with running for up to an hour before starting the engine to squirt some oil into the bearings. Our boat uses water pressure off the raw water pump to cool the stuffing box. The latter is easy to address with a "T" in the cooling lines on each side. The former could probably be handled with an electric pump in the cooler line...or periodically change to the other power plant. Our autopilot also handles the situation effortlessly.

The boat is 44', water line 39', semi-planing, 30,000 lb+. These are the stabilized test numbers I recorded along with fuel burn data taken off the prop chart, which is fairly close to actual fuel burn. Wind and seas were calm.

1550rpm x 2 9.4 mph (gps) 4 gph (total) 2.3 mpg
1850rpm x 1 9.4 mph 3.1 gph 3 mpg
1550rpm x 1 7.9 mph 1.9 gph 4.15 mpg
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Old 10-01-2012, 01:19 PM   #9
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I have a low buck plan for dealing with the eventuality of $10 fuel. Given that our boat already has shown an efficiency gain at slow speed from simply freewheeling one prop, the next step is a cruise prop on one side. The remaining engine becomes a high powered wing engine which is started for docking, maneuvering in congested areas or if I wanted a burst of cruise speed for poor weather conditions, for example. Yes, the rpm for equal thrust would be different, but so what. Swap the props every couple hundred hours to keep equal wear on both drive trains...no big deal on Great Lakes as the boat is on the hard every winter anyway.
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Old 10-01-2012, 06:28 PM   #10
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Your props have the same rotation?
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Old 10-01-2012, 06:29 PM   #11
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skidgear:

Be careful. A twin engine with one engine shut down is already in "cruise prop" mode. That one engine will probably only get you to 90-95% of wot rated rpm. Why?

The boat is going much slower with one engine at wot and the drag of the prop shaft, strut and freewheeling or fixed prop all adds up to load the working engine more.

I wouldn't put a cruise prop on a twin. Stick with a prop that will run up to max rated wot rpm with both engines running. Your engine will love you for it ;-).

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Old 10-01-2012, 07:20 PM   #12
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Flywright, No, opposite rotation, but I have a set of larger spare props that I bought back when I was thinking about overpropping both engines.

djmarchand, I'm fairly sure that part of the current single engine efficiencies I've noted come from the fact that the engine is forced to work harder in a more favorable band for the turbocharger..."overpropped" from the engine's perspective as you point out. I'm thinking it could be tweaked just a bit more with a slightly bigger (more efficient) prop while still having plenty of margin to achieve (along with the other engine) the lower top speed I have in mind....14-15 instead of 18Kts. Can't go a whole lot bigger on the prop or boat speed at idle would become a problem. But your point about overloading the engine at higher rpm is well taken. Think that can be addressed safely.

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Old 10-01-2012, 07:38 PM   #13
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Quote:
Originally Posted by skidgear View Post
ISwap the props every couple hundred hours to keep equal wear on both drive trains...no big deal on Great Lakes as the boat is on the hard every winter anyway.
There was a famous instance some years ago of a fellow who took a GB42 from Hawaii to the mainland. In order to get the range on the limited amount of fuel he was able to carry he started the journey on one engine with the prop removed from the other engine's shaft. Halfway across he stopped the boat and dove to remove the prop he'd been running on and install the prop for the other shaft. He then completed the trip on the other engine and prop. So this gave him the range he needed as well as put equal time on the engines.

It was more of a stunt than anything else but the theory did work in practice in this case.
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