Achilles 24
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Achilles 24 |
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Dropping the keel Summer 2004.
Part 5
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We lowered the boat onto the studs and did a dry run by dropping the boat onto the keel. Everything looked ok, so we lifted her enough to get the sealer on.
Sealer squeezed out when we lowered the boat again and this was quickly scraped off before the nuts were put on the studs and tightened, making a second 'squeeze'.
The nuts were gradually fully tightened over the next few days, a half turn at a time. I had been told that Butler Mouldings used a two-part Polysulphide sealer to bed the Achilles keels. The only Polysulphide I could find was one part Boatlife Lifecalk, which is flexible, can be sanded to a smooth finish, does not have strong adhesive properties and is slow to cure. Modern sealers like Sikaflex are Polysulphates which are faster curing but have very strong adhesion. If a Polysulphide was good enough for Butler Mouldings it was good enough for me, and I had been warned by the surveyor that if I used Sikaflex I would never get the keel off again. That may be ok for seasoned keel droppers but in the event of a cock up at my first attempt I needed to know I could get the keel off again for another go. So I went for the Lifecalk which cured slowly but was easily faired. Then a lick of antifouling and job done!
A few thoughts..... There was rust in the area around the aft keel bolt which was anticipated from the leak, but the unexpected rust was where the water must have been seeping aft from the leak along the keel join and so behind the fairing filler at the keels edge. I think that this was the seat of the moisture that caused the bubbles in the filler. It is impossible to say when the original leak and resulting rust occured but it looked like it had been corroding for some time and there was the distinctive whiff of iron rusting is salt water - if you have ever smelled it you will know what I mean. The good news is that the remainder of the keel was perfect under the skin of old sealer and easily cleaned to a polished finish. Also the keel bolts were all in excellent nick after 29 years and could have been re-used although the nuts were solidly rusted to the studs. Only the aft keelbolt has the micoballoon block for supporting the grp. The other three bolted down onto what seemed to be solid grp which at the midship pair of bolts was about 3 inches (75mm) thick. The aft one (left) was new in 2003 but you can see there is rust on the bottom of the thread. This is because it had been taken out a couple of weeks before the pic was taken as part of a vain attempt to vent the pressure that was blowing bubbles in the keel filler.
I did a bit of internet research and found that the Ultimate Tensile Strength of mild steel is 470 Newtons per square Millemetre with one Newton equalling 0.22 pounds of force. This calculates to a 1 inch mild steel bar being able to take a load of 23 tonnes. Of course the keel studs are threaded which reduces the cross sectional area and the threading may reduce the strength further. If we allow a reduction of 25% this still gives a theoretical ability for the four keelbolts to take a load of around 60 tons, and this to hold a keel of just over half a ton. This is static load only and a boat pitching in a seaway is bound to increase that load although I couldn't begin to work out by how much. Would it be approaching 60 tons? I very much doubt it. I also came across something called Nevin's Rule which states that you need 1 square inch of bolt for every 1500lbs of ballast, so the Achilles 24 is well in excess of that. Please bear in mind I am not a surveyor, naval architect or mathematician so the above is only a guide - if it is correct! With the class getting older the question of keelbolt
condition is quite rightly coming up, but if the experience with the
condition of my 29 year old bolts is anything to go by - I am fairly
confident they are the originals - it suggests that there is not a lot
to worry about.
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