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Slender hulls steer better in seaway

Jan 1, 2003

Nigel Calder's article on proper beam-to-length ratios ("Beam and draft," Issue No. 114, May/June 2001) for going offshore is a good start. He is to be applauded for pointing out the high-volume, beamy, light-displacement designs, which work so well in charter and for coastal cruising, have some problems offshore. There are several more factors that need to be considered.First, I believe the single most critical element in offshore comfort and safety is the ability of the design to be easily steered. This applies in the trade winds and in storm conditions. An easily steered hull can be controlled by self steering for a longer period of time, wanders off course less, and will have a more comfortable motion than a vessel that veers back and forth across its track. The narrower the hull, the simpler it is for the designer to make the boat steer easily within draft constraints that are consistent with good cruising.Calder is right when he says beamy hulls can be faster upwind, and that the stability goes up much faster than the percentage increase in beam. In smooth water and lighter winds, this does yield a quicker configuration. But as boat speed increases, the wave drag of the beamier hull typically builds faster than a narrower boat. In addition, it becomes increasingly more difficult to push the fatter forward sections of the beamy hull through the seas. This leads to a boat that is less comfortable and in the end makes slower passages than the vessel with more modest beam.Finally, Calder states that length and costs are synonymous. This is not our experience. For many years we've found that cost is typically a function of displacement first, stability second, and surface area third. A 35,000-pound, beamy 45-footer, with a righting moment of 3,000-foot-pounds at one degree, will generally be about the same cost as a 56-footer of the same displacement and righting moment. The one caveat in this equation is that the boats must be of comparable complexity - i.e., the designer, owners, and builders resist the urge to fit more "stuff" into the longer hull.Here's another way of looking at the problem. Take Calder's Pacific Seacraft 40, and keep everything the same except add 10 feet of length to the hull. To keep stability constant you will have to shave off a few inches of beam at the widest point of the waterline. However, the overall beam of the interior accommodations will net out about even, or in some cases ahead as volume gets pulled toward the ends of the hull. The increase in cost? A couple of hundred square feet of fiberglass laminate, which is offset by the fact that the builder has more room in which to work and therefore can be more efficient. The owner gets a more evenly distributed beam in which to live, a more sea-kindly and easily steered vessel, and, of course, one that is faster in a breeze and/or under power by virtue of its waterline length. The owner also picks up some nice storage space in the ends and plenty of room to easily build in fore and aft watertight bulkheads.


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