Carbon Fibre Masts

[Eric Sponberg]

Originally Posted by Nordic Cat

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

Alan,

In my opinion, for both suggestions, no. First option, Kevlar airtight tube. How much pressure do you suppose it would take to pressurize the tube to give sufficient support to the mast? Quite a lot I would think, which makes for a thick-walled tube, which makes it heavy and expensive, and which provides support on only one side of the laminate. Plus you would need tooling and laminate work for the tube that would be better put directly into the mast laminate itself. Why build contraptions when you can build a better mousetrap in the first place? Far better that you design the laminate to give the fibers the proper support that they need, and that you make the wall thick enough to support itself. The "Home Made Carbon Masts" advice on fiber split is actually pretty close to the mark. Depending on the actual design at hand, I will split the fibers orientations for 0/90/+-45 at 60%/20%/20%, or 70%/15%/15%, or 80%/10%/10%. Split in the middle of the laminate, the layer orientations should be mirror images of each other, and the outer-most layers inside and out should be +-45.

Second option, multiple tubes with an overlay. Again, a bit of a contraption. Often, wingmasts have a central tube section, either round or rectangular, made of carbon fiber. This is the main structural member of the mast that carries the lion's share of the load. The leading and trailing edges can then be made out of thinner cored fiberglass and bonded on. Fiberglass as stretchier than carbon, so however the carbon bends and stretches, the fiberglass will follow along. Make sure that you have enough so that it does not crack itself. And its wall must be thick enough to keep from buckling. The masts for my open class 60 design Project Amazon and for Wobegone Daze, a Freedom 38 retrofit, were made this way using Nomex honeycomb for the fiberglass skins.

Another option that I have used is to make a round section carbon tube as the main structural member, then hot-wire-cut foam core leading and trailing edge shapes and glue them onto the carbon tube, or more accurately, glue the foam to the mast and hot-wire-cut afterwards. Overlay a fiberglass skin over the foam, again being careful against being too thin. These are far simpler methods of construction. The foam LE/TE method is a little heavy since even 2-lb density foam can weigh quite a lot when it's all done.

Using core in the carbon structural tube usually does not make sense because by the time you figure out how much carbon you need for the basic strength and stiffness, you have generally come to the final design size and shape for the solid laminate--optimum size, shape, and wall thickness. Remember, you want the smallest aerofoil section possible consistent with strength and stiffness. Once you are at this point adding a core to the carbon laminate only adds extra weight, cost, and complexity the design. The core does not add any real structural value because you have already achieved the optimum without it.

Eric

[Nordic Cat]

Originally Posted by Eric W. Sponberg

Alan,

Eric

Hi Erik,

thanks for taking the time and effort to give us in depth answers.

Regarding the pressure in a kevlar tube, I got the idea when using a piece of high pressure tube for something completely different. It could handle 200 bars of pressure, was about 1" and had a wall thickness of around 1 mm.

Just pressurising to 7-8 bars with compressed air made it incredibly stiff - so that's what got me thinking. Maybe multiple hoses in a section, as they are quite light. Could use a diving bottle to fill them.

I need to read your post a couple of times more, to make sure I understand it fully.

Thanks again

Regards

Alan

[Archim]

Nordic Cat,

You might be interested to know that the naval architect Patrick Luscher has designed a 50' catamaran with an unstayed CF rig, the Aquablue (you will need the help of Babel Fish for translation). The drawings for the construction of the rig can be purchased separately.

Patrick Luscher is successful with French amateur and custom professional builders with his performing and no-fuss strip-planked Looping catamarans. Most of them are used by private charterers. The construction of the first Aquablue was completed last year for the architect himself, who I believe is now sailing hapilly somewhere around Senegal.

Eric,

Sorry for the question from an aficionado : is there any international standard asking for the rig to be calculated to righting moment ? E.g. most of the twin rig catamarans with unstayed masts are calculated at half the righting moment, any logic to that ?

[Nordic Cat]

Originally Posted by Archim

Nordic Cat,

You might be interested to know that the naval architect Patrick Luscher has designed a 50' catamaran with an unstayed CF rig, the Aquablue (you will need the help of Babel Fish for translation). The drawings for the construction of the rig can be purchased separately.

Patrick Luscher is successful with French amateur and custom professional builders with his performing and no-fuss strip-planked Looping catamarans. Most of them are used by private charterers. The construction of the first Aquablue was completed last year for the architect himself, who I believe is now sailing hapilly somewhere around Senegal.
?

Thanks Archim,

I read an article on this boat in the French Multihulls World last year some time.
My understanding is that the mast is unstayed, and that the jib sits on a traveller track, so you can pull it across the boat.

In the pictures it also looks like there are stays going aft, but these are probably just for getting som tension on the forestay.

The Looping cats have a great reputation in France, but I don't think they are well known outside.


Regards

Alan

[Archim]

This is correct, the running backstays are used to tension the forestay when sailing upwind or with a spinnaker, but they are also needed when sailing or motoring in short waves to prevent the mast from whipping.

The possibility of bringing the jib to windward adds efficiency, like the Aerorig but with less additional weight.

I guess the reason why the Looping are not known out of France, is that there is no available information in English. Principle is : design a cat, then extend the hulls length by 20%. This results in well balanced, light and easy cats (most of the 50 or 60' chartered Loopings are run by a single skipper), ideal if you are prepared to keep the payload to the minimum.

The website says that "the rig can be built with carbon-epoxy by an amateur builder, for the price of an high tech aluminium rig. The only inconvenient is the weight of the rig, around 320 kg for 19 meters, but with a lowered centre of gravity". "For the unstayed rigs, with evolutive sections (cylindro-conical for the part under deck, then conical from deck to top), the male plug is made with sheets of biax carbon shaped to build tubes that will be boxed-in like stove ducts. The rest of the carbon fiber is laid on this mandril. Indeed all stratification must be carried out under vacuum". Pictures here.

[Nordic Cat]

Thanks for the link Archim. I wonder what the final weight penalty is for the whole unstayed rig, compared to a traditional rig with wire shrouds and stays.

For my bi-rig, I'm planning on around the same mast height,(from the base in the hull). Preliminary estimates of weight is around the 300 kg mark including the boom, mast track etc.

I see that he sells the plans for around 4100€, so need to consider this as a possible option.

Regards

Alan

[Meanz Beanz]

Originally Posted by brian eiland

...and some notes on carbon mast

Great post Brian, saving that one. I can't give U credit, the system won't let me but thanks anyway!

Cheers
MBz

[Archim]

Alan,

I've seen the thread on your bi-rig, really nice cat !!

I've always believed the bi-rig to be the ideal solution, but I thought the rigs would have to be dimensioned at full righting moment -very heavy and expensive.
Having seen the sections of the Coolchange rigs at deck level, I suspect this is not the case. How have you dimensioned the rigs on your cat ?

[CTMD]

Nordic,

there is some good stuff on home building a Carbon mast here...

http://www.gust.ax/gallery/mast/

[Eric Sponberg]

Originally Posted by Archim

Nordic Cat,

Eric,

Sorry for the question from an aficionado : is there any international standard asking for the rig to be calculated to righting moment ? E.g. most of the twin rig catamarans with unstayed masts are calculated at half the righting moment, any logic to that ?

No, there is no international standard for determining the load on a multihull rig. It is every naval architect for himself. And I will discuss both single mast and two mast rigs.

To my mind, a racing multihull is relatively light and it is driven hard. At times the windward ama will come out of the water. When that happens, the load on the rig is at maximum. If it is split between two masts, that's OK with the understanding that the boat is always going to be riding with two sails set and both masts working. Will there ever be a time when only one sail is set and only one mast working at maximum load? If so, you might have a problem with spliting the load always between two masts.

On a cruising multihull, the weight of the boat is heavier, but it is not driven as hard, so it may be unlikely that the windward ama will ever come out of the water, therefore it is unlikely that the boat will ever see maximum righting moment--maximum load on the rig. So, for a single mast rig, you make an arbitrary decision as to what the load is really going to be. Is it going to be half the load, a third the load, a quarter the load, or maybe two-thirds the load? How do you justify your decision? For a two-mast rig, the same as the above applies--both masts are always considered working.

Engineering wise, there is logic to use the maximum righting moment for the load on an unstayed multihull mast, and for two masts, half the load on each so long as you understand that you should not sail in heavy air on one mast only. There is no engineering logic to use anything less than maximum load unless you can justify your decision. You might be able to do it statistically, saying that you have such and such chances of reaching X load.

There are also non-engineering factors. How much is the mast (or masts) going to weigh and what it is going to cost? These are really important factors to the typical multihull sailor who is always looking for light weight and low cost. So an owner can elect to arbitrarily design the mast to half the boat's righting moment with the realization that it will not be strong enough or stiff enough for extreme conditions. So he has to know where the edge is, and be prepared to back off a little bit. This decision keeps the weight and cost of the mast to something reasonable for the boat and the wallet.

Eric

[terhohalme]

Hi Eric

ISO 12215-7 Multihulls, is coming but when?

In the coming standard, the maximum righting moment define rig dimensioning when LWL is under 12 m, between LWL=12-15 m 80% of RM, LWL=15-18 m 60% of RM, LWL=18-20 m 40% of RM and LWL=20-24 m 30% of RM.

[Eric Sponberg]

Originally Posted by terhohalme

Hi Eric

ISO 12215-7 Multihulls, is coming but when?

In the coming standard, the maximum righting moment define rig dimensioning when LWL is under 12 m, between LWL=12-15 m 80% of RM, LWL=15-18 m 60% of RM, LWL=18-20 m 40% of RM and LWL=20-24 m 30% of RM.

Well, I stand corrected, and I find that curious. It may be that stability (righting moment varies with Length^4) rises faster than heeling moment (varies with Length^3), so really big yachts would be much less likely to reach the larger loads.

Eric

[BigCat]

For passenger-carrying sailing catamarans, the USCG wants to design the mast for twice the typical maximum wind speed (not storms, but in trade wind areas, strong trades as funneled in channels,) of the area the boat is to work. This is hot off the press, as I got approval for a passenger-carrying catamaran with unstayed masts last week. No reference to righting moment at all-I told them that the boat would probably turn over before the masts broke, and that was just fine with them. A broken (stayed) mast killed a passenger in Hawaii a couple of years ago, and another one holed a cat, which started coming apart as a result. (ie. the standard is that the mast breaks at twice the typical maximum wind speed, with full sail up, exclusive of storms.)

[Nordic Cat]

After reading the latest comments which are very informative and relevant for me, i would like your opinions on the following for my bi-rig fast cruising cat. I have not yet started on strength requirements for my rig.

LWL is 13.22 m so ISO defines rig strength to be 80% of RM. If I use this factor and add a safety factor of 2.0 i get 160% of RM.

This is about half of what has earlier been suggested, i.e 100%RM and SF=3.0

What if one only uses the windward rig in heavy weather, then the RM drops?

Where would you guys draw the line on this one? The call is mine, but I would appreciate any ideas. The primary considerations here are weight and safety, price is secondary on such a key issue.

I feel inclined to drop the SF to save weight.

Regards

Alan

[Nordic Cat]

Originally Posted by CTMD

Nordic,

there is some good stuff on home building a Carbon mast here...

http://www.gust.ax/gallery/mast/

Thanks Chris,

For an unstayed mast, where weight is a key factor, i imagine that an all carbon mast would probably be the best technical option. The technique shown is really neat for a home builder.
Would foam core be a better option?

Regards

Alan