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Old 10-21-2010, 12:49 PM   #1
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Operating a twin on one engine and fuel economy

I just completed my first cruise on my newly purchased boat without a maintenance issue. Everything worked and I didn't have to crawl down into the engine compartment once. With the exception of fowling the prop with the dingy painter, those big props suck a lot of water, and a minor grounding everything went well. I envy you guys in the PNW with your majestic mountain views, but while you're putting your boats away for the winter, we're gearing up for good very pleasant weather cruising down here in Florida.

With no maintenance issues to contend with I experimented with running my engines at different rpm's and comparing the speeds with the engine manufactures fuel consumption curves. I came up with some interesting data.

At displacement speed on my twin engine planning hull boat which is approx 8.3 kts and 1300 rpm, my fuel consumption according to the fuel curves is 4.9 gal/hr 2 engine, which will give me about 1.69 Nautical mile/gal (Nmpg). At $3/gal that's $1.78/Nautical mile.

At 2000 rpm I'm getting approx 17 kts and burning about 13.4 g/h 2 engines yielding 1.26 Nmpg or $2.38/Nm. So by increasing the fuel burn by 34% I increase the speed by 204%.

But I found this even more interesting. Cruising at displacement hull speed as above at 8.3 kts getting 1.69 Nmpg, I shut one engine down left the remaining engine running at 1300 rpm and my speed dropped by 1.2 kts to 7.1 kts. Consulting the fuel curves gives a fuel burn of 2.45 g/h or 2.9 Nmpg or $1.03/Nm.
So operating single engine reduced my speed by 14.4% and increased my mileage by 58%.

What I didn't do and will do next time is to shut one engine down and increase the remaining engine rpm to remain at 8.3 knots and compare the fuel burn.

So I guess what's important is how you interpret this data. Seemingly it would make little sense to cruise at displacement speeds with two engines when one engine will only decrease speed 14.4% yet increase mileage by 58%. Note: my transmission manual allows this.

I also found that cruising at low planning hull speeds did not increase mileage significantly so in my case it would make more sense if planning is desired to plane at 2000 rpm rather than seeing a very small increase in mileage by cruising say at 1600 rpm. I did not cruise above 2000 rpm, not wanting to stress the engines. The fuel curves confirm however that at speeds above 2000 rpm, fuel flow rises almost exponentially.

Pehaps other can shed some light on this single engine cruising on a twin.

-- Edited by timjet on Thursday 21st of October 2010 12:50:50 PM

-- Edited by timjet on Thursday 21st of October 2010 02:35:06 PM

-- Edited by timjet on Thursday 21st of October 2010 02:41:22 PM
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Old 10-21-2010, 01:13 PM   #2
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RE: Operating a twin on one engine and fuel economy

*The fuel burn curves don't take account of the drag from the trailing prop (spinning has more drag than fixed) or the drag induced by the rudder correction to keep on course.
I experimented with my last boat, which had twins, and found that the economy running with just one engine was actually worse than with two
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Old 10-21-2010, 01:15 PM   #3
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Operating a twin on one engine and fuel economy

Make sure your transmissions can be freewheeled without damage if you haven't already. Some transmissions can be freewheeled, some can't and some can only be freewheeled at slow speeds (like the old BW Velvet Drive).

Also make sure freewheeling a prop will not overheat the shaft log and packing gland. We cannot freewheel a prop on our boat at any speed because our shaft log depends on a water feed from its associated engine to help cool the shaft log. If the engine has to be shut down the water feed to the shaft log stops and if the prop is allowed to freewheel the spinning shaft will overheat the shaft log and packing gland to the point of severe damage in very short order. This is why we need to tie off the shaft if we have to shut down an engine.

-- Edited by Marin on Thursday 21st of October 2010 01:18:18 PM
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Old 10-21-2010, 01:17 PM   #4
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RE: Operating a twin on one engine and fuel economy

Quote:
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*(spinning has more drag than fixed)
That's actually wrong.* MIT did a study on this that was posted several years ago on one of the GB owners forum.* A freewheeling prop generates less turbulence and thus less drag than a locked prop.

*
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Old 10-21-2010, 01:44 PM   #5
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Operating a twin on one engine and fuel economy

Jeff,
I'm no expert, but the fuel burn curves from the manufacturer simply state fuel burn at specific rpm. The engine doesn't care if the other prop is free wheeling or an anchor is being dragged. If the engine is turning at 1300 rpm then it is burning in my case 2.45 gal/hr per engine. Though the anchor analogy may not be a good one because a lot of turbulence would be created and that would effect the burn numbers, but not the drag a rudder would create or a spinning prop. The drag of the rudder and free wheeling prop is taken into consideration by the decrease in speed. But that is far off set by the fuel burn saved by the stopped engine.

You make a good point because your real world data indicates no advantage to running on one engine. Perhaps someone could explain it or otherwise refute the data I collected.

Marin,
My tranny's can free wheel without issues according to the manual. I'm going to verify this with the manufacturer. My shaft logs are not water cooled, so it would seem there would be no difference whether the shaft is turned by the engine or by the prop. Correct??

PS: Just changed my avitar

-- Edited by timjet on Thursday 21st of October 2010 02:04:38 PM
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Old 10-21-2010, 02:13 PM   #6
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RE: Operating a twin on one engine and fuel economy

What's all this kilometer stuff? I guess it makes life easier for RTF but all that mixing of knots, miles, meters, and gallons is just plain weird.
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Old 10-21-2010, 02:14 PM   #7
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RE: Operating a twin on one engine and fuel economy

Quote:
timjet wrote:

Marin,
My tranny's can free wheel without issues according to the manual. I'm going to verify this with the manufacturer. My shaft logs are not water cooled, so it would seem there would be no difference whether the shaft is turned by the engine or by the prop. Correct??


I'm curious--- if your shaft logs/cutless bearings are not cooled and lubed by water, what are they cooled and lubed with?

However, if your shaft logs and the cutless bearings in them do not require a forced-flow water feed from the engine's raw water cooling system then they are getting whatever cooling and lubing they need from the water around the boat, so I would think that you are correct in that letting the shaft freewheel with its engine shut down would not be a problem.*

You can check this easy enough by monitoring how hot the shaft log gets or doesn't get while letting the shaft freewheel at speed.* On our boat, shutting down an engine and letting the shaft freewheel results in the shaft log (and shaft) getting too hot to touch pretty quickly at any kind of boat speed.* Hence the need to lock off the shaft.

*
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Old 10-21-2010, 02:35 PM   #8
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RE: Operating a twin on one engine and fuel economy

Rick, You're right it's confusing. I corrected it.
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Old 10-21-2010, 02:37 PM   #9
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RE: Operating a twin on one engine and fuel economy

Marin,
Correct, they are water cooled, not forced water cooled like yours. Thanks.
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Old 10-21-2010, 04:14 PM   #10
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Operating a twin on one engine and fuel economy

I'm not an expert either but, I think the drag of the free wheel and rudder will be reflected in the*fuel pump delivery*required to maintain the revs. The fuel burn curve assumes a particular load, this will increase when running on one engine.

An engine turning* 2000rpm at the dock uses hardly any fuel, but when pushing a hull through the water at 2000rpm*the pump has to supply many times more fuel to maintain the revs you have selected*with the "throttle".*

Re spinning rather than fixed,*think of*a helicopter in auto rotate mode. If the blades are locked, it will drop like a stone, (not much drag) but if allowed to rotate, they provide sufficient lift (and drag) to reduce the rate of descent.


-- Edited by Bendit on Thursday 21st of October 2010 04:19:04 PM
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Old 10-21-2010, 05:42 PM   #11
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RE: Operating a twin on one engine and fuel economy

Quote:
Bendit wrote:
Re spinning rather than fixed,*think of*a helicopter in auto rotate mode. If the blades are locked, it will drop like a stone, (not much drag) but if allowed to rotate, they provide sufficient lift (and drag) to reduce the rate of descent.

That's an apples and oranges comparison.* The reason the helicopter "drops like a stone" with the rotor stationary is not because of drag or lack of it but because the stationary rotor blades are producing no lift whatsoever.* When the blades are turning, they can be used to develop lift at the end of the drop when pitch is applied to stop the drop.

A stationary boat propeller generates a huge amount of turbulence behind it, and turbulence is drag.* A freewheeling boat propeller does not generate as much turbulence behind it, thus less drag.* The MIT study made this very clear.



*
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Old 10-21-2010, 08:10 PM   #12
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RE: Operating a twin on one engine and fuel economy

It seems to me that pushing a lighter aluminum hull with a single engine* and an absurd amount of fuel on board at 3gph and 8 kts is much easier then making all these calculations.
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Old 10-21-2010, 08:12 PM   #13
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RE: Operating a twin on one engine and fuel economy

*...and I never have to worry about engine synchronization!
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Old 10-21-2010, 09:02 PM   #14
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RE: Operating a twin on one engine and fuel economy

The amount of work done to push your boat through the water at 8.3 knots doesn't change, whether you are running one engine or two. What does change, is the efficiency of the hull being pushed from one corner instead of equally from two corners. Having a rudder cocked over to maintain a strait course will also cause more drag. Thus worse performance on one than on two.
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Old 10-22-2010, 04:13 AM   #15
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RE: Operating a twin on one engine and fuel economy

A stationary boat propeller generates a huge amount of turbulence behind it, and turbulence is drag. A freewheeling boat propeller does not generate as much turbulence behind it, thus less drag. The MIT study made this very clear.


What every study I have seen is that a really truel free spinning prop, (no tranny drag , no strut drag , no bearing drag) might have less drag than a locked prop.

BUT like the helocopter (at the end if its fall the drag from auto-rotation is huge) the slower than ideal speeds of the dragging prop is worse than the locked prop.
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Old 10-22-2010, 06:04 AM   #16
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RE: Operating a twin on one engine and fuel economy

Having owned twin screw boats up til now, I really am liking the simplicity of a single screw set up and it's efficiency and yes....like already said, I don't have to ever worry about syncing the engines!!
That being said...I am pretty sure the first time I almost wreck while docking due to winds or currents or whatever that I will, at that precise moment, be strongly wishing I had twin screws!
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Old 10-22-2010, 07:43 AM   #17
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Operating a twin on one engine and fuel economy

There is no doubt the drag of the rudder corrected for the asymmetrical thrust and the wind milling propeller will slow the boat down and create inefficiencies.* This amount of drag will depend on hull design, the distance each prop is from the centerline of the boat and other characteristics unique to each boat. However any engine must produce a certain amount of power that is used to run the pumps, alternator (commonly referred to the accessories) and in the case of a diesel, the heavy fly wheel and crankshaft, power that is not delivered to the propeller, but uses fuel nonetheless. Another point brought up by Jeff is the fact that fuel flow will increase on an engine under load compared to an engine unloaded at the same rpm. The question of course is how much more of a load is produced by the asymmetrical thrust and the effort needed to counteract it.
My contention at least on my boat is the fuel consumed by an engine to run the accessories exceeds the greater fuel flow needed by the remaining engine to counteract asymmetrical thrust drag.

Another issue is the power to weight ratio of the boat. My boat unlike some trawlers has a higher power to weight ratio, allowing it to plane. When I shut an engine down and maintain displacement speed, the remaining engine is producing probably less than 50% of its available power. Perhaps as the power to weight ratio lowers, the argument shifts in favor of running on 2 engines.

The argument here is not 2 vs 1 engine as this is a personal preference issue that has no definitive answer that can be based on fact. There is a factual answer to the question of fuel performance at displacement hull speeds on a planning hull with one or two engines. Perhaps this question can only be answered by installing fuel flow indicators and further testing.* But my guess is this testing has already been done, I just need to find where the answer is.


-- Edited by timjet on Friday 22nd of October 2010 07:44:57 AM
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Old 10-22-2010, 09:51 AM   #18
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RE: Operating a twin on one engine and fuel economy

Marin, what were the conditions of the MIT experiment? Because I can tell you right now there are not any emergency procedures on twin engine airplanes that tell you to leave the dead engine propeller to windmill.

And Tim, when you shut one down, did you have to advance the power lever on the good engine to maintain 1300 RPM? If not, I am willing to bet that the fuel flow remained the same as it was before....
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Old 10-22-2010, 09:55 AM   #19
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Operating a twin on one engine and fuel economy

Quote:
Baker wrote:" I can tell you right now there are not any emergency procedures on twin engine airplanes that tell you to leave the dead engine propeller to windmill."
Amen, brother.....The difference being able to feather the prop on the dead engine of course.

*


-- Edited by SeaHorse II on Friday 22nd of October 2010 09:57:23 AM
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Old 10-22-2010, 12:53 PM   #20
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Operating a twin on one engine and fuel economy

Quote:
Baker wrote:

Marin, what were the conditions of the MIT experiment?
First of all, trying make a comparison between a thin aircraft propeller in air and* wide marine propellers in water is an apples and oranges situation even if the theories are similar.* Plus the prop in a multi-engine plane is going to be feathered so it makes obvious sense that stopping it in this condition will generate less drag than locked in an unfeathered position or even free-wheeling.

It's been a number of years since I saw the MIT study, but typical for them they did all sorts of tests, most if not all in a test tank with instrumented props and shafts.* They tested at a variety of speeds and with a variety of prop configurations, and the conclusion was that a prop that's allowed to freewheel transmits less drag to what it's attached to (the boat) than a prop that is locked off.* The drag amounts varied a lot depending on the speed through the water, the friction applied to the freewheeling shaft, etc.

But the bottom line of the study was very clear.* All else being equal--- the prop diameters, number and configuration of blades, and the speed and angle*through the water, etc.--- the prop that was allowed to freewheel, even with a high drag on its shaft simulating a transmission or whatever, always produced less measured drag than the prop that was locked off.* The locked off prop produced the maximum turbulence every time.* A freewheeling prop could be slowed by shaft friction to the point where the turbulence from the blades was very high, but it was never quite*as high as the turbulence off the locked prop.

And obviously the less resistance that was applied to the freewheeling shaft, the less turbulence was generated, and the lower the drag as FF pointed out earlier.

That's about all I remember from my one-time reading of the test results.* Someone posted it on one of the GB sites during a similar discussion to this one.*


-- Edited by Marin on Friday 22nd of October 2010 12:58:03 PM
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