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From: Tom MacNaughton
Date: 22 Feb 2008
Time: 13:55:25 -0500
Remote Name: 66.252.35.152
There is no question that the “Tahiti Ketch” has very poor lines no matter where you are looking at them. Thank Heaven Jack Hanna, her designer, is not around to take exception to what I say as he was extremely combative. Nevertheless whether you criticize the “Tahiti Ketch” for poor buttocks or poor diagonals her poor performance is not due to her being a double ender. To drag a “large quarter wave” you have to have rounded water flow lines in the stern, not just rounded buttocks. Water can only follow the buttock lines in boats with no deadrise. Once you introduce deadrise water flow cannot be directly down and under the boat in line with the buttocks any more than it can be around the waterlines. It has to take the shortest distance around the hull, which will be perpendicular to the vessel’s surface. This basically means that the diagonals are the very best representation of water flow that we can develop using any simple plane cut through the hull. There are more complex methods of projection that can give flow lines slightly more representative of actual flow but they are so similar to diagonals and so much more time consuming to create that they have not proved practical to use in daily design work. Of course one can also construct refinements to correct even this water flow according to pressure differences at different transverse locations but it really isn’t worth it in straight lines development as at that point you have gotten into water flow patterns which change constantly with every change in course and heel angle and all these effects tend to cancel out. So once again we are back to diagonals as the best representation of flow. There is no question that a boat can have too little reserve buoyancy either forward or aft. This might be the case with some double enders but it would not be true of well designed ones like the Colin Archer redningskoite type which I assure you have very good reserve buoyancy fore and aft. Both the Scandinavian style canoe sterned “Northern Crown” and the William Atkin “Eric”, which is actually derived from the Colin Archer lines have really excellent diagonals as both were from work by designers who put great emphasis on really superior diagonals. I’m afraid “The Search for Speed Under Sail” is a historical work which basically records the lines of American built vessels over time from around 1700 to 1855. Aside from frequent references to “finer ends” you won’t find that it really has much of anything relevant to what we know in modern naval architecture. I would suppose that Howard Chapelle, being a rather intuitive designer himself, simply assumed that people would look at the vessels he used as examples and simply “feel” why some were faster than others. He really doesn’t give any significant analysis of the characteristics of the lines themselves.