Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1December 2012) This issue will complete the sail sizing essay. The 15 December issue will delve into hull shaping.
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.
Pete Fribance goes for a paddle in his delux Toto.
FIGURING SAIL AREA 3
...We are trying to calculate how much sail a boat can handle. First we want to calculate the force on the sail required to capsize the boat. We started with this picture of a sailing boat in balance:
We can write the basic balance as the sail force S = W x D / L where W x D is the "righting moment" of the hull and D is the distance between the center of the sail area and the center of the leeboard (or keel or centerboard, etc.) area. W is the total weight of the boat with everything on board. L is often called the "righting arm" and changes as the boat rolls and is fairly difficult and tedious to calculate. But the free program Hullforms can handle it. To use Hullforms one must first model the hull in three dimensions. In the last issue we showed the model for Picara but this essay will focus on smaller boats. This is my Hullforms model of the 14' boat Ladybug:
Given the hull lines and a weight/center of gravity information Hullforms will calculate the value of L and graph it for various values of heel as when the hull rolls. Here is the graph for Ladybug at 400 pounds weight and with the cg 24" off the bottom. This is about the lightest I would expect a loaded Ladybug would sail at complete with a minimum crew. I took a stab at the cg location.
Here Hullforms is calling the righting arm "Gz" instead of "L". The maximum value is .76' so the maximum righting moment of the lightly loaded Ladybug would be .76 x 400 = 304 ft-lb.
Here is a picture of Ladybug's sail rig:
The distance between the sail area center and the leeboard center is about 9'. So if we solve for the maximum allowable sail force we would get S = 304 / 9 = 34 pounds. If the force on the sail exceeds that amount then the lightweight Ladybug will roll past the point of maximum righting moment (at 20 degrees heel according to Hullforms) and continue to roll until the force on the sail is relieved.
Ladybug's sail area is 74 sq ft so the average pressure on the sail at this condition's capsize point is 34/74 = .46 psf.
Now the idea is to relate that sail force to real life conditions. Remember this chart which shows sail pressure vs wind speed at three values of "sail efficiency". I always advise using the C=1.5 value when using this chart.
Using the above chart it appears that the pressure of .46 psf happens at about 9 knots.
"Not a lot of wind," you say. "Almost any boat will take more wind than that." I agree. And so can Ladybug.
THE EFFECT OF HIKING...
The smaller and lighter your boat is, the greater the effect of your weight on its sailing. You can dominate the boat with your weight. You will weigh more than many of my boats.
Anyway, the the above case has its weight centered on the hull because that is how Hullforms does its math. At the point of maximum righting moment the situation with Ladybug would be something like this:
Now let's take the perhaps more common condition of the skipper sitting to windward on the bench seat. Not exactly hiking in any extreme way, just sitting on the bench on the windward side. In this example I will assume the boat weighs 200# and the crew weighs 200 # sitting to windward like this:
Now the righting moment is 552 ft-lb, up from 304 ft-lb, just because the crew has slid his butt to windward. 552 ft-lb means a sail load of about 552/9=61# for a sail pressure of 61/74=.82 psf. From our chart that happens at a wind speed of about 12 or 13 knots which is getting well up into the whitecap wave area, a visual sign that it is time to reef.
Now let's say you have an adventurous pal who wants to sail with you. He adds another 200 pounds to the effort like this:
Now the righting moment is all the way up to 952 ft-lb. Sail load required to capsize the boat is now 952/9 = 106 lbs which is 106/74 = 1.4 psf for this 74 sq ft sail. That happens at about 17 knots. Now the wind is really starting to blow! Your adventurous pal says, "No problem." and grabs a rope leading to the masthead and steps out on the rail with a trapeze, like this:
Now we are really getting up there. Righting moment with the big guy on the trapeze is now 1752 ft-lb! That figures to be 195 lbs on the sail for an average sail pressure of 2.6 psf. That happens in a 22 knot wind.
What happens if that 22 knot wind suddenly drops back to 9 knots? Most likely the big boy on the trapeze will get a big dunking. But he doesn't care. He hangs a bigger sail so he can go back on his trapeze in that light wind. How much sail does he hoist? You can figure it out. Assuming the bigger rig now has say 12' between sail and leeboard centers (just a wild guess) then the sail force at capsize will be 1752/12 = 146 lb. At 9 knots the sail pressure is about .5 psf. So the new sail needs 147/.5 = a whopping 292 square feet! Most likely you aren't going to see anything like that on today's boats but I think in the old days of the racing sandbaggers such a rig was possible.
But you can see that it is possible to figure it all out, at least approximately.
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.
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
A NOTE ABOUT THE OLD WAY BACK ISSUES (BACK TO 1997!). SOMEONE MORE CAREFUL THAN I HAS SAVED THEM. TRY CLICKING ON...
which should give you a saving of the original Chuck Leinweber archives from 1997 through 2004. They seem to be about 90 percent complete.
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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