Farfarer’s main spar is slightly softer than the fore mast, encouraging the mainsail to depower more quickly and so offset the natural tendency to weather helm with heel.
A MODERN SOLUTION – Ted Van Dusen
Composite Engineering built their first carbon fibre mast in 1978. That was also the year that Woody Stoddard and I joined two businessmen to start US Windpower, which was to become an early leader in modern wind turbines. Woody and I had just completed our PhDs in ocean engineering and had designed and built what is generally recognized as the first modern wind turbine to demonstrate the feasibility of renewable energy as an alternative to fossil fuel and nuclear power plants. Both Woody and I also specialized in computational fluid dynamics; however, Woody had experience with wake modelling of helicopter rotors so he worked on the turbine aerodynamics. I had experience with advanced composites, so developed a computer code to model the turbine blade structure and determine the stress, deflection and dynamics. Composite Engineering produced the first run of production turbine blades before US Windpower and Pearson Yachts subsequently formed the joint venture TPI, to manufacture the blades.
From here it was a simple matter to analyse sailboat masts using the turbine blade program. With the addition of a program to calculate the load on sails we soon had a design package to predict the deflection of a free-standing mast, determine the stress anywhere in the structure, and optimize the mast diameter and wall thickness to minimize the weight aloft.
The first step in designing a mast is to determine the stability of the boat. We use the maximum righting moment to determine the highest loads the mast can experience and then use the righting moment at the heel angle for optimal beating in a good breeze to determine the luff curve which the sailmaker then uses to design the sails.
Composite Engineering use both pre-preg laminate and tri-axial braiding in fabricating their hulls and spars. Most of our spars are braided with about 75% of the fibre along the axis of the spar, 12% at +/-45°, and the rest wrapped around at 90°. We braid over a rigid mandrel with as many layers as necessary to obtain the desired wall thickness to form a seamless woven structure. After curing in our autoclave the mandrel is then pulled out.
This woven structure is highly resistant to crack propagation and delamination. On the other hand, a pre-preg mast made mostly with unidirectional tape can bunch the fibres more closely, thereby using 2-4% less resin for slightly better physical properties but less toughness. The service of the boat determines the method to use. An America’s Cup boat with support crew would choose pre-preg while a long-distance cruising boat would benefit from a braided structure. The process of pre-pregging the carbon doubles material cost and more labor is required to debulk every layer to maintain good fibre alignment and eliminate voids. The cost savings of braiding may ease the budget or alternatively permit the use of more costly higher-modulus carbon.
The line of Nonsuch Catboats switched from aluminum to our carbon spars in 1992. They has lost several masts to metal fatigue and were looking for a solution as well as a marketing enhancement. The carbon masts and wishbone boom were half the weight and twice the strength. At 30′ heel the boat had 15% more righting moment so mast height was increased by nearly 1 meter. The boat came alive in light air and had increased drive when the wind picked up. The masts were designed to have the same bend characteristics so the same sails could be used.
At the same time we began working with Tom Wylie who wanted to develop higher performance catboats with a large sail plan that would depower automatically in the strong San Francisco breezes and also be able to provide zippy offwind speed. His clients wanted a fast and easily handled boat so they could sail whenever they had an hour or two free without having to organize a crew, Wylie Cats performed very well against conventionally rigged raceboats, particularly in variable conditions when automatic depowering saves a lot of reefing.
We also explored how small we could make the uppermost sections and still support an efficient sail shape. At first the cat rig seems simple; however, in the design phase it is much more sophisticated that a conventional rig where the mast bend can be tuned to match the sail’s luff round.
The boat designer, sailmaker and rig designer must work closely together to achieve the desired results. If the rig is too stiff for the boats stability the depowering is lost and if it is too flexible the sails will only set properly in a narrow wind range (and the battens will not tack in light air). When done right, downhaul, mainsheet and outhaul give good control of the sail shape in all conditions. Composite Engineering have now made free-standing rigs for a wide range of boat types and sizes.
For Farfarer and a few other of our masts we went the next step of mounting the spar in full bearings so that they can easily be over-rotated. When a round spar rotates so that the lee side of the sail leaves the mast in a smooth airfoil shape, the driving force is very close to that achieved by a well designed wing mast and sail combination. There is a small increase in aerodynamic drag but this only becomes significant on very high-speed boats.
The sail set on a round section can also be designed to match the apparent wing angle over the entire mast. It is very rare to find a wing mast that can be twisted to match the wind angle, so much of the wing is not at an ideal angle.
The structural demands of a mast also require that the fore and aft stiffness and transverse be similar, unless spreaders and rigging are added. A wing mast will be heavier than it’s round equivalent; weight aloft is so detrimental that this is a big plus for a round section unless the boat is so stable that it can be sailed with a little heel- as on a multihull. For offshore work the reduced windage aloft increases safety and improves boat control.
Farfarer’s main mast is designed to be a little more flexible in relation to it’s length than the foremast. As the wind increases the main depowers faster so the fore loads up to minimize the increase in weather helm ans the boat heels. We continue to refine the rotating round free-standing rig; expect to see more of these rigs in the future.