New Age Trawler/Motorsailer; Kite assisted PowerYacht

[Greenseas2]

Eponodyne, you are partially correct with the flap deployed on one side; however, it's more in line with a "flaperon" that can be left in the full or partially down position to crab in to the wind. What is being bordered on at this point is an air inflated sail that can accommodate the various kite/sail control surfaces desired not to disclude spoilers for fast ditching of the sail if needed. Sail or wing tip winglets might also have a controlling effect of sail/kite yaw. Not to be out done by Fast Fred this morning, you might want to put wheels on the hull too in case you can't get the sail down before beach impact. LOL

[eponodyne]

Originally Posted by Greenseas2

Don't knock the leading edge Krueger flaps (simple leading edge slots) and trailing Fowler flaps. Those devices can help keep your air foil up in very light winds. They're used in the Swiss Helio Courier and Turbo Porter aircraft for ultra slow flight and short field landings/take-off capability. Before that they were use on the German Feisler Storch STOL aircraft for the same reason and are now used on the Slepcev Storch and other STOL aircraft. The steerable parasail device is fairly technical in nature and all design consideration should be taken in to consideration. Who knows? Maybe the parasail design for your boat mightl also have some nuances that potentially could benefit the aviation world as well. Good luck!

I was talking about using a flap on the daggerboard, to help work to windward. The best you can get right now is about 50 degrees true, which isn't pointing very high at all. I thought about cutting up a couple of windsurf kites and using them as a 'hard sail' but water launches would be if not outright impossible then a straight up, old skool sonofabitch. The advantage with something like that is one could use leading edge flaps but the weight and complexity just doesn't justify the results. For keeping the kite up in light air I'm thinking I'm gonna use helium LOL.

[eponodyne]

I never have figured out the edit function on this goofy format--but I want to add that these kiteboarders already figured out that it's necessarty to able to ditch the thing in a hurry. They call it a "chicken loop," and it's a pelican-hook affair that either unsheets the kite (yes, there are provisions for sheeting in and out by varying the angle of attcak) and/or detatching it from the harness completely, and retaining the control bar by one end or another. The kite then loses lift on one side, does a pretty quick dive to the water; you swim over to the bar, reset the chicken loop, and re-launch. Not that hard.

[Greenseas2]

In the design effort of a vessel using a parasail/kite as the main source of propulsion, it might be a good idea to separate the parasail design from the vessel design. Being that the prime mover in this case is the parasail/kite, it may be more prudent to start with this design effort first, and the finished product will probably be far removed from the typical common parasail of today. Inflatable, possibly yes, helium, No....too expensive over a period of time and hard to ditch in an emergency. As it stands at this point, the parasail is a kissin' cousin to a square rigger's top gallant and top royal sails.

[eponodyne]

Oh, no. No no no no no. These aren't parasails. They're parafoils, which is completely different if you disregard the fact that both designs are made out of cloth. Google 'traction kite' for a bajillion hits and tons of info; but in a nutshell, these kites are shaped overall like a section of a sphere (so lift is roughly equal no matter where the kite is relative to you) and in section like an airfoil. And they're no joke. That 12-sq-m will pick me right up off the ground in 15 mph of wind, and I weigh 265 pounds (like, what, 125 kilo?). Naish makes a monster that has something like 27 or 30 sq. m.

I don't need to do my own research on this. Naish, Cabrinha, SLingshot, Ocean Rodeo and a whole bunch of others have people on staff they're paying good money to do research for me (Ssshhh. They don't know that yet...).

[Greenseas2]

Good show on the references Eponodyne, this is the sort of supporting data that is needed on threads such as this. Still, any product can stand improvement to meet a specific need.

[kach22i]

Originally Posted by eponodyne

I'm *just* about to order an i550 kit; I plan on powering it with a 12 sq m Cabrinha bow kite

Are you still going to have a mast?

[eponodyne]

Originally Posted by kach22i

Are you still going to have a mast?

Nope. It would only get in the way. For the very lightest of airs, Minn-Kota has some excellent selections.

[kach22i]

Originally Posted by eponodyne

Nope. It would only get in the way. For the very lightest of airs, Minn-Kota has some excellent selections.

Will the kite be mounted where the mast mounting is/would be?

A bow mounting would have a very different behavior, right?

[eponodyne]

Yes, mounting it on the bow would be a simple solution but not at all a viable one.

I'm going to tie the kite off to a C-shaped track that runs from beam to beam, and that's going to be mounted on swivels on each gunwale, so it can lift and maintain angle. Think gigantic C-clamp, sort of thing. This will, in turn, be mounted on a tracks on each side deck, each about a foot and a half long and centered on the CG of the boat. Ya folla? So the kite will be pulling from the leeward side on a reach, and from wherever else on the track everything's happiest on other points of sail; and this whole kite-track lashup in turn can move fore and aft a little to compensate for sea conditions or speed or whatever. I can do this all with off-the-shelf Harken tracks and traveller cars, and this should be well within the forces that the hardware can handle, in any sort of wind state in which I'd have this thing out.

It's important that the kite track be a true semi-circle, so I can set up a bellcrank/cam arrangement for each side of the kite-- about a 1:3 input/output ratio. You don't need a lot of force to turn these kites: you don't fly them by sheeting one side in, you steer by dumping lift off the opposite side. Anyway, if distance from the yoke to the control track is constant, then I can do this with lines and pulleys, from the yoke, below the cockpit sole, forward, and up through the deck through pulleys. I don't want to get into Teleflex push-pull cables because they're pretty expensive, but I do understand I may have to eventually. It's not that hard to move things around to suit, and that's the major reason I'm picking the i550 to do it with, as for a quick little sport boat, it is after all just stitch-and-glue plywood. It'll be easy to beef up where it needs more beef, and I can modify everything pretty easily.

Oh, yeah: kite launch and recovery: 3 Fin-Nor saltwater gamefish reels, linked together so they all run the same speed. Little tiny stout gear-reduction winches, eh?

[tspeer]

Originally Posted by eponodyne

I don't think we need to look at leading-edge slots and Fowler extendible flaps just quite yet. But if you want to work upwind to (say) port, then doesn't it make sense to provide lift to port by "lowering" the flap on the centerboard or daggerboard or whatever, let's say... 20 degrees to starboard? Smell what I'm steppin in here?...

You're right that it is a flap. You're wrong that it produces more lift on a beat, except that it increases the maximum lift so the board doesn't stall out in a tack. What it does do is rotate the hull so it points in the direction of travel.

I've pointed this out on other threads, but the lift on the board is equal to what is required to offset the side force from the sail. That doesn't change by putting a flap on the board. Instead the flapped board produces the same lift at a lower angle of attack (leeway angle). However, the path of the boat through the water is determined by the lift/drag ratio of the hull+foils and the lift/drag ratio of everything above the waterline. If the flap doesn't change the lift/drag ratio, then the path through the water doesn't change. So the boat with the flap sails with the bow off the wind compared to the boat with the symmetrical board.

Since the lift is set by the sail trim, not the board configuration, the point of adding a flap is not to increase the lift when sailing steadily on a beat, but to reduce the drag. Many modern (WWII and later) sections have a low-drag "bucket" in their drag polar that can have a profile drag coefficient less than half of what it is just outside the bucket. With a symmetrical section, the drag bucket is necessarily centered on zero lift, and unless you have an over-sized board you will be operating outside the drag bucket. But with a flap, you can move the drag bucket to center it on your operating condition.

If you're not going to use the flap that way, then you're about as well off to pick a more robust symmetrical section that has a higher minimum drag but lower drag at the operating condition.

[eponodyne]

Originally Posted by tspeer

Since the lift is set by the sail trim, not the board configuration, the point of adding a flap is not to increase the lift when sailing steadily on a beat, but to reduce the drag. Many modern (WWII and later) sections have a low-drag "bucket" in their drag polar that can have a profile drag coefficient less than half of what it is just outside the bucket. With a symmetrical section, the drag bucket is necessarily centered on zero lift, and unless you have an over-sized board you will be operating outside the drag bucket. But with a flap, you can move the drag bucket to center it on your operating condition.

If you're not going to use the flap that way, then you're about as well off to pick a more robust symmetrical section that has a higher minimum drag but lower drag at the operating condition.

But what about if sail trim can vary more widely than what can be determined by cloths on a stick? With a kite, lift is coming from a point well outside the hull itself; and with what I'm going to build, heeling moment is going to be reduced to an absolute minimum. I think this is a worthwile avenue to explore, simply because a flapped daggerboard and a traction kite have as far as I know never been used on the same vessel.



Tandem daggerboards? What do people here think about that idea?

[tspeer]

Originally Posted by eponodyne

But what about if sail trim can vary more widely than what can be determined by cloths on a stick? With a kite, lift is coming from a point well outside the hull itself...

The same thing applies - the lift on the board is determined by the force from the kite. To see why, consider these cases:

(For simplicity, assume the hull has rounded sections such that the side force from the hull is negligible compared to the side force from the board)

1. The kite is pulling the boat dead downwind, and the flap is not deflected.
What is the lift on the board? Zero.
What is the boat's heading? Dead downwind.

2. The boat is traveling to windward, and the kite is pulling mostly sideways and a little forward. The flap is not deflected.
What is the lift on the board? High.
What is the boat's heading? Pointed to weather of the course through the water by the board angle of attack required to produce the required lift.

3. The kite is pulling the boat on a straight line course dead downwind, and the flap is deflected.
What is the lift on the board? Zero.
What is the heading of the boat? Pointed to the same side of the course as the flap trailing edge, by the amount of the zero lift angle of attack of the board. This will be a negative angle of attack for a positive flap deflection.

4. The boat is traveling to windward at the same speed and course as case 2. The kite is pulling in the same direction relative to the apparent wind.
What is the lift on the board? High, and the same magnitude as for case 2.
What is the heading of the boat? The bow will be pointed to leeward compared to case 2 by the amount of the zero lift angle of attack corresponding to the flap deflection.

Note that cases 1 and 2 have the same board configuration, but different amounts of lift. The same for cases 3 and 4. Cases 1 and 3 have the same lift, but different board configurations. So the lift cannot be determined by the board configuration.

Cases 1 and 3 have the same lift and the same kite orientation with respect to the wind. Cases 2 and 4 have the same lift and the same kite orientation with respect to the wind. So the lift is evidently determined by the trim of the kite relative to the direction of travel.

For cases 3 and 4, the bow of the boat is rotated relative to cases 1 and 2. In fact, when sailing in a straight line, the difference in the boat's heading between the flap centered and flap deflected cases will be essentially the same on all points of sail and at all speeds. This is because the flap shifts the lift coefficient vs angle of attack curve, but does not change the slope of the lift coefficient vs angle of attack curve. If the angle of attack is measured from the zero lift angle instead of the plane of the board, then the angle of attack for cases 1 and 3, and cases 2 and 4, will be the same.

I suspect you'll agree with what I've said for cases 1 - 3, but case 4 comes as a surprise. It is counter-intuitive, perhaps, but I think you'll see that if you accept case 3, then case 4 follows directly from it.

And it matches what we all know intuitively - What's the purpose of the board? To counter the side force from the sail/kite. If the kite didn't produce side force, then the board wouldn't be needed at all - for the same reason power boats don't have boards. So the lift on the board has to be all about the sail/kite.

[eponodyne]

What about Case 5: Boat travelling to windward
Kite pulling mostly ahead and a little sideways
Trailing edge deflected to leeward

So the bow of the boat is deflected to windward--right?-- and if I keep everything balanced to *just* before the keel stalls, then that should net me a greater windward moment than without a trailing edge flap. I understand that this is going to cost me speed due to lift-induced drag (nothing is free, my powers are many but I can't refute the laws of physics just quite yet). You're forgetting that the kite can be sheeted independantly of direction of pull; I can hang the kite anywere in the wind window and keep it hovering just short of a stall, or sheet in and produce a truly impressive yank. Efficiency dies out toward the edges of the wind window but lift on the kite happens any time air flows over the top faster than the bottom.

I'm going to give this a try to attempt to gather some empirical evidence. Theory is a great place to start from, but I'm a belt-and-suspenders kind of guy and awful stubborn too. This keel build is going to cost me three Soss hinges, a couple Heim joints and some aluminum tubing, and about five hours of my time. I think that makes it worth a try. As I said, if it turns out to be a big pain in the ass, then I haul the boat, center the flap, and fair everything over with a couple layers of 'glass. No big deal.

[tspeer]

Originally Posted by eponodyne

What about Case 5: Boat travelling to windward
Kite pulling mostly ahead and a little sideways
Trailing edge deflected to leeward

If you can get a kite to do this, you'll have a gold mine on your hands! But I accept the premise.

So the bow of the boat is deflected to windward--right?-- and if I keep everything balanced to *just* before the keel stalls, then that should net me a greater windward moment than without a trailing edge flap.

No, the bow will be deflected to leeward. You have to consider what makes the whole system come into equilibrium so all the forces and moments are balanced.

In the wind tunnel, where you hold the board fixed in position relative to the stream, if you deflect the flap you get more lift. In order to find the angle where the lift is again zero, you have to point the leading edge down and operate the board at a negative angle of attack, so the negative lift from the angle of attack just cancels the positive lift from the flap deflection. That is known as the zero lift angle of attack.

When the board is mounted to the boat, deflecting the flap does not have the same effect as it does in the wind tunnel because the board moves the boat. Let's take something like your example, where the boat is being towed on a straight course and the boat is initially directly behind the tow. The board is producing zero lift. Now deflect the flap and maintain the heading of the boat parallel to the tow boat's heading. You will initially feel the change in lift from the flap as a lateral acceleration. Then the boat will start moving sideways.

As it does, two things happen. The sideways velocity of the boat will result in a negative angle of attack on the board, reducing the board's lift - this is known as sway damping. As the boat moves to the side, the angle of the tow will change, placing a side load on the boat.

Eventually, when everything settled down into equilibrium, the boat will be to the side and behind the tow boat, like a waterskier moving out of the wake. The angle between the towline and the centerline of the boat will be equal to the arctangent of the lift/drag ratio of the boat. The angle of attack on the board will be zero because the boat is maintaining the same heading as before, and all of the lift is coming from the flap deflection. The fore-aft component of the towline tension will be equal to the drag of the boat, and the sideways component of the towline tension will be equal to the lift from the board. But the situation will be different from the start, because now the boat is being towed to the side of the tug, not behind.

To return the boat to the original position behind the tow boat, you have to steer back, changing the heading and reducing the lift on the board. When you arrive back to a steady tow behind the tug, the bow will be pointed to the same side that the flap was deflected. The angle between the towline and the centerline of the boat will be equal to the zero lift angle of the board with the flap deflected. The lift on the board will be zero because that's what it takes to balance the zero side force from towing straight behind the tug.

When we talk about the effects of flaps on sailing performance, we're always assuming that conditions are quasi-steady and in equilibrium. But when people consider flaps or jibing boards, they often assume that the boat is fixed on its course, which is not the case. And the flap would be pointless if it were!

I understand that this is going to cost me speed due to lift-induced drag (nothing is free, my powers are many but I can't refute the laws of physics just quite yet).

No, there's not necessarily any more induced drag from the flap, if it runs the full span of the board. The total lift on the board with the flap deflected will be the same as the undeflected flap, but the leeway angle will be different. As long as the lift is distributed along the span of the board in a similar manner whether it comes from the angle of attack of the board or from deflecting the flap, the induced drag will be pretty much the same. It's only when the flap covers part of the span, thereby loading some of the board much more than the unflapped part, that the induced drag will go up.

You're forgetting that the kite can be sheeted independantly of direction of pull; I can hang the kite anywere in the wind window and keep it hovering just short of a stall, or sheet in and produce a truly impressive yank. ...

Of course, you can sheet the kite anywhere. And the leeway angle will adapt to where you fly the kite. Any time the kite is flying to the side, the leeway angle will be increased. If the kite is flying in line with the centerline, whether down low to the water or overhead, the leeway angle will be zero.

I'm going to give this a try to attempt to gather some empirical evidence. Theory is a great place to start from, but I'm a belt-and-suspenders kind of guy and awful stubborn too. ...

I suggest you put a protractor on the stern, and trail a string with a bead on it from the center of the protractor to measure the leeway angle. It'll help to have a big protractor, because for the most part the angles will be well under 10 degrees.