Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15 November 2012) This issue will continue the sail sizing essay. The 1 December issue will complete 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.
Bill Reichelt has his Mayfly14 ready for fishing with a trolling motor.
FIGURING SAIL AREA 2
...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, like this one shown for the boat Picara:
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 Picara at 2000 pounds weight and with the cg 24" off the bottom:
Here Hullforms is calling the righting arm "Gz" instead of "L". The maximum value is .84' so the maximum righting moment of Picara would be 2000 x .84 = 1680 ft-lb.
Here is a picture of Picara's sail rig:
The distance between the sail area center and the leeboard center is about 13'. So if we solve for the maximum allowable sail force we would get S = 1680 / 13 = 129 pounds. If the force on the sail exceeds that amount then Picara will roll past the point of maximum righting moment (at 24 degrees heel according to Hullforms) and continue to roll until the force on the sail is relieved.
FORCE ON A SAIL...
Now the idea is to relate that sail force to real life conditions.
I'm not sure who first figured out how to calculate the force on an object in a given wind but no doubt it goes back to about 1900 when folks started taking a scientific look at flying. Indeed many really old text books will include wind tunnel tests on sails! No doubt really detailed testing has been done by modern racers but most of the numbers I remember were seen in writings by Tony Marchez.
Essentially the force of the wind on an object at sea level is of the equation:
F = .0034 x A x C x V x V.
" A" is full sail area in square feet.
The coefficient "C" is the subject of much debate. The value of C varies with "angle of attack" which is the angle the wind acts upon the sail as shown below. But it tends to peak at about 1.5 for most good sails. Really super sails might peak at something like C=2.0 and a clunker might be down around 1.0, but when running downwind all sails will have a C of about 1.2. C = 1.5 is actually a fairly high value and few aircraft wings can operate at such a high value in real life without resorting to articulated flaps or leading edge slots.
As you see, the force on the sail increases proportionally with the angle of attack until it peaks and then decreases until the force on the sail is all drag and the coefficient is about 1.2. Tall skinny sails can reach their maximum at much lower angles than short fat sails. They can also be more efficient at low angles of attack because they have less drag and thus can provide better performance sailing into the wind (but they are harder to control that short sails so the effect may not be always real). Often tall narrow sails are less effective on other points of sail. Still, the peak value of C is little effected by the aspect ratio of the sail.
On rigs with several sails, the sail closest to the bow is often the most efficient since it gets the smoothest air. A large well shaped jib sail operating in front of a narrow main sail can be operating at a coefficient of more than 2 since the little main is working now like a trailing edge flap on an aircraft wing. The main sail will be operating at a much lower coefficient so the total might be close to 1.5. A mizzen is blanketed by those big sails up front and probably never operates at a C of 1.5.
The "V" in the equation is wind speed in knots. Since this value is multiplied twice, "squared", its effect can quickly overwhelm other factors. For example a 15 knot wind will produce 2.25 times as much sail force as a 10 knot wind. A 20 knot wind will produce 4 times as much force as a 10 knot wind. You can see that sailing in gusting winds can be a challenge. Design your boat for all out best in 10 knot winds and she will be grossly overpowered in a 20 knot gust. That's why nimble athletes make good dinghy racers and sometimes the heavier they are the better.
Sail area can be viewed as a somewhat arbitrary thing as a result of extremes in pressure resulting from fickle winds. One designer might give a skiff 100 square feet of sail and the next give it 50 and both claim they've got it right. The first might have it right in 8 knots wind and the second in 12 knots wind.
Anyway, the pressure on the sail can easily be calculated and charted out like this:
I've shown three value of C in the chart but remember that most good ones will be in the 1.5 area.
WE'RE GETTING CLOSER....
So you have another unknown in this matter, how fast will the wind blow? My feeling is that the sail should be sized such that you will need to shorten sail (tie in a reef) at about 14 knots. Why that number? Two reasons. First is that where I live a 14 knot wind will be kicking up whitecaps, a visual indication that you need to reef. Yes, it is arbitrary. Second is that the 14 knot wind produces about 1 psf on a typical good sail, a number I usually can remember.
How would this apply to the Picara example? Remember that the force on the sail required to knock down the boat is 129 pounds. So the sail area required to produce that force in a 14 knot wind is 129 square feet. In reality Picara has 171 square feet total of which 33 is in the mizzen. You might argue that the mizzen area is not as effective and the 138 square foot main is about right on. But I know the mizzen area can capsize a boat given the right conditions (as proven to me directly a long time ago when I capsized my Bolger Jinni).
Let's look at it a different way. How much wind is needed to knock down Picara in the given weight/cg condition? If you say the full sail area is effective then the wind pressure needed to capsize the boat is 129/171 = .75 psf and that occurs at about 12 knots of wind. Can you tell the difference between a 12 knots and 14 knots of wind without a measuring device? I can't. To me aerodynamics is not such an exact science that you can predict what a craft will do in real life until you build it and try it. I'm reminded of my term in the missile factory when it was planned to add four studs, about as big as your thumb, to the exterior of a missile that was about the size of an automobile in order to better support it in its launch tube. The aero engineers went through the roof with chart after chart of why it would reduce range below the minimun. But they test flew one and the studs increased the range! And then the aero guys said, "Of course! The studs acted like vortex generators that engergized the boundary layer, etc., etc., etc.". And they had plenty of charts to prove that too. They had charts to prove anything. We all did.
We'll juggle these numbers one last time with more examples.
CABIN SAILBOAT, 25' X 6', 900 POUNDS EMPTY
About 15 years ago I built a Payson Canoe and used it for several years before selling it. I replaced it about 10 years ago with my Toto double paddle canoe. Toto has the same multichine cross section as the Payson Canoe but I tried for a long lean bow which would be better in rough water and more foregiving of bow down trim. I still have that Toto, unchanged in any way since new, and still use it all summer. Amanda Johnson demonstrates:
The Toto shape worked so well that I used it in other designs like Roar2 and RB42. I tried it also in a sailing boat, the 20' Frolic2 (the original Frolic was narrower, more of a rowboat than a sailboat). Frolic2 was unballasted with a small cuddy and I hoped it would be a good daysailer and one man camping boat.
Bill Moffitt had built my Woobo design and funded a 20% enlargement of Frolic2 that would have a cabin, water ballast, and a yawl rig for cruising near the Gulf Coast.
The 20% enlargement idea went very well except that I had to deepen the hull more than that to give some serious headroom in the cabin, but it doesn't have standing headroom. Great empasis was placed on ease of use and rigging. The main mast is short and stepped in a tabernacle. There is a draining anchor well in the bow, a small storage segment under the front deck. The mast tabernacle is bolted to the bulkhead that forms the front of the sleeping cabin. There is a utility room aft of the sleeping room. Water ballast tanks are under the bunks and in the sides of the utility room floor, about 600 pounds of ballast as I recall. Aft of the cabin is the self draining raised cockpit with storage volume under the deck. Finally there is a self draining motor well across the stern. Construction is taped seam plywood.
Bill couldn't start his Caprice right away but Chuck Leinweber of Duckworksmagazine gave it a try. He has the room and tools and smarts to tackle a project like this with no hesitation. There weren't many changes from the plans that I know of, the main one being adding a conventional footwell to the aft deck which is designed to have a hatch type of foot well as with the Bolger Micro.
Chuck trailered his Caprice up from Texas to our Midwest Messabout this June and I had a chance to go over it, sail it for a couple of hours, and watch it sail from other boats. Wonderful!
Chuck tells me it takes less than 15 minutes to rig the boat to as you see here. As shown the boat has its ballast so you see it beaches very well indeed. I asked about the ballast. He can flood the tanks without power, just open the access plate, reach in and pull the fill plug and let the water rush in. Since the tops of the tanks are about even with the normal waterline he has to move his weight around to keep the tank depressed long enough to completely fill. Then you reach into the filled tank, replace the plug in the bottom, and then replace the access plate in the top. There are two tanks to fill.
Are the ballast tanks worth the building effort? On a multichine hull like this the tops of the tanks form flats that give places for bunks and storage so that is good. When full the boat should have a very good range of stabiltiy. Chuck's boat has never been in rough going as I'm writing this so the effect of the ballast remains to be proven. It has been capsized in a practice but the ballast tanks were empty and the boat was empty with no crew, etc.. But the ballast is a success from the standpoint that Chuck is able to tow his Caprice behind a four cylinder pickup truck. My idea was to pull the boat up the ramp and drain the tanks afterward by simply pulling the drains plugs. But Chuck has found it best by far to empty the tanks before recovering the boat at the ramp. So he uses a bilge pump in each tank to pump them empty. I'd be worried about water ballast tanks on a wooden boat from a rot standpoint and would be careful to open all the plugs and access panels when the boat is stored.
(I should add that I think an empty Caprice will weigh about 900 pounds based on the plywood sheet count (eight sheets of 1/4", nine sheets of 3/8" , five sheets of 1/2" and one sheet of 3/4"). But any boat like this can hold an awful lot of gear and junk.)
Caprice has the tabernacle setup that I first saw on Karl James' sharpie. The mainsail is 190 square feet, balanced lug. The mast is fairly short, stowing within the length of the boat when lowered. Chuck demonstrated putting up the mast, maybe a 15 second operation. I've been drawing these for a while on different boats but this is the first one I know of to get built and used. I'm greatly relieved that is all works so well. Before you decide to tack a tabernacle like this onto your boat, be advised that the tabernacle posts go clear to the hull bottom with big bolts all around a beefy bulkhead.
I thought Caprice sailed very well in the light winds we had that weekend. Tacked very smoothly through 90 to 100 degrees which is all you can ever get with a low tech rig. Very smooth and quiet compared to the sharpies I'm used to. It didn't seem at all sensitive to fore-aft trim. In the light winds it went 5 knots on the GPS which is certainly fast for the conditions.
Well, all in all I thought Caprice was everything I was hoping for.. Plans are $45.
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
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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