Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Feb09)This issue will finish a look at "aspect ratio". The 15 February issue will rerun the IMB capsize issue.
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.
Mike Morris's shop gives birth to a new full sized Jewelbox. Texas, of course, note cowboy hat on cabin deck.
Aspect Ratio 2
...the last issue discussed some scientific details of aircraft wing aspect ratio, a shape feature, which is about why some airplanes....
... have short fat wings. And why some airplanes...
... have long skinny wings.
Essentially the long skinny wings are more efficient from a flying standpoint (not from a structural standpoint). The reason is that the pressure on the bottom of the wing is in general higher than that on the top, thus "lift" is produced. The pressure difference causes the air at the wingtips to flow from bottom to top as it moves aft, thus it swirls around there.
The air movement there is seen (at least in subsonic flow, not too sure about supersonic flow) as a general "downwash" over the entire wing, the net effect is that the wing sees a lot less "angle of attack" into the wind than you might expect.
Short fat wings have a lot more wingtip than long narrow wings, thus a lot more downwash and in general are less efficient, at least at slow speeds. "But," you say, "really high speed airplanes all have short fat wings." Yes, they do, but like the Concorde they have power to burn. The D1000 glider has none. And...
...SAILBOATS ARE GLIDERS
Actually they are twice gliders. Their sails glide through the wind and their keels glide through the water. Both the wing and the keel (or underwater board of any sort that counteracts the side force of the sail) are subject to the downwash. So each lives in its own downwash and in both cases the downwash is going to subtract from the boat's ability to point to windward. Let's look at this with two simple example boats:
Now imagine these two are sailing together in the same wind, close hauled to sail as close to the wind as possible. Both sail and keel are brought to just below the stalling point while they do so. The skipper will note that if he tries to point any closer to the wind with his sail that it starts to "luff", that is the leading edges start to flutter and his sail power is reduced. At the same time he might watch his wake and note that his keel has not stalled and he is not making too much leeway. Both boats are the same except that the sail and keel aspect ratios are different.
Now imagine you are on one of the boats and the wind shifts slightly to the bow causing the sail to luff. The skipper must bear off a bit to compensate and keep the sail drawing fully. Now imagine instead that this "wind shift" is actually the sail's own downwash. So you see that the sail's own downwash directly decreases the boat's abiltity to point into the wind.
Now let's hang some numbers on these two boats. The aero gurus say the shift of the wind due to downwash, in degrees, is a=18 x Cl / AR where Cl is the "lift coefficient" and AR is the aspect ratio. (Actually they have lots of correction factors for this equation given different wing shapes.) A thin cambered airfoil like a sail will have a maximum Cl of about 1.5 and I'll use that for this case of sailing close hauled. So it works out that the low aspect ratio sail will have an "induced angle" of 12 degrees while the high aspect sail will have an induced angle of about 7 degrees. All other things equal the high aspect ratio sail should point about 5 degrees closer to the wind simply because it has less downwash.
(I gotta tell you that Phil Bolger argues long and hard that the lower aspect ratio sail rig can be superior in "efficiency" if the reduce cost and effort in handling it are taken into account.)
And I suppose the history of racing boats in particular is to go higher and thinner with sails. But there are limits, especially for us non racers. First of all nobody wants to make, haul and rig a mast that extends to infinity. The racers might put up with that but even then there is another serious problem with tall thin rigs. They are apt to twist more than low wide sails. After all it is just fabric and string, unlike the aircraft which use their hard skins to form efficient torque boxes to resist twist. Some sailors have tried hard skin wings, some actually rigging an aircraft wing as a sail. But tell me how are you gonna reef it? And even aircraft are eventually limited by their structure in the aspect ratio department.
Before I close out this topic I want to present this graphic in summary:
So first we have the basic sail close hauled at about 15 degrees from the wind (I'm guessing at all of these angles). It produces on the sail a total force which can be shown as the vector sum of lift (perpendicular to the airfoil), and drag (parallel to the airfoil). I'm guessing again but if the sail's general Lift/Drag is 10 then the total force will tilt back about 5 degrees from the basic angle of attack. So the total vector slats back at 110 degrees from the wind direction. Next we add a bit of downwash effect where the skipper needs to tilt the sail another 5 degrees (for example but the earlier study shows it is a bit larger) so the sails main vector tilts back 115 degrees from the wind. Finally let's simplify the picture by just looking at the resulting sail force. This is what is moving your boat when close hauled. Think of it as a rope pulling on the mast sort of like what tow path mules used to do as they pulled a canal boat down a canal.
NOW TO THAT KEEL...
I've been writing like a true mystery novelist who keeps the best until last (unlike a good techincal writer who puts the best in the first sentence because he knows no one reads past the first line). I suppose the real reason I wrote this article is to show a reason why you can't just use really shallow keels without special considerations.
Anyway, the keel has a very important job to do. Look at this graphic:
First view is just the basic sail force as before. Next we add a boat hull perpendicular to that force shown at 25deg to the wind by all my angle guesswork. So, you think, the boat can't possibly sail closer to the wind than that since now all the force is to the side. So the skipper tilts his boat slightly downwind, say another 10 degree guess, to get part of that total sail force to push him forward. The major portion of the sail force is still pushing him sideways but we'll worry about that later. At least we are moving somewhat forward. Now he is 35 degrees to the wind and I'm thinking that is about as close as any reasonable sailboat might point into the wind.
So the really important job here for the keel is to counteract all that side force.
The keel can't be cambered as a rule since it has to function equally well on both tacks. So its maximum Cl will be closer to maybe 1.0 instead of 1.5. (And I think it is far more difficult for the skipper to tell when he is stalling his keel by trying to drive it at too high and angle of attack. In general if you do that you will get a lot of leeway. Especially in waves you will note, if you watch the wake closely, that the boat is going sideways as fast as it is going forward. I've been there lots of times myself.)
The keel develops its sideforce, needed to counteract the side force of the sail when close hauled, by flying through the water at an angle just like the sail. If the skipper wants his boat to follow a heading of say 035 and his wind is from 000, he might have to point his boat say 045 to get ten degrees of angle of attack on the keel to get the side trust needed to deal with the sail forces. Like this:
So the skipper always has to aim his hull to windward of his desired course and he might think of that as leeway. Another way to look at it is in the above example he is trying to point 35 degrees from the wind and he aims it that way, but the boat's track will be 10 degrees downwind of that because of the leeway. So he is tracking 45 degrees to the wind close hauled. Mind you his hull is pointing 35 degrees to the wind but it isn't going where it is pointing due to mandatory leeway.
WAIT!! IT GETS WORSE...
...the keel also lives in a fluid and is subject to downwash! So the leeway is worse than just the basic "angle of attack", just like with the sail.
There is another difference with the keel in that there will be no spillage of the water around the top where it meets the hull (assuming no gap there or with a leeboard the water can't flow over the top of the leeboard). So in effect it is half a wing as far as downwash is concerned and the aero guys say this effect doubles its aspect ratio as far as figures are concerned. So in the example above the low rig has an effective keel aspect ratio of .5 and the high rig an effective keel aspect ratio of 8.
Let's run the induced angle numbers for the lwo examples using a Cl=1.0. For the high aspect ratio rig it calculates to be just 2 degrees. (I wouldn't notice that myself.) And for the low aspect rig it figures out to be a whopping 36 degrees! It sounds extreme and I don't know if I really believe it but I can say for sure that if you raise the leeboard on a close hauled boat you will find you aren't moving to windward anymore. I know some boats have things like keel runners and such. I've never tried them and can only say they aren't very effective by these theories. Full keeled boats of low aspect ratio have been around a long time, a lot longer than deep finned boats. They can work because they can have a huge side area, thus operate at low values of Cl all the time.
So add it all up. The high aspect keel will have maybe 12 degrees of leeway when close hauled. The low aspect keel will have maybe 46 degrees! Like this:
The penalty of the shallow keel is clear but, you know what?. Some will look at the low aspect keel and ask, "Can I make the keel just 6" deep?" And so forth.
SAILBOAT, 15' X 5.5', 350 POUNDS EMPTY
Mayfly16 is large enough to swallow up three men or maybe a family with two kids. She has two benches that are 7' long and there should be plenty of room for all. I would say that her fully loaded maximum weight might be 900 pounds and her empty weight about 350 pounds, leaving 550 pounds for the captain and crew and gear.
At the same time the Mayfly16 can easily be handled solo, although with just the weight of her skipper she will not be as stable as when heavily loaded. The boat also has two large chambers for buoyancy/storage and I can see her used as a solo beach cruiser because the floor space is large enough for a sleep spot. I've made her deep with lots of freeboard.
Mary and George Fulk built the prototype and passed by here with the prototype on their annual migration north for the summer and I had a chance to see and sail in Mayfly16 for a short bit. Weather was hot and the wind light and steady, perfect for testing. She sailed quite well I thought and everything worked as planned. It certainly was roomy and easy to rig and use.
The balanced lug rig sets on short spars and sails very well reefed, in fact can be set up with jiffy reefing. The spars are all easily made and stowed, the mast being but 14' long setting 91 square feet of sail. In addition there are oar ports for those with lots of time and little money and a motor well for those with lots of money and no time. Two horsepower is all that a boat like this can absorb without going crazy.
The motor well is an open self draining well that uses the full width and depth of the stern. It will come in handy for storing wet muddy things you don't want inside the boat, like boots and anchors. I've suggested in the plans that the rudder can be offset to one side a bit to give more room for the motor. We did not use George's little Evinrude since the boat sailed easily in all directions, but George says the sidebyside sharing on the stern of the motor and rudder works fine. There was no interference with the rudder. (As with any outboard on any sailboat, the motor has a desire to grab the sheet with each tack so you usually have to tend the sheet a bit.)
Mayfly16 uses conventional nail and glue construction needing six sheets of 1/4" plywood and two sheets of 1/2" ply.
Plans for Mayfly16 are $35.
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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