Twin Engines, Locking One Shaft

[AKDoug]

Post #2 has it right, find out these three things and you will know the answer. I towed a friend in his NT32, the dripless squealed horribly until the shaft was locked. I can only imagine the damage it would have done in the 10 hour tow without locking the shaft...

 

[Ken E.]

Always check the manufacturer AND models for freewheeling.

I know some twin Disc's models (maybe just older ones) are fine with freewheeling.

I owned a couple and many Sea Rays I ran on one engine had them.

The TD website and owners manual gives a
listing of the models that can be freewheeled. Mine are model 5050a and can be. As I recall, only a few models cannot be freewheeled. The gears' temperature has to be monitored and the engine has to be started periodically. I have run with one side turned off between here and SE AK quite a bit with no issues. And my PSS seals are both fed water water by either engine which I think is how most are set up.

 

[Per]

I have done a test on this subject.
if my one prop is locked, the force is too strong for me to keep a straight line forward at any speed, in other words I am not able to maneuver at all, the boat will turn into the bad engine regardless of rudder position.

If i allow the prop to freewheel I am able to control forward direction and even turn into the good engine.
I am unable to explain the science but this is my experience, in my boat.

 

[psneeld]

I have done a test on this subject.
if my one prop is locked, the force is too strong for me to keep a straight line forward at any speed, in other words I am not able to maneuver at all, the boat will turn into the bad engine regardless of rudder position.

If i allow the prop to freewheel I am able to control forward direction and even turn into the good engine.
I am unable to explain the science but this is my experience, in my boat.

I would have guessed it....Blackfin turning big wheels for powerful diesels.

But you are cprrect that your boat, but not all, will react that way.

 

[Rufus]

The owners manual for our Twin Disk 502 transmissions say to briefly run the engine every hour for a freewheeling shaft in order to pressure lube the bearings.

By the way there are a number of threads back in the archives on the subject of freewheeling versus locked shafts. RickB had a lot to say about it and if I remember correctly he provided test data to support the case for locking the shaft.

 

[psneeld]

The owners manual for our Twin Disk 502 transmissions say to briefly run the engine every hour for a freewheeling shaft in order to pressure lube the bearings.

By the way there are a number of threads back in the archives on the subject of freewheeling versus locked shafts. RickB had a lot to say about it and if I remember correctly he provided test data to support the case for locking the shaft.

I would love to see it if it included every vessel under the sun.

There are so many possibilities. Rick may see this and send me enough info to keep me reading till I am 6 feet under. he is good about keeping me informed.

 

[ranger58sb]

If one engine breaks down forcing you to run on a single, my understanding is that it’s best to lock up the shaft to keep from free spinning. A locked shaft will create less drag (which is counter intuitive but true), and it will also reduce risk of overheating your transmission since that stalled engine cannot oil cool it.

My question is for those who needed to lock up one shaft then running the other engine to reach a destination.
In a pinch, how did you do it? Would a large plumber wrench work, or large vise grips ?
What tips do you have to for the cruiser who needs to lock down down a shaft from free spinning until he gets to port?

Might depend on distance and what speeds you feel like you need to make. When it happened to me, I didn't lock the off gear.

For the small amount of water required to maintain a dripless shaft seal, it would be pretty easy to have an alternative as a backup plan. I would likely just make a hose to run from the seal to a 5 gallon bucket on deck. Filling the bucket once an hour would be way more than sufficient.

Ours had (have) cross-over water lines on our dripless seals, so cooling water from the other engine was sufficient for reasonable "crippled" speeds.

-Chris

 

[Hans Rovers]

Water lubricated stern shaft seals

I asked the supplier of my Manecraft stern shaft seal if I would damage the seal if I run my twin engine 40 ft Ocean Marine trawler on one engine, which I do on open waters to save fuel and to let my Lehmans run in a higher rev bracket. The answer was that the seal should never run dry, which is indeed in the manual. I was just wondering to what extend that is really an issue and wether the water present in the seal is enough to lubricate the seal.
Also, if the seal needs constant water pressure would it be OK to connect the port water supply hose to the starboard water supply hose as such that when one engine runs, both seals are supplied with cooling water. I was wondering if there could be adverse affects on the idled engine. I can't imagine what the adverse effect would be.
I looked at the manual of my gear and idling without cooling was allowed.
I am puzzled by the argument about the drag caused by locked or free running propellors. I found both the helicopter argument as the graph convincing, the latter more in line with what I would expect. My mechanical engineering brain locks up over this argument. Thanks for the original question, even if some answers, like mine, seem to be more troublesome than the question itself.

 

[cafesport]

As far as drag scenario goes much depends on shaft angle and the number of blades. The pitch on the descending blade is greater than the ascending blade and in a 4 blade scenario the athwartship force blades can be locked in the slipstream of the keel and skew. Not the same efficiency gain as you would see with surface piercing props, but not as much drag either. We have big rudders and skegs, ie more drag to start with, so for us no measurable difference either way. We do however steer much better as either of the offset props counterbalance because of opposite rotation. For us there is very little increase in rudder angle to maintain course.

Per, Your boat has a v bottom, no keel, skegs and you have frisbees for rudders. I had a 38 black fin and lost a port engine damper plate 20 miles from home in the Gulfstream. I had dripless seals, so I just let the engine idle for the long slow trip home. Fortunately I was west bound in the four knot current trying to beat an approaching front. Had it been the starboard engine I probably would have had to enter in palm beach, then pulled the prop for the ride home when the weather died down.

 

[rgano]

I asked the supplier of my Manecraft stern shaft seal if I would damage the seal if I run my twin engine 40 ft Ocean Marine trawler on one engine, which I do on open waters to save fuel and to let my Lehmans run in a higher rev bracket. The answer was that the seal should never run dry, which is indeed in the manual. I was just wondering to what extend that is really an issue and wether the water present in the seal is enough to lubricate the seal.
Also, if the seal needs constant water pressure would it be OK to connect the port water supply hose to the starboard water supply hose as such that when one engine runs, both seals are supplied with cooling water. I was wondering if there could be adverse affects on the idled engine. I can't imagine what the adverse effect would be.
I looked at the manual of my gear and idling without cooling was allowed.
I am puzzled by the argument about the drag caused by locked or free running propellors. I found both the helicopter argument as the graph convincing, the latter more in line with what I would expect. My mechanical engineering brain locks up over this argument. Thanks for the original question, even if some answers, like mine, seem to be more troublesome than the question itself.

Cross-connecting the seawater cooling between two shaft packing devices comes with a risk of back flooding the muffler of a non-operating engine unless you have a shutoff valve between the engine and the shaft seal.

 

[EngNate]

Experience: My shaft stayed stopped with a rope while being towed at 9kt. I doubled it around the end of a coupler bolt and took a few wraps and tied to a convenient rear engine mount, secured the wraps with tape. Towed for 11 hours, it didn't appear to be under much stress at all. I used 5/16" longline rope, hard, stiff and strong, no stretch. 1-1/4" shaft and 22 x 22 prop.

 

[Vinny]

On my 27’ workboat I just wrap a ratchet strap around the shaft a few times and tighten it up so the shaft don’t spin… this is how I normally tow it to job sites all over Lake Michigan…

On the larger 4000 Hp boats I wrap a chain around the shaft and tighten it up with a chain fall to stop the shaft from turning.

 

[ProMaritime]

Depending on your transmission you would normally just put it in reverse. Some transmission can be damaged by this. Boats so equipped usually have shaft locks.

 

[rslifkin]

Depending on your transmission you would normally just put it in reverse. Some transmission can be damaged by this. Boats so equipped usually have shaft locks.

The reverse to lock shaft thing only works on a mechanically engaged transmission. With a hydraulic transmission, you can put the shifter wherever you want, but if the engine isn't running, the hydraulic pump on the trans isn't pumping. Which means no hydraulic pressure to engage the clutch packs. So all 3 positions on the shifter are still neutral.

 

[DDW]

Prop stopped vs freewheeling has been tested for a variety of props and operating conditions by a UK yachting magazine. The answer isn't yes or no, but "it depends" on a lot of factors. Very hard to predict for a specific case which has less drag. A helicopter autorotating is a very different beast, very little in common with a boat prop.

 

[MYTraveler]

I have done a test on this subject.
if my one prop is locked, the force is too strong for me to keep a straight line forward at any speed, in other words I am not able to maneuver at all, the boat will turn into the bad engine regardless of rudder position.

If i allow the prop to freewheel I am able to control forward direction and even turn into the good engine.
I am unable to explain the science but this is my experience, in my boat.

Perhaps I am missing something, but the science seems straightforward -- a locked prop has more drag than one allowed to freewheel (and if somehow it didn't, it would take energy to free wheel and the prop would lock itself). It is hardly surprising that the boat turns toward increased drag.

 

[psneeld]

Perhaps I am missing something, but the science seems straightforward -- a locked prop has more drag than one allowed to freewheel (and if somehow it didn't, it would take energy to free wheel and the prop would lock itself). It is hardly surprising that the boat turns toward increased drag.

Yet the science from my experience does not say a locked prop always has more drag.

Other posts seem to support this.

 

[MYTraveler]

Yet the science from my experience does not say a locked prop always has more drag.

Other posts seem to support this.

That may be, but in DDW's case, the locked prop did, which caused it to turn (harder) into the additional drag. And if the freewheeling prop had more drag, then why would it freewheel, even if not locked?

 

[psneeld]

That may be, but in DDW's case, the locked prop did, which caused it to turn (harder) into the additional drag. And if the freewheeling prop had more drag, then why would it freewheel, even if not locked?

Sorry I may have misinterpreted the post.

There are plenty of vessels that a locked shaft does create more drag...just not all.

 

[DennisB1]

The drag non rotating vs rotating is my opinion is not an answer that can be address as either could cause a greater drag depending on the variables on a particular boat.
Drag for non rotating prop is based upon the area of the prop presented to the flow stream, the velocity of the boat thru the water (a Velocity squared, the drag coefficient, water density. The velocity can vary depending upon the boundary layer thickness which affects the velocity at the prop. D= coef x area x density x Vel^2/ (2 * gravity).
On spinning prop, it all depends upon again the velocity distribution, the amount of friction and drag in the drive train which required work/heat be generate that uses up the energy being generated by the rotating prop.
All that said, if there is very little friction the spinning prob would product less drag. If you for example added a alternator to the shaft the prop drag would increase depending upon the amount of power extracted from the alternator.

If you had to go a long distance with one of the two engines inoperable, Removing the prop would be a big help assuming you had access to a diver. Otherwise letting spin requires as noted in the posts understanding what your particular transmission requirements are for lubrication. Some transmissions don't lock. I have had sailboats that had an internal shaft lock which helped if the prop was behind structure in an aperture. Much better was a feathering prop.

 

[koliver]

That may be, but in DDW's case, the locked prop did, which caused it to turn (harder) into the additional drag. And if the freewheeling prop had more drag, then why would it freewheel, even if not locked?

To find out whether your prop has more drag when locked or not locked, move the prop through the water and see if it turns. It will always default to the lowest drag attitude. If that attitude is still, then your prop has less drag when still. If that is turning, that is where it has less drag. Physics.

 

[DDW]

????

I think you better review the physics. Almost any prop is going to turn unless the shaft is locked or has very high friction. How much drag it causes is a different issue and will be dependent on so many details that you cannot generalize. As I said this has been specifically tested.

 

[long-cours.62]

May be

If you have "dripless shaft seals" you will need water to be injected into the seals.

You could have already fitted a valve between the 2 hose who bring water to your 2 "dripless shaft seals".
If you shut down one of your engine you open the valve and your two seal continue to be "lubricated" ?
May be one possibility ?

 

[catalinajack]

This is just one more discussion of those of the same basic topic. Here is my experience, just one anecdote folks. I have Lehman 120s driving Borg Warner Velvet Drives. At 8 knots, Velvet Drives can, per the manual, be free-wheeled without any damage. Before, I had my cutless bearings replaced, shafts staightened, and engines aligned, it was near impossible to rotate the shafts by hand. It tokk a two foot lever arm inside the boat or, when on the hard, using the 25-inch prop as a lever to get my shafts to rotate. Operating on engine, with the shafts not able to free-wheel, it took 18 degrees of rudder correction to keep the boat moving in a staright line. After the drive-line work, operating the boat took only a four-degree rudder correction with my shafts rotating freely.

 

[Hans Rovers]

I asked the supplier of my Manecraft stern shaft seal if I would damage the seal if I run my twin engine 40 ft Ocean Marine trawler on one engine, which I do on open waters to save fuel and to let my Lehmans run in a higher rev bracket. The answer was that the seal should never run dry, which is indeed in the manual. I was just wondering to what extend that is really an issue and wether the water present in the seal is enough to lubricate the seal.
Also, if the seal needs constant water pressure would it be OK to connect the port water supply hose to the starboard water supply hose as such that when one engine runs, both seals are supplied with cooling water. I was wondering if there could be adverse affects on the idled engine. I can't imagine what the adverse effect would be.
I looked at the manual of my gear and idling without cooling was allowed.
I am puzzled by the argument about the drag caused by locked or free running propellors. I found both the helicopter argument as the graph convincing, the latter more in line with what I would expect. My mechanical engineering brain locks up over this argument. Thanks for the original question, even if some answers, like mine, seem to be more troublesome than the question itself.

In my attached post I was considering letting the non driven props run loose (I also have 120 Lehmans with Borg Warner, so no damage to the gears) but lubricating it from the running opposite engine via a bypass.

I followed through and I installed one way valves with 0.03 bar opening pressure to prevent cooling water from the running engine to back flood into the non running engine (thanks rgano for your comment). It works very well. I am going to make a very long trip this summer and I will save on (European priced 10$/gallon) diesel running only one engine in the most efficient rev bracket. If I run both engines at these revs I only produce wasteful wake. Attached a photo of the set up. Price? Under 60 Dollar.

 

[Hans Rovers]

The valves I installed worked well until one failed which indeed backflooded the engine that was not running. We fixed the issue by removing the injectors, sucking the water from the cylinders (lucky, it happend in a sweet water area) and thus removing the water. We blew the engine dry by using the starter for a while and after re-assembly the engine ran as before. We changed the oil to get rid of the remaining water. I removed both valves and will run on two engines until I find a better solution.

 

[C lectric]

Sorry to hear of your experience, ouch.

I looked at the pics. What I think are the valves look like check valves, correct.? The shiny things between the two beige plastic looking fittings.?

If that is the case then you should have used a good quality bronze ball valve and bronze hose fittings. Or the black Marelon glass reinforced nylon ball valves and fittings. Does not have to be Marelon by name but the glass filled nylon.

Check valves can be unreliable especially any where any debris can block them open.

With a ball valve the position can be seen positively.

 

[Hans Rovers]

Thanks for the suggestion. I am also thinking about installing servo valves with lights indicating both open and closed. Enough to think about.

 

[M. Shul]

Prop stopped vs freewheeling has been tested for a variety of props and operating conditions by a UK yachting magazine. The answer isn't yes or no, but "it depends" on a lot of factors. Very hard to predict for a specific case which has less drag. A helicopter autorotating is a very different beast, very little in common with a boat prop.

Folks let's not forget the basic physics, if the path of least resistance was the shaft spinning then the shaft will spin, if the path of least resistance is the shaft not spinning then it will not spin.



This has nothing to do with if you want to have the shaft spin, this is a totally different question, related to wear and tear, transmission oil cooling, etc.!

 

[rslifkin]

Folks let's not forget the basic physics, if the path of least resistance was the shaft spinning then the shaft will spin, if the path of least resistance is the shaft not spinning then it will not spin.

That is not inherently true. Even if stopping the shaft were to produce less drag, the shape of the propeller is going to try to spin the shaft up until the point where the resistance to rotation becomes high enough that the prop hydrodynamically stalls.