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· Mermaid Hunter
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Lets reflect on the history of naval architecture. In the days of free-hand lines and planimeters various ratios were very useful ways to make apples to apples comparisions between boats, and between requirements and the designs that responded to them. The limitations of manufacturing technology meant a lot of complex characteristics were the same because they had to be.

Time marched on and splines and ducks were replaced by CAD. Parametric analysis gave way to complex calculations enabled by computers: FEA, SHCP, SDWE, and other tools changed the game. At the same time manufacturing technology provided incredible flexibility in form.

Let's look at Mr. Brewer's comfort ratio as an example.

"This is a ratio that I dreamed up, tongue-in-cheek, as a measure of motion comfort but it has been widely accepted and, indeed, does provide a reasonable comparison between yachts of similar type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.

Beam does enter into it as wider beam increases stability, increases WL area, and generates a faster reaction. The formula takes into account the displacement, the WL area, and adds a beam factor. The intention is to provide a means to compare motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch."

The comfort ratio formula is as follows: Displacement in pounds / (.65 x (0.7 LWL + 0.3 LOA) x B^1.333). Brewer says ratios vary from 5.0 for a light displacement daysailer to the high 60.0's for a super heavy ocean cruiser.
As Mr. Brewer will tell you, length and beam affect waterline area. On the other hand a full sectioned boat like a Passport 40 and one with finer sections but greater draft can have the same comfort ratio even though their performance is quite different.

The comfort ratio doesn't even consider weight distribution. There is no consideration of moment of inertia. A boat with dinghy and engine on the back, a full enclosure, packed lazerette, and 400 ft of chain for two big anchors can have the same comfort ratio as one with the dinghy on the foredeck, a dodger, a light load, and 200 ft of chain for one big anchor.

Length to displacement ratio has the same limitations. SA/D - the same, limited by VCE and VCG being ignored. There is some residual value to ratios if you understand their limitations but they cannot be applied across the board in a meaningful way.

Weight provides inertia. The result on seakeeping is dependent on hull form and sail plan and other things. The complex function of so very many factors can lead to a boat that slams in a seaway or one that slices through the sea.

There are certainly typical performance characteristics such as those that Jeff described, but there are enough exceptions that undue dependence on out-of-date ratios may lead a boat shopper to pass up a great boat because the limitations of the ratios aren't considered.

There is still a good deal of art to hull design and sail arrangements. Environmental externals (seas and wind) are nondeterministic and evaluation of real world response is difficult. Model testing is helpful but scaling is complicated.
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