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Old 06-17-2019, 10:24 PM   #1
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Cruising On One Engine

We have a 40' trawler with Lehman diesels and the borg warner transmissions. Recently on a trip, we decided to cruise on one engine to save fuel. To my surprise sailing at 7.5 knots with both motors running at 1450 rpm, we achieved 6.5 knots running on one motor.



My question is there any ill affect with the transmission on the idle motor? It seems that with the prop is still turning that the transmission is still working, albeit not under load, and with the motor not running, the transmission cooler is also not working, Will I have a problem running on one motor?
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Old 06-17-2019, 10:27 PM   #2
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Some transmissions are fine freewheeling some are not. Do you have a manual on your transmission? If not try to get one and see what the manufacturer says. I am not sure if yours are good to freewheel or not.
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Old 06-18-2019, 03:12 AM   #3
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I have Lehman 225’s and I believe Borg Warner’s and I found similar results. I run at 1650 and get 8.5kts on both and 7.5 kts on one engine at the same rpm. American diesel said at that speed and rpm you aren’t hurting a thing. To be safe, use a handheld IR temp gun and make sure it’s not hotter than the operating transmission.
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Old 06-18-2019, 04:44 AM   #4
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Transmission bearings can overheat when free spooling. The oil inside the transmission stays in one place instead of being pumped through the heat exchanger and flowing cooler oil. If the load of the spinning prop isn't too great the temps will stay low. A heat measuring gun pointed on the case, especially near the rear bearing can guide you. If you're going to make this a regular practice, add an oil temp gauge.


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Old 06-18-2019, 05:48 AM   #5
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I don't think it's a normal practice or else more folks would be doing it.

You'll also be throwing off your normal maintenance schedule if you have one engine running and the other one off. If you make a habit of it, make sure you write down your engine hours in your log book.
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Old 06-18-2019, 05:52 AM   #6
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Quote:
Originally Posted by Comodave View Post
Some transmissions are fine freewheeling some are not. Do you have a manual on your transmission? If not try to get one and see what the manufacturer says. I am not sure if yours are good to freewheel or not.
I have a pair of 120s that power Velvet Drives. The manual clearly states that this transmission may be free-wheeled. Whether there any significant savings has been debated extensively in other threads. I choose to run on one engine only in lenghty slow zone speed or idle speed zones such as the Dismal Swamp. It's quieter and I don't have to put up with the vibrations that often come at steerage way speeds. That 1 MPH reduction in speed is important to me on a longer run adding as much as an hour to an already long day. I don't care about a bit of fuel savings. Others may. Just another personal boating choice.
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Old 06-18-2019, 05:59 AM   #7
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You might consider switching engines periodically to maintain lubrication and cooling of the transmission. Alternating each hour might be a good place to start.

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Old 06-18-2019, 06:00 AM   #8
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Quote:
Originally Posted by catalinajack View Post
I have a pair of 120s that power Velvet Drives. The manual clearly states that this transmission may be free-wheeled. Whether there any significant savings has been debated extensively in other threads. I choose to run on one engine only in lenghty slow zone speed or idle speed zones such as the Dismal Swamp. It's quieter and I don't have to put up with the vibrations that often come at steerage way speeds. That 1 MPH reduction in speed is important to me on a longer run adding as much as an hour to an already long day. I don't care about a bit of fuel savings. Others may. Just another personal boating choice.

I like this approach--will have to try it--idle speed zones are tough on our boat we are not traveling slow enough even at 1000 rpms.
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Old 06-18-2019, 07:16 AM   #9
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For reverse gear that can't be freewheeled - does anyone have thoughts on running one engine at speed and running the other engine in gear, but at idle. Obviously switching which engine was run up to speed regularly.
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Old 06-18-2019, 07:38 AM   #10
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I've rebuilt a few of the BW 72 series. I had to look up my service info on the lube path. Of course, when the input shaft is stopped (engine off), all lube oil transit stops and there is no flow thru the oil cooler. The pressure lubed bushings appear to be on the output side of the input shaft, but BW thinks that they are OK with no lube at low speeds. The planetary reduction and output thrust bearing is splashed cooled and lubed. If the output shaft is spinning rapidly, there is the condition of oil heating with limited cooling, so that is called out in the manual. Basically, go slow, watch the oil temperature and for longer or faster passages, lock the shaft. BW doesn't seem worried with lube, only overheating, so that is the critical metric. I can't imagine a 6kt boat would overheat the lube.
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Old 06-18-2019, 08:28 AM   #11
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I've rebuilt a few of the BW 72 series. I had to look up my service info on the lube path. Of course, when the input shaft is stopped (engine off), all lube oil transit stops and there is no flow thru the oil cooler. The pressure lubed bushings appear to be on the output side of the input shaft, but BW thinks that they are OK with no lube at low speeds. The planetary reduction and output thrust bearing is splashed cooled and lubed. If the output shaft is spinning rapidly, there is the condition of oil heating with limited cooling, so that is called out in the manual. Basically, go slow, watch the oil temperature and for longer or faster passages, lock the shaft. BW doesn't seem worried with lube, only overheating, so that is the critical metric. I can't imagine a 6kt boat would overheat the lube.
At 7.5 kts the tranny that was free wheeling was about 10 degrees cooler than the one that was operating.
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Old 06-18-2019, 08:51 AM   #12
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You probably wont save much fuel than if you slowed to 6.5 kts with two engines because the one engine is now doing more work and using more fuel, even at the same RPM. Be careful to not overload the one engine.
The drag of the lazy prop is substantial.
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Old 06-18-2019, 08:56 AM   #13
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Just because you are running at the same rpm with one vs two does not mean you are using half the fuel. You are now using more fuel to get that rpm on the one engine.
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Old 06-18-2019, 08:57 AM   #14
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For the record, Twin Disk recommends starting the engine on the free wheeling side every hour and running it in idle for a few minutes. The reason is that the transmission oil pump is run off the input shaft (not the transmission gear train).


Also be sure your stuffing boxes are running cool on the free wheeling shaft. Many are cooled by pressurized water supplied by the engine raw water cooling system. This is especially important for drip free seals. A few years ago I helped a gent move a twin around Michigan to Toledo the Toledo area. The prop shaft coupling on one of the transmissions sheared all of the bolts about 20 miles from the nearest suitable repair facility. The boat had dripless seals...each cooled by it's respective engine. We ended up locking the free wheeling prop shaft with a pipe wrench before proceeding on the remaining engine. As it turned out we repaired it ourselves once we got the boat in a slip and procured new bolts. The manufacturer of the dripless seal assured us that we would have destroyed the seal had we not stopped the shaft. If cruising on one, the cooling water would need to be cross connected between sides (assuming the cutless bearings are pressure cooled).

There are scores of threads on this subject in the archives.. Several have actual performance data for fuel saving on various hulls.
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Old 06-18-2019, 09:07 AM   #15
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Be also aware that many boats have a cross-connect hose between the two shaft seals. So running one engine flows water to the other. On one Viking with water lift mufflers, this filled the engine with water and forced an overhaul.
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Old 06-18-2019, 02:21 PM   #16
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Quote:
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You probably wont save much fuel than if you slowed to 6.5 kts with two engines because the one engine is now doing more work and using more fuel, even at the same RPM. Be careful to not overload the one engine.
The drag of the lazy prop is substantial.
I disagree. When Iím at 1650 and I pull one engine back to idle and shut it down. Donít have to advance the throttle to maintain rpm. Itís not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then Iíd have to push that engine up.

I believe if one runs the same rpm on one engine as two, it is half the fuel burn. But itís not twice the fuel economy since youíre going slower.
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Old 06-18-2019, 02:29 PM   #17
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I disagree. When Iím at 1650 and I pull one engine back to idle and shut it down. Donít have to advance the throttle to maintain rpm. Itís not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then Iíd have to push that engine up.

I believe if one runs the same rpm on one engine as two, it is half the fuel burn. But itís not twice the fuel economy since youíre going slower.
Depends on the engine.
On many engines the throttle actually sets the RPM not the rate of fuel consumption. You may be going slower on one engine with the same RPM, but you may also be consuming more fuel. It depends on the engine.

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Old 06-18-2019, 02:30 PM   #18
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Pretty sure if it takes more effort to keep that same RPM...no matter what the boat does, the engine is using more fuel.
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Old 06-18-2019, 02:45 PM   #19
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I disagree. When Iím at 1650 and I pull one engine back to idle and shut it down. Donít have to advance the throttle to maintain rpm. Itís not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then Iíd have to push that engine up.

I believe if one runs the same rpm on one engine as two, it is half the fuel burn. But itís not twice the fuel economy since youíre going slower.

Negative, and you just proved the contrary without knowing it. When you shut down the second engine, and the boat slowed down a bit, but engine one remained at set RPM, the prop slip MUST HAVE INCREASED. Same engine rpm/same prop rpm. That increase in slip necessarily means higher prop loading, therefore more engine loading. So, now you have a higher fuel consumption, due to that higher loading.

Yes, most diesel "throttles" are setting the RPM, not the power level or the fuel burn. Beside the point, but one could argue these diesels don't have throttles, since there is no throttle plate.
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Old 06-18-2019, 02:50 PM   #20
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It’s a subtle difference in fuel. Very little if any rpm drop and a loss of 1kt. That engine doesn’t see a difference in load. But i could be wrong....happened once before. Anyone have a tested this with boost gauges or actual fuel flow gauges? I’d be interested to see.
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