Stablity & Other Weighty Topics

The heavier the vessel, the less likely it is to be upset. This factor needs to be balanced of course against performance and ultimately seaworthiness. The target stability achievement required for CE class A is a function of the lightest operating condition likely to be encountered, mMOC, minimum crew and all stores and fluids expended. For safety sake we generate a “lightship” number as low as possible with the expectation that this number is highly unlikely to be the case.

The heaviest vessel condition, mLDC, is primarily of concern in regard to minimum angles of heel before possible flooding occurs, more of a concern to monohulls but a factor none the less. From a practical standpoint, the real upper weight limit is related to bridge deck clearance and performance impediments due to hull resistance. We use numbers for the CE calculations that try to represent a likely situation, six crew for example. These “official” load weights are not practical limiting factors. The hull was designed with sufficient hull volume to support considerable additions beyond the design displacement. The owner can therefore use his own discretion with regard to loads, considering some changes to performance as the only real limiting factor. Most vessels setting out to sea would be displacing about 26,000 lb., a weight at which they are quite comfortable and in no way compromising their safety or seaworthiness.

When the vessel was designed, a weight study document was maintained to predict the flotation characteristics of the vessel, taking in to consideration as much detail as possible. The study included allowances for everything down to the cutlery and we tried to anticipate any (reasonable) options we could think of. Carrying a dinghy was always in the plan, for example, so if it turns out to be a little heavier than anticipated, only the increase need be balanced. The vessel structures are made as efficiently and lightly as possible so that the owner may have more latitude with his additions. This does not mean that carrying excess weight has no penalty, rather that the penalty will be minimized. We would still caution an owner against taking things along for the ride, parasitic weights. Unused battery capacity and tankage commonly fall in to this category as they are very significant weights, as opposed to extra clothing or items of equipment for example. The value of the weight study is as a prediction tool and ours took into account the “growth” expected over time experienced with any project, the engine option for the Volvo 40 HP for example. So far the “extras” have added up to less than 500 lb. Against 20,000lb.+/- of vessel this is not very significant.

I have attached a link to the mast loading diagram that shows some heeling moment (stability) implications of displacement. I have also linked the hydrostatics diagram which can be used to estimate the effect of weight changes and location shifts. I takes about 1,300 lb. to put the vessel up or down an inch on the waterline. The wetted surface curve gives an approximation of the change to resistance from which a percentage change to speeds may be very roughly calculated.

The design philosophy for the vessel was aimed at owners who enjoy some extras and who’s races will be casual affairs of opportunity, (but please give Playstation a run for the money), keep global weight issues and performance in mind but enjoy the capacity provided.


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