- Status
my source is gone!
My source for ratios is off the web and out of touch. If anyone has the forumlas used to calc these things please send them along! p.
My source for ratios is off the web and out of touch. If anyone has the forumlas used to calc these things please send them along! p.
Lotsa' Web Sites
The Schell Regression Formula, along with lots of PHRF boat ratings, is located at www.well.com/user/pk/YRAphrf-sf.html A great search engine is www.metafind.com Just enter PHRF and you'll get more websites than you need! Michael O'
The Schell Regression Formula, along with lots of PHRF boat ratings, is located at www.well.com/user/pk/YRAphrf-sf.html A great search engine is www.metafind.com Just enter PHRF and you'll get more websites than you need! Michael O'
Here they are ..
The formulas are all dimensionless ratios. Using beam and DWL in feet, SA in square feet, and disp in pounds, they are: hull speed = 1.34 * sqrt(DWL) SA/D = (disp/64)^(2/3) / SA D/L = (disp/2240) / ((DWL/100)^3) capsize screen = beam / (disp/64)^(1/3)
The formulas are all dimensionless ratios. Using beam and DWL in feet, SA in square feet, and disp in pounds, they are: hull speed = 1.34 * sqrt(DWL) SA/D = (disp/64)^(2/3) / SA D/L = (disp/2240) / ((DWL/100)^3) capsize screen = beam / (disp/64)^(1/3)
WTG Been There!
Thank you BT!!! I've plugged the formulas in and they work quite well in my database app. over the next day or two I'll ammend the boat section to use the specs and calculate them on the fly. I can also put a calculator in the reference section. One question: I've always been unclear on this and can't seem to find a definition: Are LWL and DWL the same measurement??
Thank you BT!!! I've plugged the formulas in and they work quite well in my database app. over the next day or two I'll ammend the boat section to use the specs and calculate them on the fly. I can also put a calculator in the reference section. One question: I've always been unclear on this and can't seem to find a definition: Are LWL and DWL the same measurement??
More ..
DWL is "designed waterline length." I am not sure what LWL means (length at waterline?) or how it may differ. Probably the same thing. There are some other boat statistics that would be interesting. SA/WS is sail area to wetted surface, both in the same units. It speaks to light air performance. (Square feet, square meters, doesn't matter, same ratio.) B/D is ballast to displacement, same units. Vertical center of gravity, as distance above or below waterline, is more interesting than B/D. Hunter should have this latter tidbit since it is required to calculate metacentric hight and other useful things during the design process, but I doubt they will share it. BTW, there are two different notions of sail area. The standard measure uses 100% foretriangle regardless of the size of the working headsail, and the main triangle, regardless of excess or deficit roach. In other words, SA = (I*J + P*E)/2. Sometimes builders will report actual sail area, ie, the size of the main and jib that are standard for the boat. Neither is "right" or "wrong," but obviously you have to know which you have in order to interpret what it might mean. If anyone is wondering about the recurrence of disp/64 in the formulas, this comes from the fact that sea water weighs 64 lbs. per cubic foot. With areas and displacements in the square and the cube of the linear unit, it doesn't matter if the latter is feet, meters, or cubits: the ratios will come out the same. 2240 pounds is one long ton. That one is traditional. Keep in mind that boat behavior doesn't really scale as these ratios might suggest. Water and wind at the large scale behave different from water and wind at the small scale. If you really want to calculate scaling, you have to get into Reynolds numbers and all that stuff. Fluid mechanics is not for the faint of math.
DWL is "designed waterline length." I am not sure what LWL means (length at waterline?) or how it may differ. Probably the same thing. There are some other boat statistics that would be interesting. SA/WS is sail area to wetted surface, both in the same units. It speaks to light air performance. (Square feet, square meters, doesn't matter, same ratio.) B/D is ballast to displacement, same units. Vertical center of gravity, as distance above or below waterline, is more interesting than B/D. Hunter should have this latter tidbit since it is required to calculate metacentric hight and other useful things during the design process, but I doubt they will share it. BTW, there are two different notions of sail area. The standard measure uses 100% foretriangle regardless of the size of the working headsail, and the main triangle, regardless of excess or deficit roach. In other words, SA = (I*J + P*E)/2. Sometimes builders will report actual sail area, ie, the size of the main and jib that are standard for the boat. Neither is "right" or "wrong," but obviously you have to know which you have in order to interpret what it might mean. If anyone is wondering about the recurrence of disp/64 in the formulas, this comes from the fact that sea water weighs 64 lbs. per cubic foot. With areas and displacements in the square and the cube of the linear unit, it doesn't matter if the latter is feet, meters, or cubits: the ratios will come out the same. 2240 pounds is one long ton. That one is traditional. Keep in mind that boat behavior doesn't really scale as these ratios might suggest. Water and wind at the large scale behave different from water and wind at the small scale. If you really want to calculate scaling, you have to get into Reynolds numbers and all that stuff. Fluid mechanics is not for the faint of math.
LWL - load water line
I'll bet that LWL means loaded water line. This differs from design water line, especially for us cruisers who load up our boats with neat stuff
I'll bet that LWL means loaded water line. This differs from design water line, especially for us cruisers who load up our boats with neat stuff
- Status