Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(15 January 2021) We brush up our sail area math. The 1 February issue will review bulkhead bevels.
NO MORE ORDERS UNTIL MID 2021, MAYBE ???
...an update from my blueprint paper supplier, Freedom Paper in Houston, is that they were unable to locate any coating material for Diazit process blueprinting. Not in the US or Asia. So it is kaput, no chance. Looking deeper into getting scans of my mylar tracings. Chuck L sent a tally of last years sales and we figure if we scan the the top twenty or so that will cover most of the income. I am still hunkered down due to the virus and don't see it going away soon, but I think getting scans will take a lot of leg work around here, including getting trials to assure quality. When done, you will be able to order those downloads. So the plan now is to eventually continue the catalog. Right now you can download files for Jonsboat and Toto from Duckworks. These are not scans of my originals but rather are cad works done by kind and interested customers.
...AS FOR REND LAKE MESSABOUT 2021, I think it is still very much wait and see for COVID. Right now the infection rate for this county is running about 1400 new cases a week, compared to about 150 per week for last June so it is currently much much higher. The care home just a half mile from here has had 19 COVID fatalities. My old and wise pharmacist, who also runs the hardware store here in town, says he feels vaccine will not be available for the general public before March, if then. Then it takes six to eight weeks to have full effect. Anyway, if it happens, as always the messabout would be the weekend before Father's Day, June 11 and 12 this year. I will update as things go along.
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.
Wayne Reynolds has been dreaming up a new layout for Scram Pram...
1024 Merrill St,
Lebanon, IL 62254
Send $1 for info on 20 boats.
Sail Area Math
If you look at the picture below of the sail rig of Mayfly12 you will see on the sail some (fuzzy) writing (that didn't scan well) that says "55 square feet" to the left of a small circle that represents the center of that area (honest).
The center of that area is often called a "centroid" and you will see it is placed more or less directly above the center of the leeboard's area. That is very important.
As you might imagine a shallow flat hull like this with a deep narrow leeboard wants to pivot around that leeboard. If the forces of the sail, which in a very general way can be centered at the sail's centroid, push sideways forward of the leeboard, the boat will tend to fall off away from the wind. You should be able to hold the boat on course with the rudder but in that case the rudder will have "lee helm" where you have to use the rudder to push the stern of the boat downwind. The load on the rudder will add to the load of the leeboard. Sort of a "two wrongs make a right" situation and generally very bad for performance and safety in that if you release the tiller as you fall overboard the boat will bear off down wind without you.
If the centroid is aft of the leeboard you will have "weather helm", a much better situation. The rudder must be deflected to push the stern towards the wind and the force on it is subtracted from the load on the leeboard. Not only that, but when you release the tiller as you fall overboard the boat should head up into the wind and stall and wait for you if you are lucky. It's a good deal but if you overdo it you can end up with too much load on the rudder.
This balance problem is actually one of the few things about sail rigs that is not arbitrary. The type of rig and its area are pretty arbitrary depending on how fast you want to go, how much you weigh, etc. But balance is quite important and is one of the areas where backyard boaters get into trouble, sometimes changing the boat or rig with no thought of balance. So before you go doing that you should do a little homework. This essay will tell you how to figure sail area and find the centroid.
One last item: the balance situation shown for Mayfly12 is what I have found to be best for this type of boats. Boats with large fin keels don't balance that way - usually the sail centroid is well forward of the keel centroid. That distance is called the "lead". That type of boat is not within my personal experience and I'm not going to get into that. But you still would have to figure the area and centroid.
THREE SIDED SAILS...
This one is really easy. The area is just the base time the height divided by 2. Any side can be the base and the height is aways at a right angle to the base.
So when you lay out the sail you draw it up on thin paper to the same scale as your hull drawing with the leeboard (or daggerboard or centerboard) lowered. Draw a line through the center of the board straight up. Now we're going to locate the scale sail on the boat such that it's centroid falls very close to that line.
Here's how you find the centroid of a triangular sail.
Find the midpoint of each side and and draw a line from that midpoint to the vertex opposite it. The three lines will intersect at the centroid. Actually you only need to find the intersection of two lines but the third line is a good check.
That's it! Now you can take you scale sail drawing and slide it around your hull drawing until the centroid is on that line drawn up from the hull's board. Move it up and down and tilt it until you like the way it looks. But don't cheat much forward or aft of that line.
FOUR SIDED SAILS...
To find the area of a four sided sail you just divide it into two triangles, find the area of each triangle as above, and add the two together.
Now to find the centroid of the four sider. Start by finding the centroids of the two triangles that make up the four sided sail as shown above. Now draw a line from one triangle centroid to the other. The centroid of the four sider is on that line somewhere.
To find exactly where the centroid is on that line, measure the length of that connecting line. You need not use the same scale as is used on the drawing. I prefer to use a millimeter scale for this measurement. Then get out the calculator and work the formula shown in the Figure 4. Let's say for example the length of the connecting line on the scale drawing measures 120 mm (that is measurement L). Let's say the example sail has a lower triangle area of 50 square feet (that is A1). The upper triangle is 35 square feet (that is A2). So the total sail area is 50 + 35 = 85 square feet. The length L1, which will exactly locate the sail's total centroid, is L1 = 120 x 35/85 = 49.4 mm. So you take that millimeter scale and measure up from A1 centroid on the connecting line 49.4 mm and make a tick mark on the connecting line. That is the centroid of the total sail.
Another way to find the centroid, especially of a really odd shaped sail, is to take the scale drawing of the sail and cut it out. Then balance the cutout on a knife edge and mark the balance line, rotate the cutout on the knife edge about 90 degrees and rebalance and mark the new balance line. The centroid lies at the intersection of the two line.
Another way is to dangle the cutout on a pin stuck through a corner and into a wall marked with a vertical line that passes through the pin point. Mark the line that passes through that pivot corner and a vertical. Then rotate the cutout to hang it from another corner, and mark a second line through the second pivot corner and a vertical. The centroid lies at the intersection of those two lines. Back at the missle factory the designers had a favorite place, complete with pivot pin socket hole and vertical line, to hang these cutouts and that place was known as the "weighing wall". Meanwhile the super computer cranked away next door but its answers weren't to be trusted unless they agreed with the cutout hanging at the weighing wall.
RIGS WITH MANY SAILS...
Figure 5 shows the rig for Viola22. It has a main gaff sail of 177 square feet, and a mizzen sail of 45square feet. Where is the centroid of the assembly?
It's done exactly as with Figure 4. Draw a line connecting the areas of the two sails. Measure the length of the connecting line. Then run through the same equation as in Figure 4. Nothing to it.
One thing I might point out about the Viola22 rig is that the total centroid falls near the aft edge of the leeboard. By my experience the mizzen is not as efficient as its area suggests so it needs to be a bit oversized by normal rules, fudging the total centroid aft. I think in general the aft sails operate in the scrambled flow of the forward sail, causing loss of force back there.
ROBOTE, LIGHT ROWBOAT, 14' X 45", 60 POUNDS EMPTY
Robote was designed for Frank Kahr of Rhode Island strictly as a rowing boat, very light and simple and fast and seaworthy. Frank had started a few years back with my WeeVee design shown here:
What surprised both Frank and me was that both of our WeeVee's would row at 4 mph, blinding speed for a 7-1/2' boat! WeeVee has a deep V center, 42" wide and 9" deep with a lot of rocker and no twisting to the bottom panels. It's actually pretty seaworhty too but is tippy if you are not seated. It's not for everyone.
I followed WeeVee with the less extreme 12' Vireo shown here:
Vireo has a 6" V on a 42" beam so is more stable. And it has a pointy bow. The bottom planks in Vireo twist in the bow to make a wave cutting deeper V. Frank built the boat shown and rowed some long stretches with it. But he thought I was on the wrong track. WeeVee's deeper V and untwisted panels were the way to go, he said. How about a 14' boat with the same cross section and untwisted panels but with a long pointy bow? Here are the lines we agreed on:
Frank built the boat, which he called Robote, from three sheets of Okoume plywood with taped seams. It went together easily as longer boats with gentle curves often do. He said it weighs about 60 pounds, light enough that he can carry it on one shoulder for a short way. Here is Frank first time out with Robote:
Frank entered himself and Robote in the Blackburn Challenge, where one has to row about 20 miles around Cape Anne in Massachusetts, most of it on the open ocean. But it wasn't meant to be and he wrote:
"Wind was SE 15+, rising, with 2-3' chop off the ocean. I rowed about 10 miles, then ran for cover in Pigeon Cove. The alternative was several more miles of windward slog followed by more miles of crosswind. It would have been too much for me."
"The boat was dry, in good control always. It will cope with conditions in which you have no business being out."
If I had seen that forecast I would have left my boat on the cartop. I haven't yet seen any photos of this year's race, but the results of 1999 and 2000 races are posted at www.blackburnchallenge.com. Last year 2 entries scratched and this year 35 scratched! But that brings up a very good point. Good rowboats with experienced hands can handle those conditions for a while but you shouldn't set off into them if you can avoid it. You can get "blown away", especially if anything goes wrong, such as losing an oar or rowlock. And the same is true for any sort of power or sail boat - a small failure in moderate conditions can bring on a disaster. My own rule of thumb is to not venture out too far in whitecaps.
Another subject came up between us, the fact that almost any good conventional rowing boat that has no extremes will row about 4-1/2 mph and no faster, at least not in a long row. My Roar2, Sportdory, RB42 and now Robote all go about that speed. Frank adds:
"I agree with your observations about good plywood rowboats. While robote is not a real speedster, it is very pleasant and responds to greater effort with greater speed. Beaching is no problem; in calm water just lean over so one side of the V is horizontal. The boat grew on me during my 2 weeks on Cape Cod and is now beached at a town landing, to be used weekends the rest of the summer. One of my adventures involved a sudden storm with 2' chop crashing on the beach at South Monomoy when I needed to launch to return home; this wasn't pretty, but I got away on the second try.
Plans for Robote, Vireo or WeeVee are $20 each.
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.
We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...
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:
A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) This boat has been sailed and is being tested. He has found the sail area a bit much for his area and is putting in serious reef points.
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
1feb20, Bulkhead Bevels, Toto
15feb20, Cartopping, IMB
1mar20, Small Boat Rudders, AF4Breve
15mar20, Rudder Sink Weights, Scram Pram
1apr20, Two Totos, River Runner
15apr20, Water Ballast, Mayfly16
1may20, Water Ballast Details, Blobster
15may20, Mast Tabernacles, Laguna
1jun20, Underwater Boards, QT Skiff
15jun20, Capsize Lessons, Mixer
1jul20, Scarfing Lumber, Vireo14
15jul20, Lugsail Rigging, Vamp
1aug20, Prop Slip, Oracle
15aug20, Sharpie Sail Rigging, Cormorant
1sep20, Guessing At Weight, OliveOyl
15sep20, Prismatic Coefficient, Philsboat
1oct20, Figuring Displacement, Larsboat
15oct20, Choosing A Design, Jonsboat
1nov20, Balanced Lug Jiffy Reef, Mayfly14
15nov20, Weighing OliveOyl, Piccup Pram
1dec20, New OliveOyl Junk Rig, Ladybug
1jan21, Rowboat Setup, Normsboat
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
JB Builds AF4
JB Builds Sportdory
Puddle Duck Website
Brian builds Roar2
Barry Builds Toto
Table of Contents