Twin Engines, Locking One Shaft

[ca distress]

Although I hope one of my twin Cummins next shuts down, it certainly could happen based on a plethora of unexpected mechanical reasons.
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?

 

[psneeld]

Lots of ways to lock a shaft.

Depends on the vessel whether it's more efficient.

It depends on the tranny whether it's a good idea.

It depends on the stuffing box whether it's necessary.

 

[Nick F]

Letting the shaft turn will create less drag.

May or may not be acceptable for your transmission.

 

[tiltrider1]

Easiest way is to shut down the engine while in gear. This might not be appropriate for some transmissions but with most you will end up with no moving parts.

 

[psneeld]

Letting the shaft turn will create less drag.

May or may not be acceptable for your transmission.

Not on all vessels.

 

[O C Diver]

Although I hope one of my twin Cummins next shuts down, it certainly could happen based on a plethora of unexpected mechanical reasons.
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?

If it was under an hour, don’t think I would waste the time. Some transmission have an issue not from heat but lubrication depending on where the output shaft is relative to the transmission oil. The same is true to some extent on dripless shaft seals. While transmissions require some cooling depending on design and engine HP, a portion of the heat is generated by the transmissions hydraulic pump and clutch engagement. Also, heat is a direct result of HP pushed through the transmission from the engine. There will be a huge difference in heat generation between and engine in gear at idle versus a 400 HP engine in gear running WOT.

Consider also that a lot of boats are towed by companies like Seatow and TowBoatUS without locking the propeller shaft. Don't hear about transmissions dying from being towed.

Before you go crazy about locking the shaft, maybe contact the transmission manufacturer to see if it is a problem with your particular transmission. Then maybe a test where you shut down one engine and run for a few hours periodically using an infrared temperature gun to check the transmission every 20 minutes or so.

Ted

 

[Comodave]

First thing is to find out if your transmission allows free wheeling or not. If it does it may have a time allowance for free wheeling or may not have any limits. Locking a shaft probably will not be necessary with most newer transmissions. But if you really need to a chain wrench may be the easiest way. Like vice grips with chain to wrap around the item.

 

[Nick F]

Not on all vessels.

Sorry Paul - I disagree. The drag will always be less if you allow the propellor to windmill.
Regards,
Nick

 

[psneeld]

Sorry Paul - I disagree. The drag will always be less if you allow the propellor to windmill.
Regards,
Nick

You have any links to info to prove?

I was on USCG vessels with over 40 years of service and millions of miles steamed with data that said otherwise.

I have also read where it depends on the vessel all the way down to smaller sizes as tranny drag, prop size, hull design, speeds involved,, etc..etc can affect whether locking is more efficient or not.

Would love to see definitive research to prove otherwise.

 

[tiltrider1]

Sorry Paul - I disagree. The drag will always be less if you allow the propellor to windmill.
Regards,
Nick

You got this backwards. If it was true helicopters would fall from the sky every time they had an engine failure. Instead we are able to glide them back to the ground using a windmilling rotor system.

 

[rslifkin]

I would expect to see different results in locked vs windmilling drag depending on prop pitch and blade area. That will impact how the water moves around the prop in each situation. On a low blade area prop like a typical sailboat, locked may come out ahead, especially if you can hide some of that blade area behind a keel or strut. But with a high area 4 blade of moderate to low pitch, a locked prop should be about as draggy as a solid disc of equal diameter being dragged along. Windmilling has a shot at being better in that situation.

Of course, for some boats, the shaft has to be locked for mechanical reasons, so it doesn't matter what produces less drag. And if you have a controllable pitch prop and can feather the blades, locked shaft will be best.


Don't forget that air is compressible, water isn't. So behavior that applies to aircraft props doesn't translate 100% to boat props.

 

[Ken E.]

Do you have Twin Disc gears? Most can be windmilled for many hours. The owners manual explains it and to check gears' temp periodically.

 

[Nick F]

Yachting, July 2012.

I agree that the helicopter auutorotating example is intriguing. I believe the effect is different from our marine situation becuase the helicopter has forward motion - which would be sideways motion in the case of a watercraft. It may be that the helicopter autorotation lift effect would not work without forward speed.

Nick

 

[psneeld]

Yachting, July 2012.

I agree that the helicopter auutorotating example is intriguing. I believe the effect is different from our marine situation becuase the helicopter has forward motion - which would be sideways motion in the case of a watercraft. It may be that the helicopter autorotation lift effect would not work without forward speed.

Nick

And that article covers the tens of thousands of different variables?

I will bet on " it depends" versus "it is an absolute" with most things in life.

Neither explanation clearly defines what a helicopter autorotation really is. Over 5000 hours and too many practice autos to count in USCG helos gives me that expertise.

 

[Arc]

We just went from a Cummins to a helicopter. I don’t think you are answering the Op’s question.

 

[psneeld]

We just went from a Cummins to a helicopter. I don’t think you are answering the Op’s question.

I can see where I blended the 2 badly.

My first 2 comments meant the Yachting article...my last was that auto rotations are not about drag and efficiency in simple terms and have little to do with the discussion.

 

[Nick F]

Absolutely right, Arc !

 

[psneeld]

Actually the answers were in post #2.

Rebuttals afterward but no definitive answer to efficiency for his particular boat.

 

[fgarriso]

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

 

[Lepke]

Spinning one shaft while on one engine while the other is at cruising speed gives me 1 knot more speed than locking the shaft. Tested several times over 300nm ocean run. Locking the shaft also caused the running engine to use more fuel to maintain rpm. I have old manual transmissions.

If you have a mechanical transmission, unpowered spinning doesn't hurt. Most newer transmissions need oil supplied to the bearings, and that doesn't happen unless the engine is running. If you have a transmission oil cooler, lock the shaft.

 

[ghost]

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

Not necessarily. I used to have PSS seals, they only required cooling above a certain threshold. My boat was below that threshold so I did not even require the cooling when running at speed. Seems many boats that needed the cooling, would not need it at slow speeds.

Can’t talk to lip seals, I never ran them.

 

[O C Diver]

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.

Ted

 

[psneeld]

One boat isn't every boat.

Similar ones may be similar.

Easy enough to verify how "your own" does if one engine efficiency is important to your boating ( like long distance, remote voyaging.

The most critical things are tranny and shaft seal so manufacturers usually have detailed directions.

 

[rslifkin]

For transmissions, my Velvet Drives mention that it's ok to let them freewheel at "trolling speeds" indefinitely. Only problem is, they don't specify what "trolling speed" is, so I don't know if they mean 1.5 kts trolling for salmon or 7 kts trolling for tuna...


For dripless seals that have a forced water feed, with twin engines you ideally feed both seals from both engines, that way you don't lose flow if you have to shut an engine down. For dripless or conventional stuffing box installations that don't require a forced water feed, there's no such concern.

 

[psneeld]

In the early days before the stock Sea Rays had crossfeeds, we just used to pull the water feed off the dead shaft seal and let the water backflow into the bilge.... bilge pumps easily kept up and we could keep moving without messing around in the bilge too much in tight quarters at sea.

That was if we were still running the boats above I think 5 knots...but up to 8-9 knots I never thought would be much of an issue.

 

[C lectric]

Checking what the mfgr. says was mentioned several times. Do it. Find out from the horses mouth what they, the gear mfgrs , recommend . If you are wrong it can wreck the gearbox. To me this is no place for guessing.

Depending upon the gear mfgrs and the gear model:
--some windmilling is allowed for a time period and speed limit with temp. checks.
-- some require an engine startup for a few minutes every hour or so to circulate gear box oil.
--some forbid any windmilling.

You need to find out positively whether yours is allowed or not allowed to windmill and what the restriction for time, speed and so on are.

For windmilling there may be there may be other considerations such as a PSS shaft seal which needs water to be lubricated. You must also take this into account.
On twins some folks will arrange a cross over feed so either engine can supply both seals with a valve flip that normally isolates them to their respective engine.

On a single with a shaft seal you may have no choice but to secure unless willing to sacrifice the shaft seal.



Mine is NO windmilling.

I use a pipe wrench to secure it. I carry a 14" on a 2" shaft. Nothing wrong with bigger. My wrench must be mounted upside down to stop the shaft. I use Tyraps to secure the wrench to the shaft. I do mount it on the shaft but maybe yours would allow the shaft side of coupling to be used. Find out.
In my case there are too many obstructions.

I also use a piece of plywood between the wrench handle and the hull to prevent any possibility of wear. It's only about 6" square, 3/4" thick.
After about 10 minutes like that I recheck it all.

Personally I would not use Vice Grips, good as they are. THe lever action from the handle will have a big effect on the pressure at the handle end. The longer the handle the better.

 

[ca distress]

Thanks all, good info, hoping I never have 1 engine shut down, but I now have a better idea of what to do if it does.

 

[Mischief Managed]

I have Twin Disc transmissions and they need the engine running to provide lubrication to the transmission internals, so they should not be allowed to freewheel much. I have never had to operate on just one engine, but I have moored in places with fast current and "locked" my prop shafts to keep the current from turning the props. I just used rope wrapped around the shafts. Took a few seconds to implement and another few to de-implement, and worked great. I put my engine start key on the rope so that there was no possibility of starting the engines without removing the rope first. If I was in a situation where I was often moored in a strong current, I would buy Shaft Loks SHAFT LOK INC. with remote cable operation.

 

[C lectric]

Easiest way is to shut down the engine while in gear. This might not be appropriate for some transmissions but with most you will end up with no moving parts.

That would be the case if the gearbox is mechanical.

It is not true for a hydraulic box as it take the engine driving the hydraulic pump to produce pressure to lock the plates. Without hydraulic pressure the plates will freewheel.

In power boats, unless very low powered, the hydraulic units prevail, by far.

Hydraulic units are used in many sailboats also as they get bigger and the engines more powerfull. I know a couple of boats that changed from mechanicals, due to too many problems, to hydraulics.

 

[psneeld]

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.