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It depends on the specific Cape Dory, but most of Carl's Capes are not very good sea boats when viewed against similar displacement designs that are intended for offshore use.
Here's what you are missing, the reality of narrow, slack bilged boats is that they tend to have pretty good limits of positive stability, but compared to the lower vertical center, moderate form stability (not high or low form stability), shallower canoe body, longer waterline designs now advocated for seakindliness, motion comfort and ultimate seaworthiness, narrow, slack bilged boats have pretty poor stability, which also means that they are also more likely to approach their limits of positive stability. When you add short waterline to the equation you deepen the canoe body increasing the tendency toward larger roll angles and a sharper snap at the ends of the roll. And of course, with a short waterline and long overhangs you increase the likelihood of a harsher collisions with waves, broaches and pitchpoling.
As to arriving at seemingly good Comfort Index and Capsize screen formulas on long overhang boats, the point that I was explaining in one of my earliest posts above, is that long overhangs distort the results in a manner that is counter to reality. In other words, if you consider two equal length boats, with equal displacement, one with short overhangs and the other with long, all other things being equal, the boat with short overhangs would have a gentler motion and be more seaworthy, yet the formulas would suggest the contary.
And as I have pointed out before, neither the Comfort Index nor the Capsize Screen formula, really tell you a thing about the motion comfort or likelihood of capsize. Both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats that otherwise are very similar. Neither formula contains almost any of the real factors that control motion comfort or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution, and neither contains any data on dampening, all of which are the major factors that control motion comfort or the likelihood of capsize.
I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth. That is why I see these formulas as being worse than useless. This is especially true in the case of boats with long overhangs as explained above.
Respectfully,
Jeff
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