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?

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 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.

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.

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.

Comodave said:

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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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.

You might consider switching engines periodically to maintain lubrication and cooling of the transmission. Alternating each hour might be a good place to start.

Ted

catalinajack said:

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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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.

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.

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.

diver dave said:

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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At 7.5 kts the tranny that was free wheeling was about 10 degrees cooler than the one that was operating.

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.

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.

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.

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.

bayview said:

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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I disagree. When I’m at 1650 and I pull one engine back to idle and shut it down. Don’t have to advance the throttle to maintain rpm. It’s not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then I’d 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 it’s not twice the fuel economy since you’re going slower.

Steve91T said:

I disagree. When I’m at 1650 and I pull one engine back to idle and shut it down. Don’t have to advance the throttle to maintain rpm. It’s not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then I’d 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 it’s not twice the fuel economy since you’re going slower.

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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.

Ted

Pretty sure if it takes more effort to keep that same RPM...no matter what the boat does, the engine is using more fuel.

Steve91T said:

I disagree. When I’m at 1650 and I pull one engine back to idle and shut it down. Don’t have to advance the throttle to maintain rpm. It’s not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then I’d 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 it’s not twice the fuel economy since you’re going slower.

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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.

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.

diver dave said:

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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A mechanical injection pump is setting rpm and not amount of fuel going into the engine? Serious question.

Mechanical pump diesels have a high idle setting. Power lever full forward in neutral. (and not much fuel flow!) That engine speed is regulated by the adjustable governor, as is the in-between settings.

Also, this is why an overpropped diesel is smoky. It is not reaching the correct rpm for its power setting; basically, running rich, because the mechanical governor is trying to get to a certain RPM, and is giving its best shot with more fuel, that remains unburned due to lower than design rpm.

Steve91T said:

A mechanical injection pump is setting rpm and not amount of fuel going into the engine? Serious question.

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Depends on the engine. Some using a govenor set RPM and the govenor sets injection amount. As load varies, govenor increases or decreases injection to maintain RPM. Throttle set injection pumps dose per injection. As the load varies you have to keep adjusting the throttle to maintain RPM.

Ted

I contemplated this question when we first got our current boat.
Not so much as a normal operating process, more if an engine shut down for any reason and we were off shore. We routinely run off the coast.


Talked with the manufacturer of the boat. Their guidance was if it was 5-10 NM, then I was fine just running on one engine. More than that, then the non running engine needs to have the shaft secured. Either pipe wrench or a line wrapped around the shaft and tied off (they provided me with where to tie the line off).


Twin Disc transmissions, don't recall the model #.


Interesting tidbit, we discussed the cross cooling interconnect. My previous boat had cross cooling, this one does not. The manufacturer stated that previous to my model year all their boats had cross cooling. They experienced customer issues where a backfeed situation caused water ingestion. They explained the scenario, I didn't take notes - so cant offer a more detailed explanation. Suffice to stay, they stopped doing cross cooling.


One manufacturers perspective.

For the PSS brand/type of shaft seal, you only need a positive pressure inflow if you go over 12kn (at which speed the water sucks out requiring input of water to lubricate the graphite). So if you go under 12kn using only one engine, the normal water pressure from outside should be enough to lubricate the shaft seal and you shouldn’t need a cross-connect positive pressure line from the other working engine to the seal on the non-working engine. I think .

A certain amount of energy is required to move a boat at a specific speed. The amount varies with drag and drag (below planing speed) is aprox. proportional to speed^2.

Any savings from running one vs. two are going to vary from vessel to vessel. Factor in reduced speed and the drag of the other prop and it's likely a wash. With special equipment designed to reduce drag like feathering props, locking the shaft, etc... the savings could be more or less. Further complicating it is the different design of various transmissions, some are OK with a freewheeling, others are not, some are for a short duration. My twin disc specifically talks about this and recommends starting up an unused engine every few hours to lubricate the components, and recommends against freewheeling at higher boat speeds.

Accurate fuel flow gauges for each engine combined with an accurate boat speed along with an afternoon of testing will put any mystery for a particular boat to rest, without that data, it's just conjecture. My boat doesn't have flow gauges.

It would be nice to see actual before and after fuel burn data using this procedure.

Steve91T said:

I disagree. When I’m at 1650 and I pull one engine back to idle and shut it down. Don’t have to advance the throttle to maintain rpm. It’s not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then I’d 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 it’s not twice the fuel economy since you’re going slower.

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The throttle on a mechanical diesel sets the rpm, not the amount of fuel or position of the fuel rack. The injection pump will add as much fuel as is required for the engine to satisfy the rpm request. So you could be using 1% or 50% more fuel but you'd never know it.

Ken

I lost an engine once 6 hours from home. Since the engine was in gear at shut down I just left it that way. Dragging a stationary prop might not be the most efficient but it ended any concern about a freewheeling transmission.

Putting a pipewrench on your shaft is a bad idea. Much better to apply to the COUPLING!!!