Sheared prop shaft coupling bolt

[johnnysails]

I sail with friends on a 36 ft Hallberg Rassy with a shaft driven Volvo D2-55 engine. Sailing last weekend one of the four bolts connecting the gearbox to the shaft, sheared. It appears to have happened whilst under sail when reverse gear was engaged with the engine not running. Would appreciate any comments as to whether the torque applied to to shaft when engaging reverse, in those circumstances, could be the possible cause. Also appreciate any comments regarding the advisability of engaging reverse with the engine stationary whilst sailing forward at say 6 knots.

[sailorboy1]

I fail to see how 1 bolt in 4 on a coupling could shear??? What did the other 3 do, just fall out?

[Nicholson58]

Possible crevice corrosion or other type of intergranular attack. Bolts may not have been the best material selection or old or both.

Replace all four bolts

[Tetepare]

Originally Posted by Nicholson58

Possible crevice corrosion or other type of intergranular attack. Bolts may not have been the best material selection or old or both.

Replace all four bolts

What he said. Do it now do it right.

[johnnysails]

See where you’re coming from but it was just the one that sheared. They will of course all be replaced. I’m particularly interested in the torque on the shaft and gearbox when going into reverse, engine stationary and sailing at about 6 knots

[DougR]

Seems to me that pulling the gearbox into reverse at six knots with the engine stationary is very similar to a crash stop from ahead to reverse without allowing the engine to come to idle......lots of inertia that has to be absorbed somewhere.

On a shaft coupling of this size the bolts are not "fitted" to the coupling holes and the clamping action of the bolts is all that keeps the two flanges from slipping on each other. If the bolts were not sufficiently tight, or had stretched, one could have sheared.

Replace all four fasteners with high quality alloy bolts, that can really be torqued, not stainless.

DougR

[Bacchanal]

A few years ago my "new to me" 2000 Hunter 380 did a similar thing. I found a bolt in the pan under the coupling. It was actually just half a bolt. It had sheared off. Upon inspection another bolt was found to be sheared as well. These were next to each other. The other two seemed to be OK. The marina said that they were loose and torqued them down. In order to fix the problem, The boat needed to be hauled and the shaft dropped so that the coupling could be removed. All 4 bolts and the coupling were replaced, realigned and has been fine for years. Marina said the surveyor should have caught something like that. All he had to do was twist the shaft to see they were loose.

[a64pilot]

Torque applied to the shaft under sail is negligible unfortunately, if it was a whole lot, enough to shear bolts, then we would be extracting that power to charge our banks or similar.
Even putting it in gear after the boat is moving and the prop turning is no worse than putting it in gear with the engine running.
There is prop slippage both ways, so shaft RPM is likely similar.
Your bolt broke almost certainly from being over torqued and you happened to notice it then or it just fell out then.

[SeaSon]

Originally Posted by a64pilot

Torque applied to the shaft under sail is negligible unfortunately, if it was a whole lot, enough to shear bolts, then we would be extracting that power to charge our banks or similar.
Even putting it in gear after the boat is moving and the prop turning is no worse than putting it in gear with the engine running.
There is prop slippage both ways, so shaft RPM is likely similar.
Your bolt broke almost certainly from being over torqued and you happened to notice it then or it just fell out then.

Exactly this. Well written.

[billknny]

Originally Posted by a64pilot

Torque applied to the shaft under sail is negligible unfortunately, if it was a whole lot, enough to shear bolts, then we would be extracting that power to charge our banks or similar.
Even putting it in gear after the boat is moving and the prop turning is no worse than putting it in gear with the engine running.
There is prop slippage both ways, so shaft RPM is likely similar.
Your bolt broke almost certainly from being over torqued and you happened to notice it then or it just fell out then.

A good answer and maybe the right one... It certainly is possible. But on most couplings I have seen these bolts are higher grade bolts, and it would typically be really tough to overtorque them given the access most boats have to this area. Not impossible, but not likely.

I am going to suggest the opposite problem... the bolts were LOOSE. Every shift resulted in shock loading on the bolts, eventually resulting in breaking one. The next one wasn't far behind... and the next...

How are the bolts secured? Lockwashers? Nylocks? Drilled and seized with Wire? (kind of in order of my preference...)

And for what it is worth, torque on a propeller under sail can be significant. More than enough to drive a large alternator and extract a LOT of power. Look up "prop shaft alternator" and learn....

[seabreez]

Originally Posted by johnnysails

I sail with friends on a 36 ft Hallberg Rassy with a shaft driven Volvo D2-55 engine. Sailing last weekend one of the four bolts connecting the gearbox to the shaft, sheared. It appears to have happened whilst under sail when reverse gear was engaged with the engine not running. Would appreciate any comments as to whether the torque applied to to shaft when engaging reverse, in those circumstances, could be the possible cause. Also appreciate any comments regarding the advisability of engaging reverse with the engine stationary whilst sailing forward at say 6 knots.

When a bolt is torqued to proper specs the tension is on the length of the bolt and there fore not subject to side ways thrust. When a bolt is loose, it loses the longitudinal tension and the side ways thrust is augmented and the bolt is not designed for it and is susceptible to breaking. Torque the bolts to proper tension and they will last a long time.

[a64pilot]

You can get a little power from a freewheeling prop but not much, that is why prop shaft alternators are uncommon, plus of course it slows the boat some, that energy has to come from somewhere.

Shaft bolts are sometimes higher grade, but often not, but even head bolts can break, and they are usually grade 8.
Fasteners almost always fail from being overtightned, only very rarely form under, reason is people want to make sure it’s tight, so they apply German torque.
Goot-N-tight

[Wotname]

Originally Posted by a64pilot

........
Fasteners almost always fail from being overtightned, only very rarely form under, reason is people want to make sure it’s tight, so they apply German torque.
Goot-N-tight

Exactly - the more I use a torque wench, the more I realize I have way overtightened many threads in the past.

[Lepke]

a64pilot is right. The bolt didn't shear. The coupling face with most of the bolts tight takes all the torque. Many people tightening bolts do not use a torque wrench and most often overtighten and sometimes breaking or starting cracks in the bolt. Boat yards don't always have their best employee doing every job.
When I had my yard, we often found overtightened coupling bolts with cracks or ruined threads. Some failed when removing the nut.

[RaymondR]

Originally Posted by a64pilot

Torque applied to the shaft under sail is negligible unfortunately, if it was a whole lot, enough to shear bolts, then we would be extracting that power to charge our banks or similar.
Even putting it in gear after the boat is moving and the prop turning is no worse than putting it in gear with the engine running.
There is prop slippage both ways, so shaft RPM is likely similar.
Your bolt broke almost certainly from being over torqued and you happened to notice it then or it just fell out then.

Whilst it is possible that the bolt was over-torqued it is just as possible that if it was stainless steel it failed because of fatigue or the propagation of a crack which formed at a stress riser at the bottom of a thread. SS is just not a very good material when highly stressed. bigger bolts or change them out regularly as part of a preventive maintenance program.