Quote:
Originally Posted by Jeff_H
But again, the simplest way to debunk the Comfort index is by comparing comparing two boats. If we start with a CCA era 40 footer with an LWL =30ft, Beam=12 and Displacement= 20,000lbs we come up with a MCI of 33.94. If we compare that to a 44 footer LWL =39ft, Beam=13ft and Displacement= 24,000lbs we come up with a MCI of 29.8. If weight distribution and VCG, cross sectional shapes, etc were similar, the bigger, proportionately slightly narrower, heavier boat would have a significantly more comfortable motion, yet Brewer’s MCI suggests just the opposite. And missing from the formula is such critical motion impacting elements as VCG or even Ballast to displacement ratio, dampening or even draft, and height of mast and so on. Which comes back to my central point being that the MCI produces such inaccurate results as to be worse than useless as a real comparative tool.
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If you are going to "compare numbers" to "debunk" a formula; you should use scalable numbers. You can't just pull numbers out of the air and say 'yep that fools the calculator'; because it's not the case. I have a feeling that Mr. Brewer did some heavy number crunching to come up with that equation. When it comes to deriving a dimensionless number that will quantify the how comfortable the motion of a boat is; it just does not just come out of thin air and to sign your name to it as a hull designer is a pretty big commitment to it's derivation and use.
If you are going to compare numbers; start with some directly scalable values. For the 40' LOA vs 44' LOA you would have the following numbers (appx):
LOA - 40' => 44'
LWL - 30' => 33'
Beam - 13' => 13.2' (Ratio LWL/LOA = 30%)
Disp - 20,000 => 25,000 (Best guess; 25% heavier for the 4' LOA).
Those numbers give an MCR of 33.99 for the 44' LOA boat boat. If you use 24,000# the MCR is 32.63. Seems like it is directly scalable.
If you do the converse (scale the 44' boat in your example down to 40') you get an MCR of 28.17; pretty darn close to the number you calculated.
What does it mean? MCR is a measure of linear and pitch
inertia forces that the boat encounters (not roll stability, not heel). The calculator on US Sailing says the following:
Motion Comfort Ratio was developed by Boat Designer Ted Brewer. The formula predicts the speed of the upward and downward motion of the boat as it encounters waves and swells. The faster the motion the more uncomfortable the passengers. Thus, the formula predicts the overall comfort of a boat when it is underway. Higher values denote a more comfortable ride. As the Displacement increases the motion comfort ratio will increase. As the length and beam increases the motion comfort ratio will decrease.
MCR = Disp / (2/3*((7/10 * LWL)+(1/3 *LOA))*Beam4/3)
Simply put it compares displacement to hull area. The more hull surface area (below waterline) the higher the tendency to pound. If the displacement relative to the area of the hull is higher the tendency to pound is reduced. That's why light displacement racers are generally a rough ride and heavy cruisers (with overhangs) are more "sea-kindly". The MCR formula does not speak to roll stability, vertical CG, etc. It was never intended to speak to those issues.