My point, Par, was precisely that- no single factor, including displacement or displacement to length ratio will be 'terribly helpful' in answering the question. That was why I suggested the article by Nigel Calder as a good starting point. That's why I suggested that he may wish to consult with a naval architect.
If we are talking about the Endeavour 37 and the Irwin 40 of about 1982 vintage, the following additional dimensions should apply (from the 1982 'Sailboat & Equipment Directory') :Endeavour waterline 30', Irwin 31' 1"; Endeavour maximum beam 11'7, Irwin 12'2", Endeavour displacement 20,000 lbs., Irwin 16,890 lbs; Endeavour ballast 8000 lbs, Irwin 7000 lbs; draft Endeavour 4'6", Irwin 4'3".
In addition, the Endeavour narrowed considerably toward the stern while the Irwin carried more beam, much further aft. The Endeavour had considerably more rocker than the Irwin and wider side decks. Both boats, I believe, had long fin keels and spade rudders (the line drawings don't show their underbodies, so I'm going from memory and comparison to other boats from their respective manufacturers at the time). Which boat would be better in heavy seas (or 'rougher water', as was the original question)? Ignoring construction details, the rig, weather/lee helm, interior lay-out ,etc., it should be the Endeavour and not merely because of the increased displacement. The narrower beam to waterline length,narrower ends, increased rocker, increased draft to waterline length and yes, increased displacement should all make the boat more comfortable in heavy seas, especially to windward.
We should also bear in mind that the displacement figures are for an unladen boat. The weight of crew, safety equipment, water, diesel, food and clothing will add considerably to the actual displacement. What is significant is that the additional weight should be the same for the same couple taking the same trip on either boat. Adding the same weight to two boats will typically have a greater negative effect on the performance of the lighter displacement boat - in all winds and sea states. Advantage again, Endeavour.
Finally, we have to be careful in assuming that the Irwin will be a better light air boat. I don't have the relative sail areas for the two boats (using sloop rigs in both and a 100% foretriangle). We also don't know the WS (wetted surface area) figure for either boat, although we can safely assume that it is considerably less on the Endeavour based upon their respective beams, waterlines and hull shapes. The point is, of course, that SA/WS is a far more accurate predicter of light air performance and, as Nigel Calder points out in the article I referred to, "surprisingly, heavier boats with relatively low SA/D ratios sometimes have relatively high SA/WS ratios and, as a result, can have surprisingly good performance in light air." Further, the heavier displacement boat with a relatively low WS has an even greater advantage sailing in light air with a left over chop - once they get going they have momentum to keep plowing through waves. Finally, sometimes the better light air boat is so because it is over-canvassed for all but the lightest conditions. Having to reef down in anything over 10 knots may not be the best situation for a relatively novice sailor.
Where we are in complete agreement, Par, is with respect to the relevance of the type of sailing Sea Wings ultimately wants (and can realistically expect) to do. It may be that neither of these boats will suit his wants or needs.
Brad Kelneck
