Tuesday, 12 August 2014

A thought-provoking prop failure ...

I took Raasay away for three weeks in July - down the canal and than back to Findhorn via the north coast.

I'll post a bit more on this later - there are pictures on flickr at:


But they are a bit disorganised.

I had one very unpleasant experience which cost me two or three days and some lost sleep.

I came into Stornoway harbour on the afternoon of Sunday the 19th and tied up at the new floating dock.  I then had to move the boat to get nearer to a shore power point, as I inteded to stay for a day or two.

Just as I dropped the shore lines, I realised that I wasn't getting any thrust.  At first I thought it was something to do with the throttle/gearshift linkage and I spent a frantic few seconds trying to get it to engage as the boat drifted towards other moored vessels.

After a fairly gentle, and fortunately harmless, collision with one of these the harbour watch man took my stern line and helped me to get tied up again.

When I checked the machinery, I found that the gearshift was fine, and that the prop shaft was turning, but that the propellor was no longer locked to the shaft.  I secured it with a line (in case it was about to fall off), and decided to hunt down some professional advice the following morning.

In fact, I was overwhelmed with help and advice - both from other boat owners and from a local marine engineer who provided his services at a very reasonable rate.  With their help, I was able to move Raasay to a place she could dry out, and also get the problem fixed.

What was frightening and dispiriting was why it had happened in the first place.  When I removed the prop, I found that it had been fitted very badly.  In fact, the fitting was criminally negligent, given the possible consequences.

Removing and inspecting the propellor for internal corrosion (it's a J-Prop) was one of the things required by the surveyor who looked at the boat when I bought it.  It was one of a list of things which were divided up between myself and the previous owner, and was to be for his account.

He had asked a reputable boatyard for a quote for the whole list, but was concerned that the prices seemed very high, and asked me whether I would mind him getting alternative quotes for some pieces of work.  I said I had no objection, so long as he used reputable contractors.

As it turns out, I was a good deal too trusting.  Perhaps he was as well.  I have written to him about the incident, however, and also sent some reminders.  I have not yet had a reply.

Here is how the propellor was replaced:

Instead of lapping the prop to the shaft withe grinding paste, the fitter marked the shaft taper with some hard object - perhaps a hammer - to try to get the prop to engage with it.  This is the result:

Then, instead of fitting a proper key, the fitter used what I think was a old brass bolt, filed roughly square, to fit in the key slot.  The tin had corroded out of this, leaving a copper sponge which finally failed.  This is what I removed from the keyways in the prop shaft and the propellor:

A strange aspect of this procedure is that it must have taken at least as long as doing the job properly.

The engineer who helped me out in Stornoway was Tony Morrison, of Malcolm Mackenzie & Co.  He visted the boat on a number of occasions over the day or two I was working on it, made a new bronze key, helped me to fit the prop, and drilled and re-tapped part of the pitch adjusting mechanism when a sheared stainless steel bolt nearly broke my heart during the final stages of reassembly ...

I re-lapped the shaft as well as I could, with his help.  We managed to get a good 75% contact, but the real solution will involve removing it and re-making the taper on a lathe.  As well as lapping and a new key, a lot of loctite was applied ...

I have asked the owner who did the work.  I will also check through my whole e-mail correspondence with him to find out if I already have this information - I haven't found it so far.  Once I can confirm this, I will publish it here.

I can think of a number of occasions since I've had the boat when this failure could have been catastrophic, and could easily have led to loss of life.  If the person who did this work has been doing work of this standard for any length of time, it seems highly likely that it has already caused fatalities.

Even if it had failed without causing immediate danger, it might have been very expensive and time-consuming to fix.  Rum harbour would have been tricky, for instance.  Or when I was anchored at the top of Loch Shell, with no phone signal, out of range of the Coastguard VHF antennae, and out of sight of any road ...

I was extremely lucky that this happened in Stornoway harbour, in calm conditions, and within reach of help and expertise.  My most sincere thanks to all those who helped me out.

No comments:

Post a Comment