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Boat Forum / Building / December 2004mast loads, design
Does anyone have advise on how to calculate mast/sail loads? This is actually for a solid wing ice boat, but I think if I can understand traditional mast/sail loads, it should translate to my application. I don't think aircraft design is applicable here, especially G-forces. The key point for boats vs. airplanes is that you can only apply so much force, and then the boat will start to capsize and spill the air. For sake of example here are some parameters: weight: 500lbs sail area: 50 square feet center of effort on sail (wing): 6 ft high plank: 14 ft (7ft from hull to runner) estimated speed (if it matters): 80mph I don't know how to figure this out with an equation, but by simply examining movement distances of the CE and hull, it appears that the sail has a mechanical advantage so the load on the sail would never reach 500 lbs. I also have to account for loads encountered in a gust. Am I on the right track here? Can anyone recommend some reading on this matter? Thanks for any assistance, Dave Assuming chain plate base =plank: 14 ft 500 lb. x 7 ft. = (vertical chainplate load) lbs x 14 ft. max mast load compression =vertical chainplate load max mast load compression= 250 lbs. There is more going on here, but I need spreader information. > Does anyone have advise on how to calculate mast/sail loads? This is > actually for a solid wing ice boat, but I think if I can understand [quoted text clipped - 20 lines] > Thanks for any assistance, > Dave If you give me the sail CE height from the ground, I can add the sail compression load. Is there a forestay? Height? > Assuming chain plate base =plank: 14 ft > [quoted text clipped - 28 lines] >> Thanks for any assistance, >> Dave I forgot to mention that the wing has shrouds and a forestay, probably around the 8 ft mark. CE is around 6 ft. > If you give me the sail CE height from the ground, > I can add the sail compression load. Is there a forestay? [quoted text clipped - 32 lines] > >> Thanks for any assistance, > >> Dave Hi Dave, I think your on the right track. The capsize moment is simply the weight times 1/2 the "beam", but dynamic loads (wind gusts, sharp turns, bumps, etc.) are more difficult to quantify. On my last wing sail project I doubled the capsize moment to account for dynamic effects. Crude, but you have to start somewhere. The next step is to use the beam equation, Stress = MC/I, to calculate the required mast size and thickness ( I ), for an assumed stress level. If your building the wing out of composite materials its a good idea to build some samples, test them to failure, calculate the failure stress, and use a percentage of this stress ( 50% ) in the beam equation to calculate the minimum I (moment of inertia) required. SKEN'S ELEMENTS OF YACHT DESIGN has some examples of the basic process. You can get more details on my wing sail projects at: http://www.johnsboatstuff.com/Articles/rigid2.htm . Good Luck, John > Does anyone have advise on how to calculate mast/sail loads? This is > actually for a solid wing ice boat, but I think if I can understand [quoted text clipped - 19 lines] > Thanks for any assistance, > Dave Thanks John. I looked at my movement drawings a little more and I think I understand the static loads. As I said, the mast/wing does have a mechanical advantage for lifting the hull. Using the lee runner as the radius center, the capsize moment is the ratio of the hull circumference and the CE circumference. With pi factored out, the algorithim is: | | | h | | | +------+------+ | b | 500 * b/sqr(h*h+b*b) (the CE radius is simple trig) Actually the base in this example is less than b because the movement isn't actually against the lee runner, but against a line that is tangent to the hypotenuse of the triangle formed by the rear plank and the front steering spring board. This omission actually errors on the side of safety so I'm not going to worry about it. I'm ok with designing the wing if I know the loads. I purchased a spar designer that is used for designing airplane wings. The program isn't perfect for this application because it assumes a cantelevered wing (unstayed). Only the portion of the wing above the stays is cantelevered. So now I'm still left with the dynamic loads encountered in a gust. I'm ok with just doubling the loads if that works. FYI: here is a web site that I'm co-managing. I'm co-managing only because I helped instigate the forming of the group, not because I know anything about wings. :-) My alias in that group is Sllingblade. http://groups.msn.com/LandsailerandIceboatdesignandconstruction > Hi Dave, > I think your on the right track. The capsize moment is simply the weight [quoted text clipped - 37 lines] > > Thanks for any assistance, > > Dave |
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