Originally Posted by msponer
I am guessing that the box inside is the edge of a plate that pokes up from part of the bulbs internal structure. I do not think it is an extrusion, since it seems like it'd be part of the structure that fastens the bulb to the fin, and just doesn't look like an extrusion. But of course I have a limited knowledge of metal fabrication and boat construction and defer to someone (you!) whose actually made a boat. If it is a plate I am guessing it's the same marine
grade alloy as the outside of the keel
Yes, I agree completely, I think it's border line retarded that they welded the backing plate to the keel
and then drilled and tapped through both. What is even the point of the backing plate, then? Boat yards....
Why magnesium? I read a little about it (in the book mentioned above), and I believe the widely accepted thought is to always use zinc. But I do not know the details well enough to talk about it intelligently or do more than just parrot back the conclusion. I'll pay attention to that part when I read the book again.
This is a hydraulic lift
keel, so I can't use it as a fuel tank
... But isn't a problem with filling it with oil
that it would make welding on the keel later potentially explosive? I have limited knowledge, but I learned to weld from a guy that works on natural gas and oil
pipelines. He said that before they could weld on a pipe that had oil in it they had to work pretty hard to clean the inside surfaces to make sure any trace of oil was gone.
Ok unless the strong-back/stem-bar inside has been cut from thick plate it will be a piece of extruded flat-bar, from the piccies it very much looks like it, thats no real problem it's just there are more than one alloy in there.
It's my preference to use aluminium anodes covering the salinity range (fresh and salt), the most important thing is that they are attached correctly and that there are not too many.
My point of the epoxy
resin over coat is that it seals
pinholes as well and i think this is what got you down this line in the first place.
Yes your friend is correct regarding any oils and welding, however dealing with essentially a fuel tank
is not a huge issue you just need to gas free it properly and monitor
any regeneration of gases whilst welding.
The problem has only come about due to moisture entering and ill bet my last shackle pin that it entered via the bolt on anodes??? Fix the leak and you have no issue!
The best idea is to keep it dry essentially you have a battery
down there and you need to remove the electrolite.
EASIEST OPTION? Flush it out totally using a bio-degradable truck wash, dry it completely using some warmth externally as well.
Repair and get some NDT done checking for leaks
, (in the past i would cap such a keel,incorporate some bungs and pressure test it to 3lbs air pressure.)
once confident put a litre of oil in there to slosh around. If and when it needs future welding repair you can remove the oil by introducing truck wash into the keel and draining 3or4 times. Not a huge effort to do that.
Totally dry is BEST....Cheers Frank