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Old 23-10-2013, 20:27   #1
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Join Date: Sep 2008
Location: Adelaide, South Australia
Boat: Swanson 42
Posts: 3,585
One way of measuring a stern tube diameter

Hello Cruisers,

I'm posting this idea here just in case someone runs into the same problems I had measuring up for a new dripless shaft seal. I am avoiding using abreviations for critical bits (the meta data) in the hope that anyone searching will find this thread and it might help them out. Someone might already have posted a similar idea, I just could not find it.

Of the two dimensions needed, the shaft diameter was easily obtained with a set of calipers (OK, yeah, I tried to measure with a shifting spanner first, my bad, I won't be that lazy that again.)

The stern tube was REALLY difficult though. First, the boat is in the water, so I didn't really want to pull the seal off the stern tube to take the measurement. Aside from the water everywhere, I could not be confident of getting it back on again as it of an unknown age and therefore not very trustworthy.

Second, access to my sterntube is a bit of a nightmare. It is a long way down in the keel, nearly two feet below the level of the cabin sole, and there was simply no room to get the calipers close to the critical bit, regardless of the fact that the bit I needed to measure was also covered by the dripless seal and fenced off by the bolts holding the sterntube to the bulkhead.

One person suggested a piece of string wrapped around the tube tightly, to determine the diameter, but because of the access, and the old grease, I did not feel I could get a very accurate measurement that way. I may have been wrong, it just did not feel right.

So, I cut out a series of horseshoe shaped guages from a sheet of reasonable thickness tin, in incremental sizes from a size below what I estimated the diameter to be, to a size above. These were thin enough to slip between the face of the sterntube bracket and the edge of the dripless seal without disturbing the seal itself and the arms of the horseshoe were narrow enough to fit between the tube wall and the bolts holding the the sterntube flange in place.

Clearly these had to be cut out fairly accurately, and I had to make a call on whether my system was likely to be metric or imperial, pretty easy for an old Australian boat like mine, not so easy for something newer I suppose.

So, in my case I estimated it was a two inch stern tube (as I had a 1.5 inch shaft) so cut out a 1.75" (probably pointless, but I had to be sure), a 2.0" and a 2.25" horseshoe.

As it turned out, neither the 1.75" OR 2".0" would fit but the 2.25" fitted nice and snugly.

Now I have ordered my replacement PSS seal, and in a few days I will know if I was right.

(I will update this thread or hopefully remove it completely if I am wrong. What is the price paid to the mods for removing self incriminationg posts I wonder...?)

I hope this helps someone in the same predicament.

Matt
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Old 05-11-2013, 15:02   #2
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Re: One way of measuring a stern tube diameter

A quick update to this post to confirm that the technique worked and the newly ordered shaft seal fit perfectly. (A bit of a sense of relief to honest.)

Matt
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Old 05-11-2013, 15:38   #3
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Boat: Sayer 46' Solent rig sloop
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Re: One way of measuring a stern tube diameter

Well done, Matt,

When'll Manera be back in the water?

Ann
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Old 05-11-2013, 15:53   #4
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Join Date: Sep 2008
Location: Adelaide, South Australia
Boat: Swanson 42
Posts: 3,585
Re: One way of measuring a stern tube diameter

Back in the water now Ann, and just as Jim suggested, moving a bit more easily too.

8 days work, removed 14 skin fittings, glassed over all but 7, replaced the remaining 7 with new bronze fittings and seacocks, had to chop the drive shaft out with an angle grinder and have a new shaft made, but very nice to have a new unworn shaft with new cutlass and a shiny new PSS shaft seal. Drive train is much quieter.

I am still going down to check on the boat for leaks every 12 hours at the moment, as I just cannot relax about this sort of work. If Manera is still floating after a week of sailing I will sigh a sigh of relief, and get onto the next job.

Matt
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