Can a Wind Propelled Ice Boat Outrun the Wind?

[the_jerk]

Can a wind propelled ice boat have a speed higher than the speed of the wind it is propelled by??

The answer, as i have come to know is "yes" ... but i need a proper explanation... hope that someone can help me out ! thanks in advance!

 

[mgb_phys]

For a modern shaped sail, rather than an ancient square sail, the wind flowing past the sail creates a pressure difference.
The pressure * area is force. Exactly like an aircraft wing.
So the force available depends on the wind speed and the area and shape of the sail.
Then the speed acheived by this force depends on the mass and hydrodynamics of the boat.

 

[russ_watters]

Actually, ancient square sail ships could travel upwind. Otherwise, your explanation is corret. An ice boat is a sail boat.

 

[rcgldr]

Can a wind propelled ice boat have a speed higher than the speed of the wind it is propelled by?

How Fast Do These Things Really Go?

Ice boats, depending on design and class, will reach speeds up to five times the speed of the wind. How? Well, it has something to do with the low friction between the runners and the ice, and the sail shape. The sail acts more like a vertical wing rather than a sail. Volumes could be written as to exactly what makes an ice boat speed along at five times the speed of the wind. As to how fast they can go, in the right conditions, the smaller DN class achieves speeds of 50 to 60 mph. The ultra-modern class A Skeeters (the "Formula One" class of ice boating) reach speeds well over 100 mph.

From this link. The section on "apparent wind" is a bit misleading, as airfoils can't convert a "headwind" component of apparent wind into forwards thrust. Only the component of wind perpendicular (or downwind) relative to the ice boat can be deflected (accelerated) backwards by the sail. The sail exerts a backwards force on the air coexistant with air exerting a forwards force on the sail (the Newton 3rd law pair of forces in this case). As long as the force in the direction of travel is higher than the total drag force, the ice boat continues to accelerate until the thrust equal drag.

www.iceboat.org/faq.htm[/URL][/b]

High end catamarans use a semi-rigid airfoil instead of a sail, but rigid airfoils weren't mentioned on that web page.

An analogy can be made with a glider, where the vertical descent speed of the glider is the same as the wind speed of the ice boat. High end gliders with 80 foot or longer wingspans have 60 to 1 glide ratios at about 60mph, the equivalent of converting a 1mph cross wind into 60mph of forward speed. Ice boats aren't as aerodynamic, and have additional drag from the ice and runners, and can't achieve this level of efficiency.