Sailing Directly Upwind

Gonzo, the external forces acting on the sailing machine (SM) are the wind and the resistance of the towed hull which are in the same direction. Both have components pushing the SM to its port (45 deg left of upwards in the figure) which are opposed by the hydrodynamic forces B1 and B2 on the daggerboard as shown.

I have drawn the SM with a hull of its own but it is purely as an aid to understanding. However, if you look at it in that manner and replace the red beam with a towrope, you have a towing boat on the startboard tack pulling the towed boat to windward.

All that has to be done to see my idea in action is to replace the rope with a stiff beam pivotted at both ends, delete the towing boat's hull and let the beam support the rig and board.

There is torque on the red beam due to the driving force of the sail and the drag of the daggerboard, but it is static and results from the forces shown not being coplanar. The torque is passed to the towed hull which will heel to port in response.

I haven't looked at the effect of wave action; this is a simplified diagram.

 

You are not correct. There isn't a single vector of force pointing downwind. All of them are somehow pointing in the direction that helps make your dream work. If you had ever towed a boat, you would know that the tug tries to line up with the tow. You don't have the torque or the pull (backwards and not forward as you show it) from the tow.

 

Terry
The lift force on a foil is perpendicular to the direction of the wind and the drag is in line with the wind. This means the vector angle of the resultant force on the foil can never be forward into the direction of the wind.

Stating this another way, the direction of lift is not related to the orientation of the foil. It is related to the direction of the wind.

Putting this in terms of your diagram the "Drive" force will always be negative.

To get positive drive you have to bear away off the wind until the angle of the resultant force on the sail has a component that is positive in terms of moving the boat forward.

Taking the sail off the hull does not achieve much. You might be able to tack the whole sailing machine faster than a boat but you still have to pull the boat around to sail on the opposite tack. The boat cannot sustain motion directly into the wind.

Rick W

 

This reminds me of http://www.boatdesign.net/forums/boat-design/flutterby-drive-24631.html

 

Tcubed first: that's the original, there's even a post of mine on that thread. I subconsciously regurgitated the idea, Oops! My apologies to Grover if he is reading: I wonder what happend to that idea which was stern mounted.

Gonzo: my vector diagram was confusing because the wind-derived forces (D-S-B1) are shown acting on the sailing machine (SM) whereas the towing force and its components (TDB2) are shown as they act on the towing beam not the SM. I have added a new vector diagram which shows all the forces that act on the SM; the revised figure is colored and the new diagram is in green. Hope it helps. Please note the board forces are normal to the board, the sail forces are (approximately) normal to the sail, and there are 2 opposite forces of magnitude T acting on the beam.

Rick: quite true: but the SM IS bearing away from the wind. I added arrows on the hull and SM to show their directions. I added tracks for the hull and SM; I hope the figure is not getting too cluttered.

To reiterate; taking the sail off the hull achieves one thing only, the SM can tack very quickly, because the torque that rotates it is applied from the much heavier hull: the SM has no rudder. Also hull speed is not lost through the tack. The direction of the towed force is oscillating too rapidly for the hull to follow it; the hull follows the averaged-out force which is windward; refer to the track diagram.

 

Terry
You are correct. The resultant sail force is behind perpendicular so it will do as you say.

Basically it is oscillating air and water foils in combination. It is the simplest form of the propeller/turbine configuration. You need to set up some form of flip system to get it to oscillate back and forth automatically.

A better set up would be two sailing machines working on opposite tacks so the yawing of the hull is eliminated. This was a big advantage over the Hobie flapper drive over my single oscillating foil. I had high imbalance pitching forces whereas the flappers have high but balanced yawing forces - the pitch force is much lower and not balanced.

A few systems like you have were offered on the DDWFTTW thread.

The advantage of this system over a propeller/turbine is that you are not converting through a mechanical drive train. You are only reacting forces.

Rick W.

 

The vectors are still wrong. There is no vector for the negative force of the towed hull or the torque produced on the "sailing machine" as it pulls backwards from its aft end.

 

The vector "T" in the green diagram represents the hull drag: if you are enquiring about torque on the sailing machine (SM) around the vertical axis, it is virtually zero because the beam is pinned to the SM at its balance point for this point of sail; that will probably be further forward than I have shown. Torque is applied to the SM for tacking/steering purposes of course, which will cancel torque caused by beam tension while holding a tack.

On other points of sail the beam pins will need to be locked, I expect, but that will become clearer during testing. Assuming it sails at all, that is. I think you and others are right to be sceptical, I am still looking for the fatal flaw myself, but I haven't found it so far.

Keep looking! If it is hopeless I don't mind admitting it and it will save a lot of bother building and testing it. I haven't started cutting wood yet but I sorted through the stack and I have what I need. I'll have to buy some Al tubing, a couple of pivot bearings and a tarp for the sail, looks like I have the rest.

 

There is one T forward and one T backwards. If the forces are applied like you say, the tow would have to move sideways. The torque on the "sailing machine" will make it weathercock. Try towing something and then you'll see.

 

Acting on the beam are 2 force vectors of magnitude T, both along its axis, so it will not move sideways. As the sailing machine zigzags the tow will follow it but in an attenuated zigzag as shown in the figure.

You have a point with the weathercocking. If you had a towing line that is angled off the centerline of a tug it will be pulled off course in one direction or other, that will depend on where the tow rope is secured to it. Too far aft and it will weathercock, usually the case I imagine, too far forward and it could end up helplessly sailing around the tow. I will take that into account in the design; I may have to experiment to find the best position.

 

Terry
Before you start cutting you should sit down and work out how quickly it needs to tack to make way at a reasonable speed and without having a ridiculously long towing rod.

Take it a step further with a catamaran and water/air oscillating pair on each hull synchronised to work in opposing tacks. The air foil needs to be arranged so it has mechanical advantage over the water foil.

Rick W

 

One thing that cannot be allowed is to have the SM to become normal to the beam as it's speed would have to be infinite to keep up with hull movement. Most sail boats are comfortable tacking through 90 degrees so I will use that for my calculations. The beam, therefore must move over an included angle of less than 90 degrees; I will choose 60 degrees arbitrarily at this time. It seems sensible to have the lengths of each tack significantly longer than the length of the turns, so the turn arc length becomes the driver for beam length. That must be an informed guess as no data is available for this configuration.

I think the boat available for the test is unlikely to exceed 1 k VMG so using the assumptions and calculation in the figure I get beam length of 5.6 ft, minimum. For now I will let it be 6 ft and see how the design looks. The target VMG of 1 k is hardly ambitious but per post #42 it represents at least 100% improvement for the boat that will be used.

The catamaran concept is likely to outperform it and of course a bigger boat would also have merit but for a demo that is not needed and beyond my resources!

 

I think the numbers are just a fantasy. You are inventing them to "prove" a theory. If you have a towed boat attached to a rigid arm, the towing vessel will move in an arc. Also, if you look at your drawing, even though the "sailing machine" is pulling at different angles from the direction of assumed movement, the tow magically goes in a straight line. You don't need to build any fancy machine to try the concept. Tie a bucket to a sailboat and it will show you what happens.

 

Just curious, Terry, but what is the reasoning behind the suggestion of tacking through 60 degrees?

 

Chris: the sailing machine tacks through 90 deg, as is typical. It must never be at right angle to the beam or it will be pushed sideways, so the beam must rotate less than 180-90 deg range; the 60 deg limit (+/- 30) I chose seems a reasonable place to start. A larger range of motion for the beam would force me to reduce the tacking angle.

Gonzo: I guessed the time needed for the sailing machine to change tack. It is much less than any sailboat I have ever seen. The torque to achieve that is derived through a harness and reacted by the hull's weight, instead of a rudder. That is the central principle, without it all fails.