High Modulus Masts

[Neil C.]

There is another 12-foot long development class with which I may be acquainted, but which good manners prevents me from naming on this forum. The latest step forward in rig development from our Australian buddies seems to be the use of high-modulus carbon masts, with Vectran rigging. My question is, what's the difference (apart from an extra £250 on the price tag from C-Tech)? Are these masts smaller diameter, lighter or both? How does Vectran compare to conventional wire rigging? Is this something which may be a serious performance benefit to UK Cherubs?

[phil_kirk]

Not a serious benifit.  vectran or PBO will reduce the weight of the rigging a little.  Anti has a high modulus mast as do I for E-Numbers and Dan for his boat.  This appears to give the required mast tip bend with less bend lower down.  This should improve gust response whilst not loosing all the power from the rest of the main and jib.  It is difficult to compare the current Cherubs that have opted for lighter rigging and High because the ability of the sailors through the fleet are still a significant variable. This years nationals results are testiment. Like everything with rigs It has also got to be tuned right too. i don't thin kthe lower part of the mast is any thinner than the earlier masts.

It would be interesting to know the all up weight of the recent boats and compare the difference with the change in weight of rigging materials.

[Will_Lee]

HM fibres are stiffer than the ordinary stuff, (320+ rather than ~250 though I don't know whether C tech's non-hm is IM or SM (Intermediate or standard Mod).

You can have the same stiffness with a thinner wall, which is lighter; the same stiffness with a smaller diameter and possibly a bit less weight, which is what we have, or more stiffness with the same weight and stiffness.

Vectran has a mod of 75-100, and when woven it acts like it is even worse. To get the same rig behaviour you have to put on more tension. It is probably ok for forestays and possibly uppers, but I do not rate it for other stays.

We have steel forestay and shrouds (mod ~190), woven PBO lowers, d2s and uppers. I have the vectran to make a forestay but I can't bring myself to do it.

Is it worth it? Well the rope shrouds probably make more of a weight difference than the mast, and let me promise you this, there will be no place changes in any race of 12 foot skiffs in the UK due solely to the modulus of the mast material.

http://www.netcomposites.com/education.asp?sequence=35

http://www.vectranfiber.com/pdf/8Pages%20from%20Vectran_broc_final61206-8.pdf

http://www.lenntech.com/stainless-steel-316l.htm

[Neil C.]

Thanks for the references Will. If I have this correct, "modulus" is a measure of elasticity, whereas "strength" is a measure of er, strength, i.e. the load it will take before it starts to stretch or snap. But if that's right, wouldn't a low modulus material give more stiffness for equivalent weight? I'm still a bit confused (not difficult). I fully agree that the most important performance-enhancing component on the boat is the nut that holds the tiller, but in the event that we lay out some dosh I would like to get the right gear, weighed up against vfm (I am Scottish after all). 

[Neil C.]

OK, from wikipedia:

An alternative definition is that the elastic modulus is the stress required to cause a sample of the material to double in length. This is not realistic for most materials because the value is far greater than the yield stress of the material or the point where elongation becomes nonlinear, but some may find this definition more intuitive.

So the higher the number, the less elastic it is, i.e. it's stiffer.

[Banshee Ambulance]

Correct, Elasticity and modulus are essentially the same thing. Strictly modulus is stress/strain where stress is the force per unit area and strain is the extension/original length. This is if you are talking about youngs modulus and is appropriate in this case to fibers in tension.

The higher the modulus the stiffer the material. As will says, higher modulus fibers mean for the same wall thickness and tube size you can a stiffer mast. If you desire the same stiffness you can have a thinner wall (less weight) or a smaller diameter mast (less windage). As to weather any performance gain can be seen on the race course given the variations in crew ability previously mentioned is anyones guess.   

[roland_trim]

If you desire the same stiffness you can have a thinner wall (less weight) or a smaller diameter mast (less windage).

Remember that wall thickness has lots of advantages here. more knock resistance and local wall buckling resistance (seciton buckling is a function of section stiffness)

For the same weight of cloth and number of layers weight reduces linearly with diameter, but stiffness is a factor of radius cubed (^3). So for a 50% increase in stiffness you get a 1.15 reduction in diameter. Keep the wall and layup the same and end up with a mast tube 87% the mass - although not all of this is realised unless you also factor the spreaders and fittings down as well.

We decided to experiment with dyneema uppers on Born Slippy and have continued this with EJ. Having more stretchy uppers gives a longer spring range when the tip moves (and the whippy masts do alot). In real terms this means if the mast tip comes back a bit there is still nearly the same force lefty-right on the tip and it does not fall away as quickly. But it does mean we have to remember to tension the uppers or we get Graham telling us to bin it in as the mast is inverted from the kite....

[Banshee Ambulance]

Anyone done the maths for the stiffness of a tapered tube? Some kind of crazy differential i expect.
I would be interested to see it.

Off for a blast in Stanley now! 20.2 knots on the weather station!