Dismasting - Why Does it Happen - How to Prevent it

[ewsponberg]

Originally Posted by micah719

The unstayed mast seems to be the way to go. That it isn't more popular than the stayed has a lot of reasons, and I wonder how much of that is influenced more by status-quo and the better repeat-business for the rigging industry.

I had a reply all typed out, and then something happened and I lost it. I'll try to repeat:

I will withhold any further opinions on bamboo, now after reading this article:

http://bambus.rwth-aachen.de/eng/PDF...f%20bamboo.pdf

It appears, at least according to this, that bamboo is about twice as strong and stiff as spruce, the premier mast building wood. Whether it is suitable for masts is another matter. There is nothing in here about density as far as I can see. Also, the biggest diameters appear to be around 18 cm = 180 mm = about 7 inches. That might be suitable for a boat up to about 35' or so. Whether there is enough wall thickness to handle sailing rig loads is the question--what's the overall strength of the sections? So, I'll wait for the results of your trials before saying anything else as I am interest to learn what you learn.

As for the popularity, such as it is for free-standing rigs, I would say it is DEFINITELY influenced by the status quo of the market. Very few people are building them any more--the modern heyday was the late 1970s to the mid 1980s. Many of those boats are still sailing, and owners who take good care of them hold onto them for a long time because they like them so much. When they come up for sale, good ones go quickly.

I have also seen on this forum and on BoatDesign.net a growing appreciation and new examples of boats with free-standing rigs. I think interest is growing. So it only takes an intrepid boatbuilder to start some more modern designs. But the obstacles can be daunting--not only the design and tooling, but the up-hill marketing that would have to be done. But, I firmly believe that with the right design and the right builder, and the right approach to build the market interest slowly, such an effort would be quite profitable.

So, keep us posted on your bamboo mast effort. I'll be really interested to see what you can do.

[micah719]

From what I've seen in my datamining so far, density of bamboo is about the same as average softwoods....around 600kg/m3. The difference is bamboo has more useful fibres of a better configuration, but sadly only in a thin layer at its surface, and those dratted nodes.

The most useful kind is the Pseudosasa Amabilis, Tonkin Cane. Mostly used for split-bamboo fishing rods, scaffolding and furniture. The fishing rod application proves that with care, the durability can be excellent with repeated loading to significant flexing.

The techniques already present in rod-building are the bulk of my reading so far. They've done the groundwork on preparation and gluing, all I have to do is scale it up and focus more on stiffness and damage-tolerance.

Stiffness I think I can get from cross section, both area and configuration. I'll look closely at the difference between simple round tube, and tube bundle inside an outer tube. This second approach would in my mind be better at the expense of some weight, but advantages in compression, buckling resistance, and damage tolerance. Inside out fishing-rods have the splines with the layer of highest fibre density arranged so that these layers are radial. This is less sensitive to damage at the exterior surface and has shown no appreciable difference in flexing to the more typical fishing rod where these fibre layers are arranged concentrically.

As a mast/spar, I want it to be able to stand up to some abuse, mainly the regular impacts between mast/yard, but also a compression concentration at the partners, friction and shear from a boom or gaff crutch or beaded yard parrel, and the knocking about they will get from being on a boat. This is why carbon or fibreglass don't appeal to me so much, especially carbon. Thin alloy and steel, the same.

I have some ideas for reinforcing the worst wear-points, and for setting it up with interior conduits and sheaves. The best glues seem to be resorcinol or epoxy, but I'll look at Polyurethane and others as well. UV, shear, creep and wear resistance, flexibility; and on a budget too!

So far I haven't sourced any Tonkin cane, but I have been using lesser and unknown strains as a start. Quite promising results, too. The finicky bits are arranging the configuration of the splines, and not injuring myself too often. Bamboo splines are sharp and hard and tough, and if the splines don't cut me, the tools to cut it do, especially when I'm too lazy to resharpen often.

Next steps will be Tonkin cane in some quantity, an accurate oven for heat-treating, a range of glue samples, and a series of falsifiable objective tests to measure progress, if any.

Why all the trouble rather than birdsmouth timber spar? Because there is potential for a superior and new product, and I can always go the proven timber way if it turns out I've been chasing a wild goose. Nothing ventured, nothing gained!

[Stumble]

The number of problems with bamboo are pretty significant, and not something that can readily be overcome. Anything is of course possible, but the penalties and costs to do so are pretty problematic.

1) bamboo is weak in compression and tension compared to glass
2) it isn't comparable with standard resins. You would have to use something like G-Flex. That is designed to be flexible. This means a huge increase in size to achieve the same stiffness.
3) it is highly susceptible to rot, and bugs, requiring specialized poisons to treat it.
4) it swells and contracts significantly with humidity levels, making glue bonds problematic.

Yes it might be doable if you have the money and time to invest in breaking a few to figure out how, but I can't see what advantages it would have even if you get it to work.

[Jim Cate]

Originally Posted by Stumble

4) it swells and contracts significantly with humidity levels, making glue bonds problematic.

G'Day Greg,

That's an interesting statement. At the risk of showing my age, I seem to remember that all high quality slide rules* used to be made from bamboo because it did NOT change shape/size much with changes in environmental conditions.

Do I remember correctly?

Cheers,

Jim

*With the exception of Pickett rules which were some Al/Mg alloy IIRC.

[micah719]

The comp swallowed my reply, so I'll try a shorter version.

I realise bamboo has its shortcomings, but all materials do. The thing is to play to the strengths and dodge the weaknesses.

There are stronger materials but they are not as damage resistant, and usually more expensive.

The fishing rod builders have proven that laminated bamboo can be flexed considerably and repeatedly in a wet environment and last generations.

The original rods used animal glue. Strong, cheap and reversible, but not water or heatproof. Mongol bows were made with fish-bladder glue, another protein glue with excellent flex properties, but also not waterproof. We have the WEST system of encasing wood, and many other options for gluing. Laminated bamboo with the nodes removed would be even more resilient than timber because it is inherently broken up into hundreds of compartments. Damage from whatever source is localised and isolated from the rest of the structure.

The stiffness is related to the cross section shape and area, as well as the material. The rodmakers use various cross sections, some hollow build as well, but they need lots of flex whereas we want a small amount.

If all works out as the theory suggests, the next major hurdle is the economics of it. The two big hurdles there would be the cost of the adhesives and the labour. Preparation and treatment of the bamboo can be mechanised, and proven designs can be built fast with proper jigs. All it needs is a lot of thought and experimentation, and there could be a source of very nice spars at low lifetime cost.

Ideally, I'd submit a sample mast & spar to be independently performance destructive tested alongside samples of the established materials...carbon, glass, alloy, solid and hollow timber, all of them; but also run the numbers for costs of manufacture, availability, reliability, damage tolerance, maintenance, repair, and environmental effect. Some work to do in the meantime, but most of the groundwork is already done and out there.

[micah719]

Tanks have trouble going through bamboo thickets. Not just the Sherman M4, but the Centurion, the first of the MBT's. Not sure how the later versions handle it. They mow down substantial trees with less trouble. Even the alloy M113 APC's can eat trees. Not that there's much likelihood of someone driving a mobile oven over the deck of a sailboat, but it says something about the strength of giant grass!

[Sand crab]

I'd use Gorilla Glue. It's waterproof.

On second thought I would just by an ALUMINUM mast and be done with it.

[micah719]

Gorilla glue is favourably mentioned by some of the rodbuilders. Never saw an alloy fishing-rod, though.

[hogfighter]

Lost my mast. Here's what I've learned:

It usually is the chainplate. The chainplate needs to be inspectable.

If you are removing the chainplate to inspect it, replace it. Trust me, it's much more expensive to replace the rig than a piece of Stainless.

Finally, losing your rig sucks at best, sinks your treasure at worst. Prevent it by loosening your purse strings a touch. If there's any doubt, then there's no doubt as to what to do.

[stillbuilding]

Just for fun.
Temporary bamboo constructions on Cheung Chau, Hong Kong.

[Factor]

Originally Posted by micah719

Tanks have trouble going through bamboo thickets. Not just the Sherman M4, but the Centurion, the first of the MBT's. Not sure how the later versions handle it. They mow down substantial trees with less trouble. Even the alloy M113 APC's can eat trees. Not that there's much likelihood of someone driving a mobile oven over the deck of a sailboat, but it says something about the strength of giant grass!

Cant speak to the others - but I used to drive M113 A1s and wouldn't agree that they can eat trees.

[micah719]

Well......little trees....

[Factor]

Yes - little trees perhaps. But they did swim good (well okay - not good but I am still amazed they could swim at all), and they absolutely demolished VW Beetles (dont ask how I know) Had two - Borituppa and Argustuft - I didnt name them but I did like the names! You serve in Aus?

[micah719]

Yes, a long time ago...

[PooBeetle]

Dismasting - why does it happen, how to prevent it

Part 9 – Intergranular Corrosion

Corrosion and Stress Fracturing along crystal boundaries.

Corrosion.jpg Views: 212 Size: 92.5 KB ID: 45821" style="margin: 2px" />

this is not 316, but shows the exact same effect beautifully, of SCC happening along the crystal boundaries.

this is why 316 is only useful for non-stressed parts.

This is why 316 parts will look perfect and then fail suddenly and without warning

It is impossible to manufacture any 316 item without these faults occurring.
Chromium stabilises ferrite, which has marginal mechanical properties, so nickel is added to make FCC austenite the preferred phase.

But cooling after pouring, annealing or investment molding permits chromium carbide (Cr4C) to precipitate along austenite grain boundaries, where the diffusion of carbon is very fast.

The austenite next to these grain boundaries is depleted of the chromium that went into the Cr4C, and so the metal next to those grain boundaries is no longer stainless.

The formation of Cr4C depletes austenite grain boundaries of chromium, and so a non-adhering scale (surface oxide) of FeO forms, and this oxidation then destroys the crystal attachments by rapid diffusion of carbon along the grain boundaries. (and Carbon promotes rust a thousand fold)

There are many photos on the web showing these crystals detaching from any slight stress.

Salt water then enters the cracks around the crystals, and the chlorine then catalyses the destruction of the already weak boundaries, turning the very thin crystal external material into rust. This is called Intergranular Corrosion.

If you supply energy to these defects, stress, or vibrations, then the microscopic cracks pump the salt water and chlorine in and out the Intergranular Corrosion crevices where stress cracks are concentrated and growing anyway, and then we finally arrive at Stress Corrosion Cracking.

The Stress Corrosion Cracks propagate astoundingly quickly.
A big, good looking, solid metal part can fail in hours under cyclic stress in salt water. Mainly because 316 is not really a metal, just a loose collection of crystals, semi held together, much like when the sugar in a glass jar sits for too long, and forms big lumps.

To use the word steel is very very misleading.
people see 316 doing well on static non loaded parts in salt water, they hear the word steel ( erroneously used), and so they think it is ok then to use it for cyclic loads in heavy chlorine.

utterly incorrect, and 100% wrong to use the word steel; it is very very misleading. 316 is not steel. 316 uses iron for cheapness. Which is very different from using it for strength and toughness.

Steel is carbon and iron. Very tough stuff. The term stainless steel is very very misleading.

As different as Chalk and Cheese.

Cycles under stress in salt
how well will a lump of compacted sugar crystals do under cyclic loads in salt water?

A useful test method is to get a round bar, spin it at some RPM while deforming the middle of the bar with a wheel under some pressure at some % of the yield strength, while spraying it with either salt water or analogues (to simulate years).

firstly, normal steel.
lasts a very long time, but doesn't look sexy and shiny. Looks rusty.

316 fails in hours.

The Stress Corrosion cracks propagate astoundingly quickly. Hours in the lab for a solid metal round bar to break clean in half.

Mainly because 316 is not really a metal, just a loose collection of crystals, semi held together, much like when the sugar in a glass jar sits for too long, and forms big lumps.

Modern alloys designed with mathematical models, computers, actual modern scientists, understanding, theory, and endless thousands of tests last forever.

Susceptibility to Intergranular Corrosion "effectively eliminates any safe stress level for infinite life".
Please take note of that statement, as it is important latter on when we consider land based Rigging Law and Occupational Health and Safety Laws Versus alleged and alluded yacht rigging Safety Factors.

There is NO safe stress level for any alloy that suffers Intergranular Corrosion, if you do stress the alloy, then Stochastics say we can't predict when it will fail, but it will fail semi-randomly and unpredictably.

Now compare all this to modern alloys that are immune from crevice corrosion. These alloys have an infinite life. They last forever under stress. You can actually and sensibly claim and calculate Safety Factors.

(ps
Beware of vendors saying all these problems can be fixed by annealing or other highly technical treatments. It's all Bull. The problem is inherent in the stupidity of the metal chemistry. If the vendors claims were correct then no 316 rigging parts would ever fail.
)

This is why 316 parts will look perfect, and then fail without warning.