Science of Sailing Lesson VI: Sails Science of Sailing Preserving America’s Sailing Legacy * Engaging Sailing’s Next Generation

Introduction We have already learned how a sailboat uses her sails for propulsion The difference in pressure between the two sides generates lift (like an airplane wing) Low Pressure Wind High Pressure

Introduction A sports car enthusiast searches for a car with a high horsepower / weight ratio… For a sailboat, we use a similar type of measurement to determine the potential speed (or acceleration) of the boat What variables do you think we use for a sailboat? horsepower weight Faster Car

Sail Area A sailboat’s propulsion comes from the wind on the sails and is proportional to her… Sail Area Jib Sail Area 150 sq ft Measured by calculating the area of each of the sails and then simply adding those numbers together Measured in square feet Mainsail Area 188 sq ft J/80 Total Sail Area 338 square feet

Height (Mast) x Base (Boom) Area of Mainsail Although most sails are not a perfect right triangle, we will calculate the area of the mainsail by measuring… Height (Mast) x Base (Boom) 2 Actual sail measurements are slightly more complicated as most sails have a natural curvature shape Mast Boom

Height (Luff) x Base (Foot) Area of Headsail Although most sails are not a perfect right triangle, we will calculate the area of the headsail by measuring… Height (Luff) x Base (Foot) 2 A common measurement for the headsail area is the “foretriangle” Luff Foot

Sail Area – Displacement Ratio In order to compare boats, we calculate the… Sail Area / Displacement Ratio This shows how much power per lb of displacement Under this calculation, we assume that displacement is the sole limit of a boat’s speed While not completely accurate, as we have seen with the different hull designs, this ratio provides a good way to compare boats Why do you have to divide the boat’s displacement by 64? Weight of Seawater = 64 lb / cubic ft

Sail Area-Displacement Ratio The higher the ratio indicates a high performance boat designed for racing, but generally more difficult to handle Low Medium High 8 - 13 14 - 20 21 – 40+

Sail Area-Displacement Ratio Let’s do an example using the Cal 25 Sailboat Sail Area (sq ft) [Displacement (lb) / 64 ]2/3 Sail Area = 315 sq ft Displacement = 4500 lb 315 sq ft [4500 lb / 64 ]2/3 SA/D Ratio = 18.5