Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1MAY98) This issue will show the relationship between wind speed, sail area, and boat stability. You can figure sail area without guessing! Next issue, about 15 May, will close this thread with how to figure leeboard (centerboard, daggerboard, keel) area.
MIDWEST MESSABOUT NOTICE...
The 9th annual Midwest Homebuiilt Boat Messabout will take place at the Gun Creek Recreation Area at Rend Lake in Southern Illinois on June 13 and 14. Take exit 77W off I-57, head towards the golf course, and you'll see the signs. Lots of folks arrive on Friday and leave early Sunday. Electric hookups at most camps sites and the fee is now $8 a night, which includes the ramp fee at this Corps of Engineers facility. There is also a Super Eight motel at nearby Benton.
There is no schedule of events. We usually have a pitch-in dinner on Saturday evening.
If you plan to attend, drop me a line by the end of May and I'll send you an update.
A NEAT WEB SITE...
A Swedish boater put me on to this one. After I looked it up I recalled the BRIS boat on the covers of a few slick magazines about 10 years ago. But this web page itself is written by Sven Lundin himself and is totally fastinating. Not only does the man give faith to those who have lifestyles that don't fit into modern advertizing, his adventures seem truely heroic compared to what most of us can stand.
JOHN BELL'S SPORTDORY
In the 1 April issue I presented a way of determining the maximum sail force your boat could handle and not capsize. To review a bit, first you need to figure a weight and CG for your boat, model your boat on a program like Hullforms 6S complete with that weight and CG, work up a stability curve of righting moment versus heel angle. Then you find the maximum righting moment and divide that by the distance between the center of the sail area and the center of the underwater board area to find the maximum allowable sail force (which is usually also the maximum underwater board force). If the wind force on the sail exceeds that value your hull will not have the ability to handle it and your boat will capsize.
Here is the righting moment curve we worked up for Frolic2.
So for Frolic2 the maximum righting moments are 1550 ft pounds for the boat with two crew members sitting to windward, 900 ft pounds for a solo crew sitting to windward, and 500 ft pounds for a solo crew centered. The full sail area of Frolic2 is 12 feet above the underwater board area. So the first crew can have a max sail force of 1550/12=129 pounds. The second crew can handle 75 pounds of sail force and the solo centered crew can only take 42 pounds. Quite a difference!
THE FORCE OF THE WIND....
How to relate the wind speed and sail area to sail force. The equation for figuring the wind's force on a panel (sail or wing or whatever) is .0034 x S x C x V x V in normal air at sea level.
S is the area of the sail. For Frolic2 it is 114 square feet.
C is a force coefficient. It's value is subject to endless debate but tends to peak at about 1.5 for a reasonable sail. I got most of this info from Marchaj's excellent books. What was interesting about his data was that a square rigger running downwind has sails that also operating at a C of about 1.5. Really super wings operating to windward might peak at a C of about 2 but that takes very high technology, quite unlike anything I would design.
V is the wind speed in knots. But note that the value V is actually "square" , that is, multiplied in there twice. As a result, the wind speed usually becomes the most important factor in the equation.
So, for the Frolic2 example, the equation boils down to F=.0034 x 114 x 1.5 x V x V. That would be the maximum force since I'm using the full sail and the full value of C. It multiplies out to .58 x V x V. If we put in a few values for V we might get a table like this:
Looking back to the Frolic2 rightning moment data, we'd predict that with the double crew to windward she could take a max wind of about 15 knots, the solo crew to windward could take about 11 knots wind, and the solo centered crew only about 9 knots.
Notice how the force goes totally bonkers at the higher wind speeds! So if you are sailing in 10 knot winds and get a 30 knot gust you don't have a prayer, right? All you can do is to feather the sail, letting out the sheet and allow it to weathervane into the wind and hope the wind direction is steady enough that you don't get caught by the full force. Marchaj's data showed that the max sail force comes at an "angle of attack" of about 20 degrees for a typical sail so if you can keep the sail feathered less than that angle you can hope to keep the value of C well below the max value.
Actually the force on the wind can be much more subtle than that. If you were sailing the Frolic2 alone with full sail in 11 knot winds, having a great time at near max power, heeled close to 20 degrees, you might get lazy and tie off the sheet. But a gust to 14 knots, an increase you might not even feel on your face, increases the sail force by 50%! If you don't act fast you might go over.
By the way, you might place in your memory that a 14 knot wind will produce about 1 pound per square foot on a typical sail.
SIZING THAT SAIL
There's another way to maneuver that equation. You can assume a certain wind strength and figure the sail area needed to produce the desired sail force.
For example, take the Frolic2. Assume you want to sail in 15 knot winds. You are sailing solo and think you are good enough to keep your weight to windward. How much sail should you carry?
Going back to the righting moment curve, it takes 75 pounds of sail force to capsize the boat with that crew situation. So you might write the equation as 75=.0034 x S x 1.5 x 15 x 15. Solve for S, the sail area, and you get S=75/(.0034 x 1.5 x 15 x15)=65 square feet. You might measure the sail area with different reef points and make a mental note of what wind speed that would correspond to. (Actually you might find you can carry slightly more than 65 square feet since most sails reefed set lower on the mast than when full. Thus, instead of dividing the maximum righting moment by 12 you might end up dividing by 10.)
I've found this approach to be pretty accurate, given that you never really know how strong the wind is blowing.
Here's a chart of pressure on the sail versus wind speed. I've shown three values of C in case you question the value of 1.5 that I prefer.
We'll do a similar analysis for the underwater board. But we'll also show how the board's "aspect ratio" (short and far, long and narrow, etc.) affects performance.
MIXER, ROW/SAIL SKIFF, 12' X 4', 90 POUNDS EMPTY
Someday I may get to put my full catalog on the net. For now I'll put one design in each issue.
Mixer is a stab at mixing some features of my prams with the features of the original Roar rowboat. The prototype Mixer was built to perfection by David Boston of Factoryville, Pa. That's him sailing on vacation in Maine.
Mixer's beam is half way betwen that of Piccup, which has "stand up and walk around" stability, and that of Roar, which is too tippy for serious sailing. They all have a similar multichine cross section. Dave was very happy with the stability of the prototype. He reports sailing in white caps with no troubles.
The pointy bow causes a loss in carrying capacity compared to a pram bow. Mixer would need to be stretched at least a foot longer to meet Piccup's capacity. But I left Mixer at 12 feet for two reasons. I've found a 12 footer can be cartopped without bow tiedowns and few new cars have bumpers suitable for bow ties. Longer hulls usually need bow ties (except for narrow jobs like Toto). Also, the 12 footer can still be got out of four sheets of 1/4" plywood and may be a tad lighter than Piccup. At any rate, Dave's Mixer has sailed with two adults aboard and was quite happy.
Mixer's got the exact same interior layout (a 6.5' open cockpit between two large flotation/storage chambers) as Piccup for all the same reasons. So here is a cartopper with capacity for two adults or for camping one adult with a flat floor large enough to sleep on and lots of dry storage.
The sail rig is identical to Piccup's. If you look at my leeboards you'll see they pivot at a lower hull guard while their tops are braced to take loads both ways and only one board is required. Essentially they are centerboards mounted outside the hull. But no centerboard case is required and there is no need to handle the board in tacking as with loose leeboards. For anyone thinking about converting a centerboard design to leeboards, note that a leeboard should be mounted at the hull's broadest beam to be in flow parallel to the hull's motion. The sail rig then needs to be placed for proper balance.
Plans for Mixer are $20. No lofting or jigs required.
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. (If you order a catalog from an internet page you might state that in your letter so I can get an idea of how effective this medium is.) Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currancy that I can admire but not spend.) I'm way too small for credit cards.
Anyway..... Anytime a design from the Catalog of Prototypes starts getting built I pull it and replace it with another prototype. So that boat goes into limbo until the builder finishes and sends a test report and a photo. Here are some boats in that catagory (although I been leaving most in the prototypes catalog).
The Texas Scram Pram, a 16' multichine Birdwatcher type, has been sailed and photographed by Tim Webber. (Tim has his own super web page and has posted the photos there.) I hope that next issue will feature it as a "done" boat. But see photos today by clicking here.
Sportdory was not in the prototype catalog very long but there are two well underway, perhaps done by now because it is such a simple craft. One of the two is shown in the construction photo towards the table of contents and is being built by John Bell (who has his own super web page) and the other by David Burdecki out in California. Check out John's progress by clicking here .
The Way Up North AF3 project by Herb McLeod (has his own super web page) is in the painting stage, I believe. Talk about fast work! But if you read Herb's web page you will see he is very experienced. Says using the LePages glue is a large part of building fast. Get updates on progress by clicking here.
Harmonica (ex Fusebox) is having its interior fitted. Lots of detail work planned but he hopes to have it at the Midwest Messabout in June. We had an email exchange over power requirements. He has a Tanaka 1 horse and I say that should suit the boat. Should go about 5 mph as does my Birdwatcher and my AF4 with 1 horse. No, you can't go out in big water and big winds with that, but that was never the idea behind the boat.
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