Tuesday, April 10, 2012

Archimedes to the rescue

It is a truism that in DiY, you get a job right the second time you do it. Last autumn I wrote, somewhat smugly I expect,  about how easy it was to repack Herbie’s stern gland.  Well, it was, but I obviously didn’t do it too well because it has been dripping too much, so I decided to have another go.  At least I have lost my fear of water gushing in during the process, because it doesn’t.

I think the reason it was still leaking was that I didn’t cut the lengths of packing sharply enough, leaving frayed ends through which the water can seep. So this time I decided to adopt the alternative Tony Brooks method.  This I think is what Tony does himself, but as it is unorthodox he reminds us that it is “at your own risk”.  The idea is to use a single long length of packing wound helically round the shaft rather than the conventional three or four separate cut rings.  This eliminates the cut joints, but you have to do it right because the helix of packing rope forms an archimedean screw pump as the shaft turns.  Wind it on the wrong way and it could pump water into the boat rather than out.  Of course you might get some leakage when in reverse, but that isn’t usually something you do for more than a minute or two.

Anyway, that’s what I did and it does seem a lot better although I expect I’ll have to adjust up some slack after it beds in.

1 comment:

Rainman said...

If you trace the 135m contour line from the crick tunnel entrances on the OS map it makes a (very ragged) figure of eight on its side with an area of about 50 sq km. The land rises to 180m on the east and 160m on the west. Whichever way the rock strata slope, I assume rain will be running along the joints and be intercepted by the airshafts and tunnel itself.