T&T: Drivesaver

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Just a caution for those of you that expect that the Globe Drivesaver will,
in fact, save your drive. It may not. At least it didn't in my case.
For those of you unfamiliar with the Drivesaver, it is a semi rigid disc of
polyurethane with threaded inserts embedded in the molding. It is designed to
go between the flanges of the coupling and absorb small amounts of vibration
and misalignment. The main raison de etre of the Drivesaver is to be a weak
link between the reverse gearbox and the propeller driveshaft. In case the
prop hits an unyeilding object like a rock or log, the Drivesaver should
shear before the shaft bends, the keys break or the teeth in the reduction
gear crumble. At least that's what is supposed to happen.
In my first test run of the Spring I had the misfortune to hit a protruding
branch of a watersoaked log lying beneath the surface. The branch neatly
wedged between the keel and the prop of my Willard 30 and the engine stopped
immediately. This is when the Drivesaver should have sheared protecting the
running gear. What did happen is that the keys and setscrews on the shaft
coupling sheared but not before badly scoring the coupling, bending the prop
shaft and twisting the engine on the engine mounts nearly an inch out of
alignment.
After I regained my composure and limped back to the boatyard (using the tiny
Minnekota trolling motor get home drive) I took a good hard look at the
literature that came with the Drivesaver. My Perkins 4-107 with a Velvet
Drive 2.57 reduction gear supplies 162 ft. lbs. of torque at rated output.
The model 504 Drivesaver specified for the Velvet Drive output flange is
rated for 675 ft. lbs. of torque. When the prop hit the log the rotating
inertia of the engine and drive sheared the keys and bent the shaft. As far
as my driveline was concerned the Drivesaver might as well have been cast
iron. A call to the Globe company confirmed that the Drivesaver was intended
for engines in the 200+ hp. range, not the 36 hp of the Perkins.
I know a lot of sail boat and trawler owners that have employed these
coupling dampers to increase the flexibility of their driveline and reduce
vibration and shock loadings. The current crop of such dampers, made by
Globe, PYI, and other companies are intended for high power engines and are
made of polyurethane far too stiff to provide much flexibility and breakaway
safety for lower powered trawler engines. Unless we can prevail upon the
suppliers to use a softer plastic in their moldings for lower powered boats,
their use adds little benefit for trawler owners.>>
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Since I posted my original message several years ago, I have had an opportunity to speak at length to engineers at both Globe and PYI. Both sources agreed that the problem of using a single element as a mechanical fuse, a thrust and vibration absorber, and a connection to the prop require compromises that may make it unsuitable for some boats.
Globe only makes one model which can be fitted to the BW 4-71 transmission. It must be rigid enough to handle the full torque of engines normally coupled to that transmission yet weak enough to shear when that torque is greatly exceeded. I suspect that the shear point for the Drivesaver is reached when the maximum rated torque is doubled, although I could not get a definitive answer on that point. For lower powered engines the Drivesaver shear point may be four or more times the maximum torque. Remember that this is for the Model 504 Drivesaver normally fitted to BW 71 and BW 72 transmissions. Other model Drivesavers. for other transmissions, are rated from 225 to 6500 ft. lbs. of torque. 
PYI makes a similar product but offers several torque ratings for the BW 71 transmission. This is accomplished by adjusting the hardness of the polyurethane compound. The PYI product also incorporates a mechanical link which will enable you to limp home after the coupling shears, assuming that your shaft is not twisted into a pretzel.
If you use a elastomeric coupling element as a mechanical fuse bear in mind that stronger is not better. The unit should shear at about twice the maximum torque that your engine and transmission can deliver - else why bother?
In my considered opinion, the Globe 504 Drivesaver, as used on BW 71 and &2 transmissions, is simply too strong for trawler engines of less than 100 hp. They are marginal in the 100 to 150 hp range. They may break if the prop hits an unyeilding object, but then again, they may not. If your engine is more powerful, they will reliably work as expected.
Globe could easily make Drivesavers of lower torque rating suitable for the BW 71 transmission but they have chosen not to do so. The small trawler market is to small, they say.
Larry Z
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