Prop shaft brake - SailNet Community
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post #1 of 19 Old 07-04-2008 Thread Starter
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Prop shaft brake

Hi,
I have a PRM hydraulic gearbox in my boat. I have a three blade conventional prop. The prop/gearbox rotates as I sail along. It's been suggested to me this is not a good idea as it will cause wear to gearbox bearings. Should I instal a prop shaft brake to prevent wear on transmission? Will drag increase or decrease as a result? Any simple design ideas for a braking system appreciated.
Thanks
Kevin
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post #2 of 19 Old 07-05-2008
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Hi Kevin and welcome to Sailnet, I have the same tranny. It depends on how long you sail, I don't always lock mine , it won't heat up right away, but as a precaution a break is best. I looked at those breaks they sell and settled on a pipe wrench. It works just fine as long as you don't forget it when you start your engine. Many people I spoke with use this fix. Give it a try. Good luck.
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post #3 of 19 Old 07-05-2008
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Shaft brake

I have very limited experience with hydraulic gearboxes but lots of experience with hydraulics and lots with mechanical gear reducers.I seriously doubt that while saiing the RPM of the prop would be fast enough to be of any concern. However if you engaged the drive with pipe wrench in place you may have a new thru hull opening plus severe trauma to other components.
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post #4 of 19 Old 07-05-2008 Thread Starter
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Guys, thanks for your thoughts. I agree the consequence of forgetting to remove the wrench on engine startup could be quite scary.
Cheers
Kevin
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post #5 of 19 Old 07-05-2008
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Our mower has hydraulic drives (two: one for each drive wheel). Neutral is simply no drive power. To actually get the two wheels to free-wheel, I have to flip a manual release on each drive. Then they don't drive anymore, either.

It's been years and years since I've owned a car with an automatic transmission, but I thought that, in anything but neutral, the drive train was not free to move.

How are these PRM hydraulic gearboxes different?

Jim
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post #6 of 19 Old 07-05-2008
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A simple brake can be made from 2 pieces of 2x4 with a a V cut into each and clamped to gether with carrage bolts with springs and wing nuts.

Just tighten the brake enuff to stop the shaft.

If you start the motor it will allow the shaft to spin but if you run for to long the wood will start to over heat and let you know you forgot it.

Rick
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post #7 of 19 Old 07-06-2008
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Some hydraulic drives are ok to freewheel

Pullark,

some hydraulic drives are ok to freewheel. My boat has a Velvet Drive hydraulic marine transmissions from Borg Warner and it's been freewheeling all the time & the boat is almost 30 years old. You might want to do some research into your particular gearbox.

I previously found the following on another website.
A number of major gearbox manufacturers have dispelled an earlier unfounded fear of gearbox damage due to improper lubrication while freewheeling. However some gear box manufacturers do not advise leaving the prop to free wheel as this can cause mechanical problems within the box (overheating due to lack of lubrication).

Ie - From “Velvet-Drive”:
It has been determined by tests and practical experience that all Velvet Drive marine transmissions call be free-wheeled without risking damage in sailing or trolling applications. Caution should be taken to be 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.
Extended periods of free-wheeling at high speeds may cause the transmission to overheat; therefore, it is recommended that transmission sump temperature be monitored and free-wheeling discontinued whenever 230 degrees F or 111 degrees C is reached.


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post #8 of 19 Old 10-27-2008
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Prop shaft brake

I have the same issue and am considering a folding prop. Does anyone know if a feathering prop will fold while sailing without locking the prop shaft? This would eliminate the potential for the pipe wrench effect and solve the gearbox issue.
Mike
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post #9 of 19 Old 10-27-2008
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I dunno about the gearbox wear but the drag will be least with the prop locked

I sail.
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post #10 of 19 Old 10-27-2008
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Is there harm in just leaving it in gear when the engine is stopped? I don't know what engine manufacturer you have, but for my Yanmar, the manufacturer recommends putting the transmission in reverse after shutting down the engine to keep the prop from free spinning. They recommend reverse over forward.


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