Can a crack at the foot of a mast be fixed?

[Lon nan Gruagach]

I would be inclined to suggest OP buys the boat if he likes the deal and either ignores the crack or pop rivets a piece of alloy around the base and over the crack. As said it probably was caused by corrosion of a alloy plug in the bottom of the mast. This probably can not be removed and would require you cut off the mast to the depth of the plug. The corrosion in itself will not cause a big problem being surface corrosion in the faying surface between the mast and plug.
This part of the mast is only subjected to crushing forces (trying to shorten the mast) from pull of the stays. Rather than fail completely it will slowly crumple causing loss of rig tension and obvious folding out of the metal.
I foolishly mounted turning blocks with cut outs near the base of my mast all in one level near the bottom. With age and corrosion the remaining metal did crumple by a centimetre or so. The fix was to weld on additional sleeve outside. Certainly not catastrophic failure. olewill

Not so, not by a long chalk.
Centre of action of a sail is about 1/3 the way up from the boom, so 2/3rds of the drive from the mainsail is at the boom. 1/3rd of that is on the sheet, 2/3rds on the mast. Over all, somewhere in the region of 1/4 of your drive is transmitted to the hull at the mast step.

 

[PuffTheMagicDragon]

Not so, not by a long chalk.
Centre of action of a sail is about 1/3 the way up from the boom, so 2/3rds of the drive from the mainsail is at the boom. 1/3rd of that is on the sheet, 2/3rds on the mast. Over all, somewhere in the region of 1/4 of your drive is transmitted to the hull at the mast step.

What you're saying is that the weight of a truss bridge... like this one

... and whatever passes over is distributed to the various intersections of the structure and, therefore, very little is transmitted to the end supports on the banks...

Interesting...

 

[Fr J Hackett]

What you're saying is that the weight of a truss bridge... like this one

... and whatever passes over is distributed to the various intersections of the structure and, therefore, very little is transmitted to the end supports on the banks...

Interesting...

I have a vague recollection of an old physics examination question about resolving forces in such a structure, it could have been 'O' or 'A' level my memory is not what it used to be.

 

[ghostlymoron]

I had a crack in my boom which I fixed by hacksawing off a couple of inches and re-rivetting the gooseneck fitting. On the mast, as someone said, to do the same you'd need to shorten the rigging unless there's enough adjustment on the bottle screws.
I put the crack down to age and fatigue.

 

[vyv_cox]

I would be very reluctant to weld up a crack in the foot of a mast. Presumably the crack has developed as the result of some stress that was not predicted initially. A weld will still be a stress raiser, so unless the area is strengthened it is highly likely that the crack will form again. Some advice already given would be the way I would progress: drill a small hole at the crack tip and have a strengthening collar built around the area. I would say it is not strictly necessary to weld this collar in place, as the vast majority of masts are built and extended by pop-riveting. However, I suggest it would be advisable to have it done by somebody familiar with this type of repair.

 

[dom]

I would progress: drill a small hole at the crack tip and have a strengthening collar built around the area. I would say it is not strictly necessary to weld this collar in place, as the vast majority of masts are built and extended by pop-riveting. However, I suggest it would be advisable to have it done by somebody familiar with this type of repair.

+1 Many boats have doubler plates at the spreader roots (I have), boom reinforcements, etc., especially racing ones. Same for aircraft. It's straight forward enough but does require templating and pressing of the appropriate reinforcement plate and doubt that's a DIY job for most.

 

[Lon nan Gruagach]

What you're saying is that the weight of a truss bridge... like this one

... and whatever passes over is distributed to the various intersections of the structure and, therefore, very little is transmitted to the end supports on the banks...

Interesting...

Err, NO, that is exactly not what I am saying, it was what William_H was saying.
My thumb in the wind guess is that all of the foresail drive is transmitted through backstay and jib sheet. Half of the mainsail drive is via the mast step, the rest split between backstay and main sheet.

 

[ex-Gladys]

What you're saying is that the weight of a truss bridge... like this one

... and whatever passes over is distributed to the various intersections of the structure and, therefore, very little is transmitted to the end supports on the banks...

Interesting...

So what supports the weight of the bridge and the load crossing it? Sky Hooks? The total weight of the bridge and load HAVE to be supported in their entirety by the end supports....

 

[Champagne Murphy]

Back to the OP; why chance it, find out the cost of a new mast etc and take that off the price. They might end up owing you!

 

[dom]

Err, NO, that is exactly not what I am saying, it was what William_H was saying.
My thumb in the wind guess is that all of the foresail drive is transmitted through backstay and jib sheet. Half of the mainsail drive is via the mast step, the rest split between backstay and main sheet.

On an aft swept spreader rig, all of the diagonals (Ds) are pulling the mast back at various points, terminating at the chain plates which take an enormous amount of the forward strain. Add in the mainsheet and backstay (depending on tension) and I'd personally struggle to see the foot taking anything close to 50% of the pressure.

Unless I've got the wrong end of this stick

 

[Rikds]

I have recently refitted a mast to a 1906 Fred Shepherd cruising yacht. It had a galvanised steel band around the mast foot just above the tenon. The band fitted tightly and sat on the mast step. Perhaps originally it would have been bronze but she was re-rigged in 1927 as a Bermudan sloop and a new mast made (originally gaff rigged).

 

[PuffTheMagicDragon]

So what supports the weight of the bridge and the load crossing it? Sky Hooks? The total weight of the bridge and load HAVE to be supported in their entirety by the end supports....

That is precisely my point!
All of the wind forces on the sail are trying their best to topple the mast sideways. The only thing that keeps the mast up is the rigging, as long as the mast does not collapse. The head of the mast isn't going anywhere unless the shrouds part and so the mast is pushed down onto its heel. That is why designers try to beef up the foot area as much as is possible; the forces there are substantial and nowhere near 'somewhere in the region of 1/4'.

 

[Spirit (of Glenans)]

So what supports the weight of the bridge and the load crossing it? Sky Hooks? The total weight of the bridge and load HAVE to be supported in their entirety by the end supports....

Said structure is only required to stay anchored to its supports at each end. It is not required to propel them about, on the surface of the water.

 

[agurney]

My boat had a couple of vertical cracks at the base of the mast that were found by the surveyor prior to purchase.
The price was re-negotiated based on the surveyor's repair estimate - however, I simply cut about 20mm off the foot of the mast, and stepped it on a block of oak (or beech) so that I wouldn't have to adjust the rigging.
That was more than 30 years ago. No 'repair', and no inclination to sleeve it. No regrets.

 

[William_H]

Not so, not by a long chalk.
Centre of action of a sail is about 1/3 the way up from the boom, so 2/3rds of the drive from the mainsail is at the boom. 1/3rd of that is on the sheet, 2/3rds on the mast. Over all, somewhere in the region of 1/4 of your drive is transmitted to the hull at the mast step.

Yes I understand what you are saying. Yes I would agree there is some thrust in the forward direction at the base of the mast. But just how much? If 1/4 of the boats drive forward is transmitted via the base it is not much force compared to the downward thrust. Certainly not in a way that would cause a vertical crack to appear be propagated or be a problem. olewill

 

[Quandary]

My boat had a couple of vertical cracks at the base of the mast that were found by the surveyor prior to purchase.
The price was re-negotiated based on the surveyor's repair estimate - however, I simply cut about 20mm off the foot of the mast, and stepped it on a block of oak (or beech) so that I wouldn't have to adjust the rigging.
That was more than 30 years ago. No 'repair', and no inclination to sleeve it. No regrets.

Nice simple straightforward and practical solution
Tempted to enter the debate on structural mechanics but when theories are so remote from the problem it would just lead to frustration.

 

[fergie_mac66]

you could "braze" it . ally brazing rods are available . Mapp gas give enough temp to do it .

 

[vyv_cox]

you could "braze" it . ally brazing rods are available . Mapp gas give enough temp to do it .

Cosmetic solution. It has cracked for a reason.

 

[ghostlymoron]

It could have cracked due to age and fatigue like my boom. There was never a repeat of the problem (in my ownership).

 

[Poignard]

In my case the cause was corrosion. The foot of the deck-mounted mast was closed-off with a cast aluminium bung and there was no effective means of drainage so water had collected there and the resultant corrosion and swelling split the mast at its weakest points: where the rivets holding the bung in place were.