[quote=dmgbear55]Ok, so we are all right on this topic. I think there are some risks to making changes to the original designer's sail plan, but there are also some advantages. I have both spoken with and corresponded with Andrew at SLO and here is his response after reading the forum string:
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If possible, I'd like you to give me a call so I can explain how the area is calculated which will give you a better reference for the area values. Our standard pinhead main is 154 square feet while the square top is 165 square feet. Again, if you call, I can better explain what this means as the two sails are actually measured a little differently.
I read through the forum and "bruiser" brought up an interesting point regarding the aspect ratio of wings or sails. While he is correct, the aspect ratio of a square top mainsail is lower than a pinhead for the same rig. This is because aspect ratio is span (or luff length) squared divided by area. Since the luff length is fixed with the stock N5.2 rig, when you divide span squared by a larger area, you end up with a smaller aspect ratio. He is correct that modern sail plans are higher aspect than traditional rigs but that is because they are building taller rigs with shorter booms and keeping either the same or slightly larger areas.
Additionally, I think "Edchris177" summed it up perfectly in that the movement of the aerodynamic center will be small and that time on the water will be the best determination for how to set the rig up for the square top. Regarding your post about the daggerboards, you mention wanting the center of effort of the sails located at the center of the daggerboards. This is somewhat incorrect in that the rudders contribute to the center of lateral resistance in addition to the daggerboards. This means that the center of lateral resistance is somewhat behind the daggerboards. You don't want the center of effort of the sails directly in line with the center of lateral resistance, but they will likely end up being close. I look forward to hearing from you and let us know if you have any other questions.
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In my discussion with Andrew he said that the older software package that they used to design their Nacra 5.2 sails for both the square top sail and the pin head didn't take into consideration the overlap of the seams when calculating sail area. With so many more seams and overlaps and overlap width in the paneled sail the old software overstates the sail area, so it is not actually 11 sq. ft. it is something in the 7 to 8 ft. range. To get an accurate measurement they would have to design both sails in their newer software that would better sort this overlap difference out and probably not worth anyone's time.
We also talked about the lateral loads on the rudders a little more and there we got into a discussion of rudder types, angles etc. and that goes beyond this discussion thread.
I really liked corresponding and talking with Andrew who is clearly very knowledgeable, even for an aeronautical engineer. I also appreciate that SLO supports this website, so I am going to order a square top sail and will report my results.
dg[/quote]
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