Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15February 2014) This issue will take a longer look at dory stability. The 1 March issue will capsize a Scram Pram.
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.
Steve Chambers has converted several plywood designs to skin on frame. Here he works on an LFH17 conversion.
YOU MAY RECALL....
...a few issues ago where I took a Hullform model of Sportdory....
... and gave it a virtual rolling stability test. Those first results looked like this...
In general it showed that the dory, which has many similarities to the famous Bolger Light Dory, was unstable at rest. Then it would pick up a sliver of positive stability between about 20 and 40 degrees of roll and after that go very unstable.
... had built the prototype Sportdory and brought it to a Rend Lake meet. Many of us tried it and all stayed right side up. I knew that. I had a Bolger Light Dory for several years (destroyed in one of our tornados without me in it) and spent all that time right side up.
Well, John questioned the cg location I had used in the analysis. I placed the overall cg at 20" above the bottom. On a boat like this most of the weight is in the crew. A dory like this will weight between 70 and 150 pounds depending on who is doing the building. It was time for a more precise guess at where the cg of the boat with skipper might really be. And right here I will say I initially did the numbers with the cg at 24" above the floor and found the boat had absolutely no positive stability at any time.
WHERE IS YOUR BELLY BUTTON....
.... the weights guys at the missile factory always told me the cg of a human is at his or her belly button. They said that was the real reason you had a belly button - so you could locate your cg quickly. We will come back to this later.
DO YOU KNOW WHERE YOUR CG IS TONIGHT...
Time to get more serious about the human cg. I got on the information superhighway and fell into this url, www.exrx.net/Kinesiology/Segments.html , where people worry about body segments.
There are two sets of data there. The 1983 set was based on (UGH!) cutting up old corpses. No doubt an Italian film crew was grinding away as they did their work. The 1996 data was done with measurements of fine young Russian athletes. Doesn't say how that was done but I can imagine a man in a sling with a weight scale supporting all, then he is lowered slowly into a pool of water, say down to his hips, and, voila!, you can find the weight of his legs alone (sort of with enough assumptions) by noting the reduction of weight on the scale. Anyway, I know all of us boaters fit into the fine young athlete catagory more than the old corpse. Besides, the old corpse data adds up to 108% so they counted something twice.
SO I SAT ON A BOX...
...that is 7" high, the time honored height of a dory seat, and measured the locations of my segments. I looked like this...
My belly button here was 18" above the floor.
Then I made up my very own person cg calculation table using a total weight of 200#, more than I really weigh but a nice round number. Here is the table.
Towards the bottom I added in a guess at the weight and cg of the dory hull, using 100 pounds total for the boat and some junk and guessing its cg to be 7" above the bottom (based on the idea the boat is about 15"deep total, not really true but a guess).
The upshot is that the total in this condition is a total displacement of 300 pounds with a cg 15.3" above the bottom. That is well below, in dory terms, my old guess of 20". Now the righting curve looks like this...
She has positive stability to 45 degrees now and we can all relax. It still is not the most stable boat in the world and is quite sensative to that cg height. Don't get the idea you can use a kitchen chair for rowing. And as Phil Bolger warned us...."NEVER STAND UP IN A DORY!!!"
And note that my personal cg calculates to be 19.4" when the belly button measured in at 18". Not to bad a rule of belly button.
IMB, SAIL BEACHBOAT, 13.5' X 5.5', 350 POUNDS EMPTY
IMB features a "Birdwatcher" cabin, full length with panoramic windows and a center walkway slot in the roof. Everyone rides inside. This style of boat was invented by Phil Bolger in the early 1980's.
These boats can be self righting with minimal, or no, ballast because crew weight works as ballast. They sit low looking out through the windows (although standing in normal winds is quite acceptable). The cabin sides provide lots of buoyancy up high to ensure a good range of stability. IMB, which is small with a light bottom, should reliably self right from 60 or 70 degrees and in the test described above self righted from a full 90 degrees of roll.
These boats are operated from within the cabin, like an automobile. No one need ever go on deck. For boating with children I can see no equal.
These are usually cool inside. The tinted windows cut the sun's power. The crew can sit in the shade of the deck. Downdraft from the sail cascades through the walkway. (By the way, at the Conroe messabout two boaters with Lexan windows noted that mosquito spray will ruin Lexan with one application and they noted belatedly that the back of the spray can says so.)
IMB has an 8' long cabin on a multichine pram hull. The prototype was built to perfection by Gerry Scott of Cleveland, Texas. At the Conroe (Houston) messabout I got a chance to look over his boat plus the only other IMB I know of built by Bob Williams. Both boats were quite true to the plans. Both had added low inside seats which made them more pleasant to use to the point that I will show some seats on the plans. I was worried when I drew IMB that the headroom would be minimal so drew no seats thinking the crew would sit on the floor, as with the original Birdwatcher.
While I was sailing with Gerry, Bob's boat came out on the lake with four adult males and no sign of bogging down, showing that these fat pram shapes, very much like my Piccup Pram, can handle a lot of weight in the 13.5' length.
(Later they rescued a mermaid and returned to the dock with five total.)
I don't know if either boat had ever been weighed and the 350 pounds I quote as the empty weight is just a guess. One of the ideas behind the boat was that it might be towed behind a compact car and I was glad to see that Gerry tows his behind a 1500cc mini SUV.
Both men adjusted well to the lug sail/leeboard rig. Gerry's has the blueprint 104 square foot sail and Bob's uses the 114 square foot Bolger Windsprint sail available from Payson. I used to worry a bit about running a leeboard on a full cabin boat like this since handling must be done by remote control, so to speak. No problem. Both boats have the leeboard lanyard running to a cleat on the aft deck. The leeboard position is plainly in view at all times through the cabin window. In use these leeboards need only lanyards to pull them down. Once down they will usually stay down until they strike something. Then they pop up and you will need to pull them down again. I've never seen a need for a lanyard to pull the board up although I've seen several rigged that way. The Dovekie design had elaborate cam operated levers in the cabin that operated the leeboards and I thought that all very clever. But in talking to some Dovekie owners I found the internal levers are not universally loved since they can often be in the way. Anyway, my idea was not to run the down lanyard to the aft deck but rather through a small hole in the side of the boat, say 1/2" for a 1/4" lanyard, so it could be operated totally from inside the cabin.
Both Gerry's and Bob's boats used electric trolling motors. The plans show rowing ports and no provisions for a motor. A boat like this won't be a fast row boat but it might be useful in a calm. Even the 24' Birdwatcher would row about 2.5mph in a calm. But I'll admit that adding a motor to Birdwatcher makes it a much more useful thing.
IMB takes two sheets of 1/2" plywood, eight sheets of 1/4" plywood and one sheet of 3/16" Plexiglass. Taped seam construction using no jigs or lofting.
IMB plans are $30.
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
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
1mar13, Figuring Displacement, Paddleplank
15mar13, Drawing Boats 4, Frolic2
1apr13, Drawing Boats 5, RiverRunner
15apr13, Drawing Boats 6, Picara
1may13, Two Letters About Keels, Blobster
15may13, Drawing Boats 7, Roar2
1jun13, Drawing Boats 8, Polepunt
15jun13, Rend Lake 2013, Toto
1jul13, Drawing Boats 9, AF4 Grande
15jul13, Taped Seams, Mikesboat
1aug13, Plywood Butt Joints, Paulsboat
15aug13, Sink Weights, Cormorant
1sep13, Lugsail Rigging, Hapscut
15sep13, Sharpie Spritsail Rigging, Philsboat
1oct13, Modifying Boats 1, Larsboat
15oct13, Modifying Boats 2, Jonsboat
1nov13, Modifying Boats 3, Piccup Pram
15nov13, Sail Area Math, Caprice
1dec13, Stretched Stability, Ladybug
15dec13, Trailering, Sportdory
1jan14, Cartopping, OliveOyl
15jan14, Width/Stability, HC Skiff
1feb14, Hiking, Shanteuse
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Puddle Duck Website
Brian builds Roar2
Herb builds AF3
Herb builds RB42
Barry Builds Toto
Table of Contents