Trailer Sailer - Hull Shape

Hull Shape

In monohull yachts, hull shape is very much affected by measurement rules. See Hull (watercraft) In some countries the maximum width, length or weight is restricted by road regulations. Waterline length is the determinant on displacement water speed, the speed in knots very roughly is proportional to the square root of the waterline length in feet. So a 16 ft boat can do 4 knots and a 25 ft boat about 5 knots, not a lot of difference, unless you sail in tidal waters and face a 3 knot current where the larger boat will advance at twice the speed. The larger boat may also have 3 times the volume, which helps cruising comfort. Larger boats have more headroom.It is rare to have full head room in a boat smaller than 25 ft. Some trailer sailers have pop tops, a lifting canopy that gives full head room in smaller craft. The shape below the gunwales will be determined more by handicap measurement rules and these vary between countries. Also they will vary depending on construction method. Plywood kits and plans for plywood will likely be of single or multi chines construction. Fibreglass production boats will be very smooth and rounded and can include complex curves. This advantage in hull shape though can be offset by the much higher weight of glass, unless composite materials are used. The racing advantage that trailer yachts have when racing in mixed fleets is the ability to exceed displacement speed downwind on waves known as surfing. Most yachts will surf given the right conditions, often extreme, but for a light weight trailer yacht it may surf in the harbour on one metre waves and outpace larger displacement yachts up to twice their size.Lighter trailer sailers especially sports boats will plane in moderate breeezes. Measurement rules which design yachts for all round performance have difficulty with this factor. For a normal yacht the less drag a hull has the faster it will go, particularly in light breezes and upwind, and here is the compromise, this usually means the stern of a yacht will decrease in width from the midsections aft. But this is not the ideal shape for surfing or planning where a wide stern with flat run aft is best. Thus trailer sailer hull design intended for racing will compromise upwind performance and rating for surfing and rating beating ability. Larger yachts are now following this trend. Trailer sailers are usually designed to spend most of the time on a trailer. The keel construction must be strong as it bears most of the weight of the hull plus crew. Trailers must be strong with multi rollers to assist in launching and retrieval. Smaller boat trailers use a geared winch of 1:3 or 1:5 while above 20 ft most use an electric winch.