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samgary 12-12-2011 02:57 AM

repairing cracked mast with welding
 
3 Attachment(s)
Hello everyone,

About a week ago i posted regarding my mast which is cracked above the second spreaders ( and about 15 inches above the running back tangs). I came to the consensus, with advice from others including a rigger who i trust, that the cracks should be ground out, through welded and a plate welded on over the sheave box area. The plate itself ( as of now ) is an elongated diamond ( about 15 " total) with a rectangular mid section (about 3") which will be centered over the sheave box housing. The plate covers both the cracks, which spread aft about two inches on either side from the bottom two screw holes of the sheave box ( see pictures).

Before i have this plate welded on for good i would like a second opinion. The mast is a proctor, which is 47' and only 6"x4" and has a good deal of flex. Even with running backs secured, there is flex to some degree through the entire mast, and especially above the running backs while sailing hard to weather.

The mast is ~ 1/8" thick, and the plate is a good 1/16 thicker than the mast walls. I fabricated the plate out of a broken spar.

Will there be any structural issues with having the stronger and thicker plate welded over the more flexible thinner mast walls?

Would it be better to have the same thickness of aluminum for the plate?

Any other issues/advice?

Thanks in advance for the feedback and advice!

INMA 12-12-2011 04:57 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.

pdqaltair 12-12-2011 10:00 AM

Quote:

Originally Posted by samgary (Post 805597)
Hello everyone,

The mast is ~ 1/8" thick, and the plate is a good 1/16 thicker than the mast walls. I fabricated the plate out of a broken spar.

Will there be any structural issues with having the stronger and thicker plate welded over the more flexible thinner mast walls?

Would it be better to have the same thickness of aluminum for the plate?


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.

kd3pc 12-12-2011 10:49 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

svHyLyte 12-12-2011 11:02 AM

Quote:

Originally Posted by samgary (Post 805597)
Hello everyone,

About a week ago i posted regarding my mast which is cracked above the second spreaders ( and about 15 inches above the running back tangs). I came to the consensus, with advice from others including a rigger who i trust, that the cracks should be ground out, through welded and a plate welded on over the sheave box area. The plate itself ( as of now ) is an elongated diamond ( about 15 " total) with a rectangular mid section (about 3") which will be centered over the sheave box housing. The plate covers both the cracks, which spread aft about two inches on either side from the bottom two screw holes of the sheave box ( see pictures).

Before i have this plate welded on for good i would like a second opinion. The mast is a proctor, which is 47' and only 6"x4" and has a good deal of flex. Even with running backs secured, there is flex to some degree through the entire mast, and especially above the running backs while sailing hard to weather.

The mast is ~ 1/8" thick, and the plate is a good 1/16 thicker than the mast walls. I fabricated the plate out of a broken spar.

Will there be any structural issues with having the stronger and thicker plate welded over the more flexible thinner mast walls?

Would it be better to have the same thickness of aluminum for the plate?

Any other issues/advice?

Thanks in advance for the feedback and advice!

Once the cracks have been welded up as discussed in your earlier Thread, the fastening of the mounting plate for your topping lift won't make much difference so long as you remember to brace your rig with the running backs to keep the mast in column when you sail. (Not only set-up but well tensioned!) If so, the entire mast section will remain in column and in compression. As previously discussed, the cracking was likely caused by tension on the forward face of the mast due to bowing or buckling and would likely not have occurred save for the weak spots created by the corrosion around the original mounting fasteners. By the size of the plate you describe, I suspect you remain somewhat unsure about the prescribed repair to the spar--whether consciously or unconsciously--and so are leaning on the over-large plate in a kind of belt and suspenders manner. Can't hurt however. (If accurate, the mast section you describe--4" x 6"--seems somewhat light--bendy--for that length of spar. Ensure you have your rig well aligned and tuned.)

FWIW...

shogan50 12-12-2011 12:30 PM

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.

svHyLyte 12-12-2011 06:54 PM

Quote:

Originally Posted by shogan50 (Post 805689)
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.

While the foregoing is undoubtedly correct if one were talking about a full splice weld, the OP is looking at a TIG Weld repair approximately 2" long on either side of the original slot in the face of the mast that will be covered with a stiffening plate (for the reinstalled topping lift sheave box) that should function pretty much like the splice plates in a typical mast with puddle welds. If that were not the case then Spartech/Charleston Spar, Isomat and a number of other spar builders, and particularly those manufacturing welded-tapered spars, are going to be awfully surprised, No?

(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.)

Ulladh 12-12-2011 07:20 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?

samgary 12-12-2011 07:44 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.

Thanks again

jfdubu 12-12-2011 07:47 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.

John


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