Carbon Fibre Masts

Mike,

What I wonder about FEA analysis on carbon masts is, how detailed is the model, and what is the input going in for the laminate properties? If the data going in is not reliable, then the results coming out will be likewise (garbage in...garbage out). In this field, testing at full scale or near full-scale is advisable, and was that ever done? I don't know. I do know that a lot of my engineering is based on large and small scale testing. It was not as sophisticated as FEA certainly, but it worked, and I have a good record of successes as a result.

As for my writing history on carbon fiber masts and similar subjects, here is a resume of the pertinent material, most recent first:

Technical Papers:

PROJECT AMAZON: An Open Class 60 Sailboat for Single-handed Round-the-World Racing, published in Marine Technology, Vol. 37, No. 2, Spring 2000, Society of Naval Architects and Marine Engineers (SNAME).

The Quest for the America's Cup: Focus on Composites. Presented before the Society of Material and Process Engineering (SAMPE) 23rd International Technical Conference, Kiamesha Lake, NY, 21-24 October 1991.

Carbon Fiber Sailboat Hulls: How to Optimize the Use of an Expensive Material, published in Marine Technology, Nol. 23, No. 2, April 1986, SNAME.

FRP Materials--Data Sheet Interpretation and Application to ABS Standards, presented at the Powerboat Symposium--Recreational and Commercial, SNAME Southeast Section, Miami Beach, FL, 19-20 February, 1985.

Fundamentals and Practicalities of Carbon Fiber Composites for Marine Applications, presented before the SNAME Southeast Section, St. Petersburg Beach, FL, 24 September 1983.

Design and Engineering Aspects of Free-standing Masts and Wingmasts, Sixth Chesapeake Sailing Yacht Symposium, Annapolis, MD, 15 January, 1983.

Speeches:

The Quest for the America's Cup--Focus on Composites, presented before a joint meeting of (SAMPE) and the American Institute of Chemical Engineers (AICE), Rutgers University, New Brunswick, NJ, 7 February 1991.

Design of Carbon Fiber Masts and Other Spars, presented before SNAME Southeast Section, St. Petersburg, FL, 29 September 1984.

Magazine Articles:
Case Study in Lightweight Engineering, Professional Boatbuilder #79, Oct/Nov 2002. (This covered a whole boat design, but included the carbon rig, carbon keel, and the carbon rudder.)

Factors of Safety, Professional Boatbuilder #72, Aug/Sep 2001. (This applies to general engineering for boats and included some material on composites.)

Wobegon Daze--The New Cat (Ketch) is out of the Bag, Sailing Central.com, Vol. 5, No. 8, August 2001.

Free-standing Rigs Provide Major Racing and Cruising Advantages, Racer's Edge, Vol. III, ed. 3, March 2000.

Project Amazon and the Unstayed Rig, Professional Boatbuilder #54, Aug/Sep 1998.

Wingmast without Carbon, SAIL magazine, February 1995.

Free-standing Rigs: A Concept Comes of Age, Yachting magazine, June 1994.

Carbon Fiber Ideal for Shroudless Rigs, interview with Mike O'Brien, Soundings--Trade Only, April 1987.

Eric Sponberg and the Carbon Fiber Evolution, interview with David Berson, Sailor magazine #11, April 1987.

New Rigs in the Air, SAIL magazine, January 1985.

Measuring and Reducing Windage, SAIL magazine, March 1984.

Engineered to Stand Alone, SAIL magazine, October 1983.


This last magazine article was a companion piece to the technical paper above, Design and Engineering Aspects of Free-standing Masts and Wingmasts, the former being the layman's version of the more technical piece.

Eric

 

G'day,

A properly designed and built unstayed carbon mast has one part to fail, the mast itself. This can be tested before being put in the boat, and will last almost indefinitely. Compare this with over 100 small pieces on the average stayed rig, breaking any one of which results in the mast falling down. Add this to the benefits of automatic depowering, being able to raise/lower the sails on any point of sail, soft gybes and no wear and tear on the stays and trying to make a case for stays is a pretty futile argument

As to price, I built an unstayed 12m/40' long carbon mast for my boat. Weighed about 35 kgs/77 lbs. Materials cost was less than $AUS1,500/$US1,100. I built carbon strips (tapered in width and thickness) in cheap mdf moulds and compacted them at over 2 atmospheres with a vacuum pump. Resin fibre ratios were as good as I have previously got with prepreg. The material I used was carbon tow, the stuff cloth is woven from. This costs $US8/lb, less than half the cost of uni fabric. The strips were glued together in 2 halves in cheap mdf moulds, with off axis material inside and out. All worked a treat.

For some pictures of the technique see http://www.harryproa.com/building_hg/buildinghg_wk14.htm
http://www.harryproa.com/building_hg/buildinghg_wk15.htm

We have now improved this technique and are making carbon masts considerably cheaper than any others in the world that I know of. We are currently building a 13m mast, which will be followed by a 12 and a 19.

Raggi, two masts for 20m harryproas will be built using this technique in Trondheim in the next 12 mmonths or so.

Regards,

Rob

 

I'm wondering about designing for strength vs stiffness. One can get the desired factor of safety in strength by adding more material. However, this makes the mast stiffer as well. So while you can discount the strength of the material to ensure it will be strong, you can't discount the modulus and still achieve the desired mast bend. It seems to me the factor of safety needs to be applied by limiting the maximum strain and designing for 100% of the load and material properties.

Do you find you still need an accurate estimate of the load distribution in order to tailor the stiffness? Do you tend to go with a smaller cross section and more material so you can get the desired deflection under the predicted load with a smaller strain?

 

CF masts

Two atmospheres with a vacuum pump?
I never managed to get more than 0,9.
Marco.

 

Cassavecchia,

If the strips are 25mm wide, but the vacuum is applied to strips above and below them 50mms wide, then one atmosphere of vacuum applies 2 atmospheres of pressure on the job.

regards,

Rob

 

Rob,

You say "It also takes a while to build, but the work is very simple."

Bearing in mind that for the majority of people time is money , how long does it take to build and finish the mast using your technique?

Gareth

 

Gareth,

A few too many variables to be exact, but say a day to set up the moulds, a day to vacuum and clean up 3 strips (14 needed for a 180mm dia mast), a day to join a half mast, another day for the inner laminate, same for the other half, then a couple of days to join them together. Fittings depend on the rig, but they can all be composite, fairing and painting as long or as short as you like. Probably not a method for professionals due to the time, but for amateurs who do not cost their time, it is a pretty cheap mast. If you want to pay someone to build it, then our modified technique is the cheapest you will find.

Regards,

Rob.

 

Answer to Tom Speer,

Yes, you do have to tailor diameter and wall thickness all along the mast, and then calculate the mast deflection by integration.

The process begins with the determination of the loading, and for that I work with the righting moment of the boat and not the aerodynamics of the rig. The load on the rig is balanced by the righting moment, so if you know the righting moment, you have a much better starting point. You also know that by definition, the moment at the top of the mast must be zero. Therefore, in the simplest case, the moment distribution along the mast is a straight line from maximum moment at deck level, to zero at the masthead. Likewise, the load from the deck partners to the heel, by definition of a cantilever, must be linear with the moment equal to zero at the heel. An argument can be made that the load distribution from partners to masthead to be something non-linear, like parabolic, elliptical, or whatever, and I have designed masts that way, but I have also found that a linear distribution is just as effective and does not make much difference in the overall weight of the mast by the time you work out the practicalities of the composite lay up.

The next step is to determine the diameters of the mast at various sections along the length. Pick whatever you want, most masts have smaller diameters at the top than at the partners, so this is obvious. I like to use what is called an entasis taper which gives larger diameters up high that what a normal straightline distribution of diameters would give.

So you know the load, the strength of the materials you will be using, and the outside diameter. You have only to calculate moment of inertia and section modulus at each section, and that depends on wall thickness. So the spreadsheet program calculates the required section modulus, and you cycle through the spreadsheet with different wall thicknesses to determine the actual section modulus at each section.

A second part of the calculation determines deflection. Now that you have a trial thickness to satisfy strength, you need to satisfy deflection as well. I have worked through, over the years, appropriate guidelines for how much deflection I want, so I have only to confirm that with the deflection calculation.

Deflection is the double integral of the term (M/EI) over length. E is constant for the laminate, but both M and I change along the length, but they are known from the strength calculation. It is possible to complete the integration (by brute force calculation, as I call it) and come up with a deflection of the mast all along its length. If the deflection at the tip is too much or too little, I go back to the beginning and change diameters until I get it where I want it to be. I have found that the entasis taper works remarkably well this way, and Project Amazon's masts had entasis taper and worked remarkably well. So did the mast for Copernicus, a Spencer 42, a stayed sloop converted to a free-standing mast that is up in your area, by the way, in Vancouver.

The method of construction, therefore, is tailored to each mast design, and I try to stay away from using already existing mandrels, the vast majority of which use straight tapers. However, if I am stuck with it, depending on the situation, I will use it. Other factors going into the laminate schedule are whether the builder is using prepreg or wet lay-up, because the laminate thicknesses will vary with the materials and the process used. This has to be known beforehand, obviously. Knowing the required laminate thickness, I then develop the laminate schedule with the materials that the builder has available to work out the laminate schedule. All of this is then drawn up in construction plans.

This takes a considerable amount of engineering and design, and that makes the whole process expensive. And that is just for the mast tube itself. Then there are the fittings for the masthead, the gooseneck, the hounds if there is a headsail, the bearings if the mast rotates, the partners, the heel, the halyard blocks and fittings, the boom, and the reefing lines. All these acouterments add to the details of the design, and therefore, the cost. But I have had many successes, and done properly, they work very well.

The final step, of course, is to make the sails, and with any rig, there is a bit of trial and error. While I can predict the actual curve of the mast bend, to try to get the sail fabric conform necessarily requires cutting the sails, trying them out, and fine-tuning them. It usually takes no more effort than a normal sail order. What sailmakers usually fail to understand, however, is that the mast bends differently for a free-standing mast than for a stayed rig, and so they have to cut the sails very flat. Because a stayed rig does not rotate, the normal cut of sailboat sails is to put in lots of draft into the sail in order to fool the wind, particularly in beam reaching and broad reaching, that the mast-to-mainsail transition is really something akin to an airfoil section, when in fact it is not. It is actually a very poor shape. But a rotating free-standing mast has an exceedingly good mast-to-mainsail transition--the mast always turns to the best angle of attack into the wind, regardless of boat heading--and so the need for draft is almost totally eliminated. It usually takes a sailmaker a few tries at these free-standing rotating rigs to understand this.

Eric

 

Carbon Mast Design and Costs

We are given to understand that each and every carbon mast needs to be designed specifically for each particular boat. Surely the need for individual design continues to increase the costs of a mast which needs to be manufactured from expensive exotic material anyway.

Yet, other threads on this site and indeed throughout the internet continue to expound the fact that the calculation of mast forces is such an inexact science except perhaps for AC syndicates. It seems that most put in a rather large factor of safety to take care of these inexactitude.

If this is correct, what is the scope of incurring higher costs for the custom design for a carbon mast? Wouldn't the design of a few standard layup schedules be good enough? Most people seem to get by quite well with aluminium masts of standard profiles. If nothing else this may reduce the costs somewhat.

regards

 

Farjoe,

In theory, you are correct. One would think a few stock sections would satisfy a broad market. But so far in my experience, there has not been enough of a demand to go through the engineering and design of such sections and expect to make any money at it. I deal with a few mast builders who have a variety of mandrels on hand to make suitable masts--so in a sense they are prepared to make some standard sections. And maybe they even have some standard designs per each mandrel of their own. But they don't want to start lay-up and tie up all that money in carbon and epoxy in the hopes of selling a bunch of masts. The demand has simply not been there to date. So they wait for an order to come in and tailor the laminate accordingly.

Maybe that will change now as more and more people are inquiring of free-standing masts. I certainly get a lot of traffic on the subject.

My focus of recent years has been on wingmasts, and that adds complication to the design. I have not yet found a consensus of design sizes that would lead me to develop a standard series of mast designs. The boats are of so wide a variety (weight and size) and different in rig layout (sloop, cat ketch, conventional ketch, etc.) that I have not detected any pattern to what the demand would be. I could do a bunch of design work toward a standard series, but it would still take a long time to make a profit off the effort.

Eric

 

Hello all! A "new guy" here. Mr. Sponberg; I have been perusing a Gary Hoyt designed, Freedom sloop with a CF, free standing mast. It is a 1986 vintage, and what you all are discussing here are exactly the causes of my hesitation. What and how does one go about inspecting a 20+ year old mast. Surface peeling in my opinion be a dead givaway I expect. And at the cost of a "free standing" rigged boat (I don't know why they seem to be more on the expensive side of used boat's) I certianly wouldn't want to re-man's the mast. All said, I remain facinated with the unstayed rig. The full battened very large main and fractional foresail seem to be a comfortable set-up for an newbe like me. As with anything "new" CF seems to be the future when costs come down, and not having the clutter of standing rigging snagging a toe, appealing.
One question, that I hope is not stupid! Is foam filling, with suitable passage ways for internal halyard's and wiring, any kind of an advantage?
Thank You

 

nseal1,

You are right that surface cracking and peeling is very obvious. If you try to slip a knife blade under one of the cracks, see if you can peel it up. Often the cracks are very fine, you can't get a knife blade in there, and there is no worry for the integrity of the mast. As the mast ages, the cracks will get wider and the exterior gelcoat may peel up. Then the cracking is serious and should be repaired.

I'd like to state again that this cracking phenomenon appears only on Freedom and TPI-built masts. The cracking is a function of how the mast laminates were built. If the cracks are attended to in a timely manner, they can be successfully repaired, and the masts can go onto long and happy lives.

As for your question about foam filling, as a matter of fact, just today I have updated my website to include a story about a New Zealand cat ketch named Pooh Sticks II, for which I designed the masts back in 1985. The owner used a foam mandrel which remained captive in the mast. This boat has been sailing successfully for 16 years. Click on the NEWS item for Pooh Sticks II, and it will take you to the story.

http://www.sponbergyachtdesign.com/News.htm

While you are at it, have a look at the other news item for the Scandinavian Cruiser 40, which is a new production 40' daysailer that I am working on. It will have carbon fiber lifting keel, lifting rudder, and free-standing wingmast sloop rig. It will also be a "boat in a box", shippable out of mainland China in a shipping container.

Eric

 

I have decided to go for unstayed masts on a 49 ft catamaran, and have looked at the relative complex methods required to design and build them.

I have 2 ideas - they might be goofy -anyway here goes:

1.) If using a round mast section, why not put an airtight tube in it (kevlar) and pressurise this? Surely this will solve some of the buckling issues?

2.) For a wingmast, why not use a central round section that can be made on a machine, and then add smaller round sections around this. All the CF tubes can be fixed in wingshaped "templates" at calculated distances up the mast. Finally cover this with a thin surface to achieve the good aerodynamic shape required. Some of these tubes can be sealed to ensure that the mast is buoyant.

Surely much cheaper to build, and probably lighter.

Are these viable solutions?

regards

Alan

 

Cruising Catamaran Rigs

What do you think about this analysis Eric? I was looking thru some older material I had from Chris Mitchell
http://www.aes.net.nz/Rig%20Design%20Commentary.html

Crusing Catamaran Rigs

It is common for light weight racing catamarans to 'fly a hull' and use 100% of the available righting moment and at the same time reducing hull drag by removing a hull from the water. The overall "effect" is well known and the righting moment is generally the displacement multiplied by half the beam from hull centreline to hull centreline.

The situation changes radically for cruising catamarans. Instead of a displacement of some 3 to 4 tonnes, a similar sized vessel may have a displacement of 12 to 15 tonnes. A four to five fold increase in displacement is not uncommon. The vessels are normally bridgedeck which affords a large volume for the nice things in life and each hull can contain various chest freezers and washing machines for live aboard comfort. This is in contrast to a racing catamaran with trampoline netting between two mostly empty hulls containing a few sail bags.

So it follows that the righting moment of cruising multihull is some four times larger than a similar sized racing version and that the mast and rigging might become four times larger and more expensive. Not only this, but it takes considerably higher windstrength to fly a hull and that it would be outrageously dangerous to fly a hull in such a vessel at those windspeeds.

So, this raises a number of interesting issues not the least of which is the refusal of owners to spend so much money on an enormous rig and the risk of capsize. We understand that in France it is common for rigs to be designed to withstand 60% of the righting moment. Given a factor of safety of 2.75x it would follow that if one were to fly a hull this is exceeding the working load (effectivley 166% of the working load). It can be shown this means that yacht flys a hull with only a small factor of safety of 1.0x at hull fly. Thus the rig should be close to falling down, but not quite... Hmm, anyway this is what we had heard. This is something of a compromise position which may work best for a catamaran which is relatively light in respect of cruising catamarans.

NZ designers take a number of stances. One stance is to design for a wind range and whether this be 20% of the righting moment or 60% of the righting moment is irrelevant. Thus the rig is reefed at say 25 knots apparent windspeed, regardless of the displacement of the vessel. Whilst the windspeed varies between designers and projects this is in fact the same approach which is taken on multi-masted monohulls such as the huge sailing yacht Phocea (ex-Club Med). The argument is simple enough that where the righting moment is effectively infinite, then we might as well ignore righting moment; eventually at some windspeed the sail will shred and there is simply no point in carrying around enormously heavy and expensive masts simply to be able to fly a hull in (for example) 50 knots of wind.

A further refinement of this is to put forward the proposition that it is most important that the vessel does not capsize. Therefore it is actually safer if the mast falls down prior to flying a hull. Thus rather than floating upside down offshore, the crew are left with a yacht floating the right way up albeit without a mast. For an offshore cruising catamaran of high displacement we think this approach has some merit and often engineer the rig accordingly under direction from the naval architect. Provided the crew reef the sails at the appropriate windspeeds the rig will never be in danger. However if full sail is left up the rig will fail prior to lift off. There may be a situation where due to mechanical failure full sail is left up unintentionally. Insurance companies of course take a dim view of rigs which are designed to fall down, and in general the wide variety of factors of safety and rig configurations make it hard to access which vessels represent a poor risk or indeed which are seaworthy at all..

The end result is that it is important to have good communication between owner, crew, designer, mast builder and insurer if the vessel is to be operated in a safe window with an insurance safety net to cover events which truly can not be foreseen. As if these issues of windspeed and righting moment were not enough, there is a similarly wide range of options for the actual rig configuration. This ranges from simple three stay rigs, either with or without multiple diamonds to keep them in column, to rigs which have really long swept spreaders in the style of the America's Cup yacht KZ1 finally through to rigs with no spreaders or diamonds at all but instead have multiple sidestays and forestays. This last class of rig was to our knowledge used most famously on the english cat APRICOT, fifteen or more years ago. Recently the racing 60's have moved back toward such spreaderless rigs. The cruising yachts do tend to follow the racing ones though it has to be said that many of the cruising yachts do not have rotating wing masts, so the reasons given for copying the racing versions are sometimes misguided. Owners of mono-hull vessels are equally susceptible to this. The long spreader mono-hull KZ1 style rigs put massive loads on their spreaders and the spreaders sections then become almost as heavy as the mast itself, which quickly reaches a point where it becomes self-defeating.

The good news is that with wide staying bases these rigs are always lighter than their mono-hull equivalents when operating in the same load regimes and the greater number of rig options provides for great flexibility to create an optimal rig plan for almost any type of sail plan on almost any catamaran configuration.

 

Home Made Carbon Masts

...and some notes on carbon mast

Home Made Carbon Masts
Various home builders have asked for help building their own masts. Obviously, there is a wide range of techniques the professional mast builders use and for the budget builder the costs and the risks need to be carefully thought out indeed. Insurance of your finished boat is a topic in itself. Assuming that you have covered all the bases and want to do it yourself then you need to cover the basics. Here are some rough rules of thumb given in good faith with no warranty of any kind. Find an alloy mast extrusion which you know is safe and conservative and recommended by the yacht designer. This establishes a section size and wall thickness. Your carbon mast will be the same size and same wall thickness! It should be a tad stiffer made in carbon and it should be a lot lighter too. This is a very simple approach to avoid [evade?] doing the job professionally. Next step is a laminate, normally 1000gsm of carbon will laminate up to be about 1mm thick. So, if your mast is 3mm thick in alloy, then you need 3000 gsm of carbon. Carbon normally comes as 300gsm unidirectional, 200 gsm woven cloth 0/90 degrees, and double bias which is 400 gsm plus and minus 45 degrees. In general, the rule of thumb is that you use 60% of the laminate as a zero degree fibre, 30% 45's, and 10% at 90 degrees. Next you need to have it balanced so that inside and outside plies are the same as each other, Next you need to try to distribute the 45 and 90 plies through the laminate. Generally, you start and finish with a 200gsm woven cloth because it is easier to drill and cut holes in the mast afterward if this is the ply on the surface. Given that you use the plain weave inside and out then you add the others as you need them to get near to 60/30/10 mark whilst keeping the laminate balanced and evenly distributed as possible. The double bias 45 is normally quite expensive if available at all. it is possible to make the mast entirely from unidirectional and this is often done. But keeping the outside and inside plies in plain weave cloth is recommended even if you use glass plainweave in order to save money and you don't included the glass in your percentage figures. If the mast needs stiffening in some areas, you can add extra unidirectional locally to go up to 80% unidirectional. Conversely, if you have a lot of holes at deck/spreaders/stays then you might be wise to wrap the finished mast in some triaxial to really boost the off axis content in those areas, in addition to adding compensation patches to make up for the holes themselves. Masts that have long swept spreaders will sometimes need more 45 fibres (plus/minus) since alloy masts are in general far stiffer in torsion than carbon ones. Hopefully I have not confused you. There are some other issues to be wary off, and I hesitate to try to write a full design manual, but in particular local buckling can happen if the wall is thin/flat, but if the alloy mast extrusion you are targetting is NOT extreme, then all should be fine. Obviously if your mast is small the costs and risks are low and if it breaks you can stick it back together and add more carbon next time too. If on the other hand the boat is big the costs are large, the risks are large and the relative cost of professional engineering from AES or any of the other design professionals is relatively low by comparison to the cost of getting it badly wrong. Yes, I know the Titanic was designed by professionals, but a number of people have misjudged the work involved in building a carbon mast and with hindsight have wished they had thought it out better. So, research the whole thing thoroughly from every angle before you start and then it will either go very well or you wont attempt it at all. Both of those two outcomes are fine if they result in many years of trouble free sailing where everyone has a great time.

Finally, a confession, I don't strictly practise all that I preach, it takes a little interpretation. I sail a Farr 3.7 and my current boat has rather an old carbon mast homebuilt by one of the former owners, at least three owners prior to me. The entire boat cost NZD 2000 (bottom of the range and good for a learner). The mast is made from several windsurfer masts in the 80's era. This mast has clearly broken before I owned it, to date it has broken three times in my tenure, not solely due to operator error; but mostly. The mast has never broken at the same place twice (so far) and is developing a patchwork-quilt look... Having sleeved and joined it back together three times, including coving on a whole new bolt rope groove once, it has to be said that repairing a carbon mast is much easier than repairing a broken aluminium mast and I am getting quite good at it. I race once a fortnight and it takes one week of lunch-times to effect a repair to a big break. Working for AC teams (mostly NZ) various masts have broken, thankfully mostly in training mishaps. In each case the teams have not had insurance (it is not realistic for this kind of operation) and in each case the mast was repaired with almost zero weight increase or change in performance characteristics. A lot of prattle is spoken over the yacht club bars of the world about this sort of thing. The truth is that it is cheaper, faster and better to repair a broken mast nine times out of ten, assuming the mast was basically fit for its purpose. Owners and insurance assessors tend bring along a lot of baggage when negotiating over this kind of problem. All that said, my own dinghy mast is a bit of a poor example, and should be put out of its misery; I only keep repairing that one out of interest: a bit like a 'cat playing with a mouse'. If you enjoy your sailing and you enjoy experimenting with your rig, go for it. Best advice I can give you is to choose a boat/class where the costs are within your 'leisure budget', that is to say, DO NOT buy a boat that you can't afford to actually have fun with because you paid so much for it that you daren't spend any more on it or because you are afraid to break it. Buy a boat that costs only half of what you wanted to spend, and don't spend the other half unless you can make a genuine case for its improvement.

http://www.aes.net.nz/Rig%20Design%20Commentary.html