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March 2005
Pocket Boom improves mainsail handling

Designed to make yacht mainsail handling simpler and easier, GMT Composites' (Bristol, R.I.) Park Avenue Pocket Boom doubles as mainsail foundation and sail container. Unlike conventional mainsail booms, the Pocket Boom, as its name implies, is a open-ended, three-sided structure with a flat-bottomed "U" shape in

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Posted on: 3/1/2005
High-Performance Composites

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Designed to make yacht mainsail handling simpler and easier, GMT Composites' (Bristol, R.I.) Park Avenue Pocket Boom doubles as mainsail foundation and sail container. Unlike conventional mainsail booms, the Pocket Boom, as its name implies, is a open-ended, three-sided structure with a flat-bottomed "U" shape in cross-section, open on the topside to provide a 355-mm/14-inch deep sail storage chamber. According to GMT's marketing director Ben Sprague, the unusual design "offers the ability to reef on any point of sail, and it is a mainsail system that can't jam. The sail flakes itself as it's lowered into the boom cavity, and the sail cover can be zipped closed and you're done." (A "flaked" sail folds neatly as it lowers, in such a way that it can be easily raised. To "reef" a sail is to take it in to a degree, to reduce the sail area exposed to the wind.)

The boom is manufactured with standard-modulus unidirectional and biaxial carbon, from SP Systems (Magog, Quebec, Canada), wet out in GMT's automated resin impregnator with Pro-Set 135 resin (Pro-Set Inc., Bay City, Mich.). "We lay the inside skin over a male plug of the boom," says Sprague. The 12.2m/40-ft long plug allows GMT to fashion Pocket Booms in a variety of lengths from a single tool. The inner skin is vacuum bagged and cured at ambient temperature. Additional carbon doublers are applied in high-load areas. Following the initial cure, Divinycell H80 high-density foam from DIAB Inc. (DeSoto, Texas) is dry fit and shaped to conform to the boom. The outer skin is subsequently laid over the foam to the prescribed laminate schedule, vacuum bagged and cured. The entire part is postcured in GMT's instrumented oven at 60°C/140°F for eight hours. Following postcure, the boom is ready for pre-assembly of metal fittings that anchor components, with colorful nautical names, such as the vang cylinder tang, hydraulic outhaul ram foundation, outhaul track and turning block, and inboard end boom gooseneck attachment. Once pre-assembly is complete, the boom is faired, sanded and sent to GMT's paint facility for additional fairing and a five-step finishing process, using Awl Grip two-part polyurethane coating from US Paint (St. Louis, Mo.). The painted boom then returns to GMT for final assembly, quality control inspection and shipping.

Already aboard several yachts -- a Swan 56, a Baltic 55, a Dijkstra 65 and a Fontaine 60 -- the Pocket Boom's strength has been tested in Mediterranean Mistrals, trans-Atlantic passages and slogs through the Gulf Stream enroute to Bermuda.

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