Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1April2012) This issue will continue an essay about underwater board shape, in particular it will try to explain the "why" behind it all. The 1April issue will cover rowing gear.
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...
MESSABOUT NOTICE:THE REND LAKE MESSABOUT WILL TAKE PLACE ON JUNE 8 and 9 AT THE PINTAIL LOOP OF THE NORTH SANDUSKY CAMP GROUND AT REND LAKE IN SOUTHERN ILLINOIS. USUALLY THE SANDUSKY CAMP GROUNDS MIGHT BE FULL FOR THAT WEEKEND. BUT SOME OF OUR BOATERS HAVE RESERVED SOME OF THE SITES THERE SO YOU (AND I) MIGHT PASS THAT WAY FIRST SINCE SEVERAL TENTERS CAN SHARE A CAMPSITE. IF NOT, THEN I WOULD TRY THE WAYNE FITZGERRELL STATE CAMPGROUND WHICH IS ON THE MAIN CAUSEWAY LEADING TO SANDUSKY. THEIR WEBSITE SAYS IT IS STRICTLY FIRST COME FIRST SERVE, NO RESERVATIONS ALLOWED. IT IS A VERY QUICK DRIVE FROM THERE TO SANDUSKY. HOPE TO SEE YOU THERE!
Gary Blankenship's veteran Frolic2 one more time, shown here leaving the start line at the Everglades Challenge. Looks like a deep reef, with one more reef line left, with John Wright at the Tiller and Gary getting the leeboard down as they work away from the beach.
Underwater Board Shape 3
...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 sail 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 045 and his wind is from 000, he might have to point his boat say 035 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.
Marksbark, Fast Rowboat, 18' X 3', 90 POUNDS EMPTY
This long narrow boat was designed a long time ago for Mark Bustemonte who thought it might be possible to have a boat for fast sliding seat rowing, for slower fixed seat rowing with a passenger, and maybe for use as a canoe by kids.
I agreed with the idea. I had already once rigged Toto with outriggers and oars. Toto is too short to take a sliding seat. She was a bit faster under oars than with paddle - you can get lots more muscle behind oars and I think the oars are less tiring. But the low slung oars were a problem in rough water and I suspect that is a problem with all low sided rowboats. Also I didn't think the extra speed made up for the extra gear involved and having to watch my own wake instead of seeing what's up ahead. For sneaking up on wildlife and such, the double paddle or kayak has it all over any type except for maybe an electric. Anyway, with the extra length of Marksbark, I think she'll hit 6 mph in good conditions and cruise forever at 4 mph.
It will be interesting to see if Marksbark is stable enough for normal kid rough housing used as a canoe. She's already wide for a sliding seat boat.
Marksbark was drawn well before I put a simple straight plug in the middle of Toto to get a Larsboat. Today I would suggest simply putting a 5' straight plug in the middle of Toto to get the length of Marksbark, long enough for a sliding seat. Instead back then I stretched out the Toto which will probably look more elegant but the prototype builder will have to keep a close eye out to see if the pieces all fit since all this was done before I figured out how to use a PC to give the shapes of the expanded panels. He ought to do what I did with the original Toto - stick it all together except for the bilge panels and then trace the shape of the bilge panel hole onto cardboard and transfer to plywood. The Marksbark plans do give the shape of the bilge panel but as with all multichine shapes it pays to cut that bilge panel well oversized and trial fit to see if the lines are correct. The thing is the last piece of the puzzle catches all the previous mistakes of the builder (and designer!) and a trial fit before cutting to the line is always a good idea.
Marksbark plans are $20 until a prototype is built and tested. Needs four sheets of 1/4" plywood with taped seam construction. No jigs or lofting required.
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 out West.
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 Battista's of www.breakawaybooks.com, printer of my book and Max's old outboard book and many other fine sports books. Beautiful job! 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. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.
And a Deansbox seen in Texas:
The prototype Twister gets a test sail with three grown men, a big dog and and big motor with its lower unit down. Hmmmmm.....
And the first D'arcy Bryn is ready for taping. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
And the first Brucesboat is in the water for testing. A full report soon.
OK, so he found a major league goof in my plans on fitting the bilge panels. He did some cut and fit and did a great job of salvaging the work, but I have corrected the drawing for the aft end of the bilge panel (I drew it in upside down!!)
And a Hapscut goes together in Texas. He has scarfed some material on the stern to finish the boat with a built in motor well like Laguna. Good idea:
And here is a custom project going together so quickly that I am going to hold releasing prototype plans, just release "done" plans in a few weeks. He also already has a crew...
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)
Puddle Duck Website
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds Toto
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