Power Sailing - Useful or just fun?

[mastreb]

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

[DaveC426913]

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.

Yes, agreed. But your scenario starts with 3 knots of actual wind, which we can do something with. The current point of discussion was about whether we can extract any energy from apparent wind - at least that component generated from forward motion - the same way we can extract energy from actual wind.

[mastreb]

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.

Yes, agreed. But your scenario starts with 3 knots of actual wind, which we can do something with. The current point of discussion was about whether we can extract any energy from apparent wind - at least that component generated from forward motion - the same way we can extract energy from actual wind.

Force vectors will tell you: No, you cannot. Apparent wind in a 0 knot true wind has a force vector that consists of 100% drag and 0% lift. The energy we can extract is from lift only. So this is a mathematical way of saying "There ain't no such thing as a free lunch".

[BOAT]

You guys are driving me crazy.

[seahouse]

You guys are driving me crazy.

It's easy, BOAT.

The maximum forward thrust vector on the sails provided by the motor occurs when the throttle is in full reverse!

- B.

[BOAT]

You guys are driving me crazy.

It's easy, BOAT.

The maximum forward thrust vector on the sails provided by the motor occurs when the throttle is in full reverse!

- B.

I ALREADY TRIED THAT right there at the dock in front of the Jolly Roger. After three or four beers I almost rammed the dock!

I got a sail boat because I SAIL! I am not a "motorboat" captain. I only have that big motor as a point of convenience because out here on the Pacific Coast everything is always at least 25 miles apart:
It's 25 miles to nearest harbor
It's 25 miles to the islands
It's 25 miles BETWEEN the islands!
Heck! It's 25 miles just to get to the end of Point Loma at the opening of San Diego Bay from Imperial Beach!! That's 25 miles in a bay just to get to the damn ocean!!

Unless your retired with all the time in the world all those 25 mile legs really add up so I got that big motor to get me to all the best sailing spots.

That does NOT mean that I LIKE the motor - I'm not a big fan of motor boats.

As for the aft thrust vector blah blah bla stuff I can't follow I will just bring mastreb with me the next time I tank up at the Jolly Roger so there is a guy there that can hold his beer better than me to drive me off the dock. Once I have the sails up I'm fine - I can sail totally drunk no problem - it's the motoring part that stumps me.

(Also there is another "fuel dock" (bar) on the other side of Oceanside Harbor now where I can park next to a MAC X boat and Captain Nick of that X boat will always give me a proper shove off so i don't need to use the motor).

Motors are frustrating.

[Three Gypsies]

On our cruises we were doing a lot of motor sailing , using the jib only . It was on a furler and could be controlled from the cockpit .
The jib , on average added about one to two miles an hour , seldom more than that .
It probably increased fuel economy by a very small amount .

The reason we converted our Gypsy to a trawler was that the problems associated with a mast and boom were not worth the small amount of fuel savings .

If we used our boat offshore more , it probably would be worth it , but we are coastal cruisers , preferring the ICW to the big water . This wasn't the best sailing but it was the smoothest . By taking the ICW we were saving fuel by not having to fight the wind and waves if they weren't favorable for sailing .

[dlandersson]

Is it 25 miles down?

[DaveC426913]

The jib , on average added about one to two miles an hour , seldom more than that .

On what point of sail? Broad reach Beam reach, close Reach, Close haul?