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...
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...
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.
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.
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.
...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.
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.
I did, and several responses indicated a significant fuel savings.
At any rate, have a nice evening.
The horsepower needed to drive a vessel with a displacement hull is 2/hp per ton (2,200 lbs) of displacement.
Garry
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
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?
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...
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 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?)
Did you lock your marine gear or let it freewheel - probably culprit!
What is Trail?
Here is an interesting article. Strategies for long range cruising on one engine
What is Trail?
Here is an interesting article. Strategies for long range cruising on one engine
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.