The wonders never cease in the world-famous Sailing Anarchy Forums, where people say the strangest things. Like yesterday, when Anarchist ‘bgytr’ calculated the real-world efficiency of the fantastical sail-powered crude carrier we featured earlier in the week. Do his numbers check out?
As a nav. arch. student, I did a thesis paper on wind assist for commercial ships. Basically the price of fuel needs to be so high for it to work that it is not feasible. Also, tankers routinely take green water over the deck. Those little sails would have to be pretty dern heavy to stand up to that. And just everyday industrial conditions on a tanker are pretty friggin rough. Maitenance costs of another system onboard- not reasonable.
A simple computation, under ideal beam apparent wind at 20 knots:
Assume those sails are roughly 80 feet high by 20 feet chord. = 1600 ft^2
Lift= 0.5 r*v^2*A*C
C: lift coefficient (give a fairly generous C of 2.0)
r: air density taken as 0.0023769 slug/ft^3
v: 20 knots = 33.8 ft/s
A: area = 1600 ft^2
Force from one sail = 4300 lb of thrust per sail. x 20 sails = 86000 lb of thrust. A typical tanker of that size will crank out 80000 hp or more at about 17 knots => 1.5 million lb of thrust.
Multiply the sail thrust by about .7 since the leeward sails will be in the wash of the windward sails => 60,000 lbs of sail thrust under ideal conditions.
60,000 / 1,500,000 = 0.04, or a 4% thrust benefit. Not worth it when you factor the drag under headwind conditions, maintenance costs, weight, etc. And that calc was for pretty much ideal circumstances. Actual thrust benefit under a range of conditions would be way less.
