Wingtips?

[JeffM]

My imagination has been sparked by David Nichols' new Working Guide to Traditional Small Boat Sails (Duckworks). (That's also got me looking at the variety of hull shapes, etc. that Phil Bolger has designed.) I'm particularly intrigued by sprit sails (which Nichols thinks to be at least the equal of bermudas for pointing) and lug sails, both low-aspect rigs.

My understanding of the chief reason low-aspect anything is inferior to high, is the increased vortex off the end of the air (or water) foil: The "leakage" of fluid over the end of the foil (top of sail/bottom of cb or rudder) from higher pressure "underside" to lower pressure upper side. (You might be able to tell that I get these ideas from airplane wings.) Since a broad, low sail has a lot of "end," it loses a lot more of its effectiveness to this "leakage" than a tall narrow sail does. Now in the case of airplane wings and fixed rudders and keels, the problem can be reduced by putting an end plate on it to block this leakage. (If you've flown at all recently, you may have noticed wingtip extensions.) [i've often wondered, by the way, how difficult it would be to arrange something similar on centerboards or kick-up rudders.]

I'm wondering--in a purely thought-experiment kind of way-- whether a balanced lug sail with a level gaff (I suppose that's just a square-rig?) could have an end plate built right into the gaff. In other words, the gaff would be broad and flat, with the flat surfaces horizontal. I suppose it would be very cumbersome, but might it work well enough to be worth a try? The advantages of a short, powerful, unstayed rig might then significantly outweigh the disadvantage of inefficiency.

[PAR]

End plates rely on minimum resistance within the flow, at the same time impeding the "bleed off" from the high pressure side. On a fixed wing aircraft, the end plate remains minimally oriented to the flow, while providing a dam. On fixed keels and rudders, you can also use this principle to good effect, in some sailing conditions. For the racers, who's wind strengths may be governed to a great degree by rule, they can be effective. In conditions that may cause considerable boat movement, like pitch and yaw, then they start hurting performance, acting like a drogue.

On a centerboard or kickup rudder, the relationship with the flow must be maintained, with the flow (minimal aspect), which means it will have to vary it's angle of attack, depending on relative board position. With the board at 1/2 down on a reach, the end plate would have to adopt the correct angle, to align with the flow. This could be as simple as a pivot, permitting the end plate to adjust to the flow from it's own forward movement (also pitching motion). It would probably need some dampening method or flutter could result, creating more drag and tip disturbance (lose of lift).

A dagger board and fixed rudder could use this feature and foil boats like the Moth and others, do in fact incorporate wings to permit very high speeds. These are usually in concert (not necessarily so) with a rudder mounted foil. They work because the foils don't move much from their minimum aspect to the flow. Most of the real speed freaks have to incorporate trim tabs on the foils to control pitch when up on the foils, but these speeds (an average foil Moth 20 MPH, all out racer 27 MPH and the bigger boats now pushing 50 MPH) with sails on the water, are short lived and the boats delicate.

This subject can get very involved with theory, math, physics, speculation and rumor. There are a number of sites available on the web. Most are chocked full of graphs, theory and healthy, experienced speculation. On craft like the Moth or IC, where costs can be reasonable, experimentation can be tossed about in a garage, tested on the local puddle and re-engineered after you drag the wreckage back to the garage.

The gaff, being at an angle to the flow, would generate huge drag numbers (like they don't suffer from this already) with an end plate or thick cross section worked in. A thick boom would be effective to a small amount. The sprit, lug and other hoisted spar rigs will also suffer the same fate as the gaff. If a wing was used, one that could maintain it's penetration ability into the flow without causing too much drag, then you'd have something. It's been tried, but adds a great deal of complication to the rig, not to mention weight in the worst place it can be, farthest away from the CG. From a technical stand point a bunch of little end plates could be rigged along the gaff, but the small advantages would be spoiled (I suspect) when the gaff fell off to leeward, or when the boat pitched. You could also fool around with vortice generators and other such devices to play with flow efficiency. I had a lot of fun with this sort of thing, when I was younger and willing to swim the broken craft back to the dock after a failure. I now prefer to stay in the boat (I have gators here, which think we taste like chicken) and drag models of design concepts. They're cheaper, much less likely to embarrass you upon your arrival at shore with the half sunken, wreak of a boat in tow. They're much faster to build and gives you something to do when the fish aren't hungry for the style lure you've got rigged that day. I've learned much from two models with slight configuration differences dragged on a balance beam, along side a boat.

[JeffM]

Paul, fascinating business! I'd never thought about the possibility that an end plate on a cb or rudder could simply pivot on its own in response to flow! That's a thought worth having! Of course, the Bolger designs that incorporate rudder end plates have reduced draft in consequence, with the idea that they therefore don't need to swing. I'd worry about a shallow rudder coming out of the water, though. In the case of a cb, I'm not sure how such a thing could be retracted into the trunk.

On a sail, I was picturing a completely horizontal upper spar (are there any?) so that drag would be less of a factor. But I'd forgotten about the issue of weight high up.

Paul, it sounds like you've had a lot of fun playing with stuff like this. Pity the gators have put a bit of a damper on! I wasn't at the point of even modelling, myself. Just daydreaming, and trying to breathe a little life into the Design forum, which deserves more posts now that we (most of us) are entering the armchair season.

[PAR]

I get more work done in the cool months of our winter, then in the 9 months of summer heat. I just can't hang out in the heat all day like I use to, so I look forward to the mid 70 degree days of winter.

I've got a couple of concepts I'm playing with (that's what she calls it when I head down to the lake with a few models in the truck). One is a spin off of the "box" keel which Atkins spent considerable development effort on. The other is a wild hair sort of thing that is an attempt to dramatically reduce eddy making drag and reduce the size of the "hole" a boat carries along with her underway. The wave train the model makes gives you the speed/length ratio, the wake and wave formation give you an idea of efficiency and when coupled with another model of similar configuration, you can see which is more so, at a glance or at different S/L's. I usually conduct several runs, then make adjustments to trim, heel, etc.

A horizontal spar would be very difficult to "stand" properly, with the lousy purchase on the peak halyard. It would also likely slat around in light air a bunch more then a peaked up gaff. You'd also loose useful luff length, which on a gaff is also true, but additional area can be found above the luff, farther aft on a peaked gaff.

Sail efficiency can be increased many ways, but how much do you want. Once you get to a certain point, efficiency increases become quite small, maybe not enough to improve boat speed. There's no discounting that in 100 years a retractable hard wing will replace our soft sails. Clean, free of external rigging, seamless construction will make these very efficient. The technology is in place, the prices coming down, materials and engineering capable of the tasks. Digital controls that can alter sail shape, aspect and incidence as the conditions require will make it literally a button pushing affair. We have 18' skiffs doing 35 MPH now, Volvo 70's topping 50 and that's just the mono hulls. There was a time, not long ago, that suggested multi hulls were the only way to go fast on the water, but many monos now are very fast, scary fast. This is efficiency, with a capital E.

Sail models are more difficult to analyze then hull forms. I've built primitive tunnels and pressure testing devices, but mostly relied on obvious efficiency improvements, like windage reduction, leading edge efficiency, camber shape and location, etc. for gains. I have to admit I've toyed with laminar flow, vortice generation and other ploys to see what would happen, but generally you end up with a lot of work for a 100th of a knot difference in ability, which just isn't worth the trouble.

From your aircraft knowledge, you know what would make a good sail plan, which will likely be quite similar to a sail plane wing. Okay, work that into a practical sail (or appendage) on a boat and you quickly approach what AC boats and other speedsters have on their craft. Dramatically truncated versions will provide truncated results, usually . . .

[JeffM]

Well, it was just a thought, anyway. For sure once we hit hard up against the law of diminishing returns its time to pack in the science and just enjoy the ride. I'm the low-tech type (if not much stuck on tradition), and a push-button-controlled flexible wing sail is not really my idea of fun. Give me wind and sea, a good hull, a little cordage, some dacron, and enough mechanical advantage that I can handle things myself.

By the way, are you telling me that there are sailboats -- monohulls --no bigger than mine than can do 35mph?! I never knew such things existed! I've got to have a look, just to satisfy my curiosity.