Re: Power Sailing - Useful or just fun?
Posted: Thu Jun 26, 2014 10:35 am
It's pretty simple guys:
Wind incumbent upon a sail is divided into two force vectors: The proportion of force that goes to lift and results in forward motion, and the proportion of force that goes to drag and results in heel. The vectors must sum to the total wind force, so anything you do that reduces heel will increase speed, and anything you do to increase heel will reduce speed. This simple math is why cats and tris are faster than monohulls: They don't heel, so the wind force must go to speed.
Consider the boat at a dead stop, and consider only the true wind. Let's say the wind is 3 knots at 45 degrees to the bow. This wind is going to result, with properly trimmed sails, in a force vector that has about a 1.5 knot component going to lift when properly trimmed, and a 1.5 knot component going to drag. It's not enough wind to get our boats moving. It's very difficult to keep the sails full in these winds, and it's not enough current under the boards to keep the helm on course, which means the drag component will rotate the boat such that 100% of wind goes to drag and zero goes to lift. You can't sail in these winds, they are useless.
But, when we add a motor, we improve the region of efficiency that the boards and sails are operating in. The sails now have enough wind to take on properly trimmed shape, and the boards have enough hydrodynamic force to do their job: keep course and reduce heel. We can now get the full lift component of the wind, which we could not do without the motor.
The only thing that's really occurring is that the motoring gets the boat to the speed where it sails most efficiently, and then the actual true wind force lift vector can be summed with the motor's propulsion vector because they are collinear; You can now use that 1.5 knots of forward wind force, which goes to improving fuel efficiency.
Yes, you can think about it in terms of apparent wind and you can think of it in terms of aerodynamics and also come up with the correct solution, but I think force vectors are the simplest math to understand what's going on.
Matt
Wind incumbent upon a sail is divided into two force vectors: The proportion of force that goes to lift and results in forward motion, and the proportion of force that goes to drag and results in heel. The vectors must sum to the total wind force, so anything you do that reduces heel will increase speed, and anything you do to increase heel will reduce speed. This simple math is why cats and tris are faster than monohulls: They don't heel, so the wind force must go to speed.
Consider the boat at a dead stop, and consider only the true wind. Let's say the wind is 3 knots at 45 degrees to the bow. This wind is going to result, with properly trimmed sails, in a force vector that has about a 1.5 knot component going to lift when properly trimmed, and a 1.5 knot component going to drag. It's not enough wind to get our boats moving. It's very difficult to keep the sails full in these winds, and it's not enough current under the boards to keep the helm on course, which means the drag component will rotate the boat such that 100% of wind goes to drag and zero goes to lift. You can't sail in these winds, they are useless.
But, when we add a motor, we improve the region of efficiency that the boards and sails are operating in. The sails now have enough wind to take on properly trimmed shape, and the boards have enough hydrodynamic force to do their job: keep course and reduce heel. We can now get the full lift component of the wind, which we could not do without the motor.
The only thing that's really occurring is that the motoring gets the boat to the speed where it sails most efficiently, and then the actual true wind force lift vector can be summed with the motor's propulsion vector because they are collinear; You can now use that 1.5 knots of forward wind force, which goes to improving fuel efficiency.
Yes, you can think about it in terms of apparent wind and you can think of it in terms of aerodynamics and also come up with the correct solution, but I think force vectors are the simplest math to understand what's going on.
Matt
- B. 
