Nai'a

FREE-STANDING MASTS

NAI’A

(Note: The following is the original text for an article I wrote about this wingmast design for the February, 1995 issue of SAIL magazine.—EWS)

The owners of a 110’ steel motorsailer, used for SCUBA diving charters in Fiji, wanted to replace the yacht’s two-masted junk rig with a more efficient and easer-to-use wingmast rig. Wingmasts are usually found on ocean racing multihulls, and most are built of carbon fiber composites. To put a wingmast on a big, fat motorsailer was unusual, to say the least. But one of the owners was a former avid multihull sailor, and he much preferred to have any kind of wingmast over the two 22” diameter, 14,600-lb. round steel pipe masts that were originally built onto the boat.

Carbon fiber masts, however, are expensive to build because of the material costs, and they require an environmentally clean facility, expensive molding machinery and tools, and highly skilled labor for the best-quality lay-up. The owner only had access to a beach-front warehouse, basic boatbuilding tools, and a semi-skilled labor force, which fortunately did know a lot about wood-epoxy boatbuilding. Therefore, the motorsailer’s wingmast would be built with Fijian Kauri timber and plywood by local shipwrights. It would carry the same sail area in a mainsail and yankee as the original foresail and mainsail of the junk rig. The new rig would not make the motorsailer a speed machine, but it would improve sailing performance measurably, and it is what the owners wanted.

Over the last 15 years I have designed over 40 different composite free-standing masts and wingmasts (now in 2001, the number is 54). One technique I developed for one-off mast construction utilizes a wood-epoxy core structure overlaid with a carbon fiber laminate. This time we would leave off the carbon fiber, so the mast sections and wall thickness for the motorsailer’s wingmast had to be much larger. Kauri is a dense, light-colored, straight-grained pine indigenous to the Western Pacific. The Fijian wood industry had remarkably good published test data for Kauri’s strength and stiffness for both timber and plywood, which made designing the mast possible.

The wood structure design was the easier job to do. Much more difficult was designing the stays and their fittings which would hold the wingmast, yet still allow it to rotate. The original steel pipe mainmast went all the way down to the keel, and inside the mast section between the keel and deck were ventilation pipes that, because of cost and complication, could not be removed. Therefore, once the original mainmast was cut off at deck level, the new wingmast would be stepped atop the remaining base section. The owners desired only three stays to keep equipment cost and maintenance low: one headstay for the yankee and one stay each port and starboard. This is the minimum number of stays required to hold a deck-stepped mast; if any one let go, the mast would come down, potentially killing someone.

Since rigging wires almost invariably fail at their fitted ends, the least expensive solution was to fit each wire with four ends—the two primary ends which were simple thimbles, then a spliced-on section of wire at each end with a duplicate thimble. Duplicate tangs were fabricated in stainless steel and installed on the wingmast to accept the two thimble ends from each wire. Later experience moved the owners to accept the higher cost to duplicate all the wires for safety’s sake, making six wires in all holding the mast.

When floated across the harbor and lifted by crane onto the yacht, the wingmast weighed about 3,000 lbs. and was 99.2’ (30.235 M) long. This is the tallest wood-epoxy wingmast in the world and 6^th tallest wingmast of any type for yachts. The tallest is the 110’ (33.53 M) carbon fiber wingmast of Stars and Stripes ’88, the 1988 America’s Cup defender. (These records may have been broken by now.—EWS)

The success of this project proved that wood (miracle fiber “W”) is still a suitable engineering material when sized properly according to the loads. However, a simpler and safer rigging arrangement would be to have no rigging at all, making the mast free-standing. Without rigging wires, their cost and threat of failure simply disappear. In addition, the wingmast would rotate more than it can with stays, making it a more effective sailing device. But a free-standing arrangement necessarily requires that the wingmast be stepped on the keel and be held by a bearing at the deck. In this particular situation, the constraints of the existing boat structure prevented using the keel-stepped, free-standing option.