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|12-12-2011 09:13 PM|
Originally Posted by jfdubu View Post
|12-12-2011 08:47 PM|
Ok, My two cents, My guess is that this is the box section at the front of the mast where the spin halyard sheeve mounts. My second guess is that the crack is a fatique result of cranking the backstay.
The fix, first you must drill stop holes at the base of the crack. If you can't find the base drill the past where you think it is and cut the crack to the stop hole. If you don't, the crack will continue to propagate. After drilling and cutting /cleaning the crack, tig weld it up.
I would then fab plate to pick -up the entire area above and below the box, an inch or so on either side and weld it on. If there is concern about the weld temper, rivot it on.
In order to fix this you need to understand where the stress that caused the crack comes from and either eliminated it, spread it on a larger area, or make the part strong enough to handle it.
|12-12-2011 08:44 PM|
Thanks everyone for the feedback,
I have contacted a few welding shops who claim to be competent. I know very little about welding, what should I ask them to reduce my uncertainty, and increase my odds of getting a welder who knows what he is doing. All claim to have welded on masts before.
I have fabricated the plate, and plan on showing them.
|12-12-2011 08:20 PM|
Looks like with a bit more mast flexing the crack may travel and you will lose the mast.
i would suggest taking the mast to a welding shop with marine experience, cut at the corroded section and insert a internal sleeve, at least 1ft above and below the cut.
Do you need the open slot?
|12-12-2011 07:54 PM|
Originally Posted by shogan50 View Post
(While it may or may not be particularly relevant, I am a licensed Structural Engineer--in multiple venues--with 37 years of experience and we "ain't" talking rocket science here.)
|12-12-2011 01:30 PM|
|shogan50||As a licensed mechanical engineer with a little experience in welded aluminum structures, this sounds risky if you aren't having a professional at least evaluate your methodology. Welded aluminum is typically 1/4 as strong as the material you are welding to. This is because your mast gets most of its strength from heat treating, which is destroyed (annealed) around the weld. I don't know what alloy your mast is, but a common industry alloy for that type of thing is 6061-T6, where the T is the temper. It has a yield strength of 35Ksi. When you weld it, it is closer to the T0 condition at 8ksi. Successful repairs can be made despite this, but the repair plate and weld geometry needs to be considered. When designing welded aluminum structures, there is typically a 1" wide zone around the weld that is considered to be T0 condition.|
|12-12-2011 12:02 PM|
Originally Posted by samgary View Post
|12-12-2011 11:49 AM|
the cracks, their direction and the oxidation on the mast would make me change my idea, to a new mast...
the plate will provide a temporary fix and all but guarantee that the mast will break above the patch.
I would suggest stopper holes drilled at the end of the crack, but without sonic/xray, you will not KNOW where the end is...grinding out and welding the crack will also provide some temporary peace of mind, but given the pics....
new mast or at least a complete unstep, inspection, sonic and then see....this is a stress/flex point you already know.
I think it is too far along to safely repair for any length of time, of course YMMV
|12-12-2011 11:00 AM|
Originally Posted by samgary View Post
I do know that all repair standards for tanks and presure vessels require that the plate be the same thickness as the base material. In some odd cases where this is not possible (reinforcement plates), then the patch plate is ground to match at a 6:1 taper. That would be easy to do. I would be at least 10:1, since aluminim is more crack prone than low carbon steel.
Also it is very important to round all of the corners of the plate. I'm guessing a 1-inch radius should be the minimum; for large tanks the minimum radius is 6 inches, but that's not to practical here, and that is for 1/4- to 1-inch plate.
|12-12-2011 05:57 AM|
Those cracks will keep growing unless something is done to stop them and repair the areas.
The patch is a good idea but the position and shape of the welds will need to be done in a way to avoid stress concentration under fatigue loads.
In addition make sure the area of repair is able to be inspected for future cracks, particularly from the new welds.
I have not designed welding for rigs. In other work in steel we used plug welds to avoid the problems of fatigue damage at stitch welds.
The rigger should be better able to advise on a proper plate. Don't worry about the cost of a proper repair, it will be a lot less than a new mast.
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