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