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Displacement weighs too heavily in comfort ratio?

Jan 1, 2003

Dieter Empacher's recent article on yacht ratios ("By the numbers," Issue No. 85) makes several useful points regarding the characteristics of different types of boats. However, his motion comfort ratio with displacement in the numerator asserts that added displacement increases comfort. This correlation between displacement and comfort is not consistent with my experiences.

This past summer my wife and I sailed our Able Apogee 50 from San Diego to Alaska (Glacier Bay area) and back to Seattle. We found her motion to be quite easy and predictable. She does not exhibit the plunging and exaggerated motions that heavier boats often have when sailing to windward under the boisterous conditions encountered going north on the West Coast. In addition, the fact that we did not have leeward and forward decks frequently awash added to safety aboard. Interestingly, the Apogee is a boat Mr. Empacher cites as not having a very comfortable motion since she is relatively light.

After sailing in a number of different hull types over the years I have an explanation as to why many people find moderately light displacement boats to be at least as comfortable as their heavier counterparts. The ratio used to calculate comfort does not recognize the effect of the moment of inertia that accompanies added displacement. When displacement increases, so does the moment of inertia. A heavier hull will have a hard time climbing over steep waves without being overswept with green water since the added inertia will drive the boat into the wave. On emerging from the top of a wave that a heavy hull has "punched through," the momentum will cause the hull to continue out the back of the wave until it is quite far out over the water. The subsequent fall and dive into the trough is comfortable only if slowness of motion is the sole criteria for comfort. I think that "comfort" is more than slowness of motion.

The overall seakindliness that can go with a well-designed lighter displacement boat provides an environment for the crew that is often preferable to the heavier boat. A related characteristic of many modern lighter designs that contributes significantly to their comfortable motion is their long waterline with little of the boat's weight in the overhang. Other beneficial effects on crew comfort of this design type include smaller and lighter sails to handle and a greater speed potential to avoid inclement weather.

The equation used in the article may provide a way to compare boats of a similar type but it does not seem capable of predicting overall comfort of heavy short-waterline sailboats versus lighter longer-waterline designs. Unfortunately, there may never be a formula that everyone will agree with. It is much like asking whether the Cadillac Sedan de Ville or the Mercedes 500S is the more comfortable car. The formula Mr. Empacher uses clearly favors the soft but pendulous Caddy ride. I and a number of other sailors prefer the Mercedes for overall comfort and safety.

Thomas Wadlow lives in Seattle, Wash.

Dieter Empacher replies: The questions from Mr. Wadlow concerning his boat performance are exactly what I have heard over the years many times. Several years ago, I started to collate simple ratio formulas, easily understood by yachtsmen who otherwise only knew their boats through personal experience. The ratios used in the article are basic design calculations and the design numbers used all come from a source using the original design data. After working with the ratio numbers, I separated the boats into four categories because of the totally different kinds of designs available.

Motion comfort ratio is probably the least commonly used of the formulas because of its more subjectective nature. In my article I used the simplified formula for motion comfort as developed by naval architect Ted Brewer. This formula deals with displacement, length, and beam. It was used for all boats listed in the article and each boat is treated equally.

I agree with Mr. Wadlow when he says that there can never be a formula that will satisfy every boat owner. However, naval architects have not given up on the further development of a formula dealing with motion comfort.

This new formula will probably look more complex because of many factors that will have to be added. For example, some designers, when using the formula for motion comfort, have added sail area, which also relates to a boat's sail-area capability.

Also, a yacht's underbody shape, no matter what its length or displacement, plays a large role in its motion.

No matter what a vessel's displacement, any extreme acceleration or subsequent slow-down over an extended period of time is not something most sailors will be happy with.

One more suggestion is for each owner to check his actual displacement and use that number for the motion comfort ratio.


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