Ok, I’m convinced to putt along at Hull speed, but

[socalrider]

But those are much smaller non-hydraulic gears built with that in mind. They have no oil pumps, the lower shaft and gears sit in the oil sump and splash up to the upper shaft. We also commonly put the gear in reverse to stop freewheeling, that cuts down on noise and wear. That doesn't work on a hydraulic gear.

Yeah, and a lot of sailboats have feathering or folding props which don't freewheel.

 

[mcarthur]

The other thing to check is your shaft seals. Some of them are lubricated via sea water from the motor - our PSS seals are like this. However, even if they are so lubricated, it may not be a problem: our PSS seals can run without lubrication (ie. free wheeling) up to 11-12kn, which is far above most of our boats' hull speeds.

 

[koliver]

That makes sense. One of the articles at the time said my boat got 2 mpg at 8 mph and 1 at 20, with a top speed of 24mph. My goal is to enjoy nice seas and get to port fast to avoid the bad ones.

You may wish to measure your fuel consumption at different speeds, just to make a data sheet for yourself.
From the photo in your first post, I doubt that you have enough keel to move your boat at less than a crabwise angle while running on a single engine, so you will have a lot of rudder required to go where you want to go. That will increase your fuel consumption, likely well past any savings from the reduction of friction losses by running only one of your two engines.

Weebles recollections bring back my own recollection of his example. In the same era, or close, we had a GB 42 owner on this forum or its Passagemaker predecessor, who ran on one and provided a data set, comparing single efficiency to twin, with his own results showing more fuel consumption on the single to achieve the same speed.

My own data set, accumulated when I was forced to spend a season on one, showed me that, for my own boat only; the single could not achieve the usual cruising speed comfortably; The speed achieved when running at the usual rpm was just below hull speed, where the fuel consumed by the single was unchanged from its consumption while the twin was running at that same rpm, just the speed through the water decreased.
The most indicative bit of data was the height of the stern wave. In order to make a wave of the height I make cruising at 8 knots, the single couldn't do it comfortably. Making the wave at a slower speed could be done on significantly less fuel, but there is no huge difference whether running one or two.
Pushing the boat from the aft corner required a lot of countering rudder, so more fuel consumption than keeping the rudders strait, this in a full keel design. Yours without a full keel would require more rudder still.

 

[Owen Christensen]

You may wish to measure your fuel consumption at different speeds, just to make a data sheet for yourself.
From the photo in your first post, I doubt that you have enough keel to move your boat at less than a crabwise angle while running on a single engine, so you will have a lot of rudder required to go where you want to go. That will increase your fuel consumption, likely well past any savings from the reduction of friction losses by running only one of your two engines.

Weebles recollections bring back my own recollection of his example. In the same era, or close, we had a GB 42 owner on this forum or its Passagemaker predecessor, who ran on one and provided a data set, comparing single efficiency to twin, with his own results showing more fuel consumption on the single to achieve the same speed.

My own data set, accumulated when I was forced to spend a season on one, showed me that, for my own boat only; the single could not achieve the usual cruising speed comfortably; The speed achieved when running at the usual rpm was just below hull speed, where the fuel consumed by the single was unchanged from its consumption while the twin was running at that same rpm, just the speed through the water decreased.
The most indicative bit of data was the height of the stern wave. In order to make a wave of the height I make cruising at 8 knots, the single couldn't do it comfortably. Making the wave at a slower speed could be done on significantly less fuel, but there is no huge difference whether running one or two.
Pushing the boat from the aft corner required a lot of countering rudder, so more fuel consumption than keeping the rudders strait, this in a full keel design. Yours without a full keel would require more rudder still.

This makes sense .

 

[MYTraveler]

Hate to burst your bubble Pete, but the sail-cats have been running on one engine to conserve fuel for a long time.

Perhaps those sailing cats have feathering props, and perhaps that sufficiently reduces drag to increase efficiency when running on a single-engine?

 

[MYTraveler]

The other thing to check is your shaft seals. Some of them are lubricated via sea water from the motor - our PSS seals are like this. However, even if they are so lubricated, it may not be a problem: our PSS seals can run without lubrication (ie. free wheeling) up to 11-12kn, which is far above most of our boats' hull speeds.

A slick setup to avoid lubrication problems is to connect the two shaft seals. Mine came that way from the builder. If either engine is running, both shafts are get water.

 

[Owen Christensen]

Nice, good idea.

 

[Hippocampus]

you know, sailboats run around all the time with an off engine freewheeling

Most have either feathering or folding props. Very few free wheel

 

[Simi 60]

A slick setup to avoid lubrication problems is to connect the two shaft seals. Mine came that way from the builder. If either engine is running, both shafts are get water.

I was going to run an inline aquarium pump on the powercat I was building.
12v brushless models are about $15.

Fixed 4 blade prop freewheeling wasn't a problem on other similar builds

 

[smitty477]

On all of the twin engine boats we owned achieving hull speeds with one engine was quite easy at reasonable rpm. These boats all have been able to easily plane with two engines, so the props were appropriately aggressively pitched and loaded.
I had made the charts below a long time back on one of our past boats...twin engines, 310Hp diesels, 47' overall, and maximum rpm of 3,100 (chart only shows lower rpm range).
Although we rarely cruised on one engine many of these same types of boats cruised extensively on one engine for 1,000's of hours with no issues reported.
Depends on your transmissions and goals in mind. Fuel savings from various owners was another 10-15% on one engine vs twins at lower speeds averaging near the square root of the LWL (not 1.34 X sqrt LWL)

RPM Speed (knt) EGT Boost (psi)
800 5.4
1000 6.5
1200 7.4
1400 8.3 400
1600 9.3 550 1.5
1800 10 650 2.5
2000 11.2 750 3.5
2200 12.3 850 5
2400 15 900 8

Then with one engine (starboard) only.

RPM Speed (knt) EGT Boost (psi)
800 5
1000 5.9
1200 6.6 400
1400 7.4 475
1600 8.1 575 1
1800 9 750 2.5
2000 9.8 900 5

 

[DennisB1]

I have a GB 49 and I have tried to understand on my boat what happens on one engine. I can easily get 8 knows at about 1450 rpm range which is about 200 rpm higher than with both engines running to obtain the same speed. One one engine I have to crank in about 10-12 degrees rudder angle. Boat turns easily in one direction and harder in the opposite direct. It would be harder in a following sea to steer a strait course. Post 15 mentioned an article in Passage Maker magazine on some one removing a prop -- not practical in my opinion for normal operation, but would demonstrate how the boat would handle if your could affort feathering propellors. I would expect the rudder angle would be near zero without the prop and the rpm near that same as with the 2 engines running if you are below hull speed. I don't have any numbers on fuel burn but would expect the fuel pump would be near half the twin engine. I have seen test data where different props were tested a 6 knots in a test basin with accurare data collection. 12" 3 blade 150 lbs drag, 12" sailor 2 blade 100, 3 blade feating < 5 lbs and and folding less than 3 lbs. These were small props suitable to a 30 ft saillboat but show the impact of fixed vs feathering. On my boat I have 30 inch wheels and the drag would be much higher at 6 knots with the fixed none rotating prop. Last time I checked a 30 inch feathering prop was in the area of $10K so out of my range for our limited cruising. On my sailboat I had a 4 blade feathering in a aperture made a big difference speed and rudder angle under sail verses the three blade fixed prop.
The other benefit it you cruised using one engine and switch say on a daily basis, the accumulation of engine hours/oil changes would be reduced along with fuel burn. With a feathering propeller you don't have to worry about shaft rotation and gear problems. Negatives is greasing the propellor hub on a regular basis.

 

[Johnboyrtw]

I have a Grand Banks 42, cat 3208’s, 375 hp. Last year I started using one engine for cruising at hull speed, 8 knts. I burn 4 gallons an hour. With both engines at 8 knts I burn 7 gal/hr. I do rotate the engines to get good lubrication. My owners manual says that its ok to do this. Saving lots of fuel doing this. In and out of marinas and passes I obviously use both engines. The off engine free spins. I use to cruise at 15 knts, 24 gal/hr, those days are over with fuel costs so high. Retired now, so slow and easy for now.

 

[Owen Christensen]

Interesting, thanks

 

[mmullins]

Very informative PM article:https://www.passagemaker.com/technical/feel-the-burn

 

[DennisB1]

A comment on Johnboyrtw: I have the same engines on my GB49 classic. I burn with both engines about 4 gallons/hr. At 9 knots both engines about 6 gal/hr and at 10 knots 10 knots/hr. At max rpm 14 knots about 38 gal/hr. Big difference is the water line length and a much heavier vessel approx 75,000 lb scale wt. So good performance below hull speed and much worst about hull speed.

 

[rgano]

Freewheeling a prop will almost certainly damage the gears and the shaft log. Locking the prop will almost certainly counter the effort of the remaining engine. In other words, too much drag.

Even in an ideal situation you will not save any fuel. It costs a fixed amount of energy to move a boat. Using only one engine will use twice the fuel.

Just run both engines

pete

Pete, there was quite a discussion here years ago about whether trailing a prop locked or freewheeling created more drag. Surprisingly, the consensus was that a spinning fixed pitch prop created more drag.

 

[Hippocampus]

I have a GB 49 and I have tried to understand on my boat what happens on one engine. I can easily get 8 knows at about 1450 rpm range which is about 200 rpm higher than with both engines running to obtain the same speed. One one engine I have to crank in about 10-12 degrees rudder angle. Boat turns easily in one direction and harder in the opposite direct. It would be harder in a following sea to steer a strait course. Post 15 mentioned an article in Passage Maker magazine on some one removing a prop -- not practical in my opinion for normal operation, but would demonstrate how the boat would handle if your could affort feathering propellors. I would expect the rudder angle would be near zero without the prop and the rpm near that same as with the 2 engines running if you are below hull speed. I don't have any numbers on fuel burn but would expect the fuel pump would be near half the twin engine. I have seen test data where different props were tested a 6 knots in a test basin with accurare data collection. 12" 3 blade 150 lbs drag, 12" sailor 2 blade 100, 3 blade feating < 5 lbs and and folding less than 3 lbs. These were small props suitable to a 30 ft saillboat but show the impact of fixed vs feathering. On my boat I have 30 inch wheels and the drag would be much higher at 6 knots with the fixed none rotating prop. Last time I checked a 30 inch feathering prop was in the area of $10K so out of my range for our limited cruising. On my sailboat I had a 4 blade feathering in a aperture made a big difference speed and rudder angle under sail verses the three blade fixed prop.
The other benefit it you cruised using one engine and switch say on a daily basis, the accumulation of engine hours/oil changes would be reduced along with fuel burn. With a feathering propeller you don't have to worry about shaft rotation and gear problems. Negatives is greasing the propellor hub on a regular basis.

Don’t believe feathering props are as efficient as fixed. Even tunable excellent ones like a max prop. Would think if efficiency is what you’re chasing you’d be better off with CPP. Be interested if anyone could chime in with hard data.

On this site many have pointed out engine wear seems more related to how much fuel is used by the engine rather than hours it’s on. That being the case would probably be a good idea to shorten the hours between oil changes if running on one frequently.

Rather than just considering rpm increase would think about how much load the engine is under when running just one of two. In a M1 or M2 rated engine probably wouldn’t matter but think most of our boats have lower times for constant higher loads.

 

[OldDan1943]

Use a chain strap wrench to hold the unused stationary?

I only have a single engine, single shaft so maybe I should not comment?

 

[Marco Flamingo]

I only have a single engine, single shaft so maybe I should not comment?

If knowledge was a prerequisite of posting on Trawler Forum, there would be no threads longer than 14 posts.

 

[C lectric]

Use a chain strap wrench to hold the unused stationary?

I only have a single engine, single shaft so maybe I should not comment?

No, your comments are valid. Most of us will be towed at some time, me twice. Same comment stands about allowing freewheeling .
You need to KNOW what your gear box needs.

 

[bkcorwin]

Several people i am aware of have rigged pumps to circulate oil on the free wheeling shaft. Its not that uncommon.

Also a thing to consider beyond just fuel consumption is engine loading and accrual of engine hours.

Some of the twin engine N76s run with a single engine as it allows them to run the motor at a higher load range than if they ran two engines.

Also consider the rate at which engine hours accrue when you run one engine at a time vs both engines. A single engine boat driving for 6000 hours would have 6000 hours on the main. A twin engine boat where you alternated engines and only ran one at a time would have 3000 on each engine. For engines with costly service intervals it can push out the time to major service appreciably.

 

[Owen Christensen]

Well I was out for a couple weeks. I ran both engines at 900 rpm’s doing 8 mph. Got great mileage 2.25 with the gen running too.. I even spent a little time on the plane. I couldn’t be more pleased.