Wing mast wall-buckling

My limited experience agrees that wing masts do bend.

One serious factor not yet mentioned is twist in the mast which can bring the whole thing down as I know from sad experience. Twist is the result of lateral luff loading. Diamonds or similar devices designed to resist off center loads are effective. A single web at "b" was used partly for assembly issues as well as loading from diamond spreaders and column buckling. Luff groove was an aluminum awning extrusion with the web cut off. This was epoxy/glassed to the mast and the slot was then cut out.

I think that a wing mast stiff enough to resist twisting and side loads would be a lot heavier than using diamonds.

 

Idkfa, I have to ask, first, why do you want to make the skins so thin in relation to the width of the section. A thickness ratio of even 2% is too thin. My guess is that you have probably deduced that a larger foil section with very thin walls is lighter than a smaller section with thick walls. For such a small boat, the weight savings that you get will be miniscule, and the risk of the mast failing goes way up the smaller the t/D ratio is.

The other factor that figures into this is the size of the mast section. Even with an aerofoil section, larger sections have more drag than smaller sections, so there is always an incentive to make the mast as small as possible.

I have found in my engineering of wingmasts that a t/D ratio should be at least 3% of the minor diameter. So you should do some size comparisons to find out which size of section will give you enough strength and stiffness with at least a 3% t/D ratio. That will be the most weight efficient and aerodynamically efficient section.

There are also the practical considerations of molding or building the mast. First, you will need at least one shear web, and your B position will suffice. The double shear web will give you more shear and torsional strength and stiffness, but if you go for thicker skins, the need for a second shear web goes down because of the built-in shear and torsional stiffness of the thicker skin. One shear web is easier to build than two.

Also, you have to consider what the actual lay-up is going to be to get that very thin skin. Ideally, in a round section mast, about 80% of the lay-up by weight should be unidirectional carbon. For a wingmast, I have gone to a 70/30 split between UDR and off-axis fiber (±45° and 0°/90°), and even as low as 60/40 split. UDR carbon alone has practically zero cross-fiber strength and stiffness (provided only by the resin holding the laminate together) and it buckles easily because there is little holding the fibers in column. This is very localized buckling--not Euler column buckling. UDR in a mast has to be sandwiched between other layers of fabric, and on a small mast like this, a single layer of carbon 0°/90° cloth will suffice. That is, the other 20% of the laminate should be carbon cloth, with 10% by weight on the outside surface, 10% on the inside surface, sandwiching the UDR in the middle. Figure out what the thickness for that lay-up will be--you can vary the amount of UDR accordingly, and do iterations to determine the best section using the 3% t/D ratio. That is, you want the smallest section possible consistent with overall strength and stiffness.

Making the skin thicker by using a core is false economy. It will improve buckling performance to a degree, maybe. But if you split your already thin skins in two to insert a core, your skins are going to be even thinner and more fragile, and they can still buckle locally. And all you have added is the weight of the core and the resin to glue it to the skins. Cores in mast walls are not weight and strength effective. Even if you double the laminate by placing your basic laminate on the outside of the core, and duplicating that with the same laminate on the inside of the core, then your total carbn skin thickness has already achieved the minimum 3% that is desirable and there should be no reason to have the core then. So it is easy to come to the conclusion that a cored skin mast achieves nothing--it only adds weight, which is the very element you are trying to minimize. Composite masts should be only solid laminates with the appropriate mix of UDR and cloth fibers. On larger masts, I generally include both ±45° and 0°/90° layers sandwiching the UDR, and they are mirror images of each other through the center plane of the laminate. If the laminate is very thick, which must be molded in stages, then I will intersperse other off-axis layers throughout the laminate.

Finally, if I may, I shy away from female molded masts because of the cost and effort used to build the mold. Boatbuilders and sailors are notoriously cheap--that is, they do not want to spend any more money than they have to--so to build a mold first is to put money, time, and effort into bding something that is later going to be thrown away. For a one-off mast, that is total waste. For production masts, it is a different matter because the cost of the mold can be amortized over a production run to pay for itself, Therefore, the least expensive way to build the mast is over male formers and structures that are part of the mast anyway. It is not hard to do this. At the very least, it is possible to use foam male formers which are cheap and easy to make, and consititute a very small part of molding cost, even if they are discarded later.

Gary B. and Petros had the best advice above, which I endorse.

I hope that gives you some additional insights.

Eric

 

A typical 60mm profile with a 3% wall thickness is more than adequate for such a light weight boat. The increase in inertia in going to a 90mm profile allows for walls to be less than 1% (theoretically - my estimate), super light but then we have local wall-buckling to deal with.

The main reason for choosing the larger dia, is immersed volume, hoping it will stop the cat from turning turtle in a capsize. (will seal top 25%) The penalty is more drag, but if the leading edge is "smooth" it should also work at high angles of attack. For this reason I have constructed a female mould 60% the length of the mast. I'll make (4) half-profile sections, 2 60% and 2 40% long. Join the 2 60s together to make the full length, with a 20% overlap in the middle, (with good access to bond on insides) then add the 40s.

If the mast fails, ie a section beaks hopefully the delamination will halt at the bond line and I can cut that section out and replace it. Carbon masts being very repairable?

I hope to use no spreaders or lower stays. A 90mm 2% is "same" weight as 60mm 3% but lots stronger/stiffer and so I was also hoping that the web at B was not needed either. Will use a 60/40 fiber lay-up,, employ some of the "extra" vertical strength.

Placing a web at A or C, I think is easier than at B: Slide a thin-wall galvanised fence pole (cheap) through finished mast. Tape paper rings on flat of shear web (the web is constructed with correctly angled bond tabs), butter tabs with thickened epoxy, orientate vertically and slide along fence pole using rings (so the epoxy is not disturbed). Once in, turn horizontally and lower to correct position, when cured remove fence pole. B is the max width making harder to get in position without disturbing the epoxy?

 

Well if you are dealing with a cat and captsize, you need to look at the shockloads of a crash. I've seen a 49er snap a 3mm thick section in a high speed crash.

As for immersed volume giving you inversion protection - odds are it will not, particularly with a cat in a "worst case" scenario (crash to leeward in waves). remember that its not just the weight of the mast and rig that you have to keep floating. you also have to resist the sail area of the tramp and weather hull

F = 1/2 rho x v² x A x C where F is the force of the wind load in pounds, rho is the air density, v is the wind velocity, A is the surface area of the panel and C is a dimensionless drag coefficient (assumed to be 1.0).

Rho for air is roughly 0.75. On a typical beach cat A = 9m²

so this gives you 2.25 v² a 10knot breeze is 5 meters/sec.

so a 10knot breeze on a 3m x 3m tramp will give you a force of 56kg/m/sec of force.

and this will be operating with a force couple of roughly 1.5 meters (media distance of surface area from the immersed hull). So you will need roughly 80 kg of floatation. ie 80 liters of water. Not gonna happen with your mast

 

The 56Kg rotates the pivot (the hull) with force of 80kg. Where as the lifting moment acts near the end of the mast 7m away, lets say center of lift at 6m,

Mast vol 170 sqcm*175cm=29.7Liters

6*29.7=178kg floation

 

Yeah but you have to add in the weight of the mast operating at that same force couple distance. The mast is going to weigh at least 5kg. plus sails you probably are at 7kkg at 3.5 meters so that's 25kg of immersion force, add in one hull at say 30kg at 0.5 meters (15deg leeward cant and you now have 80kg+25kg_30kgs ==135kg and this is before the wave force both on the sail in the water and the hulls.

real close.

 

Made a 1ft test part. It needs a shear web, will place one at B.