Carbon Fibre Dinghy Mast

I mostly lurk in this forum but I have run into a question that has me stumped.
I'm setting up to make a carbon fibre mast for my 15 ft CLC Skerry. This is really just playing and exploring but I plan to later use this mast for my next boat so I want to make it longer than the Skerry specs. I'm hoping to end up around 17 feet.
I think I have a workable method and a source of materials BUT I'm having trouble finding info on how to determine how thick the build up should be.
How do I test if the mast is strong enough. Do I just sit on it and measure deflection after each layer has cured? Is there a table that magically says for a 17 foot mast with 90 sq ft of sail the mast should be 3/16 inch thick? I realize that there are a lot of variables but I expect to overbuild a bit.
As you can tell I'm a bit of a beginner but I am careful.
Maybe the way to approach is to figure out maximum load using maximum sail area and maximum expected wind and then just put a weight on it and say it's strong enough. I simply don't know where to start.
Thanks
Chris

 

I've seen a chrub video/ website that talks you through laying up a carbon mast. I'll try to find the link. I understood that the stiffness of a mast is usually defined by sailing characteristics rather than the breaking point. If your using a circular profile I’d start off with hand calcs pull out your old copy of roarks.
I'd be careful only laying up part of it as it could de-laminate between layers. With glass and polyester we work on a 12 hour window. This is maximum time between any two layers. Any longer than this and you can’t be sure it won’t delaminate between layers. If your using epoxy I’d aim to layup the whole mast in one hit. Then either sand it back if it’s too stiff or lay on another layer. You can buy some epoxy resins that etch into the surface of polyester or epoxy designed for repairs that bond a lot better but are more expensive. (west systems sell one)

 

Hi Laurencet
Thanks for the reply.
I've never thought of delamination as an issue with epoxy particularly since I usually sand a bit between layers anyway. I'm not using polyester.
Do you think I could laminate the whole mast out of fiberglass rather than carbon fiber? Carbon has quite a reputation but really what I am looking for is fairly low tech.
<If your using a circular profile I’d start off with hand calcs pull out your old copy of roarks. >
Where could I get my "old copy of roarks"?

I appreciate your comments, information at my level is hard to come by.
Chris

 

depends how highly stressed it is also what resin system you are using. there's a collection of tricks of the trade like sprinkling it with chopped strands to reduce the problem.
I'm not sure without running the calcs what weight saving of building a grp mast would be. Might be alot easier to buy a second hand alumium mast from ebay. you could build a short sample and see if it has the characteristics you want. I know some of the cheaper windsurfer masts use a carbon glass mix.

There's a few dubious PDF versions of the roarks floating around the internet. Probably best to buy a copy.
http://www.amazon.co.uk/gp/offer-listing/0071210598/ref=ed_oe_p_olp/280-6224888-9705026
have you tried to get hold of a set of plans for a similar mast. At least then you have a good starting point.

 

Chris,

What you are asking for is a Dummies Guide for Building Masts. There isn't any such animal. The problem of determining the loads on a mast, and then figuring out the size of the mast section, in concert with determining what the strength and stiffness properties of the laminate will be, is a complex engineering problem that requires many calculations and iterations using first engineering principles. Roark's is one such guide ("Formula's for Stress and Strain" by Raymond J. Roark and Warren C. Young) but it generally cannot be used alone. You have to know basic engineering mechanics first. Also, Roark's applies to homogenious materials, not composite materials. So you also need an understanding of engineering of composite materials in conjuction with the principles that are laid out in Roark's. You also have to take into account that if the mast is stayed, it behaves as a column. If the mast is unstayed, it behaves as a beam. If the mast is tapered, there can be an infinite number of solutions to the design because the wall thickness depends on the size of the section. And there are limits--too small a section will have too thick a wall and will be to bendy (generally) and too big a section will have too thin a wall and will buckle. So you have to have an engineering understanding of why all that happens, what controls what else, and how to avoid problems.

In general, masts are engineered structures that are very sensitive to a number of variables, and each is customized to the boat at hand. This is why I never standardize my mast designs because too many variables would get out of my control. Lay-up schedules are complex as well, and must be made to the materials at hand. Carbon fiber in Europe is not the same as carbon fiber in the states, particularly when you consider the variables just in the fabric weights and construction of the weaves. All these things play a factor in the engineering of the mast laminate schedule.

You have a few options:

Learn engineering principles and composites, and design the mast yourself--a very long process.

Consult a professional who does mast and composite design regularly and pay for his services. At the size of your mast, the design fees will generally cost way more than the mast.

Consult a mast builder who does this kind of stuff every day and see what they may have in the way of stock designs that you can use. Harry Proa (www.harryproa.com is one such company).

Build a mast by trial and error. The easiest way to start is find a mast design in aluminum that is typical of that size of boat. Note the size and shape of the mast section, and the wall thickness. You can duplicate that section shape dimension for dimension in carbon fiber and epoxy resin, and you will be pretty close. The mast should be stronger, about as stiff, and nearly half the weight. You'll have to use vacuum bagging techniques for the layup, and you should post cure the mast after the normal cure. Try it out. If it works, great. If you want to improve it--make it smaller in section or lighter--you know enough from the first build to attempt a second build.

I hope that helps. Good luck.

Eric

 

Hi Eric
You are dead right, I am looking for the dummies guide to mast building. I enjoy tinkering and exploring materials and techniques. Your advice is extremely well put.

Fortunately for me it is a small boat with a small sail area and the unstayed mast is not subjected to huge forces. The current mast is a Birdsmouth mast and it works well. Another experiment that turned out really well. It is just a bit short.

If the carbon fibre mast doesn't work out not much is lost on such a small project. If its better then the good guys win again.

I will as you suggest, look at existing aluminium masts and plan my layup on these. Luckily for me the Toronto Boat show is at the beginning of January, good timing.

Thank you so much for taking the time to reply to an obviously very inexperienced builder.

Christine

 

You can always sell it off as a mike boom if it doesn't work out

(Just kidding!)

Good Luck!

Jimbo

 

Heck I always need good tomato stakes. These would be a cut above the rest.
Thanks for the help.
Chris

 

Chris,

Another idea to consider. A Flying Dutchman is approximately 6 meter in length. They allowed carbon masts for the fleet a few years ago.
Several manufactures went through the learning curve you are considering. After several failures in the masts they now seem to have worked out the bugs including the necessary thickness to prevent mast failure.

Look a the the web site for Ballenger Spars listed below. Send Buzz Ballenger an email or give him a call.

http://www.ballengerspars.com/

He, I believe, could possibly sell you a carbon mast for a flying dutchman dingy at a good price. If you want a blank to trim out yourself he probably could help you in that area too. Flying Dutchman is a high performance 6 meter dingy. There mast are high performance and the weight is pertty minimal too.

If anything mast are his business and he could help out in the right direction.

Duane

 

Hi Duane
Thank you for the information.
I think these masts are a bit overkill for me to buy at this moment BUT the lead about Flying Dutchman is great. It will give me another avenue of research.
For my first mast I will do it myself and accept that the mast will be less than perfect.
Ballenger has an interesting website thank you for passing this information on.
Chris

 

Eric, as usual, has given good information and advice.

There is one more thing to agonize about. When you finish building the mast, it will have its own unique bend characteristics. That means that you will need a sail that is cut to accomodate those particular bend characteristics. If you can find a sympathetic sail maker just take the mast to him or her and have them do the deflection tests that will determine the sail cut. You can have a good mast and a good sail that do not agree with one another and then you will think that either the mast or the sail is a dog. Not necessarily so. They just need to work in harmony.

 

That's a very good point. Thank you.
I had planned to put a balanced lug on the mast and I will now wait till I can check the mast before I get it.
Chris

 

I used to work for a mast maker. We made one offs or very small batches for sailing boats from about 10m length up to the largest sailing superyachts. Some were ally, some were carbon. Since I dont work for them any longer, and the company has gone bust anyway, I can let you into a secret. It was a relatively long established company and we had a data base of thousands of masts we had made. For each new customer we advised that we would need a few days to do some special calculations for his boat, then we went away and looked through the data base to find one that had worked before for a boat similar to his one and that was our design, with minimal mods. Occasionally we did actually do some calculations, but really we only did that out of curiosity, I dont think we ever believed the calcs even if they were right. We would base the calcs on the maximum righting moment of the boat, the width of the shroud base and the compressive load for buckling.

btw, You dont need Roarks to predict the strength or stiffness of a circular section tube!!

Eric above is absolutely right when he says that if you make the carbon/epoxy wall thickness and section similar to an aluminium mast that already suits the boat then you will have something quite close to the right stiffness and strength with less weight than the aluminium mast. I did check the actual bending modulus of some of our carbon masts by loading them in bending between two trestles and working back through the calcs to get the modulus. By theory, the modulus should have been higher than for aluminium, and perhaps an aircraft manufacturer could achieve that, but in reality for our carbon masts the modulus was generally very similar to that of aluminium. The weight was of course less so you do get an advantage.

I did also make a carbon dinghy size mast as an amateur project before I happened to find myself doing it professionionally. For my dinghy mast I made a styrofoam male mould, covered it in parcel tape, supported at intervals along a bench, applied a little carbon to the side away from the supports, enough to give it some strength, took the supports away and applied carbon all over then vaccum bagged while curing.

 

Chris; When I wrote the bit about sail and mast compatibility, I was making assumptions. (assumptions are the devils playtoy) When the sail is attached to the mast, as in a bermudian sail, the mast bend is in direct contact and attached to the luff of the sail. So sure there are some influences at work. With a lug rig, the luff is not directy attached to the mast and therefore the behavior of the mast has but little influence on the way the combination works. You might need to consider the bend charactristics of the yards, not the mast in this case. If you use a lug rig, the mast can be really crude without a problem. Something as sophisticated as a CF mast paired with a lug sail is like making a silk purse out of a sows ear.

One of the factors that favor lug rigs is that you can get a lot of sail on short spars. They can also be easy to reef and easy to douse. When reefed there is but small change in helm. Old time fishermen would furl the sail around the yard and hoist the bundle above the deck so that they had unobstructed space to do their work. Within the list of disadvantages, there is the weight of the yard at the top of the rig, good tack/bad tack unless you dip, sometimes difficult to tack smoothly, especially in a light boat, and most seriously their tendency to oscillate on dead downwind runs. The oscillations have a quaint name; death roll. Many peak sprit type sails have the same malady.

Fair winds

 

Hi John
Re using tried and true proportions is a good way to go. Harder to sell I guess but safer. You know it actually works.
Calculations are only as good as the data you plug in.

I have started making my core.

http://www.christinedemerchant.com/carbonmast.html

I think this way will take more time than I had thought but will work.

Chris