Dismasting - Why Does it Happen - How to Prevent it

[ewsponberg]

Originally Posted by micah719

Thanks Cap'n Sandcrab. I've come across Mr Sponberg's work before, it's interesting stuff. Unfortunately, he's more into composite, alloy or wood masts, and the link about the mast failures makes me more wary than ever of composite. Some very interesting stuff in there that would be good in the lightning thread. It's not absolutely certain yet that I'll make my masts of steel...alloy is the main alternative. But, I have somewhere in my archive an article (wish I could find it, need to do some cleanup in there) about the relative merits of the various materials. From memory, designing to the kind of loads of the size boat & sails I intend to build, I have the option of wood (solid or hollow), alloy, or steel. The timber loses out because it would have to be solid to get anywhere near the strength, and would be awfully heavy and very thick indeed....windage. The alloy would come out at walls 8mm thick, and the same weight of steel, 2.6mm thick. Corrosion makes me leery of such thin steel, but I've tried compensating for that with fully sealed and well coated masts; halyards running in recessed tracks, with a sealed internal tube from below deck to masthead for electrics such as lights and antenna.

Thanks Cap'n Zeehag....I had intended keel stepped masts, just raising the deck penetration four feet higher than the deck and keeping the mast winch & blocks attached to the extension. In my mind that keeps the stress points off the mast, and reduces the lever length between the wedges and the step, and thus lessening the stress on the mast yet again as well as reducing the amount of water the deck penetration will have to try and keep out. A recent thread supplied me with the idea of a solid cast-in-place epoxy collar rather than wedges. If the horrifying thing happens and I lose a mast, the extension is an ideal stump for a jury rig. The downside is, I can't lower the masts easily to go under bridges, but I wasn't keen on that anyway.

I'm getting closer to the stage where the number crunching really begins, and I'll be building a 1:16 model of her as well for fiddling everything into its shape and place. Weight aloft does concern me, and I don't want a dog in light airs either, though I understand I won't be able to have a stunningly fast boat without compromising non-negotiables. It is working out to be a boat that will really begin to shine when the winds are getting up, when the light fast folks hitch up the skirts and go home. This fits in with the inevitability of getting caught out in gales...and as a shorthanded/singlehanded cruiser I want a boat that is friendly when the sea isn't. With the Junk rig the stress on the rigging is less and there are so many more options, and it's cheap and easy to maintain. A big plus. There will likely be a fair bit of ballast and with the large amount of sail I anticipate an initially tender boat that firms up. Ok, we'll be heeling, but that's life. Otherwise I'd have gone for a multi and be done with it.

It is great to have a lot of salt-stained veterans to bounce ideas off...I'm trying to listen to the experience and advice without being bullheaded about my unusual ideas, or caving too easily and going along with the crowd for the wrong reasons. It seems to me that a lot of modern boating methods are driven more by convention and convenience, and the profit motive, rather than what really works and is necessary. I'm open to criticism and the wisdom of folks who've been there & done it all...but to be able do the unusual means I'll need thorough convincing. A good way to learn, though it has driven many folks to exasperation ever since I pestered mum & Dad with my questions. Fair sailing!

Micah,
When you build your model, or even when you build your boat, you may find that for the size and weight required, steel is going to be just too heavy. Can the stability of the boat support it? I have scanned through this thread only lightly, so I don't know if somewhere you mentioned the size of the boat you intend to build. A couple of thoughts:

First, aluminim is usually the prefered mast material for free-standing masts on smaller boats because reasonably sized mast sections can be purchased in aluminum. They are relatively easier to buy in tapered tubes, and they are less expensive, generally, than carbon fiber masts. Carbon fiber, of course, is a lot lighter than aluminum (40% or so) and that is why it is attractive for using in masts. Nonsuch boats are perfect examples of good cruising boats with aluminum unstayed masts. Wyliecats are perfect examples of reasonable cruising boats with carbon fiber masts. Freedom Yachts, over time, had first aluminum, then carbon fiber masts.

Second, in your example of determining the required wall thickness for aluminum and steel masts, you must be aware of a free-standing cantilever mast's propensity to buckle if the wall thickness is too thin. That is, on carbon fiber masts, if the thickness to diameter ratio, t/D, is less than 0.03 (3%), then the mast may buckle in bending long before the stress in the wall reaches its design value. That is, the mast will fail too soon. I have tested this phenomenon a lot in my continuing work with the engineering of composite flagpoles. With aluminum masts, which have the same material stiffness as carbon fiber (modulus = 10,000,000 psi) I would expect similar behavior, although in truth the ratio could be a bit lower, say 0.025 (this is an educated guess, I have not tested it). With steel, which has 3 times the stiffness of aluminum and carbon, I would expect the ratio to be lower still, but at what value I just don't know. But this is something that needs checking. I don't know what mast diameter you are contemplating, but you should at least check this ratio for the loads that you are contemplating. You might want to check with another professional who has direct experience in bending tubes and buckling.

Third, your concept of having a stubmast built into the boat, over which the tube mast fits, is very sound. I use exactly this concept on my rotating wingmast designs, for the very reasons you cite--to seal the deck against water getting down below. The bearings between the stubmast and the wingmast are located at deck level and at the top of the stub. Generally, my stubmasts extend up into the wingmast about 10% of the wingmast height above deck. It is adequate to engineer the mast that way.

Finally, for everyone, there are a lot of words expended on this thread regarding fittings, their materials, metallurgy, corrosion and maintenance, all as possible causes of dismastings. How true. It takes an eagle eye to look after and keep track of the maintenance of all those fittings. It takes time and effort. This is why free-standing rigs, with no standing rigging whatsoever, make so much sense--If it is not there, it cannot break! If it is not there, you don't have to maintain it. If the rigging is not there, the measure of safety goes up by literally hundreds of times.

I have been designing free-standing masts for over 30 years. A lot of my masts are on a lot of different boats, and they have cruised many thousands of miles, some around the world. I have yet to have a mast break under normal sailing conditions. Collisions, yes; lightning strikes, yes; car and truck accidents, yes; and being driven over by forklifts (surprising how often that happens), yes; but never (touch wood) under normal sailing conditions.

As you can expect, I would vote for free-standing rigs as a preventative against dismastings.

Good thread!

Eric

[zeehag]

i removed my 316 cracked chainplates and installed 304 to one side--we see how long it takes them to suffer failure..LOL... the 316 were toast but still usable in dire emergency for something.....but then the alleged 316s were from taiwan.....

[micah719]

Thanks very much for your reply, Eric....wasn't aware you were on this forum.

The boat I am designing (more like frankensteining from other people's ideas) is in the 40+ foot range...I settled at max 14m, or 46', LOA from the tip of the stempost to the tip of the sternpost. I wouldn't go longer, but quite likely shorter, but probably not less than 40'. Still fine-tuning what else will be aboard, and where to put it...once I've got that settled, I'll squeeze her down a bit while playing with exact shaping and rig numbers. I love the Valhalla 44 from Kasten Marine,
The 44' VALHALLA - A Pelagic Sailing Yacht
but the plan price is astronomical...not sure whether he really wants that much for it, or if it's merely for discouragement of purchase. There are a number of changes I'd want anyway....shorter and narrower cockpit, centreline aft companionway, hard dodgers fore and aft, nav station combined with seaberth shifted forward to starboard of galley, galley sink on centreline under ladder, smaller enginespace (probably no engine, though genset and ancillaries) etc etc.

The raised mast partners.....I was figuring on having one-piece masts from truck to foot, keel stepped, not rotating. Instead of a hole in the deck at deck level, I wanted to extend a tube about 4' above the deck; less exposure to water, longer lever arm between partners and keel, greater surface area for partners, no need to compromise mast tube walls for winches and fittings as they can go on the extension. The masts themselves...an internal tube from close to keel to truck for nav lights and antenna, halyards in recessed external channels (junk rig can't have any projections) which will strengthen the tube somewhat and permit easy access to the halyards. Some reinforcing webbing inside the mast low down, decreasing in taper and weight higher up. The buckle problem is prominent in my mind...the 2.6mm was more of a demonstration of equvalent to a solid oregon spar or an aluminium one with 8mm wall. Still trying to re-find the article....atm I'm chucking stuff over the shoulder into files for the time when I sit down and really crunch numbers and get to fining hull lines. Actually building will be tedious by comparison. First up would be a 6m beach cruiser; the Caldonia Yawl appeals to me....versatile, easy to build, and an ideal test-bed. Dipping lug main and standing lug mizzen, and split junk rig later for tuning and testing....especially sail shape.

Agree with the freestanding concept wholeheartedly...aeroplanes dumped wires a long time ago, about time boats did too. I expect to approach 95% of the performance of a bermudan with the dipping lug on the beachboat, and over 80% out of the junk rig on both (no spinnakers or headsails or kites figured in to that, yet).....will take a lot of head work, but well worth it. Bang for the buck, and much less chance of a little thing dumping the whole rig overboard. Minimum 3mm wall thickness steel tubes, with reinforcement...have to actually hit something with them, like a bridge, or a freighter, or a violent pitchpole/360, to lose those sticks....wind would just knock her flat or rip the sails. That is more easily preventable...

Of course, it would never take off as a production type; too unfashionable, and not enough repeat business for the marine industry. No racing in mind, but a safe floating go-anywhere home for a small crew.

One thing that had bothered me, especially being steel masts....corrosion, corrosion protection, and chafe. In my lurkings around wooden boat forums and kayakers, it seems kayakers use graphite mixed into epoxy to allow the hull to slip easily over snags and branches and whatnot....how about graphit/epoxy on the mast to prevent the corrosion protection being worn through? Ok, potential for nasty black stuff wearing off, and regular maintenance....but my sails won't be lily-white anyway, and maintenance on a boat you love and built isn't so bad. Less friction must be a good thing. Perhaps other things than graphite....teflon powder?

[Cheechako]

Originally Posted by rebel heart

How is 316 likely to last six months when I had a stem bracket that was 36 years old? There was corrosion in the lower sections and I had a new one re-fabricated since I had removed it already, but saying that stainless chain plates last six months is nuts.

Spend an hour walking around looking at multi-decade old chainplates and explain how they're still holding load, if they break at six months.

Additionally, every pressure vessel that contains the nuclear core on a Navy submarine is made out of 316.

Yeah, you can do all the reading you want , but maybe it's easier just to walk out to the dock and look a t all t he 30 year old boats with their 30 year old rigs!

[micah719]

Just a thought.....threads in high-strain applications are often rolled rather than cut.....has this been tried with SS parts for boat rigs, seeing the danger from crevice corrosion?

[Cheechako]

Good thought. I dont know if any of the turnbuckle manufacturers do this... I would guess not. OTOH, I dont know that I've ever seen a broken turnbuckle threaded shaft. Seems like the body breaks more often...
I think there is a lot to be said for unstayed masts for cruising boats... if the mast material can be made to last without deterioration. Seems like the few older carbon fiber (?) ones I've seen look pretty bad... if looks mean anything. They bend and dump some of the excess wind out when needed..not bad for cruisers, this can be a performance issue for racers I suppose, but for cruising.... wow.
alot of weight saved aloft, and no wires in the way! Lets say a 45 foot boat has a rig that weighs...600 lbs total and is 60 ft tall. the moment of 600 lbs at 30 ft up = 6tons!

[ewsponberg]

Originally Posted by micah719

Thanks very much for your reply, Eric....wasn't aware you were on this forum.

I love the Valhalla 44 from Kasten Marine,
The 44' VALHALLA - A Pelagic Sailing Yacht
but the plan price is astronomical...not sure whether he really wants that much for it, or if it's merely for discouragement of purchase.

AND

One thing that had bothered me, especially being steel masts....corrosion, corrosion protection, and chafe. In my lurkings around wooden boat forums and kayakers, it seems kayakers use graphite mixed into epoxy to allow the hull to slip easily over snags and branches and whatnot....how about graphit/epoxy on the mast to prevent the corrosion protection being worn through? Ok, potential for nasty black stuff wearing off, and regular maintenance....but my sails won't be lily-white anyway, and maintenance on a boat you love and built isn't so bad. Less friction must be a good thing. Perhaps other things than graphite....teflon powder?

Micah,
First, I see that Michael Kasten's Valhalla design is brand new, and the prices that he is charging for the design drawing and the patterns is about the same that I would charge for a custom design of that size. So, he has done the design and is probably hoping for one order to recoup the cost of his doing it. Once he sells one or two sets of plans and patterns, the price will come down.

As for stopping corrosion, I recommend that you consider Chlor-Rid, which you can see at their website: Chlor*rid International, Inc.. This is a very watery liquid that is used to wash steel prior to priming. One reason that steels, even painted steels, corrode is because there are still salts on the steel surface that normal preparation does not get out. Chlor-Rid does, it really washes away every last trace of impurities prior to priming. We used this on some of our steel Moloka'i Strait motoryachts and it seems to work really well. For your steel masts, I would consider washing the inside and out with Chlor-Rid, then priming and painting. Remember, most steel structures corrode from the inside out, not so much from the outside in, so priming/painting the inside of the mast will be as important or more important that doing the same on the outside.

I hope that helps.

Eric

[micah719]

Thanks again, Eric...that Chlor-rid is one for my files. What got my attention was that gritblasting wouldn't necessarily leave a perfectly clean surface...

I was figuring the sealed part of the steel mast would not corrode inside once the oxygen was depleted...but was planning on coating it internally anyway by pouring in epoxy paint and rolling it, and burning an oily rag in it just before sealing it shut. The remaining worries then would be the outside where halyards/yard/battens/boom would chafe, and especially the antenna/light conduit on the inside. It's not something that would be opened often, but I'd like to have easy access to it (as easy as climbing a 18m steel mast can be, anyway...). Still going to test the graphite/epoxy mix, or teflon or something similarly slippery/tough, and keep looking. Might even go for alloy after all, but something keeps pushing me back to steel.

[j dragon]

who do you go to ...to have youe rigging inspected ? Local boatyard ?

[ewsponberg]

Originally Posted by j dragon

who do you go to ...to have youe rigging inspected ? Local boatyard ?

If you cannot do it yourself, you should go to a qualified rigger. Some boatyards may have one, but it is not guaranteed. Spar builders have riggers, because that is what they do besides build spars, they rig them. A spar builder may be able to recommend one in your area. Spar builders also have rigging inspection guidelines which you can usually download off the internet, and these guidelines will walk you through what you should be looking at. Some marine surveyors are also qualified to look at rigging.

Eric

[Sand crab]

Still waiting for the details on the boat with the failed chinese made mast.

[PooBeetle]

Dismasting - why does it happen, how to prevent it

Part 4 – 316 Wire Rope

So now you've read the Metallurgy, and understand that no other alloy is as bad as 316 in resisting work hardening, brittleness, metal fatigue, pitting growth, cycles to failure under load in salt water.

so how well do you think such a brittle alloy is going to perform as a wire rope?

Such a brittle, fragile alloy that cracks so easily, formed as small thin wires, under big stress, in salt water, while being constantly bent in moving wire ropes.

Thrumming
Vibrating under heavy loads. Wire stays thrum. The loads go up and down depending on the wind and waves while all the time the rope is thrumming. So we know the wire rope is going to break, but where?

Density Changes
When you add a swage to a rope, the density changes. There is a boundary. Vibrating metal free to move suddenly hits a solid metal swage. The density edge is where all the energy gets transduced into work hardening. Brittleness and fatigue happen right here.

Speed of crack propagation Versus the size of a single wire strand
will SCC break a small 1mm wide part faster than a big solid part like a chainplate? A question to ponder.

SCC
Temperature will play an important part. Lots of europeans and cold water people ie (US, england etc ) never have problems and so get blase` about 316 inspections, but then they sail somewhere warm. With the hot tropic sun heating up wet salt in tiny crevices SCC takes zero time and you are suddenly dismasted in the tropics like so many others. Exactly this happens all the time.

You should read up on what effect temperature has on 316 in chlorine @ 35000 ppm at 40 degrees C under stress.

Cascade Failure.
Could this dismasting have been prevented by inspection? No. There is no way you can see up inside a swage.



It can clearly be seen that some of the more stressed wires broke first a long time ago from Stress Corrosion Cracking, and so more and more load was transferred to the remaining strands, which started breaking one by one, each suffering higher and higher SCC until the remaining strands all then suddenly broke at once.

This is 316. This is exactly who 316 is. How it fails every single time.

Swaged versus swageless fittings
Swageless fittings last much longer. The huge hydraulic press deforms 316 past it's yield, and so induces brittleness and micro-crevices everywhere inside the swage. Heaps of pics of old swages cracked all along their length. The same pressure has to work harden the wires.

Tensioning your rig
The Selden manual says 10mm 316 wire rope has a Breaking Load of 8800Kg, and you should tension such a stay to 15% of BL which equals 1320 Kg. Some riggers go to 20% of breaking load. The Safe Working Load of 10mm rope is 1300 Kg. Can you see any problem?

316 240 MPa yield strength suggests your 10mm part is permanently deformed after a 1480 Kg load is applied.

Average rig tensioning tools have an accuracy of 5%, and then add load cycles in chlorine, wind shock loads, plus the loading from waves.

Does anybody else see that a problem actually exists? (or am I all alone)

[PooBeetle]

Dismasting - why does it happen, how to prevent it
Part 5 - Capitalism

An invisible economic paradigm that influences, permeates and stains ALL objects created by it.

The Lowest Quality
The tone comes from the top. Air is invisible, yet it exists. Because it is invisible and we are swimming in it, and rarely notice or think about it. (I say because they hide under rocks avoiding the cold hard light of day)

If you charge more for a product that looks exactly the same as your competitor, you will go out of business.

Survival of the fittest then produces winners in the race to produce products that look GREAT but are made using ever cheaper materials, ever lighter construction, as cheap as they can get away with.

So;
Capitalism Always produces A MacDonald’s Hamburger Patty.

they say 100% beef,
and in reality? They tell the truth, as a collection of lips, testicles, and eyeballs etc. IS 100% beef.

Marketing – Convincing People That the Lowest Quality is Actually the Highest Quality
Companies spend as much money as gives maximum return brainwashing people into thinking that the "as low as they can legally get away with quality" is actually the highest quality. For most companies this is just under 50% of their profits spent on brainwashing you. Go to the central library of any university and look at the scientific journals on marketing and brainwashing. Endless. I repeat endless. Is this the biggest single section? I think so. It gives an indication of how much money is spent. Compared to the amount of space devoted to engineering journals etc.

So this is their job.

They have a LEGAL obligation to make as much money as they can for their shareholders. A legal obligation. They are bound by law to do this. They want to do this. This is their job, and they work very hard every single day doing all these things. Competing with each other.

The stock standard plain vanilla way this world works is that companies always produce cheaper and cheaper goods, of less and less quality, all the while spouting how good it is. If they don't do this, they go out of business, because there are other guys out there competing harder.

To oppose this downward spiral in quality, governments often produce legislation to enforce minimum standards. ie for pollution, work safety, toxic chemicals in the home, in food, engineered building codes etc etc. The companies then always go right up against this line. As hard as they can.

Ice Cream.
I no longer eat ice cream. (thank you Margaret Thatcher et al)

Margaret Thatcher was an ice cream scientist. Her job was to produce ice cream that had less and less cream, and more and more cellulose gell, water and air.

This is fine for the first ten years, but after the new accepted norm becomes slightly sweet water with a white crayon briefly dipped in for colour, it's then in the second ten years of capitalist survival of the fittest that causes the problem.

Government Legislation was introduced for ice cream, because the capitalists started being extra silly. To put a stop to the downward spiral process. To put a floor of minimum standards.

Car Fuel Tank Placement
In 1971 a Ford engineering report determined that the cost of a design change for moving the fuel tank above the rear axle would have been $ 9.95 per vehicle.

There have also been petrol tanks installed behind the front engine and no firewalls. FORD et al knowingly killed thousands of people, HORRIBLY, and they refused to stop due to making money.

Government Legislation was introduced to put a stop to this downward spiral process. To put a floor of minimum safety standards.

Again, it takes legislation to make them stop killing people HORRIBLY.

This what they will do to you and your family, and please note for how much.

For $9.95

The Same Forces of Capitalism Acting on Yacht Mast Rigging.
3 cents of molybdenum extra turns 316 into an alloy with acceptable SCC resistance.

So people have died for the want of 3 cents extra of moly. Yay! capitalists.

The company you are buying your 100% BEEFY yacht mast rigging fittings from is exactly the same as MacDonald’s, Ice Cream, and Ford. The $9.95 principle. The 3 cents principle.

It is exactly the same never changing story here, Capitalism demands a downward spiral of quality. And then Legislation puts limits on them. There's just fewer deaths and injuries involved is all, so we have the unrestrained, unregulated stupidity of using 316 in salt under stress..

The first legislation touching stainless steels though has just been done. stupid old 316 has been outlawed in one place.

All countries will follow suit as they always do on safety laws. Some faster, some slower, but they generally always follow in safety laws.

Will marine codes be forced to follow normal building codes? Don't know. But there will be no change until legislation is introduced. Despite the deaths and injuries from falling masts and rigging.

Most small yachts are toys that last a couple of years before being sold anyway and so if the 316 rig fitting last only a few years, nobody really cares. It's the new owners problem. Who wants expensive proper land based building and engineering codes to be followed just for toys?

Mast rigging fittings will continue to be as cheap as the marked can stand, before the customer goes elsewhere.

$9.95



where did my 316 bolt go? it just vanished.

[PooBeetle]

Dismasting - why does it happen, how to prevent it

Part 6 - The Mythical Composite Sandwich meets the Demands of Capitalism.

Sandwiches.
Everybody here understands what a composite sandwich is.
It's marketing dribble!

There's Peanut butter in your Sandwich
Based on a concept of a cubed increase in stiffness due to increasing the thickness of the core.

However;
The properties of the structure is totally dependent on the stiffness of the core.

If you use peanut butter in your sandwich core, you will not get the mythical and yet forever spouted cubed increase in stiffness, you will get a linear increase in your sandwich stiffness. you will get deformation under load cycles appropriate to the linear equation, not the cubed.

A Composite Sandwich is basically two layers of glass either side of a filler. The thickness of the filler increases the stiffness and strength to the cube of the distance between them. Most mass produced cats are built from foam and polyester resin. Much faster to build, but you can pull polyester fiberglass off plastic foam with your hand. Waves slapping the underside of the boat have delaminated this stuff often. Being raised out of the water on a lift in a boat yard makes these boats dimple, as the sling crushes the side of the hull. When people call their hull a Composite Sandwich, they are implying a cubed stiffness relationship,

Cyclic Loading Under Stress
Glass in micro scale is above ten times stronger than steel wire. Wood is kilo for kilo stronger than steel. Billions of years of evolution have created a microscopic fractal order that has the highest resistance of load cycles to failure. If you stress aluminium, foam, steel etc and then run them through 10 million load cycles, nothing else comes close to wood. Epoxy is stronger than wood. When you glue wood with epoxy, the wood always breaks first. Glass is ten times stronger than steel in tensile strength.

So a composite sandwich made from wood, epoxy and glass is amazing.

A composite sandwich made out of Polyester and foam or balsa is also awesome. it's just that merely tightening the rigging can deform them permanently. they will break, and delaminate as soon as they hit a wave. and they will turn sloppy if they are ever sailed. Awesome.

A True Composite Sandwich versus a Ronald MacDonald Sandwich.
Why is fiberglass used so much for building boats? - because it's 10 to 15 times stronger than steel - weight for weight. So 1mm of (micro-scale)glass in tensile strength is equivalent to 1 cm of steel.

If you have a composite sandwich with 1mm of glass/epoxy on either side of a core material, you get a radical increase in strength and stiffness of the whole due to the loads being distributed evenly to either side of the structure, transmitted over a very long distance in the glass plane, and any stress is then dissipated over a very wide area due to the tensile strength and stiffness of the glass.

Composite sandwiches are basically based on beam theory. If you increase the height of a beam, you get the cubed stiffness. ie double the height you get 8 times the stiffness. (the real engineering formulas are as complicated as you wish to take them ie how powerful you want your mathematical modeling to be; some firms draw the line on simple formulas, other people like governments that have endless money employ ace mathematicians and use mathematical formulas ten pages long (my old job); but as formulas go for utility and comprehensibility, this is a good simple analog. not correct, but comprehensible.)


Increase the core two fold, and you get 8 times the stiffness. 3 gives you 27. In engineering terms, this is astounding.
but:
Just like ice cream, the reality of competition forces capitalists to adopt the downward quality spiral in order to survive.

Production cats have a capitalism perverted composite sandwich hulls. They call their hulls composite sandwich, which is perfectly true in layman’s terms, but they know they are implying an engineering cubed stiffness relationship of this quality, and this is not true. They use foam as a core, which is like using fruit jelly sandwiched between fiberglass. They do this to keep their build times extremely low, ie it takes one day for a hull in a mold. They simply must do this to compete with other people who are also doing this to stay in business. If you use peanut butter between two layers of heavy glass, your sandwich stiffness shows only a perfectly linear increase, ie add one unit more glass or core and you get one unit more stiffness. The two sides are simply not really connected and i've seen photos of modern hulls being buckled just by lifting them out of the water in slings for a standard bottom clean.

Sloppy Glass
The lack of fixity between the two stiff layers of glass on either side of the foam results in movement which starts to stress and break the glass in micro scale. Even attached good glass on foam loses around 10% of its' stiffness in the first year. There is always a bell shaped curve of stress. As all the tiny little glass fibers are not stressed exactly the same. Some are stressed more than others. So when the hull hits a wave and flexes, then the very few that are taking most of the stress break first. On the next wave, the same thing happens. Over and over the whole boat gets sloppier and sloppier.

The same effect as wearing normal hard heeled shoes on your fiberglass deck.

All engineers know the sandwich core. I've read foam web sites. All they ever do is imply a cubed stiffness relationship. They imply a true Composite Sandwich effect, but sloppy sandwich fillings can give somewhere between a linear and squared relationship. Not cubed. And not under heavy cyclic loading for years. BIG dynamic loads. ie big waves hitting the entire boat for years.

So how good your sandwich is depends on the filling. To get a true cubed stiffness increase, your sandwich core needs to be stiff, and needs to stay connected.

Polyester and Foam
There are guys from boat yards on this forum that have said many times that some of the foam cats are just fine for their average use. ie sitting at the marina, and going for a day sail once every now and then in good weather. But as soon as they go for a big hard sail, delamination from the foam core happens.

Even tightening the rigging deforms foam cored hulls. You get a single wave, and how much extra stress is put on the rigging as the boat loads and unloads? deforms from side to side?

Yet if a boat yard tried to build out of wood and fiberglass sandwich, the boat would take many times longer to build, and cost many times more. This boat yard just couldn't compete, and would undoubtedly go broke. Many boat builders have gone out of business.

On this forum, right here, we have boat builders from boat yards that have said 100% of all polyester and foam cats that come in have delamination of the Polyester delaminating from the foam. I used Polyester when i first started making model boats, and the fiberglass has just delaminated off in sheets.

I say Polyester is NOT GOOD ENOUGH to even build MODEL boats. Same with epoxy over balsa. I've pulled glass off of balsa models fairly easy. (only in this case it's the balsa itself that fails) i recommend you do exactly this at home first before whinging and whining in a post here about how great the stuff is. Just build some model boats first, and see what happens BEFORE you send your post here complaining that this stuff is JUST GREAT for building REAL boats.

and this is fricken model boats i'm talking about.

The Properties of Your Polyester and Foam Sandwich After Delamination Occurs
The engineers crunch the numbers, on their safe, dry, and comfy chair, so far removed from huge waves smacking into your hull a thousand miles from shore, while you are puking your guts out and dry retching endlessly hour after endless hour.

So they poke the calculator once, beep. and then again, beep. Beep beep beep. =
beep. Ping etc.

Until finally they decide that if they use the cubed stiffness equation for polyester on foam, and never talk to the boat yard workers ( the guys on this forum who maintain that 100% of these boats have delamination, and polyester cracks and crazes ), then all the maths appears perfect. And a new shiny boat get pulled from the mold and sold to you.

(JFYI, the reason you bought it was because it was so shiny. (due to a random confluence of human eyeball visual information pre-processing artefacts. (as the human eye neuron wiring actually does signal pre-processing to lighten the data transmission load and cognitive processing load, and so is hardwired to deliver 10 times as much 3D information cues into your brain as 2D information. And this is part of the reason why you are hypnotised by SHINY objects.))

so you bought the polyester and foam yacht both because you could afford it, and because it was so shiny.

So whatever the alleged and alluded stiffness properties the foam/polyester sandwich had, it looses some stiffness the first time it goes sailing.

Life Expectancy and the Demands of Capitalism
Racers
A formula one engine lasts one race, and revs at around 20000 RPM (though limited on the track to 18000)

Hyundai and Mitsubishi
Hyundai recently carved out a huge chunk of the car market against all the very clever japanese car manufacturers who were so focused on quality etc. An Excel engine lasts about 100 000 K before it dies. It revs 5000 max. They carved out a huge chunk of the car market by being cheaper than everybody else (by using cheaper metals in their engines, they saved 3 cents on the alloys compared to others). The marketing blurb says “Manufactured from high quality materials for long life”, when it has the lowest engine life known to man. To quote my mechanic “if it wasn't for Mitsubishi, I'd be out of business” and yet the car magazines always give them glowing articles.

Heavy Cruisers
Awhile ago I read a marine diesel engine web page that had 26 different variants of exactly the same marine diesel engine. The life expectancy was the difference; the MTBF. The top marine diesel was designed for continuous running. ie you turn it on and let it run for 50 years. 500 RPM These guys spent the extra 3 cents on the metal alloy used.

The Boat Analogy
Racers
Life Expectancy? one Australia at the America's cup trials broke in half and sank. I'd employ that engineer again. Exactly what you want. Finish the race and fall apart. Heaps of very lightly built catamarans out there. Shockingly lightly built foam and balsa things..

Hyundai and Mitsubishi
Life Expectancy? Not forever, but OK. They will have a shorter life if sailed hard. Just fine for the money you pay. Average polyester and foam catamarans are the Hyundai's and Mitsubishi's of the boating world. They are perfectly suited to sitting in the marina. Just go to any marina and you will notice that every berth is always full. They are fantastic boats at very cheap prices and perfectly suited to the demands of the average customer. They will last numerous years and give nice service, but they will last less time if you beat them hard.

Capitalism demands you compete or you go out of business, and so they are all exactly the same, and all made from the same materials. If they swapped the manufacturers name plate over nobody would know the difference.

And the verbiage is always the same. “NASA envy us our technology”. “We use Ultra-BAM! Miracle goo in OUR hulls, and that is why we are so good.” There is “Miracle Ingredient X42” in our sandwiches. (cheap salmonella infected peanut butter from China).

Safe? Nobody is as safe as us! as Fast!, as Cheap!, as Light! Quality! The words are totally unrelated to reality. Just whatever works to separate you from your money. The sales blurb never varies and the magazines parrot off exactly this never changing verbiage. All these cats are exactly the Mitsubishi and Hyundi of the boating world. And I am all in favour of them making cheap toys.

If they cost twice as much, they'd sell ten times less. And I am all in favour of people being able to afford toys and go sailing.

Heavy Cruisers.
Life Expectancy? Forever. Built to take a beating without even blinking. Will still be sound after 50 years of heavy sailing. Many of you would know the custom built german catamaran Fallado, Bottom is 5 layers of 8mm red cedar strip plank in heavy glass/epoxy. Bit different in inertia from a bit of cheap foam and polyester. Still Stiff in 50 years. Probably forever.

So the Rigging Starts Getting Hull Loads When the Polyester and Foam Hulls Start Flexing.
What is the inertia of 10 tons of wood or steel versus the inertia of 500 kilos of foam sandwich?
A better strength to weight ratio than steel versus Ronald MacDonalds' ********.

So;
Dismasting - why does it happen, how to prevent it?

A good question is What is your hull made out of? What are the exact properties of your Polyester foam sandwich? Any Polyester cracks and crazes yet? There is definitely delamination, so the sandwich alleged cubed stiffness has suddenly evaporated to shockingly low linear stiffness. How much extra load will be hitting your chainplates and mast base due to your linear stiffness delaminated foam sandwich flexing on waves is a good question.

I suggest tons of extra wave induced cyclic loading forces over a steel, good solid glass, or wood and glass/epoxy hull. And remember, there is no alloy worse than 316 in taking load cycles in salt water.



p.s.
Don't get me wrong;
non-cubed stiffness normal foam sandwiches are good enough for rock and roll.

They give cheap affordable toys to millions of people who otherwise couldn't afford the better quality forever life expectancy. Most boats are toys that never move out of the marina, so the Polyester/foam is perfectly suited to building these low life expectancy toys.

But a delamination patch will turn a cubed stiffness structure into a linear stiffness structure, and that means flexing hulls that will contribute tons of extra cyclic loading into the rigging compared to a true epoxy sandwich hull, or a solid glass hull, or a wood hull, or a steel hull.

[Sand crab]

Poo beetle. Do you have blisters on your fingers? So many words, my eyes hurt.