Monday, March 26, 2018

The Hidden, Never-resting Enemy

If your boat has an inboard engine, chances are that it has a shaft log - you know, that place where the prop shaft exits the hull.  This cannot be a mechanically rigid seal because it needs to accommodate movement of the engine on its mounts, and movement of the prop shaft in the cutlass bearing.  Typically then, regardless of the seal type, the shaft log includes a short length of rubber hose, one end of which is clamped onto the rigid hull fitting, and the other end onto the actual seal housing.

OK, so it's below the waterline and therefore needs to have two hose clamps on each end:  four hose clamps in total.  Also, typically, the shaft is slanted down, towards the aft end of the boat.  And it's in a typically hard-to-access location.

Add all this together and what do you have?  An opportunity for the bane of all boaters to work, silently, unobserved, trying to sink your boat.

No seal is perfect; this means that there will be the occasional drop of sea water exiting the seal.  Because the shaft is slanted aft, this lone drop will travel aft, along the bottom of the seal assembly and then the bottom of the hose.  In it's passage, it will travel over the bottom side of all four of those hose clamps. 

The bottom side of the hose clamps.

Where corrosion can work it's evil, undetected.

The NEW hose clamps

Last weekend I changed those four hose clamps on Eolian's shaft log.  All four of them showed corrosion; two were terrible.  And all four hose clamps looked almost new from the top, the only side that you can see.

If both hose clamps fail on one end of the hose (typically the aft end), there is a good chance that the torque applied to the hose via the seal assembly will cause the hose to rotate and eventually come off of the hull fitting.  When this happens, water starts coming into the boat.

How fast?  Well, that depends on your boat's construction.  On Eolian, the cutlass bearing is embedded in the hull, just aft of the hull fitting that the hose attaches to on the inside.  Therefore in our case, the water flow would be minimal - just that little bit that can squeeze past the clearances between the shaft and the cutlass bearing (and its lubrication grooves).

If, on the other hand, your boat has its shaft supported on a strut, then the flow area is huge - the difference between the shaft diameter and the hull fitting diameter - probably not less than a half inch.  There's gonna be a LOT of water coming in!

Due to the seriousness of a failure, and because visual inspection cannot disclose incipient failure, your shaft log hose clamps should be changed on a schedule, regardless of their appearance.

What frequency?  Well I can't tell you that.  If you haven't changed them in a while, go do so.  If they look bad, you waited too long. Cut your maintenance interval in half for the next time.  And if it's been more than 5 years since the last replacement, your should go change them now!



Rick said...

Good reminder, Bob. My boat's out of the water right now, getting ready for recommissioning - and I'll check the hose clamps.

Deb said...

PSS seals are also not immune. Read this story.

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