Hello All,

I am planning on building a carbon wing mast for a catamaran, the section shape will be a 5% mast according to the method presented by Tom Speer in the paper:
http://www.tspeer.com/Wingmasts/teardropPaper.htm
The question I have concerns mast bend. I've read about dynamic/automatic gust response, in Frank Bethwaite's High persformance sailing and on homepages from various spar manufacturers. However none of these sources offer anything of more substance in the way of furmulae or numbers.

The taper on a a-cat mast (marström), was from a circumference of 17,5 cm to at the hounds to 14cm at the masttip. This would (with same laminate stack) result in a reduction in stiffnes to 64% of stiffnes at the hounds.

I don't know if this is at all representative, and I don't know how it scales.( I plan to have a 27sqm unarig on a 4m wide doubletrapping cat) Could anyone please give me some hints and or comments.

Regards
Patrik Elfving

Hello Patrik,
If I were you, I would not attempt the project. A 5% mast section: the max thickness is 5% of the chord. This means if the mast is 20cm on the chord measurement, it is 1cm thick at max thickness. This is way too much difference in fore and aft stiffness and sideways stiffness. For 27sq meters of sail area, this mast would be about 40ft or 12 meters tall. A 12 meter mast that is 1cm thick at max thickness will not stand even if it were solid carbon.
I think you should forget this project.
Bill

Bill,

Did you look at the link in the above post? It would appear that in context the 5% refers to the mast chord as a portion of the total chord of the airfoil (mast + sail) and that he is planning to build a typical teardrop shaped mast with a cord of roughly 5% of the sailplan.

It does not appear that he is asking about building a teardrop shaped mast with a cross-section equal to only 5% the chord of the mast as you imply.

Hello again,
Yes Jamie, thats correct
Anything would really help, such as mast mesurements from "good" masts, even from masts with less or more SA. Hopefully I could, given enough data and some imagination, figure out how to best scale the properties.

Regards
Patrik

Hi Jamie,
No, I did not look up the link. In the world of aerodynamics, starting with areo 101, the % chord of an airfoil is defined as the thickness to chord ratio times 100%. Wing masts were tried and passed over about ten years ago. To be effective, they are large. When they are large, relative to normal catamaran mast, they are very stiff because of their moments of inertia and heavy because of their large surface area. Being stiff, the mast plus sail is set at one camber. There is no flexability in sail shape/control.
I built a wing mast once. I built a plug and then I built a mold. Then I built two masts. The mast section was 12 inches long on the chord and 5 inches max thickness and 45 ft tall. I spent one winter doing this project. I have never worked so hard for no improvement in my life.
Bill

Hello again,
I probably need to make myself a wee bit clearer,
The term "wing mast" might be used broadly by me but the section I'm talking about is something very (for me impossible) hard to distinguish from a normal a-cat mast. It's about 6cm * 15cm,
I'm not shure if this is semantics or not, but it seems as the question at hand is missed.
What are the mast bend characteristics of masts with dynamic/automatic gust response?
Given that I will figure out wether it will work or not on the section I plan to build.

Regards
Patrik Elfving

Patric,
I would sugest you involve your sailmaker from the very begining. You can go either of two ways 1) you specify the deflected mast shape and he/she optimizes a sail to fit or, 2) he provides you a recommended deflected mast curve and you design the mast to comply with the curve. Either way once you have established the deflected shape you are after, it can be obtained by either tapering the section, tailoring the layup, or a combination of both. Because of the directional properties of carbon it is possible to build a non-taperd mast to deflect to any curve you choose by selectively positioning plys of fiber. There is a lot of software out there that will calculate lamiate properties based on the orientation of the individual layers of carbon uni fibers. This allows you to try a lot of different concepts before investing time in the real thing. I do the calculations by hand but it's tedious! god luck,

Kevin

I would like to 2nd Kevin Cook's reccomendation, Involve your sailmaker from the beginning.

I am doing something similiar on a smaller scale. I am turning a broken Marstrom A-Cat into a mast for a F14. I talked to my sailmaker from the beginning and his help has been invaluable.

Also be carefull about using Marstrom A-Class Mast's for scaling. They are much thinner and stronger than anything a homebuilder would be able to create. In cross section they look like thick paper.

A mast for a C-Class or an RC-30 would be a better starting point for what you are looking for

Hello Patrik,
My advice is still forget this project. Someone is leading you astray. For example; tapering a mast from 17.5cm circumference at the hounds to 14.5cm circumference at the top does not reduce the mast stiffness 64% at the hounds. Someone is telling you homespun physics 101.
If you look at the Bethwait mast/sail, you notice the leech of the mainsail intersects the top of the mast at nearly 90 degrees because the mast is built with severe aft curve in it. This causes any increase in leech tension, due to a puff for example, to pull directly down on the top of the mast and induce more mast bend which flattens the sail and opens the leech.
On a typical catanaran rig with a relatively straight mast, the leech load from the mainsail intersects the top of the mast at about 15 degrees. (The tangent of the head angle equals the foot/luff for a straight mast.) This means that 25% of the leech load goes into bending the mast and most of the load increases the axial compressive load travelling down the mast. In contrast to this the Bethwait rig puts 100% of the leech load into bending the mast and the mast responds. With only 25% of the leech load going into bending the mast, the bend response would be much less. To build a mast with built in curvature like the Bethway mast, requires two molds, one left side and the other right side. This is a big project!!! Too big for a hobby.
Bill

Two curved mould halves indeed sounds as a large (too large) commitment. But as long as the mould halves are straigt I actually believe I should be OK. I work at a company making carbon fibre composites for aero and auto inustry, so I have acces to equipment beyond the normal hobbyist. I am naturally going to discuss this with a sailmaker, but would still like to have some ballpark guess to be able to understand and ask him the right questions.

BR
Patrik