Volvo Penta S130-D saildrive snapped, why

[Mrflip]

I have a 2021 Fontaine Pajot, 40 foot ISLA 40 sail catamaran, Volvo Penta 30HP Diesels with S130-D sail drives and folding props. Down here in St Petersburg FL, work was done to the sail drives (replacing shaft seals and welding/grinding the shafts to remove some scoring). Shortly after getting her back in the water, we lost one prop, hub, and line cutter. Looking into the remaining prop, we found the folding tab lock washer was not folded, and the prop not was barely finger tite. Almost lost that prop too. Everything was repaired, nuts and bolts torqued (calibration checked) with thread locker, and locking tabs folded. All should be good, right? Nope, made it about 5 feet and the shaft snapped just aft of the hub spline section where the shaft narrows for the prop nut threads. The prop nut, locking washer and bolt are all still intact, attached to the snapped piece of shaft.
The big question is why. Is it possible that the prop nut being loose allowed the hub to act like a slide hammer when shifting forward/reverse, or perhaps overtorquing the prop nut much earlier could have streatched and weakened the shaft.
Any thoughts would be greatly appreciated, anything to keep this from happening again.

[skipperpete]

Welcome to the forum Mrflip…… The answer to your big question is YES, a loose folding prop could do what you describe, but so could shifting between ahead and astern with an engine that has the idle speed adjusted too high. This forum has pages of stories about all the ways Volvo and Yanmar saildrives shed folding propellers but you‘re unique for losing one and almost losing the other in the same time frame.
Yes, overtorquing the prop nut ( or cone nut on a regular propeller) will fracture the propshaft thread especially upon removal if thread locker was used there. It sounds like you didn’t have the long bolt that screws into the centre of the propshaft, often not possible with folding props. Losing folding props at inconvenient times is not at all an uncommon event, yanmar made a shock absorbing joiner to connect the top gearbox with the lower leg for folding/ feathering props and I put a few in with maxprops but never got to hear about how successful that mod was. The good news is that the propshaft can be bored out and a stud installed ( complete with centre bolt thread if needed) but if you’ve already had to weld the seal tracks , it might be better to buy a pair of new shafts.