Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1December13) This issue will look at the scaling/stability issue. The 15 December issue will discuss trailering issues.
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.
We're not in Kansas anymore....we are in BEAUTIFUL BRAZIL! Andre Basso and friends have a Piccup launching beach party! Read about it at http://umveleirosimples.blogspot.com.br (Not that Kansas might not be beautiful, maybe.)
LET US GO BACK...
... two decades to when all we had was a pencil and a paper. The problem to be solved for the still young designer was howinhell to calculate the stablity of a boat, especially in roll, such that a fellow might think then about how much sail a boat might carry and also how much ballast a boat might need to be self rightious. No computers allowed back then.
Then we might fast forward to last year when the much older designer, giving a talk in front of some big time boat nuts, says off the cuff that he didn't think a boat's length had anything to do with stability besides making a boat heavier. Then he peeked at the faces in the group to see if he had maybe said something stupid to which there would be objections. None came forth although the looks were somewhat worried.
I probably admitted that I had never really done any analysis to back up the statement and now is a good time to do that. So the question is...
DOES A BOAT'S LENGTH HAVE ANYTHING TO DO WITH ITS ROLL STABILITY???
...anyway, back to the early pencil and paper. Starting with basics I imagined a square stick sitting on its side like this...
If you start to "roll" it it will start to lift up on one edge. The "righting moment" at the start will be the weight times half the width of the stick. Then as you start to roll it the "moment" arm between the support corner and the center of gravity gets shorter to the point where, at 45 degrees of roll, it goes to zero. Beyond that point the stick will try to flop the other way over, it "capsizes".
Well, its sort of like a catamaran, isn't it, since the cat will have its support point all on the corner once it starts flying a hull. And by this time I already knew that the classic sharpie hull, in this case my old Bolger Jinni, would capsize at very close to 45 degrees of roll (I was way to busy to measure the exact angle at those times). And then I read that Howard Chapelle considered "self righting" for a sharpie hull to be the ability to recover from a 45 degree heel. Right away I felt I was on to something. But note,,,the length of the rolling stick has nothing to do with the results.
IN REAL LIFE....
...a monohull doesn't roll around its outer chine like the stick does. Instead it "mooshes" (a technical term I don't have the time to explain now) down until the weight of the entire boat is countered by the weight of the water displaced by the abovementioned mooshing, so its righting arm will be less than the above simple example. Like this...
(...and keep in mind that the center of gravity of the real boat isn't going to be at the middle of the "square' - it could be about anywhere given the ballast, rig, crew, etc, but the designer can calculate it and control it to some degree).
It is possible to do the analysis of the above with pencil and paper for a given cross section in the old fashioned way. But you will need to redraw and calculate the "righting arm", the horizontal distance between the weight center and the supporting displaced water, at each angle of heel to get a handle on it. Still it can be done, and I did it.
BUT, YOU SAY,...
...a real boat is usually a shape that varies in cross section all along its length. Calculating the righting arm at just one section (hint - use the maximum beam section) doesn't account for the normal tapering of the ends of the hull. To use just one section is to think in two dimensions when the real hull has three. Well, it is possible to do the analysis shown above in three dimensions. First you will draw the heeled waterline on the end view of the hull sections and determine the heeled view at various sections along the length, do the righting analysis at each section and then integrate it all together to get a realistic answer. A lot of work. In one of Chapelle's books he talks about doing this in the old days with a wonderful old gadget called a planimeter that figures out areas mechanically, and so forth. I got the impression the work was so tedious it was seldom done, except maybe for large ships and even then maybe only for a few roll angles and only after the real boat had sunk and the lawyers were trying to pin the blame.
But I plugged away at it with a simple sharpie shape. I think I got it right, but....
....it was clear afterward that the great lion's share of the roll stability of a real boat is in its center area, where the beam is maximum. If you did the analysis there alone you would get the right answer probably to within 90 percent. So I still thought that the boat's length had almost nothing to do with its roll stability (besides affecting the hull weight). The roll stability is essentially a factor of the boat's beam and cross sectional shape and the weight and cg issues.
It's deceptive because the roll stability really determines how much sail you can hoist. I think almost anyone who takes a WAG at how much sail his hull can hoist will take a side view of his boat and start drawing in sails to fit. And yet what I was seeing is that the side view had nothing to do with it. Look at the maximim cross section of the boat to figure the sail area, not the length. The length has nothing to do with it.
...OR DOES IT???...
...enter the computer, about 1995 for me. A nice toy for word processing for most of us. But even my old Apple2 was taught to expand hull panels so folks could make instant boats without lofting. After a while I got on the internet and found the free "Hullform" program. I have always carried a link to that website. You can find it somewhere below if you scroll far enough. I noticed that the link has changed and you will be redirected to the current download location, but it still is there.
I don't know how Hullform works but I have never found it to tell me a lie. Even in the old days of 1 megaherz processors it would spit out stability and displacement details as soon as you clicked "enter". It was and still is amazing. Now, it needs to know all about you boat of course. You need to tell it the shape by typing in offsets at different sections along your hull. (And by the way, I have found that the earliest versions of Hullform are easiest to use for doing that. Later versions, like most programs, get so many fancy options that you can eventually fail to see the forest for the trees.) And it will want to know the weight and cg location. So it needs to know all this important stuff just as you would for a hand analysis, and I suppose any program that doesn't ask for such isn't really up to the task.
So, at long last twenty years on, let's use Hullform to see if there is any truth to my guess that the length of a hull has nothing to do with the roll stabitily. I found in my old files my Hullform model of Musicbox2. Here is the program's graphic of that old file...
Then I stretched the basic design simply by respacing the sections so that the initial 15' design became a 20' design - the beam stayed the same. Like this...
Then I let Hullform do all the work and got righting moment curves for both designs. I did this for three examples - the first with the 15' hull at 1600 pounds total...
... and then for the 20' hull also at 1600 pounds...
and then for the 20' hull at 2000 pounds total with the idea that the longer boat will be heavier. The vertical cg was the same in all cases...
Hullform is giving the stabiltiy solution in the form of Gz, the righting arm. So the righting moment is Gz times the weight. Anyway, it shows at 1600 pounds, the Gz is the same for the 15' and 20' hulls so the length has almost no effect on stability. Note that for the 2000 pound 20' hull that the Gz actually goes down compared to the lighter boats. I would say that is the effect of the heavier boat mooshing deeper into the water and, when rolled, cannot be supported as well by the chine area as the lighter boats.
No, just stretching the hull from 15' to 20' made no appreciable change in the stability. The length just doesn't matter all by itself.
Well, everybody knows a good scientist can use numbers to prove any damn thing he wants.
LADYBUG, SAILBOAT, 14' X 5,5', 250 POUNDS EMPTY
Ladybug is a lot like Woobo which was one of my first designs. There was a Woobo near here for a while. I never got to sail it but was told it would really fly. (That boat was made of Lauan plywood from Home Depot which fell to pieces after rainwater filled the boat over the winter.) Ladybug is a bit shorter and wider and deeper and has bench seating, much more suited for older legs. Both boats have a small motor well. The best motor for something like this is the 2hp Johnson/Evinrude which weighs 25 pounds. Even that is overkill since 1hp will push this hull at top speed, anything more just makes bigger waves. Here Sandra Leinweber pushes into a stiff breeze with a 2hp Honda.
This shape of hull with multichines and a plumb bow seems to be a good all around thing with rough water abilities. I think if there is a problem it is that it has almost no lateral drag and unless the boat has a big skeg or keelson or something like a leeboard or daggerboard or centerboard it would just as soon go sideways as straight. I've given Ladybug a keelson and when using power you should keep the leeboard down just enough so its tip drags the water.
Ladybug's hull has the layout I like the best - a 6-1/2' cockpit between two storage/buoyancy boxes. It would be a great solo camping boat. The buoyant wooden spars prevent it from turning turtle. You bring it upright with weight on the lowered leeboard. Then you must climb back on board and a slot in the rudder seems to be the best boarding ladder around, bringing your weight back on board where it least affects the boat. You will have to bail some water.
I've kept the same sail as with Woobo, a 75 square foot balanced lugsail. It hoists on a 13' mast with 11' yand and boom. All very low tech built with common materials.
Chuck Leinweber of Duckworksmagazine.com built the prototype and brought it to our Rend Lake messabout so I had a chance to go for a long ride in it. One thing that impressed me was how large it was for a 14' boat. In the opening photo you see it sail with three men on board, all comfortably sitting to windward on the bench seat, and I'm told it has sailed with four men with no effort. I do think it would be quite suitable for a family of four say with lots of room for all and storage space for all their junk. Here Sandra Leinweber sets things up at a recent campout on the Texas coast:
Here is the same campout from a distance with the mast folded to support a tent. I'm told the tent is not done yet. Chuck has modified the bench seats so that his expand towards the center and meet in the center thus making a 6' x 5' sleeping platform. Turns out a small commercial camping tent can be set up on that platform and that is what they are using for now.
Ladybug uses taped seam construction. Six sheets of 1/4" plywood, one sheet of 3/8" plywood and two sheets of 1/2' plywood.
Plans for Ladybug are $40.
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 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:
Another prototype Twister is well along:
And the first D'arcy Bryn is taped and bottom painted. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
AN INDEX OF PAST ISSUES
THE WAY BACK ISSUES RETURN!
MANY THANKS TO CANADIAN READER GAETAN JETTE WHO NOT ONLY SAVED THEM FROM THE 1997 BEGINNING BUT ALSO PUT TOGETHER AN EXCELLENT INDEX PAGE TO SORT THEM OUT....
THE WAY BACK ISSUES
15dec12, Hull Shaping, Sportdory
1jan13, Bulkhead Bevels, OliveOyl
15jan13, Drawing Boats 1, HC Skiff
1feb13, Drawing Boats 2, Shanteuse
15feb13, Drawing Boats 3, IMB
1mar13, Figuring Displacement, Paddleplank
15mar13, Drawing Boats 4, Frolic2
1apr13, Drawing Boats 5, RiverRunner
15apr13, Drawing Boats 6, Picara
1may13, Two Letters About Keels, Blobster
15may13, Drawing Boats 7, Roar2
1jun13, Drawing Boats 8, Polepunt
15jun13, Rend Lake 2013, Toto
1jul13, Drawing Boats 9, AF4 Grande
15jul13, Taped Seams, Mikesboat
1aug13, Plywood Butt Joints, Paulsboat
15aug13, Sink Weights, Cormorant
1sep13, Lugsail Rigging, Hapscut
15sep13, Sharpie Spritsail Rigging, Philsboat
1oct13, Modifying Boats 1, Larsboat
15oct13, Modifying Boats 2, Jonsboat
1nov13, Modifying Boats 3, Piccup Pram
15nov13, Sail Area Math, Caprice
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Puddle Duck Website
Brian builds Roar2
Herb builds AF3
Herb builds RB42
Barry Builds Toto
Table of Contents