"Drivesaver" devices on propshaft

[Jim Legere]

I notice Johnb had a picture of his transmission (in another thread about changing tranny oil on a 3QM30) with a "Drivesaver" or similar coupler device fitted between the shaft coupling hubs. I have always meant to try one of these - I even purchased one, but it was the wrong size. What models have others used and what's the opinion on them?

 

[Ed Schenck]

My shaft saver is one of these Jim( http://www.clrmarine.com/462m4/boat-systems/shaft-saver.html ). If you needed to know exactly which one I might be able to find the receipt. I installed it along with the new Yanmar. Somewhat flexible it probably requires less precision in alignment. But it is comforting to know that it would shear before tearing up the tranny or pulling the strut out of the bottom.

 

[Blaise]

Ed,
I have the same 1" flex coupling on my engine. It will NOT shear any more than the standard coupling because is still uses the same bolts. It is mainly to reduce vibration. If you have figured a way to attach it to the transmission without through bolting it, I would like to know.

 

[Ed Schenck]

No Blaise, mine is not thru-bolted. The shaft flange is bolted TO the shaft saver, not through it. In the picture the saver is already bolted to the crank flange. I don't know the actual forces required to shear it. I could probably find that somewhere.

 

[Ed A]

I used one on my 44 hp yanmar and i liked it for the same reasons.

 

[Jim Legere]

My shaft saver is one of these Jim( http://www.clrmarine.com/462m4/boat-systems/shaft-saver.html ). If you needed to know exactly which one I might be able to find the receipt.

Thanks for the offer Ed but I suspect that the shaft couplings on your new Yanmar may be different than those on my old 3QM30 - or does Yanmar use one coupling for all 1" shafts? Anyway, I can just measure my coupling and go from there (should have done this the first time, instead of trying to get the dimensions from drawings on the web!).

I can't see any reason not to put one of these "Drivesavers" in there and it may even reduce vibrations a little.

 

[Maine Sail]

But it is comforting to know that it would shear before tearing up the tranny or pulling the strut out of the bottom.

I have direct personal experience that shows they do not always do this on small engines. My neighbors replaced a prop, shaft and had a gear box rebuild after the DriveSaver failed to do anything when they wrapped a lobster pot. Shaft was bent, prop bent and the gear box toasted but the DriveSaver was as good as new. I suspect they would blow apart on a high HP motor but did not on their boat.

 

[Maine Sail]

I can't see any reason not to put one of these "Drivesavers" in there and it may even reduce vibrations a little.

I have removed more of them than I installed mostly due to improper installation. The can actually induce vibration/shaft whip when your shaft moves back nearly 1" and have more shaft overhang than is recommended especially when used in conjunction with folding props that already move the blades aft when compared to a fixed prop. The couplings still need to be aligned to .003" max before installation or you can still create shaft whip.

 

[Ed Schenck]

Well so much for shaft savers. But Maine does make a good point about the "overhang". My system was designed by a local prop shop. They faced and cleaned up the coupler and made the shaft for the "saver". In other words an inch shorter.

 

[Maine Sail]

Well so much for shaft savers. But Maine does make a good point about the "overhang". My system was designed by a local prop shop. They faced and cleaned up the coupler and made the shaft for the "saver". In other words an inch shorter.

If that is the case then yours was properly installed, very few are. The problem now becomes that if it ever did do what they say, if you hit something, you can't pull your shaft forward and re-connect. But either way you'd be dead in the water.

You also lose your shaft ground which is an ABYC standard requirement for new boat builders and a good idea for all boaters to not interrupt this ground path but this can easily be fixed.

If "properly" installed they certainly can't hurt anything but they are rarely properly installed. I tend to like the PYI R&D Flex Coupling better than the red one.

 

[Jim Legere]

Wow, thanks Maine Sail - I guess you just saved me $$$ ! I had considered shortening the shaft - the keyway on mine is long enough to lop an inch off - but I had not considered the issues of of bolt torque and alignment.

Surely its possible to design a "Drivesaver" that is soft enough to shear off to protect a 1" shaft and gearbox? I guess companies like Globe or R&D just sell more to the fishing industry and haven't done the design work on the pleasure craft versions...

 

[Ed Schenck]

Interesting that the shaft and prop MUST be part of the ground. Guess I have just been lucky all these years. Just too old to worry about it. Going to the lake today. Actually going to sail the boat this year. But I will check the alignment when she splashes. Last I checked she was perfect.

 

[Jim Legere]

Interesting that the shaft and prop MUST be part of the ground. Guess I have just been lucky all these years. Just too old to worry about it. Going to the lake today. Actually going to sail the boat this year. But I will check the alignment when she splashes. Last I checked she was perfect.

Its easy enough to rig a bit of brass strap and some wire to resolve the bonding of the prop shaft (or you can buy a commercial version thereof). It would be interesting to know whether anodic erosion of the shaft and prop in a "stray current" situation, like you sometimes see at marinas with bad AC wiring, would be better or worse with the shaft not tied to the boat's ground. (I am refering to electrolyic erosion of the shaft and prop, not the sacrificial shaft anode).

 

[Ed Schenck]

Boat and shaft have been in no fewer than five marinas since the new engine was installed in 2005. Picked up a few barnacles in Florida but shaft still looks like new.

 

[Maine Sail]

Surely its possible to design a "Drivesaver" that is soft enough to shear off to protect a 1" shaft and gearbox, but uses collars or some other means to ensure variance in torque does not cause mis-alignment? I guess companies like Globe or R&D just sell more to the fishing industry and haven't done the design work on the pleasure craft versions...

I'm sure it is possible and perhaps the R&D unit will fail. I do know that on low HP sailboat AUX engines that the Globe does not always fail. I tend to like the design of the R&D a little better. As I said nothing wrong with them IF installed correctly, but very few are..

 

[Jim Legere]

Boat and shaft have been in no fewer than five marinas since the new engine was installed in 2005. Picked up a few barnacles in Florida but shaft still looks like new.

My guess is that an un-grounded or electrically isolated shaft and propeller would be less likely to suffer from stray current corrosion, since they would be at the potential of the water itself. Your comments would seem to bear it out.

I haven't read the ABYC practice that mandates grounding the shaft, but I suspect it is about establishing a reliable path to ground for safety reasons. On a lot of boats there's not much real metal in the water (especially with encapsulated keels on some boats).

 

[Don12364]

I wanted to fit a rope cutter to my prop but this would mean moving the propshaft back about 1 1/2". This provided an ideal opportunity to fit a flexible shaft coupling.

I had read lots of uninformed comment on various websites regarding alleged problems resulting from the additional shaft overhang so I did the calculations and it was okay for my 1" diameter shaft length and maximum prop/engine speed.
Bought the "Stripper" see:- http://www.ropestripper.com/strippers.php
and the R & D Flexible Coupling see:- http://www.randdmarine.com/flexiblesc.asp.

Since then I picked up a big rope and some netting which stalled the engine but without damage to any of the sterngear. Years later ran over a floating fisherman's landing net with a 1" diameter aluminium handle. The cutter just chopped it into pieces!

I see no problems with the equipment I selected. In fact I put the same on my previous boat making, to date, over 28 years of satisfactory service including several occasions where we have picked up lobster pot ropes, an agricultural fertiliser sack as well as the two items noted above. In no case were we immobilised, though remnants of the netting did remain on the prop afterwards and we needed a diver to clear them on our safe arrival home.
Note the Flexible Coupling makes NO claims to be a drive saver.
What it does, being slightly flexible, is to much reduce the need for precise alignment of the gearbox and shaft flanges - and, of course, makes an ideal spacer for the Stripper.

 

[Ed A]

take a look at drivesavers web site. It lays out what they are designed to do.
ED S is correct they do isolate the engine from the shaft. Thats a good thing.