One problem I can imagine is when you want to go downwind. As the boat approaches windspeed, the relative windspeed of the boat would theoretically approach zero, though I suppose water resistance might prevent the boat reaching windspeed. This would keep the turbine going, but it would obviously be the slowest direction of travel for the boat.
But, this is just my imagination working. IANANaval architect or engineer.
Nope, the sail was not an airfoil, instead it was a rotating cylinder. The “Alcyone” magnus-effect craft.
Search on
http://www.google.com/search?q=%2BCousteau+%2B"magnus+effect"
I suspect one problem with all props is that in very high wind speed they might become dangerous? At least with a sail you can take it in.
Another problem, it seems to me, is that a windmill needs a cantilevered mast with no rigging because you need clearance for the blades. It would be significantly heavier than a conventional mast which consists of a thin pole supported by multiple cables.
Here are some ruminations on why it isn’t such a hot idea and hasn’t become popular yet.
Large rotating objects have nasty gyroscopic effects.
Consider the typical land-based wind power turbine: a fixed vertical tower with a rotating horizontal base plate atop the tower. A generator with a horizontal shaft is mounted on the rotating base plate. Then a blade set like an airplane propeller is mounted to the shaft with the rotating blades forming a circular disc in the vertical plane.
Rotating the base plate in azimuth to point the turbine axis into the wind generates some hefty dynamic gyro forces that the tower structure has to absorb. The good news is that although the wind changes direction continuously, it generally does so slowly.
Now put that same assembly on a boat: Rather than being absolutely stationary, the base plate is swaying and coning in 3D. Yowza! The gyro forces go off the scale.
And since the gyro forces are 90 degrees out from the masthead motion you have very bad effects on the boat’s stability. As a wave makes the boat, say, pitch bow downwards, then the gyro forces will create a heel to the left, or the right, or maybe even increase the pitch-down, depending on the wind direction (ie turbine azimuth in boat-centric coordinates).
So I’d argue that a turbine like that is a loser. A drum turbine with a vertical axis might be a little better.
Now let’s turn to power output. For some good learning, try www.awea.com
Here’s a simplified formula for power output from a propeller-style wind turbine that I derived from one of theirs. I converted the units from metric to american and assumed typical-good values for a couple of efficiency factors.
P = R^2 * V / 115, where P is output power in horsepower & R is turbine disc radius (blade length) in feet & V is windspeed in mph.
So a turbine with 1 foot blades, i.e. 2 foot diameter, in a 10 mph wind produces 1^2 * 10 / 115 => 0.08 HP. A 10’ diameter prop in a 20 mph stiff breeze produces 5^2 * 20 / 115 => 4.35 HP.
And that’s HP at the turbine hub, ignoring bearing friction in the shaft, any losses in the power transmission system to the water-prop, and the coupling efficiency of the water prop to the water. If the turbine turned an electrical generator that powered an electric motor to drive the water prop, you’d be looking at about .9*.8*.8*9 = 52% efficiency for the bearings, generator, motor and bearings again. And electrical is probably the lowest-loss transmission technology available, certainly better than mechanical, probably better than hydraulic.
So for the 10’ diameter turbine producing 4.35 HP, you’d have about 2.25 HP delivered into the water prop.
So much for power delivered, now how about power required?
I used to have a 27’ sailboat. That’s a typical family weekend sailboat for lakes, bays and near-shore ocean sailing. It’s certainly not a yacht for the rich or famous, nor is it big enough to do any useful cargo hauling as a working boat.
The aux motor was an outboard that supposedly put out 8HP. At full throttle that motor was sufficient to move the boat to “hull speed”, the fastest that hull shape can push thorough the water without the drag curve going all-but vertical.
If we assume the 8HP figure was the truth and was measured at the motor’s prop shaft, then we can work out how much turbine we need to duplicate that awe-inspiring amount of power.
We need a turbine where R^2 * V = 1769. So a 1’ blade-length will be enough if the wind is blowing at 1769 mph, i…e Mach 2.6! A 5’ blade length will be enough in a 71 mph wind, i.e a baby hurricane. A 10’ blade length will be enough in a 18 mph wind.
A decent modern sailboat can hit hull speed in about 12 mph of wind. That needs a turbine with a 13’ radius. And just like a conventional sailboat, power & hence boatspeed will be less in less wind.
So we need at least a 13’ blade length, 26’ diameter wind turbine to drive our sailboat as well as conventional sails. And as others have said, the wind the turbine “feels” is the vector sum of the wind motion over the water and the boat motion over the water. So traveling downwind you’d want a much larger turbine since the faster the boat goes the slower the wind feels.
Assuming you don’t want to get decapitated, you’d want that mounted on a mast at least 23 feet above deck level. That’s about the same height as typical sailboat mast for that sized boat, but you also can’t have any support rigging since the turbine must be free to rotate in azimuth. So it needs to be real stout compared to a conventional mast.
Last of all, 100% of the forces on that mast are applied at the very tiptop. On a typical sailboat, the sails taper and so most of the force is applied near the bottom.
So we’re talking about needing a MUCH more rigid and stout mast structure to carry the extra loads due to lack or rigging multiplied by the lever arm effect from all forces being applied at the top, multiplied by the gyro issue I mentioned at the outset.
That’s a tough set of problems to solve. But it might be kinda fun.
But how is this any different from a sailboat? If you sail directly downwind, you eventually approach zero velocity relative to the wind. Diminishing returns and all that.
So what’s the difference between sails and windmills in that case?