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Old 24-11-2013, 10:10   #1
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Do Aluminum masts have a "working life"?

I'm in the process of buying a used mast for my Pearson Triton. Long story, I think I've covered it before, but the original mast has a pretty big repair and the rigger is advising me to replace it.

I located one that appears to be in pretty good shape and I'm going to have it trucked here. Total cost for the mast + shipping looks like $575. So much cheaper than a new spar I think. The rigger advised me to go for it.

Anyway, I'm just wondering, do Aluminum spars "go bad" or have a working life if they are not damaged? This is a spar that's about the same age as mine...i.e. 50 years or so. I'm not aware of people just replacing their masts because they are old. But mine would not be being replaced it if had not been damaged in the first place.

Thanks
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Old 24-11-2013, 10:42   #2
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Re: Do Aluminum masts have a "working life"?

Plenty of fifty -sixty year old masts out there doing fine .
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Old 24-11-2013, 10:56   #3
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Re: Do Aluminum masts have a "working life"?

In general, alloy masts become unusable because of corrosion. Corrosion can be controlled by proper maintenance during the mast's life. So, if the mast has had good care, it will be fine. Fortunately, this sort of damage is visible to the eye and an inspection will reveal its usefulness.

good luck, mate

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Old 24-11-2013, 12:00   #4
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Re: Do Aluminum masts have a "working life"?

Sully,

In a technical sense everything made from aluminum does have a fatigue life, so yes a mast will eventually wear out.

In a practice I have never heard of this happening to a mast. The number of cycles (loading and unloading) is dependent on the strength of the mast relative to the loads. For a mast I would expect the fatigue life to be measured in the billions of cycles, which just doesn't occur on a boat.
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Old 24-11-2013, 12:13   #5
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Re: Do Aluminum masts have a "working life"?

I have seen cracks in a mast. I would call that fatigue caused by localized stress. Check the inside of the mast for corrosion.
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Old 24-11-2013, 12:22   #6
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Not being a rigger or even a sailor to be honest I can't be sure, Masts are in compression and not extension? I ask because metal fatigue is really only present in materials subjected to extension, not compression and if a mast is like I suspect mostly in compression then it should have an almost infinite fatigue life. Meaning of course like it has been said that damage and corrosion will determine useful life.
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Old 28-11-2013, 07:53   #7
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Re: Do Aluminum masts have a "working life"?

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Originally Posted by a64pilot View Post
Not being a rigger or even a sailor to be honest I can't be sure, Masts are in compression and not extension? I ask because metal fatigue is really only present in materials subjected to extension, not compression and if a mast is like I suspect mostly in compression then it should have an almost infinite fatigue life. Meaning of course like it has been said that damage and corrosion will determine useful life.
One side of the mast will be in compression while the other side is in extension. On the other tack, the two will be reversed. The cycles repeat every time the mast is flexed, so every wave that hits the boat affects the mast at least a little bit. (Try being at the masthead when a launch goes by...) As said above, masts tend to be over-engineered, wearing out only after billions of cycles. Failures due to corrosion or other issues like a broken shroud are something else.
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Old 28-11-2013, 08:00   #8
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Quote:
Originally Posted by psk125 View Post
One side of the mast will be in compression while the other side is in extension. On the other tack, the two will be reversed. The cycles repeat every time the mast is flexed, so every wave that hits the boat affects the mast at least a little bit. (Try being at the masthead when a launch goes by...) As said above, masts tend to be over-engineered, wearing out only after billions of cycles. Failures due to corrosion or other issues like a broken shroud are something else.
Your right of course, I had not though out the fact of course that a mast is holow
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Old 28-11-2013, 18:48   #9
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Re: Do Aluminum masts have a "working life"?

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Your right of course, I had not though out the fact of course that a mast is holow
Doesn't matter if the mast is hollow or not. One side is in compression and the other is in extension even if it's a solid mast.

This discussion makes me think that the poster in another thread who's eager to replace his rod rigging every six years should probably replace his mast, too. This thread seems a lot more realistic about the longevity of sailboat spars & rigging.
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Old 29-11-2013, 06:42   #10
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I had thought it was the rigging that would keep the mast in compression, apparently not.
I'm trying to learn this sailing thing, my thoughts about fatigue came from aircraft, where the lower wing spar is in extension, the upper in compression. The lower spar cap is subject to fatigue and usually has a replacement interval, the upper will last until it corrodes. In an attempt to continue the aircraft analogy, externally braced wings, (wires or struts) wings don't usually fatigue, the wires or struts carry the load and therefore they fatigue. Of course a one time extreme overload can and will fail a wing regardless of age or fatigue, exceed the design limit, and your a test pilot
I thought a mast on a sailboat may be similar, I guess not, I know it has to vary widely as I'm sure some masts are braced better than others and not all rigging is kept properly tensioned, but it there a suggested replacement interval for a mast? I assume most are never replaced until they fail, but mast failures not due to extreme overload events are exceedingly rare? Or are they like rudder failures, as in there are a lot more than I would have thought?
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Old 29-11-2013, 10:11   #11
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Re: Do Aluminum masts have a "working life"?

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Originally Posted by a64pilot View Post
I had thought it was the rigging that would keep the mast in compression, apparently not.
I'm trying to learn this sailing thing, my thoughts about fatigue came from aircraft, where the lower wing spar is in extension, the upper in compression. The lower spar cap is subject to fatigue and usually has a replacement interval, the upper will last until it corrodes. In an attempt to continue the aircraft analogy, externally braced wings, (wires or struts) wings don't usually fatigue, the wires or struts carry the load and therefore they fatigue. Of course a one time extreme overload can and will fail a wing regardless of age or fatigue, exceed the design limit, and your a test pilot
I thought a mast on a sailboat may be similar, I guess not, I know it has to vary widely as I'm sure some masts are braced better than others and not all rigging is kept properly tensioned, but it there a suggested replacement interval for a mast? I assume most are never replaced until they fail, but mast failures not due to extreme overload events are exceedingly rare? Or are they like rudder failures, as in there are a lot more than I would have thought?
For racing some mast designs really push the envelope for a light weigh mast. There are failures, but usually from exceeding the design loading, not from fatigue. I have a book on Rigs and Rigging by Richard Henderson and though I would give an easy answer to your question, but not possible because the stresses are just about impossible to determine, what with everything that a sailboat is put through. However, there are some formulas that help, some of them determined by experience.
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Old 29-11-2013, 11:07   #12
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I've gotten from a few that the opinion is that a cruising boat fixing to undertake a long voyage should most probably replace the standing rigging if it's ten years old. I know that a rough average, there are some boats I'm sure with 20 year old rigging in excellent condition, and some at 2 years old may have considerable fatigue. But it's a figure anyway.
Just wondered if masts had a recommended replacement interval, or is it usually corrosion that forces replacement prior to anything else
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Old 29-11-2013, 11:17   #13
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Re: Do Aluminum masts have a "working life"?

The common thing I see is all the stainless fasteners will eventually corrode the surrounding aluminum to the point that any machined threads fail and the bolts/screws will have questionable hold and are subject to sudden failure with shock loads. Its such a bad idea to connect stainless to aluminum in the salt environment makes ya wonder why more steps arent taken to not do this;
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Old 29-11-2013, 11:52   #14
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Re: Do Aluminum masts have a "working life"?

A64,

Take a look at the S/N curve for 6064-t6 aluminium which is a very common mast material.

Stress amplitude,σa = (σmax − σmin )/ 2 (MPa)
So σa for a mast looks something like

(1,000-500)/2*360 = 500/720 = .7

When you put this on the S/N graph at around 1x10^10 fatigue cycles. The exact same calculation is made for aircraft wings, but there the cycle loads are much higher, the number of cycles are higher, and the wings are inspected frequently by experiences people. Plus every ounce of weight results in large additional fuel burn over the lifespan of the aircraft, so it's cheaper to fix damage than it is to never break over the lifespan of the part.



Stainless rigging is a completely different issue, since the lifespan is dictated by localized corrosion, not by fatigue.

Masts don't really have a lifespan. Long distance racers may be an exception, but they break their masts regularly for other reasons anyway, typically because of sail handeling failures (running backs are a b*#%)
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Old 29-11-2013, 12:00   #15
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Re: Do Aluminum masts have a "working life"?

There is a reason mast extrusions have bending moment data in 2 directions X and Y ..... In lay man's terms, we'll call it "resistance to bending - or stiffness" Rigs are moving (bending) all of the time under sail, although for the majority imperceptibly ... Racers and cruisers alike bend sticks on purpose all of the time with a backstay tensioner....

Fatigue life of a spar is a non issue if it has been designed to be very low in the working load range of the application... Fortunately gut feeling made picking the right size tree more common than not! (i.e. "Yikes Timmy... That mast looks scary slim")

The place where people get in trouble is "modifying" a mast... A hole is the place where a stick is going to be coming apart if it does....
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