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Old 05-17-2013, 04:14 PM   #1
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Freewheeling main shafts

During sea trials after some upgrades, we stopped the port engine to check a water pump leak. In passing, the boatyard captain advised we carry a large pipe wrench to affix to the 2" main driveshaft to prevent shaft freewheeling of non operating engine which will damage the ZF IRM 220 transmission.

Boatyard lore or good advice ?
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Old 05-17-2013, 04:26 PM   #2
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Depends on the transmission and the shaft log. Some transmissions can be freewheeled, usually at lower speeds. And some shaft logs (like ours) require a cooling water flow from the main engine(s) raw water cooling system to lube and cool the forward cutless bearing.

Our boat has BW Velvet Drive transmissions that can be freewheeled at slow boat speeds but our shaft logs require cooling water from their associated engine. So shutting an engine down and then going somewhere requires that we tie off the shaft.

I don't like the big pipe wrench idea because it will most likely mar the shaft fairly severely. When the original shaft tie-off points on our boat were blocked by a new fuel filtering system we had our diesel shop install, I mounted strong tie-off points in the very heavy cabin sole beams which happen to run directly over the shafts.

If we have to finish a run on one--- which we've had to do a few times since owning the boat--- I tie off and wind a stout line around the shaft coupler multiple times and then tie it to the tie-down directly above it.

The force applied to a lock-off device be it a line, wrench, or whatever, from a shaft that wants to freewheel is surprisingly strong, so whatever system you decide to incorporate had better be able to take it.
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Old 05-17-2013, 04:40 PM   #3
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I personaly not like a shaft Freewheeling. Put the box in reverse to stop Freewheeling (can damage the box). If you want to go long distance on one engine replace the prop on the standby engine for a folding one.

My 2 Centavos.
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Old 05-17-2013, 04:41 PM   #4
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Marin, thanks for reply and input. I fully agree on the downside of big wrench, and will look for alternate method. But remember, we boat in LA (lower Alabama), so big wrench/ bigger hammer/red headed wrench standard order of repair !
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Old 05-17-2013, 04:57 PM   #5
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Well, I certainly agree about not using a pipe wrench.

It sounds like you are just interested in temporarily running on one engine while and until you can fix a problem with the other.

I just read a discussion on another forum, maybe boatdiesel, that said that the ZF 220 transmission can freewheel for 8 hours as long as the transmission temp stays below 90 deg C, about 195 F.

And as someone else noted the cooling of the shaft logs is another matter. The way to handle that is a tee from one engine's raw water system over to the other engine's shaft log so it always gets cooling water.

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Old 05-17-2013, 05:30 PM   #6
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Check with the manufacturer of the trans to get the best info on their product.

I do like the idea of connecting the cooling lines from one trans to another....but the transmissions we have can be freewheeled without problem.
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Old 05-17-2013, 07:29 PM   #7
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A single engine/propeller boat doesn't have this issue unless being towed. For me, I'd put the transmission "in gear" since the shaft is water-lubricated via the engine.



By the way, my prop doesn't turn in neutral while anchored in 2-to-3 knot currents. How much speed is necessary to free-wheel a propeller anyway?
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Old 05-17-2013, 07:42 PM   #8
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Depending on the type of transmission, putting it in gear may not lock it. The BW Velvet drive, for example, does not lock anything if you put it in gear. Even with the shifter in forward the prop and shaft will still freewheel no differently than if the transmission is in neutral. But there may be some marine transmissions that do lock the shaft if they're put in gear.

As to how much speed is required to freewheel a prop, it will depend on how much friction there is in the drive train. Tight cutless bearings can keep a prop from freewheeling until the boat is moving several knots. Other people on this forum in past discussions have reported that their props freewheel when anchored or docked facing into a mild river current. So they have less friction in their drivelines.

As I recall a long time ago when we had to shut an engine down at the very end of our delivery trip from Tacoma to Bellingham due to a leak that had developed in the port engine's coolant pump, the shaft started freewheeling at about three knots or so. That was with very tight cutless bearings: the shafts could not be turned by hand when the boat was stationary in the slip with the engines off.

About six or seven years ago the cutless bearings were replaced in conjunction with a shaft replacement on one side and a straightening on the other. The shafts were very easy to turn after that so probably would have started freewheeling almost immediately if we were running the boat on one engine. Today the bearings have tightened up (they swell over time). I can still turn them by hand but it takes a fair amount of pressure to get them started. Once started they are easier to turn.

So see how hard your shaft is to turn by hand, Mark. If it's tight it will take more speed to get the prop turning than the tidal currents you've been experiencing. If it's very easy to turn by hand then I have no idea why the tidal currents going past your boat at up to three knots don't turn it.
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Old 05-18-2013, 06:41 AM   #9
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What about the loss of HP if your props don't turn freely or are tight in car racing it is called rolling resistance and a big goal is to reduce any and all friction so the HP loss from the crank to the wheels is minimized! This can cost more HP (fuel) and change the GPH rate of burn!
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Old 05-18-2013, 07:00 AM   #10
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Quote:
Originally Posted by Marin View Post
The BW Velvet drive, for example, does not lock anything if you put it in gear.
But, would the BW Velvet be damaged by freewheeling? If so I'd probably want to have a strategy in mind to lock the shaft if I ever had to get home on 1 engine.
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Old 05-18-2013, 07:15 AM   #11
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A locked prop has far less drag than a freewheeling prop.

While the wrench is not very elegant , a quick hit of reverse will drop the wrench when the engine is needed again'.

IF you are contemplating long more efficient passages pulling the prop on one engine is the norm.

There are prop lock mechanisms , mostly for sail boats that will do the job , and oil pressure on engine start releases the lock.
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Old 05-18-2013, 07:34 AM   #12
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A locked prop has far less drag than a freewheeling prop.
http://www.google.com/url?sa=t&rct=j...SnaYjA&cad=rja
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Old 05-18-2013, 08:02 AM   #13
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A locked prop has far less drag than a freewheeling prop.
Every sailor know this... nothing new on the horizon. Standard we put the gearbox in "Reverse".

Cees
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Old 05-18-2013, 09:36 AM   #14
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Interesting report. Page 49 shows data for 3 blade fixed prop, which is the most relevant of the tested configurations to the "trawler" community. Seems to conclusively settle the fixed/free wheel drag debate....take a look.
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Old 05-18-2013, 11:53 AM   #15
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Very interesting report. Thanks, Rick.
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Old 05-18-2013, 12:06 PM   #16
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Very interesting report. Thanks, Rick.
Yes, it is. I did some tests a couple summers ago to investigate the fuel consumption advantage of shutting down one engine on our boat. The tests were done with the prop free wheeling. Performance numbers showed about a 25% improvement in overall fuel burn at slow speeds (your results may vary). Timjet saw similar numbers when he did the test. I have a do list for every "long" crossing of Lake Michigan as it's normally fairly steady state for six or seven hours. An item for this summer was to perform a comparative test with the shaft locked. After reading that report, I won't bother.
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Old 05-18-2013, 02:48 PM   #17
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A locked prop has far less drag than a freewheeling prop.
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From CaptTom: But, would the BW Velvet be damaged by freewheeling? If so I'd probably want to have a strategy in mind to lock the shaft if I ever had to get home on 1 engine.
FF--- According to MIT, that is an urban myth. An MIT study circulated years ago on the Grand Banks owners and other forums showed definitively that a freewheeling, fixed-blade prop creates significantly less turbulence-- which is drag--- than a locked prop. I no longer have a copy of the study so I don't have the numbers anymore. But whatever they were, they proved conclusively that a locked, fixed-bladed propeller creates not just a little bit, but a lot more drag than the same propeller allowed to freewheel.

So it was interesting to read the report Rick posted and see this confirmed--- that a locked prop produces over three times as much drag as the same prop allowed to freewheel.

To me, this is just common sense and logic. When we have had to bring the boat home on one engine, which requires us to lock the non-powered shaft off, we know that locked-off prop is producing a hell of a lot of turbulence which means drag, by a) the buffeting against the rudder which is clearly felt at helm (we have cable-chain steering so anything the rudder does or feels is felt at the helm-- hydraulic steering does not have this feedback) and b) simply standing on the aft deck looking at the water behind the boat the turbulence coming off the locked prop is clearly visible.



CaptTom--- Regarding freewheeling the BW Velvet Drive marine gear, if you read the operator's manual it states that the Velvet Drive can be freewheeled at slow speeds. Unfortunately instead of giving an actual speed figure they simply define slow speeds as "trolling or sailing speed."

In talking to our diesel shop, which also does or vends out transmission work, the speed figure they gave us was three or four knots.

However.... since our boat has shaft logs that are lubed and cooled by water fed to them from each engine's raw water system, we have to lock off our shafts if their associated engine is not running and thus pumping water to them even thought the VD marine gears themselves can be freewheeled.

The reason is that the unlubed/cooled shaft log and shaft will get incredibly hot in a very short time. I know this because we did it on the delivery cruise of our boat from Tacoma to Bellingham. At the very end of the trip we had to shut the port engine down due to a leaking coolant pump. We called the diesel shop and given the short distance remaining they said bring it in with the shaft freewheeling, just go slow and monitor the shaft log temperature. While we did not overheat the log and shaft, I was amazed at how hot they got even at four knots or so in a very few minutes. It was obvious that freewheeling the prop at any speed at all for a longer period of time would most likely severely damage the log and bearings and possibly the shaft, too. Hence the heavy-duty tie-off system I fabricated in the engine room.

It has been suggested here that one could create a hose and valve system by which the running engine could send water to both shaft logs and thus leave the un-powered shaft to freewheel thus causing less drag on the boat. I thought of and discussed this years ago with the diesel shop we use, right after we got the boat, actually, and experienced what I just described.

The consensus of the shop guys was that while one could easily do this, the water flow to the logs from our FL120s raw water cooling systems is not much. Pretty small hose, picked off from one end of the main heat exchanger. It's fine for feeding to one log--- ours run dead cold with no drips--- but they felt that splitting this feed-- and there would be no way to know if the feed was being evenly split between the logs-- could deprive BOTH logs of sufficient cooling and lube water so they would both run hot. So they did not recommend it. However, they said that on systems with a higher flow to the logs it could certainly work.

We have since replaced the stock FL120 raw water pumps and drives with new pumps with a significantly higher flow rate. However the issue of the small hoses to the shaft logs and where they get their feeds remains. The system could be re-designed,probably, to send more water from each engine to its log and thus accommodate a split in the feed.

However an engine shutdown is a very rare occurrence. We've had to do it four times in the last 14-plus years, which is a lot in my opinion. But one case was the leaking coolant pump on the delivery trip so who knows how long that problem had been brewing. Another was totally my fault during a fuel transfer when I allowed an engine to get a big slug of air that stopped it and rather than take the time to bleed the system in the rough-ish water we were in we elected to tie off the shaft and come home on one. The other two were restrictions in the raw water hull intake, one a growth that had started before we bought the boat and the other a slowly increasing blockage on the external screen from barnacles.

The point is that given the normally very rare requirement for a precautionary engine shut-down, it's way easier to simply tie off the shaft than go through the exercise of redesigning and fabricating the water feed system to the shaft logs. For our engines, anyway. With different engines and a different shaft log cooling system, it may be pretty easy.
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Old 05-18-2013, 05:45 PM   #18
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freewheeling main shaft

Great info on free wheeling.The BW lubricates from the main engine via pump ontranny.In engine failure (TOW) always chain off transmission coupling.CCG will ask you to do this.Best to know if you are right or left handed before tow because it can become dangerous after tow starts.Usually the bolts on coupling will snag the chain.Dog chain or larger is all that is required
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Old 05-18-2013, 07:00 PM   #19
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The reason is that the unlubed/cooled shaft log and shaft will get incredibly hot in a very short time.
Yep
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Old 05-18-2013, 08:03 PM   #20
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FF Regarding freewheeling the BW Velvet Drive marine gear, if you read the operator's manual it states that the Velvet Drive can be freewheeled at slow speeds. Unfortunately instead of giving an actual speed figure they simply define slow speeds as "trolling or sailing speed."
Thanks, I don't have the manual. My logs aren't lubricated from the engine, so I assume I'd be OK freewheeling at low speed. Sounds like 4-6 knots would work. I don't expect it would be often, but it still makes sense to keep some chain in the engine room.
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