Mast engineering stresses

[roaringgirl]

Hi All, I have a 40 yr old Selden mast on a 40ft blue-water cruiser. I recently pulled the mast out and it's lying beside the boat on the hard. We cracked one of the spreader brackets getting the spreaders off, so I had a rigger look at getting me another one. After looking at it, he suggested that I might want to consider a new mast. I now have a quote for that, and given the cost, I'm trying to make very sure that it is necessary before going ahead.

There is no physical damage to the mast section at any point as far as I can tell, no dings or dents. The main concern is galvanic corrosion - at the join between the 2 sections just above the spreaders and at the rivet holes where the un-insulated steps run up each side. Obviously if aluminium corrodes forever, all the metal turns to powder and the mast disintegrates, so I must try and assess how much material has been lost to corrosion and how much loss is acceptable. I'm currently trying to find some good rule-of thumb calculations for rig-loading and bending, which I could perhaps use in conjunction with the section's I-value, size and wall-thickness.

The two separate riggers who have looked at it so far have ummed an erred and (understandably) not wanted to give a firm decision either way about whether or not a new mast is required.

Assuming that I can get an endoscope inside the mast and have a good look around, can anyone give me some practical advice about what is ok and what is not ok? Can anyone give me any good practical sources of information for mast-load calculations?


Thanks

Charlie

 

[wombat88]

Hi All, I have a 40 yr old Selden mast on a 40ft blue-water cruiser. I recently pulled the mast out and it's lying beside the boat on the hard. We cracked one of the spreader brackets getting the spreaders off, so I had a rigger look at getting me another one. After looking at it, he suggested that I might want to consider a new mast. I now have a quote for that, and given the cost, I'm trying to make very sure that it is necessary before going ahead.

There is no physical damage to the mast section at any point as far as I can tell, no dings or dents. The main concern is galvanic corrosion - at the join between the 2 sections just above the spreaders and at the rivet holes where the un-insulated steps run up each side. Obviously if aluminium corrodes forever, all the metal turns to powder and the mast disintegrates, so I must try and assess how much material has been lost to corrosion and how much loss is acceptable. I'm currently trying to find some good rule-of thumb calculations for rig-loading and bending, which I could perhaps use in conjunction with the section's I-value, size and wall-thickness.

The two separate riggers who have looked at it so far have ummed an erred and (understandably) not wanted to give a firm decision either way about whether or not a new mast is required.

Assuming that I can get an endoscope inside the mast and have a good look around, can anyone give me some practical advice about what is ok and what is not ok? Can anyone give me any good practical sources of information for mast-load calculations?


Thanks

Charlie

Not the same application but I have an old Merlin Rocket (1950s) with an original Proctor alloy mast that presumably was made in the first few years of metal mast production.

There is some debate as to whether it is anodised. The outside is scarred and there is evidence of the white oxidisation that happens with age (like a lamp post). I am told that this acts a sort of skin...hmm.

I asked some professional rigging 'experts' about the mast and their opinion was that the inside, being more or less weatherproof, would not show too much oxidisation and is likely to be OK. Not worth the bother of the endoscope. Indeed I removed the heel and had a peak as best I could. Some wildlife but nothing scary. Is yours full of foam?

Any stainless fittings look as if they have been isolated from new and the rivets etc are all alloy. I do NOT have stainless halyards that can rub against the mast and cause significant damage over time. I've replaced two stainless self tappers that had made a mess of things.

So in my case a new mast would probably cost twice the value of the boat and since I don't sail this boat out to sea I will carry on regardless and paddle home if it all falls apart.

Your blue water cruising makes for a very different requirement and peace of mind is part of it. I don't know if crack or gauge testing can be done on alloys masts but guess it would be £££s.

Are many older boats advertised for sale described as having a new mast? I haven't noticed.

I fear that no one will give a clear cut answer to your question...and I suspect peace of mind would become the decisive factor...but at one hell of a price!

Unhelpfully

Michael

 

[Birdseye]

There is no practical answer to your question. No one knows how many load cycles your mast has suffered over 40 years. No one can calculate what the effect on the mast strenght is cause by all the little holes that hold fitting and then there is the issue of corrosion as you point out. Whats more if you now go sailing and the mast falls down after you have been advised to replace it , the insurance company will rightly reckon you havent been prudent and have exposed them to risk they werent aware of.

For some reason we accept that boat engines have a useful life span and even that standing rigging does, but we expect masts to go on for ever. Life isnt like that.

 

[lw395]

My concern might be the sleeve inside the join being corroded.
A good TIG welder might sort the holes from the steps?

The mast is likely to have been over-specified originally.
I suppose you could estimate the % of weakening, then compare the I-values with a new mast for a similar boat?
Or you could talk to an alloy fabricator about rebuilding, probably with a new sleeve?
It might be possible to add an external sleeve?

 

[AntarcticPilot]

I'm no expert! But my concern wouldn't be for gross erosion of the wall thickness of the extrusion; that's probably not an issue. As others have said, aluminium is protected from that kind of corrosion by a layer of oxide which forms in contact with air, and the inside of the mast section is a more benign environment than the outside. What I would be concerned about would be galvanic corrosion round rivet holes, and/or cracks developing at stress concentrations such as rivets and the join between the sections. Without removing some rivets, that might be hard to quantify. However, drilling out a few rivets might provide a better idea of the amount of corrosion than an internal examination of the mast.

I'm sure others will be able to say more about the potential for fatigue in a mast section.

 

[sarabande]

the inside of the mast is likely to have the same sort of exposure to moisture, and even sea water, as the exterior. If you see degradation of joints and loadbearing holes for the steps on the outside, the same will happen internally.

A new mast will hit the wallet, but will give you peace of mind, as mast design, material, and fabrication standards are very much higher today. You may save some weight as well - which is always useful. Masthead and spreader lights wiring can be renewed at the same time - perhaps to LEDs.

Mainly, if you are an offshore sailor, it is about reduction of risk, and being able to say to the insurance company, "I look after the boat".

If you can afford it, do it. You feel much happier and enjoy the sailing more without mast worry.


Oh, what about the rigging, does that need renewing ?

EDIT
Oh2, if the mast is in reasonable condition, you should be able to sell it to someone whose operating conditions are not as demanding as yours.

 

[lw395]

the inside of the mast is likely to have the same sort of exposure to moisture, and even sea water, as the exterior. If you see degradation of joints and loadbearing holes for the steps on the outside, the same will happen internally.

A new mast will hit the wallet, but will give you peace of mind, as mast design, material, and fabrication standards are very much higher today. You may save some weight as well - which is always useful. Masthead and spreader lights wiring can be renewed at the same time - perhaps to LEDs.

Mainly, if you are an offshore sailor, it is about reduction of risk, and being able to say to the insurance company, "I look after the boat".

If you can afford it, do it. You feel much happier and enjoy the sailing more without mast worry.


Oh, what about the rigging, does that need renewing ?

All of these fine sentiments apply equally to most components of the boat of course.
e.g. Rudder, steering gear, winches, boom, keel bolts etc etc.
The joy of old boats.


Out of curiosity, roughly how much is a new mast for a 40ft boat?

 

[Martin_W_Brown]

I'm not a mast expect, but from a general engineering view the problem is crack propagation. Cracks start at stress concentrations such as holes or corrosion and get bigger every time the structure flexes. Once the crack is big enough the thing breaks.

Given enough load cycles Aluminium will always form cracks. The key thing is where they are and how big they are.

How about Dye Penetrant Testing? This would show up whether there are cracks radiating from any areas of concern - joints and holes. You and your rigger can then take a view on whether you replace or not. Kits are available for not too much money, particularly if you are only testing key areas.

 

[ibbo]

In one of the magazines they x rayed a mast repair to check it for strength. Is that feasible for the whole mast?

 

[npf1]

roaringgirl - whereabouts are you located?

 

[Concerto]

Recently I had a fitting pull out of my spinnaker pole. I believe it was original from 1981. The galvanic action between the rivets and tube was the cause. Luckily I have a very similar diameter section of stainless steel tube and will cut a massive washer to fit inside. This pole will never now be used at anywhere near the loads found under racing conditions, but I did enquire how much a new pole was. Not that expensive, so if it fails again I will replace it. Luckily my mast was replaced in 1996, so in far better condition than the pole.

My advice would be, change it if you are intending to travel any distance from the UK and sell the existing mast. If you are intending to remain in northern European waters, then get a dye penetrating kit and check around all the fittings. If no damage is found, then inform your insurer that the test has been completed and enjoy your sailing once the spreader has been fixed.

 

[doug748]

I had my doubts about some stainless pads on my mast and they had corroded right through when I had a look see. So I would 100% re bed and rivet those dodgy steps. If they are very bad underneath, it may help make your mind up.

If you do the same with all external fittings, tangs and brackets you will soon get a feeling for the overall condition of the spar.

Taking the joint apart may cause more damage than is acceptable but it might be considered, as could drilling out the old rivets and going a size up - if this is possible.

I would be inclined to keep the mast, it was probably a substantial thing when new, and my impression is that most mast failures are down to rigging.

 

[charles_reed]

Catastrohic mast failure is caused by major overload (shroud breakage) most commonly. The second most common cause is from fatigue failure due to cyclic action. Having experienced both (only one of which I had to pay for) I speak from bitter experience.

If you really want an informed opinion I'll ask my daughter if we can approach her colleague who's associated with the Wolfson Unit and pronounced so pithily on my mast section, now being used as an example of fatigue-failure for 2nd year engineering undergrads.

If yours is a masthead rig, it's very unlikely that it will fail due to local corrosion at a joint-splice. A bendy 3/4 rig is a different matter.
I'm not surprised the riggers weren't prepared to give an informed opinion, it's probably a situation they've never come across before.
Personally I'd keep an eye on it and not bother to try and correct the ravages of years - you could drill out and check some of the rivets - I find I have to regularly replace aluminium rivets on shock-loaded parts like goosenecks and gas-strut connections.

 

[PeterR]

I've never understood why Seldon produce masts by riveting sections together. I've never seen anyone else do it and its an obvious point of weakness - even if it does take 40 years to become problematic. One of my criteria when buying a yacht was to avoid anything with a Seldon mast for just that reason. Given that you will have double the mast thickness in the riveted area with the inner sleeve there is probably still adequate sound material to take compression loads but in tension (i.e. when bending) - who knows? As the previous post implies bending forces are more likely to be a problem in a fractional than a masthead rig. As an extreme example I also sail a laser dinghy and their masts are always failing at rivet points where corrosion has set in.

 

[roaringgirl]

Edinburgh

 

[vyv_cox]

Cutting holes in stressed components (to save weight) has been done for hundreds of years. The strength effect of rivet holes in a mast section is pretty much negligible. When the holes become enlarged due to corrosion it may pay to consider their location.

At rest a masthead rigged mast is universally in compression, in which case the effect of any holes is almost zero. Tightening the babystay may put the forward side in tension, although probably not a lot as the force is over-ridden by shroud tension. Under sail, laterally the mast is in compression with the upwind shroud in tension. Fore and aft the front face of the mast is almost certainly in tension, the remainder in compression. So I would be wary of any holes or major loss of material on the forward face, not too bothered about the rest. The area at about the centre of the mast height is under most tension, so be very careful there, probably about the height of the spreaders.

 

[npf1]

Edinburgh

Ah ... too far away for the person I was going to suggest.

 

[William_H]

Firstly X ray of a mast would be a waste of money. The preferred crack detection method woukld be dye penetrant although a far better methode is Eddy current detection. This is like a mineral detector in miniature that detects lack of electrical conductivity (in a crack) rather than electrical conductivity in minerals. An aircraft maintenance shop may be able to put you on to an expert. This method is used for detecting cracks in alloy wheel rims as an example. Very quick and easy. However general checking for cracks in a mast is more tedious. Dye penetrant method requires fairly smooth clean surface so may also be tedious. Mark one eyeball is still the primary crack detection method.
I find it surprising one poster refers to fatigue failure as a mast is mostly just in compression without much actual bending. Critical parts become the areas between support ie between the stays so for a cap shroud and single intermediate stays at 50% height the critical areas are 25% and 75% of height.
Masts get a lot of holes drilled in them for various things so crack propagation is not so critical.
If you calculate the area of the mast wall at any point. ie the thickness of the wall multiplied by circumeference of the mast walls then at any point where there is corrosion and loss of metal you can calculate loss of material as a percentage. Compare that to loss of material as a percentage where holes have been drilled. Obviously not as simple as that yet this will give you an idea of the significance of corrosion damage. I suspect that you will find there is not a real loss of material and you can go ahead and remove corrosed material stop the corrosion and reassemble the components.
It is hardly surporising a rigger will not stick his neck out to say it is OK. he could be liable if it fails and if you decide to replace he might get the job. So he offers a very conservative "Rolls Royce" service hang the price.
Of course it will also mean that you must be confident and satisfied with the mast when you are far from land and the wind comes up. Yet all the masts I have seen fail have been from rigging failure not the mast.
As a matter of interest I had a mast on the little boat which had fitted 2 turning pulley type boxes right at the bottom of the deck stepped mast.(one each side) These got quite corroded and the loss of metal at the one level weakened the mast so that it crumpled in compression. ie the mast simply got shorter by a centimetre or so. and the sheave boxes crumpled. it took a while till I noticed that rigging was a bit slack. So in this case it was compression that caused the problem not bending.
Just as a mad alternative to a new mast you might consider more stays if you want real safety. ie more intermediate stays between the existing stays with more spreaders as well.
I built a catamaran long ago. Quite heavy and had several mast failures simply due to under strength mast. I ended up with a heavier mast and diamonds supporting the top half and bottom half to keep it all in column compression. Perhaps a solution for you.
Just an opinion good luck olewill

 

[Yacht Yogi]

Speaking as a Chartered Engineer specialising in aircraft structures and stress analysis and as the owner of yachts with aluminium masts, I would like to offer the following opinion.

Firstly, aluminium alloys do not deteriorate just because of age. In any case 40 years is nothing for an aluminium structure. There are plenty of aircraft, large and small flying around carrying passengers and performing at air shows that are considerably older than this. The RAF has Hawk, Tornado and its newest "RC-135 Rivet Joint" airframes are all around this age or older.

Aluminium structures, as noted in previous posts have two issues of concern; metal fatigue and corrosion.

As remarked in a previous post, a yacht mast is almost completely working in compression reacting the tension in the shrouds and fore/aft stays. In this state fatigue is not generally a problem. There may be local areas particularly around attachment fittings where out-of-plane loads might cause small cracks. These should be visible by eye or checked with dye-penetrant inspection (aviation or motor-sport services could provide this for you). Small welded patch repairs are perfectly acceptable and reinforcement of the attachment region would prevent a recurrence.

Corrosion is a more likely concern. The biggest problem is galvanic action between dissimilar metals such as stainless steel fittings or rivets. Oxides are usually visible at corrosion sites and if a fitting or rivet is becoming loose then black fretting residue will be present. This kind of damage should be repaired because it will have weakened the structure or the attachment. The joint should be dismantled and all corroded parts and fasteners replaced. Damage to the main mast sections can be repaired with reinforcing plates and larger rivets fitted if space permits to clear out corrosion from the holes. Bare Aluminium parts and holes should be painted with Alodine (available from LAS Aero Ltd or other specialist supplier) and the joint reassembled "wet" using JC-5 or Duralac compound to prevent corrosion starting again. This is boatyard stuff really, rather than DIY. I'm afraid I don't know who might be able to help in your area.

Best regards,
Chris

 

[ean_p]

Small welded patch repairs are perfectly acceptable and reinforcement of the attachment region would prevent a recurrence.

Hi Chris

Would you suggest the need for any post welding heat treatment or should a'standard' cruising mast be good following welding without any form of post welding treatment ?