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From: Tom MacNaughton
Date: 30 Jan 2008
Time: 14:23:31 -0500
Remote Name: 66.252.38.154
It is true that a few British designers have struck all diagonals at 45 degrees. I have tried this on only one design and found that it was at least better than designing without emphasis on the diagonals. You are correct that 45 degree diagonals could not follow the planking run, nor would they be much good at representing water flow. A very large amount of information in our Yacht Design School main curriculum covers the proper use of diagonals for developing really excellent lines. I attribute a great deal of our great good luck with the performance of our sailing vessels to very careful attention to waterflow, primarily through attention to the diagonals. It would be impossible to summarize everything we know and teach about diagonals here but let me just give a brief look at how to draw really good lines by working back and forth between diagonals and sections: Basically everywhere in the wave making region, which basically means everywhere in the hull other than the keel and rudder foils themselves, the water flow is best if you can construct diagonals normal to, that is at right angles to the surface. These diagonals will have the largest practical radius amidships that you can manage and still have the radius continue to increase toward both ends, ideally until it is as close to infinite, that is straight. It is astounding when you research this how many of the truly great designers put an enormous emphasis on diagonals, while often not talking about it at all, as they probably felt it was a good trade secret. Among those who clearly placed great emphasis on good diagonals have been Nathaniel Herreshoff, Olin Stephens, Knud Reimers, and several others. I often suspect that many designers whose lines don’t even show much in the way of diagonals develop their lines as we do and then do not show the diagonals on any drawings shown to the public so people won’t realize what they are doing. Diagonals solve a lot of very esoteric problems very simply. For instance I have come across many varying theories on whether you should use fillets at the roots of keels and rudder skegs and if so how they should be shaped. Interestingly enough if you just build your fillets so that you get perfect diagonals through them you automatically get very nice looking fillets that you can easily see will give very good water flow. This same process will show you that flat bottom boats do not need fillets so it is a great tool. It tells you when you need a fillet and whether it will be perfect. Great stuff.