The Offshore Voyaging Reference Site

Carbon Fibre Masts, Amateur Boat Design

We had an interesting to-and-fro with a reader who thinks that the benefits of carbon fibre for cruising yacht masts are trivial and that the same benefits could be realized in easier and cheaper ways. To understand why this is not so, and why carbon fibre as a mast material delivers such astounding increases in performance and comfort, we need to delve into a little simple—it better be simple since I’m really reaching here—engineering and yacht design.

When designing a voyaging boat the naval architect must balance, among many other factors, the following needs:

  • Resistance to capsize, to be safe offshore, as well as quick and sure righting after a knock-down or inversion. (Counter-intuitively, these two needs have somewhat different and conflicting solutions.)
  • Enough sail area in relation to the boat’s displacement, preferably without the use of overlapping headsails, to perform well in light air—that is, if the owner wants to sail and is not really looking for, as many are, a disguised motorboat.
  • Reasonable draft for the cruising area envisioned. Over the years we have found that the sweet spot, at least for us, is 6 to 6-1/2 feet (1.8 to 2 meters).
  • Sufficient stability to stand up to the necessary sail area required to go up-wind or close-reach well into waves offshore.

Leaving out the stability provided by hull shape, to satisfy the above needs the designer is faced with balancing the force of the wind on the sails together with every ounce of weight above the center of gravity (hull, decks, fluid in tanks, mast, rigging, and on it goes) against the weight and depth of the ballast in the keel and any structure or fluids below the vertical center of gravity. Wait, it gets worse: as the designer adds more sail to get good performance he or she must make the keel heavier (we are assuming a fixed draft), and that then makes the keel bigger and drives the boat down in the water, increasing the drag from wetted surface, which in turn requires a bigger rig, which requires more ballast, which requires…you get the idea, a vicious circle.

However, going to carbon fibre for the mast is a virtuous circle since the boat gets both more stable, without adding ballast, and lighter, by the weight saving in the mast. The benefits are dramatic, and more than one might expect, because the effect of weight on stability is multiplied by its distance from the boat’s vertical center of gravity. For example, Morgan’s Cloud’s new carbon mast, with its weight saving of about 600 pounds (270 kilograms) over the old aluminum mast and its lower center of gravity, is like adding about one and a half tons to the keel, but without the above-detailed diminishing returns.

Or, to look at it another way, to make the reader’s suggestion of saving weight by buying a water maker and carrying less water equal to the effect of the new mast, it would require (assuming the water was stowed under the cabin settees) getting rid of an amazing seven tons or 1400 gallons (5300 liters), nearly six times more than we carry! (In fact Morgan’s Cloud’s water supply is in tanks in the keel stub below the boat’s vertical center of gravity and so having them full actually has a positive effect on her upwind speed and stability.)

Photographs of carbon fibre masts being built are courtesy of Hall Spars, and no, we have not sold our souls—we truly believe Hall Spars made us a great mast.

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