Prop's at Anchor

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Andy G

Hospitality Officer
Sep 20, 2010
Vessel Name
Vessel Make
IG 36 Quad Cabin
Just got back from an overnight jaunt to a little bowling club restaurant on*Dangar island. We anchored off on the narrow side of the island, and the ebb tide was pretty strong, you certainly could not swim against it.

At about 2.00am we were awakened by a low*rumbling noise coming from below the boat, checked all the usual suspects, bilge pump etc, took me a while to figure out it was the ebb tide turning the screws* through the water. just wondering how common that is. Obviously the gears should be left in neutral I guess.*
My first thought, having encountered rumbling before myself, Shrimp is that it is the sound the anchor makes if dragging a bit, or maybe just a loop of redundant chain, dragging over rock. I find it hard to envisage enough current to turn props, but I suppose it might if strong enough, but would they rumble - I doubt it? It matters not whether in or out of gear. If the engine is not running, and the drive side not turning, there is not enough oil pressure in the tranny to stop the prop and shaft turning, which is why you need to lock the free shaft in a boat where one engine has failed and you are motoring on the other. Could you actually see the shafts turning...?
Old Stone wrote:
Trying to remember acurately, but did I not see somewhere that freewheeling one prop MAY cause tranny damage?
*I think it*depends on the tranny.* If it is hydraulically operated, then free wheeling is a no no, or at least that is what my Twindisk manual says.* It has a way to lock it if you're towing the boat so this doesn't happen.
Yeah I have experienced tha same thing in Tidahapah.
When anchored up at Yellow patch just north of Gladstone.
Tide changed and we were asleep thought the noise was the anchor chain at first and this did add some of the rumbling but upon further inspection found the shaft turning over and a low
grumble comming from the gearbox.
My thoughts are that the noise is promoted due to low revs and lack of oil flow.
Not enough to worry me re damage.
Once I knew the source it was back to sleep.
Have experienced this a couple of more times since.
Old Stone wrote:
Trying to remember acurately, but did I not see somewhere that freewheeling one prop MAY cause tranny damage?
*Depends on the transmission.* The BW Velvet Drive used on a large number of trawler-type boats in the 70s can, according to the operators manual, be freewheeled but only at slow speeds which the manual defines as "sailing or trolling" speeds.* At higher speeds the BW VD transmission must be prevented from turning, usually by tying off or locking the shaft.

The other consideration with regards to freewheeling is if the shaft log has a water feed from the engine's cooling system or not.* If it does, letting the shaft freewheel (assuming the transmission permits it) means it will be turning in the log with insufficient cooling/lubricating water and the shaft and the log can heat up to the point where damage to both can occur.

Our boat has water feeds from the engines' raw water cooling systems because the double cutless bearings in the logs of many GBs does not allow sufficient cooling to the forward bearing from the water outside the boat.* So if we have to shut an engine down and run on one we have to tie off the shaft of fhe shut-down engine.

On one such occasion we didn't have far to go so after calling our diesel/transmission shop on the phone for advice we let the shaft freewheel for the couple of miles we had left to go.* It was impressive how fast the shaft and log got hot.* On the advice of the shop we finished those last miles at a slow speed so the temperature of the shaft and log did not reach the danger point, but they were plenty hot by the time we got to our slip.

I have since fabricated brackets and shackles on the floor beams over each shaft coupler to facilitate tying off a shaft should the occasion arise.*

We also wanted to know exactly how long it takes for our prop shafts to stop turning both at idle rpm and when moving forward through the water at cruising speed and power is pulled off and the transmissions shifted to neutral.* The idle timing told us how long we need to wait in neutral while shifting between forward and reverse while maneuvering.* So we lifted a hatch in the floor and my wife timed the shafts while I manipulated the throttles and shifters.* So I can tell you that a freewheeling shaft very definitely has a rumble to it :)

FWIW here is the text from the BW Velvet Drive manual regarding freewheeling....

"3-4 Freewheeling.* It has been determined by tests and practical experience that all Velvet Drive marine transmissions can be freewheeled without risking damage in sailing or trolling applications.* Caution should be taken to make sure that proper oil level is maintained prior to freewheeling as well as normal running.* Freewheeling one propeller of a twin-engine boat at trolling speeds will not cause damage to the transmission connected to the freewheeling propeller."

-- Edited by Marin on Wednesday 6th of July 2011 01:42:02 PM
I had that too and some people on this forum had me convinced it was the ghost of Edwin Monk
"It has been determined by tests and practical experience that all Velvet Drive marine transmissions can be freewheeled without risking damage in sailing or trolling applications."

Which, unfortunately begs the question, "what is normal trolling speed...?"

I ask this not to be mischievous but because, for example, trolling for trout via lead-line and lure, 1 - 1.5 kn would be trolling speed, but trolling out in the ocean for Spanish Mackerel is about 6-8 kn, for marlin more like 10kn I think.

Sailing boats have similarly, a quite wide operating speed range depending on conditions.* However, the original thread query was can current driven prop turning make a rumbling sound - the answer would appear to be...yes.

-- Edited by Peter B on Thursday 7th of July 2011 05:15:53 AM
Peter B wrote:

Which, unfortunately begs the question, "what is normal trolling speed...?"
Peter-- Very valid question and one that I think has to be asked with the era in which the transmission was designed in mind, which would be the 1960s I believe.* So put yourself back then and ask the same question.* I suspect that for the boats these transmisisons went into originally--- like the old GBs and the like-- a trolling speed would be considered two to four knots.* That's how fast we trolled for mahi mahi, ono, ahi, and marlin in Hawaii back in the 1970s.* And for downrigger trolling for salmon two or three knots is about right.*

Sailing speed for the sailboats these transmissions might have gone into back was probably four or five knots or less.

I have found over the years that the trick to longevity in older equipment is to operate and maintain it as though it was the same year or era in which it was designed.* This is as true for the Pratt & Whitney radial engine in the floatplane I fly, an engine designed before WWII and built during WWII, as it is for the 1950s-design Ford Lehmans in our 1973 Grand Banks, and my 1973 Land Rover.
Well, if they're spinning at 2knt current, you can be the alignmet is pretty good, or the cutlas bearings are shot
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