Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15Jan09)This issue will take a look at "aspect ratio". The 1 February issue will continue the topic.
THE BOOK IS OUT!
BOATBUILDING FOR BEGINNERS (AND BEYOND)is out now, written by me and edited by Garth Battista of Breakaway Books. You might find it at your bookstore. If not check it out at the....
ON LINE CATALOG OF MY PLANS...
...which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Then Duckworks sends me an email about the order and then I send the plans right from me to you.
"Dear Jim, I've got to tell you - that little AF4b of yours is a marvel. I've logged close to a thousand miles on her so far, through streams, rivers, canals, bays, sounds, and even one unplanned foray into the wide Atlantic*; in blazing sunshine, driving rain, wind speeds over 25k, and temps as low as 30º. Never once has she let me down." So says Rene Vidmer. Beautiful picture and beautiful job of building and using a boat.
ASPECT RATIO ....
...is all about shape. In particular we will take a look at the overall shape of a sail or underwater board. Aspect ratio, let's call it AR, has always been a concern to aircraft designers all the way back to the Wright brothers who did a lot more than just fly the first airplane. They were also pretty good scientists, I think, and in the Air Force Museum I recall seeing a wind tunnel built by the Wrights, very state of the art back then.
Anyway, AR is defined as AR=bxb/S where "b" is the span of wing in aircraft terms and "S" is the total area of the wing. Look at this:
What this is supposed to show is two airplanes from a top view. They have the same wing area and if they had the same weight and wing cross section they should have the same stall speed and so forth. But they would fly totally different from each other. The plane with the long narrow wing might be a modern sail plane. High powered jets might have a short wide wing. So what's the deal with the shape?
I GOTTA TELL YOU...
...that aerodynamics seems at times to be like witchcraft, with correction factors piled on high to theories that never seem to explain everything. Luckily a hundred years of research has left us with many choices of correction factors. Hey! One day I'm sitting at the missile factory and the boss is agonizing over having to add little studs to his missile so it will fit a new launcher. "They will ruin the range with extra parasitic drag, everybody knows that," says the aero department. "But we gotta have them," says the boss. "We'll build it with studs and fly it and see what happens," he says. The test goes very well indeed, the studded missile outflying the nonstudded. "The studs acted like vortex generators and energized the boundary layer thus reducing drag, everybody knows that," says the same aero department. Well, they have lots of majic wands in aero but they know their aspect ratio effects. To start take a look at this:
This is an airplane viewed head on. The thing flies because the air pressure acting on the underside of the wing is higher than that on the top. But in real life the air near the tip will try to escape from the bottom to the top by flowing around the tip, being pushed by the difference in pressure there. It swirls from bottom to top as the plane flies forward making a vortex. You can see them under certain conditions as they cause swirling clouds to appear at the wingtip.
Now, this doesn't happen in wind tunnel testing of wing sections because the wing model usually goes across the tunnel so that air can't swirl around the end. That testing is supposed to be independent of "3D" effects so the wing section test data is reallty for a hypothetical wing of infinite span, no tip losses. So adjustments are made to calculate the effect of real life wingspan. The effect is I think mainly in increased drag and I don't recall ever seeing any wingspan related adjustment to maximum lift.
Great efforts have been made to reduce the swirls such as twisted wing tips, endplated wingtips and lately elaborate winglets on the tips.
But clearly a long narrow wing has less wingtip than a short wide wing.
NOW TO THE BOAT...
When a sailboat is close hauled, that is sailing close to the wind, both the sail and the boat's underwater board are acting as airfoils, the sail through the air and the board through the water. Here is an end view....
The wind pressure on the sail causes a vortex at the top and bottom of the sail, just like an airplane's wing held vertically. I've never checked for them in real life but I suppose you could with tufts or ribbons that will align to the air flow there. Or maybe a good excuse to start smoking cigarettes could be made in saying you hope to view the vortex at the bottom of the sail. Or if you are say an Englishman you might sail in fog a lot and actually see the vortex.
The sideforce on the underwater board is a reaction to the sideforce made by the sail in a close hauled sailboat. The board also acts like an airfoil and actually flies through the water (which is about 1000 times as dense as air). One difference here is that the board has no gap to the hull at the top. There can be no vortex there. Same with a rudder or leeboard which extends past the water's surface. So it has half of the usual vortex effect.
Hey! If you have a leeboard boat you can sometimes see the vortex at the end of the board! I've done it with Piccup Pram. Raise the board a bit to get the tip closer to the surface and effectively reducing the aspect ratio. Get the boat close hauled in such a way that you can peer over the side at the tip of the board. There it is! Looks like a white rope twisting off the end of the board.
Now, you ask, what if the sail also extended to the deck? Would that not eliminate one vortex. Yes and some sails have been made to do that. I think large headsails of racing boats in particular can be made this way. But they are deck sweepers and clearly are a big bother on the usual day sailer. I remember reading somewhere that the gap needed to prevent the vortex is quite small, say one tenth of the sail's width or less. The sail on my Bolger Birdwatcher might approach this. You can sort of get away with this on a Birdwatcher because you are always supposed to be in the cabin. But even here the decksweeper is a very dangerous thing. So a decksweeper sail is a feature I try to avoid.
So far I'd be tempted to say "So What!" The short wide wing has bigger tip vortices than the long narrow wing. But the aero guys point out that the swirl produces a "downwash" over the entire wing, not a good thing at all since the wing is supposed to be pushing us up. The aero guys simply add the downwash to the real wind like this:
So what we have here is that the pilot sees he is flying at say 10 degrees to his flight path. But the wing tip vortices are swirling the wind on the wings downward by say 5 degrees. So he is really flying at 5 degrees to the wind the wing is experiencing. That might not be a huge factor to a pilot who can correct by changing the aircraft's pitch and get the lift proper simply by finding the pitch that gives level flight. He might find that the real limitation here is that on final approach to the runway he is pitched so far upward that he can't see the runway! I suppose the Concorde airliners were the best example here. Short wide wings are great for supersonic flight, like say the Space Shuttle, but they produce a downflow such that the required angle of attack on landing resulted in the design of a folding nose to give pilot vision over the bow.
Now, according to the airfoil section data his wing might stall at say 15 degrees angle of attack. The plane might appear to have a flight path of 30 degrees in very slow flight but the airflow local to the wings is actually below the stall angle once you correct it for downwash. I am guessing at all these numbers but I hope you see how it works.
Let's contrast this with the sailplane with the wing aspect ratio of 25. His wing tips are tiny and so are his wing tip vortices and so is the resulting downflow on his wings. He is approaching 2 dimensional flow like the section tests in the wind tunnel.
Of course the glider pilot's idea of "level flight" is different from the Concorde's pilot. He is slowly gliding downward but the angle can be quite low, maybe 50 to 1 in a super glider (I'm guessing). I am going to guess again and say maybe his "cruising" speed is twice his landing speed, unlike the Concord which might cruise at ten times its landing speed. So the glider may not see downwash as a huge part of its life. He flies and lands flat. The downwash issue has been designed out when the long long high aspect ratio wing was designed in.
WHAT! MORE NUMBERS......
When the pilot tilts his plane upwards to correct for downwash a nasty thing happens. See below...
Same drawing as before but note that by adding pitch to account for the downwash the wing's force also tilts backward and creates an added drag component "induced" by the downwash. It is proportional to the aircraft's lift and to the downwash.
The aero expression for calculating the induced drag coefficient (to be added to the basic parasitic drag coefficient to calculate total drag) is fairly straight forward. Essentially it is just (Cl x Cl)/(3.14 x AR) where Cl is the lift coefficient and AR is the aspect ratio. And right off the bat you can see that in our very first example the short winged plane with AR=4 always has about six times as much induced drag as the long winged plane with AR=25.
Induced drag is at its worst when the lift coefficient is high such as when landing or high G maneuvers. It can become very high for short wing airplanes. It can overwhelm other types of drag at times. A low speed glide angle might be quite steep in the downward direction. Or if thrown into a high G turn the short winged pylon racing airplane will slow down a lot and lose his lead to the racer with the long narrow wing. But the Concord cruising at mach 3 is at a very low value of Cl and induced drag is of no concern then.
I suspect the close hauled sailor is always operating at a high value of Cl or at least at the best lift/drag ratio his sail can produce. And for the close hauled sailor that extra induced angle of attack comes right out of his sail's ability to point into the wind. We'll take a closer look at that next time.
SAILBOAT, 15' X 5', 400 POUNDS EMPTY
This is Jewelbox Junior, a 15' version of the original 19' Jewelbox built a while back by Karl James in Texas. That Jewelbox went all over the US and parts of Canada and I understand was sold a few years ago to someone in Florida and replaced by a larger sharpie that Karl had designed. Here is a photo of the original Jewelbox.
JB Jr is also narrower than the original boat, the bottom now planked with just two sheets of 1/2" plywood. Perhaps a good comparison of the two boats would be that Jewelbox needs 16 sheets of plywood and JB Jr needs 9. In particular I hope that JB Jr could be towed behind a small car. Two protoypes of JB Jr were completed last fall. One by Vern Stevens in Idaho and the other by Erwin Roux in Pennsylvania. Vern had a chance to take this photo before winter hit:
And Erwin sent quite a few great photos taken on a beautiful autumn day.
Here you see that JB Jr has that Birdwatcher cabin. The idea behind the Birdwatcher cabin, invented by Phil Bolger in his Birdwatcher design, is that the crew sits low inside the cabin looking out through watertight windows.
The crew weight thus acts like ballast. The boat becomes more stable with extra crew where a normal raised deck boat becomes less stable. I did some paper studies of the self-righting abilities of Junior. With it lighter bottom planking, Junior is bound to be less in that department than Jewelbox, which Karl says has righted from having its windows submerged. By my studies Junior should self right from up to 65 degrees of roll. Beyond that and she would roll another 15 degrees and become stable on her side. She won't flood due to her Birdwatcher cabin. If you couldn't rock the boat back upright you would have to exit, right the boat from the water by stepping on the leeboard, and climb back in. And you would need a reliable step to do that in any high sided boat. These are just paper studies. I would expect my IMB design to behave the same way. Larger heavier designs, like Jewelbox and Scram, should self-right from a full knockdown. More weight on the bottom, especially another layer of plywood there, would be a good investment if you could tow the extra weight.
I've shown Junior with a sharpie sprit rig, although you could substitute a lug sail as Stevens did.
JB Jr plans are $35. Simple nail and glue construction with no jigs or lofting.
Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currency that I can admire but not spend.) I'm way too small for credit cards.
I think David Hahn's Out West Picara is the winner of the Picara race. Shown here on its first sail except there was no wind. Hopefully more later. (Not sure if a polytarp sail is suitable for a boat this heavy.
Here is a Musicbox2 I heard about through the grapevine.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans. Should have a sailing report soon.
And the Vole in New York is Garth Batista's of www.breakawaybooks.com, printer of my book and Max's book and many other fine sports books. Boat is done, shown here off Cape Cod with mothership Cormorant in background, Garth's girls are one year older. Beautiful job! I think Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat.
And the Leinweber's make another prototype! This one by Sandra, an Imresboat shown here on its first outing. They are taking it on a "cruise" so more about it later.
And a new Down Under Blobster, now rightside up for final finish. Looks like another beautiful job....
A view of the Caroline prototype showing a lot of the inside, crew on fore deck. Beautiful color:
I gotta tell you that on the Caroline bilge panels I made an error in layout and they are about 1" too narrow in places on the prototype plans. I have them corrected but it always pays, even with a proven design, to cut those oversized and check for fit before final cutting.
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED BY DATE
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds Toto
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