Jim Michalak's Boat Designs

1024 Merrill St, Lebanon, IL 62254


A page of boat designs and essays.

(15 March 2017) This issue will continue the leeboard shape discussion. The 1 April issue will be about capsize lessons.

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.

Left:

Bud Brinkley's Jonsboat project has the basic materials in the background. (The ladder frame isn't required but can be nice.)


Contents:

 

Contact info:

jim@jimsboats.com

Jim Michalak
1024 Merrill St,
Lebanon, IL 62254

Send $1 for info on 20 boats.

 

 

Underwater Board Shape

Aircraft designers have been trying to predict the performance of wings since the very beginning. The Wright brothers not only developed the airplane but made some of the best wind tunnels of their time. And from the very beginning they noticed that long skinny wings usually performed better than short fat ones. And so it is with the underwater board of a sailboat. In its own way it "flies" through the water producing a force that counteracts the side loads produced by a fore-and-aft sail rig.

Designers quickly developed the term "aspect ratio" to measure that feature of a wing. The aspect ratio of an underwater board is AR=2 x B x B / S, where B is the length of the board and S is its area. Figure 1 shows how you measure those elements. (Aero students will note that the above equation is twice the usual given for an aircraft wing which has two tips while an underwater board has but one tip.)

Next, for a board with a symmetrical cross section (so it can function equally on all tacks), the board must go through the water at an "angle of attack" to generate force, as we discussed in the last issue. Figure 2 shows how the situation applies to a close hauled sail boat. The skipper wants to go to on a certain couse but because of the angle of attack required by the board he must head his boat upwind by the angle of attack. So he might think the boat is "slipping" and producing "leeway" equal to the angle of attack. One other important thing to notice here is that any angle of attack of the underwater board must subtract from the angle of attack of the sails. A bad situation. Sort of a double penalty.

Now look at Figure 3. Here is the big message: High aspect ratio boards (deep and skinny) reach their lift coefficients with less angle of attack than do shallow fat boards. The figure is sort of idealized but Marchaj gives examples of tests of different actual boards that agree with the figure.

Low aspect ratio keels can be successful if they have sufficient area to allow them to always operate at a low C. The Micro has I think about 14 square feet of keel, about twice the value I recommended of 4% of the sail area for a deep skinny fin. Large low aspect ratio keels also make for steadier boats. Marchaj points out that they have superior damping which makes for a safer boat in rough going.

As an example, let's use Frolic2 again. Her board has 4.4 square feet of area immersed and it is 3.5 feet deep. So the aspect ratio is 2 x 3.5 x 3.5/ 4.4 = 5.6. Now let's say we wanted to guess at leeway on a close reach in 10 knots wind, and we'll assume she is going 4 knots at the time. Frolic2 has 114 square feet of sail and 10 knots produces about .5 psf on a typical good sail, so that is 57 pounds of sail force. We'll assume that all of that force is counteracted by the board. So the overall pressure on the board is 57/4.4 = 13 psf. Now we need to calculate the value of C for the fin. Remember that the psf on the fin is = 2.86 x V x V x C. Using V = 4 knots, we can solve for C and get C = .28. Now get into the aspect ratio figure, guess where AR = 5.6 might be and it appears that the leeway angle would be about 4 degrees.

Here's an interesting "what if". What if the Frolic2 narrow board were instead mounted like a shallow keel such that it still has 4.4 square feet of area but now has a length of 3.5 feet and a depth of 1.25 feet? Now its aspect ratio = 2 x 1.25 x1.25 / 4.4 = .71. Going into the aspect ratio chart you would see the leeway for this fin would be about 15 degrees under the same situation.

Let's get one step fancier and say we have the original board set up as a centerboard that folds into a similar sized shallow stub keel as shown in Figure 4. This is a pretty common arrangement. Now the two areas will share the side load, but in what proportion? One thing we can say is that they will operate at the same angle. Let's guess that the angle of attack is 4 degrees. Then the deep skinny board is operating at a C of about .3 while the shallow fat board is operating at a C of about .1. They have the same area so the deep skinny board is producing three times the lift of the stub keel! It's doing nearly all the work. And it will continue to do so until the angle of attack reaches about 15 degrees at which point the deep skinny board reaches C max of about 1.2. Then the stub keel is working at a C of about .3 and will continue to be effective until it reaches an angle of maybe 50 degrees. You wouldn't want to sail at a 50 degree leeway angle although there might be times when it would be advantageous. I can't really think of such a situation.

This sort of analysis flies in the face of some designs. As mentioned earlier, the shallow keel can work very well if given enough area. But I've seen designs with "keel runners" or "chine runners" which I think are just small projections off the bottom. Given what we have seen here I don't see how they can work effectively. I've sailed boats with skids and external chines which approach those things but my boats always had leeboards or centerboards too. If you think the skids and chine runners are effective, you need only take one of my boats out, get close hauled, and raise the leeboard or centerboard to see what happens. You'll slide right off!

I'd really like to try or see a good test on Matt Leyden's Paradox. That boat has chine projections and nothing else. I met him at Sail OK a few years back and saw his boats in action. I wrote an article about them. The chine runners are fairly short and stout. The boat is quite heavy for its size in the usual way of thinking - a 14 footer might weight a thousand pounds with all its ballast. As a result it sits fairly deep in the water and Matt said the object is to get a draft of at least 25% of the beam of the flat bottom, but that could be caused by heeling the boat also, which puts the chine deeper down by quite a bit. There was also speculation, Matt did not mention it, that the rudder is large and deep and that the rig is set up to put a large lifting load on the rudder. Bolger also got heavily involved with "lifting" rudders with boats like Cartopper. Anyway, it would be cool if a Paradox owner would hang a pair of old fashioned Dutch leeboards on his boat and tell us about the difference resulting.

One last comment is that I think "leeway" is tough to measure and I should give some thoughts on this in a future essay.

Harmonica

Harmonica

HARMONICA, MINISHANTY, 13' X 5', 400 POUNDS EMPTY

Harmonica is a tiny shanty boat that sleeps two in its cabin. There is a porch up front suitable for lounging and a small room in the stern for the kitchen and the water closet. I think it is arranged so that two people could wait out an all day soaker without feeling too pressed. For protected waters only. I've been asked more that once if Harmonica would be OK in the Ohio River, etc.. My answer is no. In calm weather you might get away with it but you always run the risk of huge wakes, especially from large motor yachts. The prototype Harmonica was built by Chris Crandall of Lawrence, Kansas.

This boat was originally called Fusebox. It was intended to be an electric boat for the wonderful little conservation lakes we have around here. But later I thought that the electric scheme was strained because few trolling motors could push this box on a windy day. And because I noticed that none of the local conservation lakes have electric plugs at the docks, so recharging the batteries there would not be possible. Putting a gallon of fuel on board is a lot easier than taking an 80 pound battery home. Chris used a 1 horse Tanaka and that was about the minimum required, pushing the boat at 2 or 3 mph. At the same time I would say that 5 hp would be a reasonable maximum. You don't need much power or speed because you never can go more than a mile in any direction in the lakes I'm thinking of.

I also heard from John Applewhite, of Gainesville, Fl., who built the Harmonica shown in the photo above. He had quite a bit of shantyboat experience and also some electric experience. He used a 3 hp Minnkota and two golf cart batteries, probably about 200 pounds of gear. John said this electric is used nearly all the time with great satisfaction but he also has a small gas outboard on the transom as a backup.

John also wrote, "...The entire family (two adults and two children) has spent the night on Steel Will. I put 1x2's between the slats in the two forward bunks and created a single bunk that is five feet wide. We have found that the thick cushions sold to cover lawn furniture very adequate mattresses for boats."

Harmonica soaks up four sheets of 3/8" plywood and six sheets of 1/4" plywood and uses simple glue and nail jigless construction. Blueprints with keyed instructions are $25.

Contents


Prototype News

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:

The first Jukebox3 is on the (cold) water. The mast is a bit too short - always make your mast too long. A bit more testing will be nice...

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.

Contents


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

1apr16, Capsize Lessons, RiverRunner

15apr16, Wood Vs Aluminum Spars, Mayfly16

1may16, Scarfing Wood, Blobster

15may16, Prismatic Coefficient, Roar2

1jun16, Figuring Displacement, Mayfly14

15jun16, Rend Lake 2016, Mixer

1jul16, Ballast Calculations 1, Dorado

15jul16, Ballast Calculations 2, Robbsboat

1aug16, Ballast Calculations 3, AF4

15aug16, Taped Seams, Cormorant

1sep16, Butt Joints, Vireo

15sep16, Old Outboards, Philsboat

1oct16, D'Arcy Ballast, Larsboat

15oct16, D'Arcy Ballast 2, Jonsboat

1nov16, D'Arcy Ballast 3, Piccup Pram

1dec16, Sail Area Math, Ladybug

15dec16, D'Arcy Thoughts, Sportdory

1jan17, AF3 Capsize, Normsboat

15jan17, The Weather, Robote

1feb17, Aspect Ratio, Jewelbox Jr

15feb17, Aspect Ratio 2, IMB

1mar17, Normsboat Capsize, AF4Breve

SOME LINKS

Mother of All Boat Links

Cheap Pages

Duckworks Magazine

The Boatbuilding Community

Kilburn's Power Skiff

Bruce Builds Roar

Dave Carnell

Rich builds AF2

JB Builds AF4

JB Builds Sportdory

Hullform Download

Puddle Duck Website

Brian builds Roar2

Herb builds AF3

Herb builds RB42

Barry Builds Toto



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