Thursday, October 25, 2012

Sometimes it's the little things

Over on s/v C'est la Vie Jeff has been working on a big project: refurbishing his engine room.  Now this certainly does not qualify as a "small" boat project.  But in the midst of this, he has found one of those small things that can be more important than the big ones. 
One of the few items remaining in C'est la Vie's engine compartment is the drive shaft.  The drive shaft is a solid stainless steel rod that connects the engine inside the boat to the prop outside the boat.

C'est la Vie's shaft and shaft seal were replaced in 2007 
Traveling from the prop towards the engine, the point where the shaft passes through the hull it is supported by shaft log.  The log is a bronze tube with rubber bushings.  The log relies on water from outside the hull to serves as a lubricate thus reducing wear and friction. Directly forward of the log is our shaft seal.  The shaft seal allows the shaft to spin freely yet prevents water from entering the boat.   In 2007, we replaced the shaft, shaft log, and the shaft seal (here is a link to the photo album - Replace Shaft, Log, and Seal - fall 07

During the 2007 project, we purchased a PYI-PSS shaft seal and had the boat yard complete the installation work.  The shaft seal uses a rubber bellows as a spring to press a carbon flange against a spinning stainless steel rotor.  The carbon flange and stainless steel rotor are very smooth and when pressed together create a water proof seal.  As with any hole in the hull of the boat a failure of this system can cause the boat to sink.

the newly installed shaft seal - Fall 2007

Since all the components of the shaft system were new in 2007, I left them in place and did not intend to replace any parts.  My experience over the weekend has me questioning this decision.

C'est la Vie's Shaft Seal - Sept 30, 2012

While sanding between coats of primer in the bilge, I noticed a paint drip on the aft most of the shaft seal's four hose clamps.  Using a scotch bite scrubbing pad, I reached back to remove the paint drip. Instantly the hose clamp broke in my hands. 

the  stainless steel hose clamp that broke in my hands after 5 years of
service on our shaft seal.

Alarmed I grabbed a nut tool and attempted to loosen the other hose clamps for an inspection.  The very next hose clamp I touched, the forward most clamp, broke as I torqued on the hex head.  Yikes! Failure of these clamps can sink the boat.  The clamps used on shaft seal were provided by the manufacturer, but are not of the quality I expect for critical through hull fittings.  Not all hose clamps are created equal.  

Without a doubt all hose clamps on board a boat should be stainless steel, but even among stainless clamps there is a broad spectrum of quality.  Clamps used in critical areas (i.e. through hull fittings, engine coolant lines, engine exhaust lines, black water systems, etc.) must be ABYC approved  316 stainless steel with a non-perforated band.    

an ABYC approved 316 SS hose clamp with a non-perforated bans will replace
the bands on our shaft seal.

How does one identify the various quality of clamps?  The easiest test is to run a magnet across all hose clamps on the boat.  If the magnet is attracted to the clamp then the clamp contains ferrous metal that will easily rust. Any clamp attracted to a magnet needs to be kicked off the boat ASAP.  I keep a magnet in my tool box expressly for testing hose clamps, bolts, screws, etc.  If any of these items do not pass the magnet test then off they go.

Hose clamps that will pass the magnet test are appropriate for non-critical boat systems (i.e. tank vent lines, fresh water plumbing, etc.).   If the clamp is destined for a system that is considered critical then it must pass two additional tests.  First is the band non-perforated?  A non perforated band is stronger, better resists rust, and provides more uniform pressure around a hose.  Secondly the hex head on the clamp must be 7mm.

I'm unsure if the 7mm hex head is a brand specific standard or if it is set by the ABYC.  All high quality AWAB Hose clamps have a 7mm head so I look to this as an identifying feature of high quality clamps.

If your using the 7mm head as standard of quality then I recommend purchasing a  flexible shaft 7mm nut tool  to compliment the clamps.  

It is worth noting an exception to my hose clamp rant.  As hose diameter increases to 2" or greater and/or if the hose has a wire for additional strength (e.g. engine exhaust systems), then t-bolt hose clamps are recommended due to their ability to provide increase pressure on the hose. 
Inspecting and replacing hose clamps is a "small" boat project we should all perform, annually at the least.


  1. This is a scary report as I just installed a new PSS shaft seal a couple days ago. However, the one I replaced had been in for an unknown length of time (over 5 years at least) and was still in good shape as were the hose clamps. But now you've given me something new to worry about. And the only solution is to get a bunch of the "preferred" hose clamps when we're back in the US next summer as I'm sure that they're unavailable n Mexico. I can get stainless steel clamps but the ones w/o perforations and certainly the T-bolt style are likely to be very hard to find down here.

  2. You and Jeff are kind of working in parallel on this...

    Steve, I'd say that whatever hose clamps you used were fine going in. Your concern should be with their longevity... so if you inspect them on some kind of schedule and carry spares you should be fine.

    I know - inspecting clamps at that location is not easy, and like with Jeff's (and mine!), the failure will probably occur on the bottom of the clamp, where it is hardest to see.


  3. Gawd. Last week our shaft seal failed, probably while we were backing down to tie up for lunch. I heard something strange, but passed it off with a shrug. We returned from lunch to see our 1500 gph bilge pump squirting a stream out of the stern. There was a foot of water in the boat. Our 3750 gph bilge pump, when switched on, overcame the influx and the boat was dry in about 45 minutes. My first move was to tighten the hose clamps on the shaft seal. Next, I installed a float switch for the 3750 gph, which will remain on at all times. The interesting thing is that the old hose clamps were fine, just very loose. I have never tightened them and infer that they were either always loose or had worked loose in the 12 years that we have owned the boat. I feel lucky and chagrined.


Related Posts Plugin for WordPress, Blogger...