The double skin construction developed by Sadler is unique. It is exceptionally strong and produces a boat, which is quieter, drier, warmer and more comfortable than any other form of construction can provide. Why? This is because of the double-skin hull with polyurethane foam injection, which provides outstanding thermal and acoustic insulation properties.

The outer hull is laid up in a similar manner to that of a conventional yacht, and then a one-piece interior moulding is positioned inside the outer hull. This interior moulding provides locations for the bulkheads and forms the basis of the accommodation layout. The inner and outer mouldings are solidly bonded at key stress points and the space between them is then filled with closed-cell polyurethane foam. The result is an exceptionally strong hull with a smooth, easy-to-clean gel surface inside the boat instead of the usual rough glass-fibre matt finish. The foam also provides a large amount of positive buoyancy, which has proven sufficient to keep the Sadler 26, 29 and 34 afloat, even when holed or with the seacocks opened and the boat flooded.

As a result of the lack of condensation, a Sadler is a good deal more comfortable to live on than most single-skinned craft – cushions stay drier and the whole "below deck" environment, remains fresher and more pleasant. Insulation properties of the double skin also keep the boat warmer below when it is cold outside and vice versa. Sleeping is easier, particularly for the crew down below on a passage, because of the reduction in water and wave noise transmitted through the hull. The age-old problem for the cruising yachtsman of bilge water finding its way into a locker is banished – lockers are formed by the interior moulding and are totally separate from the bilge area. On the Starlight 35, 39 there is also an unusually deep bilge sump, which keeps the yachts accommodation dry, should bilge water accumulate for any reason.

The Starlight 39 uses a system of ring frames to bond the inner moulding to the outer hull. These frames run from gunwale to gunwale, thereby distributing the keel and rig loadings over the whole hull. As the boats become larger, the loadings from rig and keel are greater and design engineering increasingly important. The keel stubs on the Starlights are a superb example of this. The stub is laminated as part of the hull and reinforced with plywood transverse members (floors), which spread the load from the base of the stub up to the bottom of the hull, from where the ring frames take over. Special high-strength glassfibre cloths, such as tri-axial fabrics and uni-directional rovings are used in the keel area.

The final assembly operation is to inject closed-cell polyurethane foam (in liquid form) through suitably positioned injection holes, so as to fill the space between hull and interior moulding. Almost instantly the liquid begins to expand and generates a foam which progresses upwards and completely fills every cavity. The foam is "free-flowing" and has a density of about 2.5lbs per cubic foot. This is achieved consistently within the cavities. The total time for expansion is only about one to two minutes and by this time, it achieves an initial state of cure and becomes "firm" between the hull and interior moulding, binding the whole structure together. This creates an incredibly rigid hull, so desirable in a well-found yacht (and achieved by few!). Most yachts creak and squeak to some extent when sailing – not so on Sadler and Starlight yachts, where very little of the noise is produced by the boat! Until you have experienced the benefits, it is difficult to appreciate the difference between this and any other boat.

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W/B/Construction9/11-Apr-99