Running on 1 engine for fuel savings?

[psneeld]

Or did it get damaged from the ding?

 

[Nomad Willy]

The amount saved by running single varies a lot depending on many variables like how far apart your props are.

But like jungpeter says .. “You want to save money on fuel? Slow down.” It would be hard to argue otherwise that this is probably the best advice for ERTF.

This is profoundly an indication the OP has the wrong boat. Selling the twin (that is probably overpowered) and w the new knowledge and experience shop well for a more fuel efficient boat.

 

[Jimintheair3]

Another Waste of Time

What's the deal with all the grumpy old man snark??

I asked a simple question.

And it is a fact that most trawlers are overpowered for displacement speeds. Not saying it isn't nice to have the power & the twins for certain situations...but it's not even close to NEEDED for slow speed icw cruising....your calculations will tell you that.

Now whether it's bad for the transmissions (I have borg-warner) is out of my range of knowledge. But that's why I specifically asked about any mechanical risks in the OP...

SO, if it out of YOUR "Range"? Why don't you ask the Borg-Warner team?

You asked a "simple" question and then dismissed everyone's advise!

IMO, Jim

 

[gc9773]

I have 44ft Marine Trader with twin Cummins 210hp. The other day I had a warning light for the port engine, so i turned it off and continued on with just the starboard. I was surprised that I felt no difference and didnt even need to increase rpms to maintain the same 7ish knt speed.

Does anybody here with twins ever intentionally just run on one engine for fuel economy? Are there any mechanical risks from long term use (due to the uncentered nature of forces?)

A little late in the game here, and I will have to admit this is for larger ships, but yes, you can do this, provided the following:

- The strut bushing and any bearings for your propeller shaft (at least any part that will be moving, are lubricated (if necessary).
- There is a clutch or neutral to either disconnect your shaft from the transmission and/or engine.

I would check the manual for your transmission. Not sure what kind of shaft seal you have (dripless? It seems these need water for lubrication/heat reduction).

Other than that, you should be fine. And yes, you should see a SIGNIFICANT reduction in fuel consumption, and this was for gas turbines/steam. We used to do this ALL THE TIME, particularly on long ocean cruises aboard ship (my first two ships were twin shaft).

We would typically shift shafts during the mid-watch, port to starboard, starboard to port, you get the idea, maintaining 15 knots. You might want to do the same, not sure how long you are cruising, but could maintain a log to even wear. Not to mention the reduction in hours on your engines!

Obviously, the scale is different, but the laws of physics still apply!

Garry

EDIT: One minor note, you will need counter rudder to offset the loss in propulsion for one shaft/drag. Not a big deal, but something to remember...

 

[gc9773]

One more note, I assume the displacement of your trawler is approximately 40,000 lbs, give or take. Considering this:

40,000 / 2200 = 18 tons
18 x 2 (2 hp/ton) = 36.4 hp

In other words, you only NEED 36.4 hp to move that vessel at hull speed (8.5 knots for a waterline length of 40 feet) in calm water, barring any adverse conditions. Throw in another few hp for the trailing shaft, you are still well under 50 hp! Obviously, this is the ideal and not necessarily "Real World" conditions, but even at 100 hp on whichever engine you are using to maintain propulsion, I don't think your logic is far off at all.

Garry

I would imagine the fuel savings would be considerable!

 

[catalinajack]

Not a big deal to counter rudder? When one of my shafts parted from the transmission and would not freewheel in the water, my rudder correction was 18 degrees. The drag was tremendous, hand-steering was a constant over-correction even for small changes in course, and the autopilot was rendered useless.

A little late in the game here, and I will have to admit this is for larger ships, but yes, you can do this, provided the following:

- The strut bushing and any bearings for your propeller shaft (at least any part that will be moving, are lubricated (if necessary).
- There is a clutch or neutral to either disconnect your shaft from the transmission and/or engine.

I would check the manual for your transmission. Not sure what kind of shaft seal you have (dripless? It seems these need water for lubrication/heat reduction).

Other than that, you should be fine. And yes, you should see a SIGNIFICANT reduction in fuel consumption, and this was for gas turbines/steam. We used to do this ALL THE TIME, particularly on long ocean cruises aboard ship (my first two ships were twin shaft).

We would typically shift shafts during the mid-watch, port to starboard, starboard to port, you get the idea, maintaining 15 knots. You might want to do the same, not sure how long you are cruising, but could maintain a log to even wear. Not to mention the reduction in hours on your engines!

Obviously, the scale is different, but the laws of physics still apply!

Garry

EDIT: One minor note, you will need counter rudder to offset the loss in propulsion for one shaft/drag. Not a big deal, but something to remember...

 

[gc9773]

Not a big deal to counter rudder? When one of my shafts parted from the transmission and would not freewheel in the water, my rudder correction was 18 degrees. The drag was tremendous, hand-steering was a constant over-correction even for small changes in course, and the autopilot was rendered useless.

I don't believe you read my reply... We called it "Trail Shaft" and it was ALWAYS used on open ocean crossings, of which I did more than a few as a part of the bridge team as either, early in my career, as a helmsman, EOT operator, and lookout, and later, as a member of the navigation team.

You state, "When one of my shafts parted from the transmission and would not freewheel". I would not be surprised to see 18 degrees of rudder correction to compensate for the drag owing to a FIXED SHAFT.

"Trail Shaft" means exactly what it implies, you "Trail a Shaft". Disengaging the propeller shaft from the reduction gear via a clutch, in our case. It was a massive affair, and operated off high pressure air, thus the shaft was allowed to "trail", or spin freely while underway. We had to compensate from the standard engine orders on the driven shaft to maintain speed, which typically was 15 knots.

Now, there can be efficiency issues associated with trailing a prop, which are typically associated with propeller pitch, among other factors. My first ship was 6,000 tons with a steam plant and a FIXED propeller pitch. Yet, we still "trailed shaft" as a fuel consumption reduction strategy. My second ship was 10,000 tons with a gas turbine plant and had a set of VARIABLE PITCH PROPELLORS. Thus, we would modify the pitch on the trailing prop to reduce the drag.

We also had TWO rudders, which can also affect ship handling in an obvious manner. I don't know how many rudders the initial poster has on his trawler. While the SCALE in my case was larger, the same rules should still apply, or at least hold more or less true.

While "trailing shaft", at 15 knots, I never needed more than 5 degrees of rudder to compensate (and no, we did not have an autopilot on my first ship, on my second we had an analog based autopilot which worked fairly well but we were rarely allowed to use it), and this was on blue water. The initial poster did not mention any issues with having to compensate, no doubt he was aware, I just did not want to give him bad information without considering all the variables.

I will state this again, TRAILING SHAFT IS an effective fuel management strategy, at least that was our experience, providing all factors in the "drivetrain" are accounted for (shaft seal, reduction gear/transmission, prop shaft bushing lubrication).

I would NEVER use it in restricted maneuvering or a "high risk" evolution, coastal piloting, etc. (unless, as in the case with initial post, an emergency presented itself), and neither did we.

If you have a 44 foot trawler and the resources necessary to maintain one, an autopilot would be in order, thus negating some of the heartache associated with maintaining course on open water, provided this is how you are using the vessel...

Hope this helps...

Garry

 

[psneeld]

Wow, someones boat behaved differently than others? Or ships behave differently than small vessels?


Holy cow!!!!!!


Who would'a thunk?

 

[gc9773]

Well, it would seem to me that on ANY boat WITH 2 shafts, and one is LOCKED, a significant rudder correction would be involved, but what would I know, it was only my job...

 

[psneeld]

Well, it would seem to me that on ANY boat WITH 2 shafts, and one is LOCKED, a significant rudder correction would be involved, but what would I know, it was only my job...

I wasn't being too specific...just overall comment on many TF debates....


Can you deny that all things "ship" don't necessarily scale down to the small boat version?


I get what you are saying...heck I had the privilege of riding an icebreaker on one shaft from near Iceland to near the North Pole and all the way back through Europe to it's homeport in Wilmington, NC, USA...it and the engineering details were often the talk of the wardroom dinner table....heck it took over 2 months.

 

[tiltrider1]

I would like the definition of significant in the terms of actual numbers. No doubt shutting down the internal drag on a large ocean going ship will result in a significant savings by pleasure yacht standards but this does not mean a pleasure yacht can achieve significant savings by shutting down an engine.

 

[gc9773]

I wasn't being too specific...just overall comment on many TF debates....


Can you deny that all things "ship" don't necessarily scale down to the small boat version?


I get what you are saying...heck I had the privilege of riding an icebreaker on one shaft from near Iceland to near the North Pole and all the way back through Europe to it's homeport in Wilmington, NC, USA...it and the engineering details were often the talk of the wardroom dinner table....heck it took over 2 months.

That is the stickler, I don't have any experience on trawlers, but ships were formerly designed and tested in tanks with small "models" to test hull efficiency, etc. so I suspect it would be the same, or at worst, similar. One fellow chimed in here (real world experience with a trawler) with a 50% reduction in fuel savings, I was not surprised. Having larger engines REALLY HELPS with currents, etc. but the initial poster was correct in my opinion regarding that many vessels are "overpowered". The counter argument to this is "overpowered" in IDEAL conditions, consider the currents in the Strait of Juan de Fuca, for example. I would want the additional power.

We were "crabbing" in the Strait one morning (3 am) and took buoy "SA" down the port side, and this was a lighted (4 second) buoy. We tried to explain to a certain individual that visual navigation was superior to radar navigation, but apparently, listening skills were not taught at his institution of higher learning, needless to say, he never made that mistake again!

My biggest concern with "trailing shaft" on a trawler would be ensuring the propulsion system can handle it.

We had fuel curves for "trail shaft" and running both in the standard configuration. My memory is a bit hazy, but as I recall, it was somewhere in the area of 35% - 40% less fuel used compared to running both. I suspect it is also dependent on speed/engine percentage. I would experiment with it, figuring out what works best. But it could REALLY increase your range, even with a modest 30% fuel savings, which should be easily achieved.

Never been on an icebreaker, must have been quite an experience going that far North!

My last ship was 4,000 tons, single screw. One of the same class lost a propeller in the Arabian Gulf and had to get back to port with bow thrusters, I was glad I was not on that one!

Garry

 

[gc9773]

I would like the definition of significant in the terms of actual numbers. No doubt shutting down the internal drag on a large ocean going ship will result in a significant savings by pleasure yacht standards but this does not mean a pleasure yacht can achieve significant savings by shutting down an engine.

You are not shutting down the internal drag of a ship... The maximum speed of a vessel with a DISPLACEMENT hull is the square root of the waterline length multiplied by 1.34, you can find this in Bowditch, basically a nautical "bible", or at least we considered it as such while I was engaged in maritime employment...

The horsepower needed to drive a vessel with a displacement hull is 2/hp per ton (2,200 lbs) of displacement.

So it becomes obvious that most vessels are OVERPOWERED for MOST CONDITIONS. I use MOST here in reference to AVERAGE SEA conditions. In other words, not heavy weather or strong currents, etc.

So considering a 40,000 lb displacement trawler (yacht, whatever) WITH a DISPLACEMENT or SEMI-DISPLACEMENT hull (I am using generalities here), you would NEED 36.4 horsepower to drive the vessel to hull speed, a SIGNIFICANTANTLY SMALLER fraction of the horsepower provided in the design. In which this case is 2 x 210 hp engines = 420 horsepower.

Basically, the trawler has nearly 11 times the power necessary to drive it to hull speed in CALM conditions with supplied engines.

Most of the time. I am assuming most of the time you are not battling 10 knot currents or a driving hurricane? So why not save the fuel? And put it into better places, like beer, or steaks, for example???

ASSUMING a modest 30% savings in fuel with a trail shaft configuration and say, for example, a 1,000 gallon fuel tank, and with diesel, at say, $2.90 a gallon, one could EITHER save $870 or EXTEND the range of your trawler 30%. Not a bad deal either way...

Not to mention the REDUCTION on the hours of your engines!

We used this technique, I was there, this was my job... The initial poster figured it out, quite by accident, but on ocean going passages, saving fuel was significant considering the amount we used. So basically, an exercise in economics.

We were REQUIRED to use this technique, by the bean counters... I replied earlier that we saved an average of 35% - 40% fuel. Irrespective of the power source, whether that be a diesel engine, a gas turbine, or a steam plant, the numbers work out more or less the same.

Hope this helps...

Garry

 

[Insequent]

...snip...

Hope this helps...

Garry

It would help if you read all of the thread, not just the first post.

Our little boats will often use more fuel, although not very much, when running on just one of the twin engines.

 

[gc9773]

It would help if you read all of the thread, not just the first post.

Our little boats will often use more fuel, although not very much, when running on just one of the twin engines.

I did, and several responses indicated a significant fuel savings.

My C&L 44 is on the same hull as your MT 44. I also have twins, Volvo TAMD41s rated at 200hp each.
I had a serious failure at the beginning of my summer cruising a few years ago. My mechanic suggested bringing it in when he was less busy, like nearer to the end of the cruising season. I was OK with that, so ran the rest of the season, about 150 engine hours, on the single. I ran at the same rpm as I would have with both engines running. My speed dropped from 8knots to ~6.5. My fuel consumption remained where it had been for that engine, so I saved 50% over the summer. I learned how to dock an off centered single, in cross winds and tricky currents. Making prop walk my friend was key to that.
I know this doesn't answer your question, but I think pushing from the corner, with corrective rudder, has to use lots more fuel than pushing from the back of the keel.
I have BW Velvet Drive trannies, so freewheeling is no problem. I did put a pipe wrench on the shaft of the dead engine for a while, but I know I needn't have.
I also have a high output alternator on only one engine, so having that engine down meant paying closer attention to battery State of Charge.

I don't think you actually made the same speed without any extra push on the operating engine. For that to be the case, your non-operating engine would have been making no contribution at all to your forward speed. Physics says that wouldn't have been happening.

At any rate, have a nice evening.

 

[Insequent]

I did, and several responses indicated a significant fuel savings.



At any rate, have a nice evening.

I'm glad you did read it all, because it seemed that you had not. In many cases (including the example you quoted above) the fuel savings arose from slower speeds when on one engine. He went from 8kn to ~6.5 kn.

The graph I included in post #20 indicates that if I slow from 8 kn to 6.5 kn I also save 50% fuel.....

 

[markpierce]

If having a twin-engine boat, I would operate with one engine if one engine/shaft/propeller was inoperable.

 

[tiltrider1]

The horsepower needed to drive a vessel with a displacement hull is 2/hp per ton (2,200 lbs) of displacement.


Garry

So if you are driving the boat at hull speed using your 2 hp per ton which is .1 gallons per ton per hour, how do you magically reduce the hp needed to 1.5 hp per ton and continue on at hull speed?

 

[catalinajack]

Garry, I do have an autopilot. It was useless with the 18 degrees of rudder correction I needed with one shaft locked and dragging a propeller through the water. As far as your experience on large ships goes, very interesting but hardly relevant to our vessels but I did enjoy the reading. Thank you.

I don't believe you read my reply... We called it "Trail Shaft" and it was ALWAYS used on open ocean crossings, of which I did more than a few as a part of the bridge team as either, early in my career, as a helmsman, EOT operator, and lookout, and later, as a member of the navigation team.

You state, "When one of my shafts parted from the transmission and would not freewheel". I would not be surprised to see 18 degrees of rudder correction to compensate for the drag owing to a FIXED SHAFT.

"Trail Shaft" means exactly what it implies, you "Trail a Shaft". Disengaging the propeller shaft from the reduction gear via a clutch, in our case. It was a massive affair, and operated off high pressure air, thus the shaft was allowed to "trail", or spin freely while underway. We had to compensate from the standard engine orders on the driven shaft to maintain speed, which typically was 15 knots.

Now, there can be efficiency issues associated with trailing a prop, which are typically associated with propeller pitch, among other factors. My first ship was 6,000 tons with a steam plant and a FIXED propeller pitch. Yet, we still "trailed shaft" as a fuel consumption reduction strategy. My second ship was 10,000 tons with a gas turbine plant and had a set of VARIABLE PITCH PROPELLORS. Thus, we would modify the pitch on the trailing prop to reduce the drag.

We also had TWO rudders, which can also affect ship handling in an obvious manner. I don't know how many rudders the initial poster has on his trawler. While the SCALE in my case was larger, the same rules should still apply, or at least hold more or less true.

While "trailing shaft", at 15 knots, I never needed more than 5 degrees of rudder to compensate (and no, we did not have an autopilot on my first ship, on my second we had an analog based autopilot which worked fairly well but we were rarely allowed to use it), and this was on blue water. The initial poster did not mention any issues with having to compensate, no doubt he was aware, I just did not want to give him bad information without considering all the variables.

I will state this again, TRAILING SHAFT IS an effective fuel management strategy, at least that was our experience, providing all factors in the "drivetrain" are accounted for (shaft seal, reduction gear/transmission, prop shaft bushing lubrication).

I would NEVER use it in restricted maneuvering or a "high risk" evolution, coastal piloting, etc. (unless, as in the case with initial post, an emergency presented itself), and neither did we.

If you have a 44 foot trawler and the resources necessary to maintain one, an autopilot would be in order, thus negating some of the heartache associated with maintaining course on open water, provided this is how you are using the vessel...

Hope this helps...

Garry

 

[gc9773]

So if you are driving the boat at hull speed using your 2 hp per ton which is .1 gallons per ton per hour, how do you magically reduce the hp needed to 1.5 hp per ton and continue on at hull speed?

I think we have veered significantly off track here, and I am not suggesting any "magic". The initial poster stated that it was his belief that many vessels were significantly overpowered. I agreed with this from the standpoint that 420 horsepower was NOT NECESSARY to propel a 40 foot (waterline length) vessel displacing 40,000 lbs (18 tons) but additional power was DESIRABLE to address adverse conditions and "Real World" situations and will leave it at that.

I suspect the "Law of Diminishing Returns" has crept into this thread (at least for me) but it did elicit some positive memories, and for that I thank you...

 

[gc9773]

Garry, I do have an autopilot. It was useless with the 18 degrees of rudder correction I needed with one shaft locked and dragging a propeller through the water. As far as your experience on large ships goes, very interesting but hardly relevant to our vessels but I did enjoy the reading. Thank you.

How many degrees of rudder do you believe you would have needed should the shaft have spun freely? Hypothetically?

At any rate thanks for having me here...

 

[catalinajack]

I can tell you not hypothetically, a real world experience. The cause of my running with a locked shaft and requiring an 18-degree rudder correction occurred BEFORE having my cutless bearings replaced. I had fractured a DriveSaver coupling which had parted the shaft from the transmission which forced me to run 40 very slow miles on Christmas Eve, 2018. Note that before bearings were replaced, my shafts were so bound up that, when on the hard, even using its 25-inch propellers as lever arms it was extremely difficult to rotate the shafts.

After the New Year, I had to run another 25 slow miles to a yard for repairs. The cutless bearings were replaced (six), shafts straightened, and the engines realigned. Afterward, being curious I ran for a bit on one engine. The rudder correction was only four degrees and the autopilot could be used. Quite obviously, on my boat, that locked shaft was a huge drag in the water. No way that was not costing extra fuel. My boat has Borg Warner transmissions which may be freewheeled with no risk of damage. For those boaters having transmissions that cannot be freewheeled and must lock the shaft in order to run on one engine safely, I just don't see how a fuel penalty is avoided. Anyway, the rudder correction on my boat is 18 degrees versus 4 degrees. To me that is a profound difference.

I have no idea of whether running on one engine with just four degrees of rudder correction saves any fuel for me. But, I have no interest in bothering to finding out. I prefer to run hot, straight, and normal.

How many degrees of rudder do you believe you would have needed should the shaft have spun freely? Hypothetically?

At any rate thanks for having me here...

 

[gc9773]

I can tell you not hypothetically, a real world experience. The cause of my running with a locked shaft and requiring an 18-degree rudder correction occurred BEFORE having my cutless bearings replaced. I had fractured a DriveSaver coupling which had parted the shaft from the transmission which forced me to run 40 very slow miles on Christmas Eve, 2018. Note that before bearings were replaced, my shafts were so bound up that, when on the hard, even using its 25-inch propellers as lever arms it was extremely difficult to rotate the shafts.

After the New Year, I had to run another 25 slow miles to a yard for repairs. The cutless bearings were replaced (six), shafts straightened, and the engines realigned. Afterward, being curious I ran for a bit on one engine. The rudder correction was only four degrees and the autopilot could be used. Quite obviously, on my boat, that locked shaft was a huge drag in the water. No way that was not costing extra fuel. My boat has Borg Warner transmissions which may be freewheeled with no risk of damage. For those boaters having transmissions that cannot be freewheeled and must lock the shaft in order to run on one engine safely, I just don't see how a fuel penalty is avoided. Anyway, the rudder correction on my boat is 18 degrees versus 4 degrees. To me that is a profound difference.

I have no idea of whether running on one engine with just four degrees of rudder correction saves any fuel for me. But, I have no interest in bothering to finding out. I prefer to run hot, straight, and normal.

I am definitely not disagreeing with your assertion that running with a LOCKED shaft would incur a fuel consumption penalty, not to mention excessive over steer to compensate.

My suggestion was simply that allowing one shaft to trail might save fuel for some people with some boats... And at least for the ships I was on, it was normal operation. Whether that scales to trawlers or not, I do not know. It would seem that for some people here, it does save fuel and run time on engines.

The initial post was from an individual who had experienced an engine failure and running in this configuration was necessary. His question being whether fuel could be saved and if there were risks to the propulsion system associated with operating in this manner.

Some folks here would likely not prefer to try this, but if there no harm to components and it did save fuel, it might be useful for some individuals, particularly those that make long runs.

I would not be surprised if some folks here were doing this as standard procedure.

I was honestly surprised to see the "push back" in many of the posts, particularly considering that others had chimed in reporting a fuel savings.

My intent was just to illustrate that, at least in our situation, it was normal and considered prudent.

 

[Rufus]

This subject comes up fairly often. More than a decade ago forum member Timjet and I ran some real world experiments. My procedure was based on engine prop charts, which track almost perfectly to well established twin engine fuel burn over long runs at constant power. I believe Timjet used fuel flow meters.

Test proccedure. Ocean Alexander 44. Calm day, no wind. current, or waves (unusual day on Lake Michigan). Set GPS speed at 8.2 knots, twin engine. Record engine rpm. Shut down one engine (prop freewheeling). Increase power on remaining engine to achieve same GPS speed of 8.2 knots...steady track. Record rpm and rudder angle. Using the same speed eliminates one variable.

Look up fuel burn for both twin and single engine rpm as recorded (twin engine times two). Compare. I found about 5% savings running on one engine. I don't recall the rudder angle, but the Will Hamm autopilot did fine and appeared to have plenty of excess capability had the ambient conditions been more demanding. The OA has relatively large rudders.

Timjet saw larger fuel savings on his 36 Carver. I do not recall if his test procedures were the same.

 

[gc9773]

I have 44ft Marine Trader with twin Cummins 210hp. The other day I had a warning light for the port engine, so i turned it off and continued on with just the starboard. I was surprised that I felt no difference and didnt even need to increase rpms to maintain the same 7ish knt speed.

Does anybody here with twins ever intentionally just run on one engine for fuel economy? Are there any mechanical risks from long term use (due to the uncentered nature of forces?)

As a side note, one of the larger vessels I was on had a waterline length of 529 feet, giving a theoretical maximum hull speed of 31 knots, the fastest I had ever seen her go was 35 knots, and it felt like the stern was going to fall off, and this was at flank, with the throttles at max and max efficient prop pitch...

The ship displaced 10,000 tons with 114,000 total installed brake horsepower, 80,000 shaft horsepower. This works out to 11.4 brake horsepower per ton.

Given the specs for a 44 foot Marine Trader with a waterline length of 40 feet, displacement of 40,000 pounds, and 420 total installed brake horsepower, 294 shaft horsepower, one gets 23.1 brake horsepower per ton.

Is your trawler over powered? We probably were... For a safety margin this was likely necessary. But I suspect your observations correlate well with the notion that one engine is probably more than adequate to drive your vessel to hull speed, possibly well beyond...

Did you lock your marine gear or let it freewheel - probably culprit!

This, I suspect, is where much of the confusion here arises... There is a difference between running LOCKED and TRAIL, and Mako hit the nail on the head...

 

[SteveK]

What is Trail?
Here is an interesting article. Strategies for long range cruising on one engine

 

[gc9773]

What is Trail?
Here is an interesting article. Strategies for long range cruising on one engine

That was a comprehensive and well written article! I found it interesting that locking the shaft was suggested, at least for some vessels, so apparently, it is not the demon I believed it was...

I suspect this is only an issue for cruisers who make long trips, so it is an "economy of scale" item. The writer also mentioned the necessity to ensure transmission temps were monitored, provided the manufacturer allowed for this sort of operation as one means to reduce fuel consumption.

One item I have not seen mentioned as much is the reduction of hours on an engine provided some strategy of alternating the shaft being driven, which for a long range cruiser, would be considerable...

And were one cruising long range, I would imagine installing a system where the transmission for either shaft could be cooled independently of the engine driving it, along with the shaft seal, provided it were necessary, would be recommended...

 

[rslifkin]

How much drag you suffer from locking a shaft vs freewheeling will depend on prop size and prop design. In general, locked adds more drag, but some setups don't have a choice, as the trans can't safely handle freewheeling.

As others have pointed out, the gains (or not) will depend on how much drag you gain from the dead shaft, rudder offset, etc. compared to how it changes where you sit in the BSFC curve of the engines as well as changes in efficiency of the prop. Even then, on a given boat, it may show a benefit at one speed, but not at another.


Now, if you had variable pitch props, you could lock the dead shaft, feather the blades on that prop and then optimize the pitch on the other prop and you'd likely see more significant gains than most fixed pitch setups.

 

[gc9773]

What is Trail?
Here is an interesting article. Strategies for long range cruising on one engine

I apologise, this was "corporate lingo" or "cultural vernacular" for allowing one shaft to spin freely, I forgot it might not be used in the trawling community. Basically comes from:

"Trail: 2(b) : to lag behind : do poorly in relation to others" from the Merriam-Webster dictionary...

Here is a link to a .PDF describing how it is implemented:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.onr.navy.mil/-/media/Files/Funding-Announcements/BAA/2007/07-029_Amendment0001.ashx&ved=2ahUKEwjnwdyn54fuAhUFI6wKHXqSCUMQFjAFegQIChAB&usg=AOvVaw1V7nWpNhCjn1FvpanEUfFx&cshid=1609953915806

If you read page 6 (I've attached a screenshot), one will note a 77% reduction in fuel consumption over full power operation, however, it is only used up to 18 knots, which is a reduction of 12 knots from a maximum theoretical hull speed of 30 knots given a waterline length of just under 500 feet (60% of maximum speed)...



I don't know of how much interest there is here in this, but thought I would share it...

 

[gc9773]

By the way, for that ship, there is a total of 150,000 brake horsepower, 105,800 shaft horsepower, and about 8,400 tons displacement, giving about 18 brake horsepower per ton... And it has a displacement hull...

I'd be very interested to see "real world" tests here from folks similar to the post by Rufus:

This subject comes up fairly often. More than a decade ago forum member Timjet and I ran some real world experiments. My procedure was based on engine prop charts, which track almost perfectly to well established twin engine fuel burn over long runs at constant power. I believe Timjet used fuel flow meters.

Test proccedure. Ocean Alexander 44. Calm day, no wind. current, or waves (unusual day on Lake Michigan). Set GPS speed at 8.2 knots, twin engine. Record engine rpm. Shut down one engine (prop freewheeling). Increase power on remaining engine to achieve same GPS speed of 8.2 knots...steady track. Record rpm and rudder angle. Using the same speed eliminates one variable.

Look up fuel burn for both twin and single engine rpm as recorded (twin engine times two). Compare. I found about 5% savings running on one engine. I don't recall the rudder angle, but the Will Hamm autopilot did fine and appeared to have plenty of excess capability had the ambient conditions been more demanding. The OA has relatively large rudders.

Timjet saw larger fuel savings on his 36 Carver. I do not recall if his test procedures were the same.

EDIT: My calculations were incorrect, the ship has a total of 105,000 installed brake horsepower with 73,500 shaft horsepower, total displacement of approximately 8,400 tons, giving 12.5 brake horsepower/ton.