Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1feb07) This issue willl rerun the IMB capsize article. The 15feb issue will return to estimating power.
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.
The Burtons again with the Mayfly14.
IMB Capsize Test
WE CAPSIZE IMB....
The IMB design has been around a while. It was designed for a contest put on ten years ago by International Marine Publishing. So I called it the "International Marine Beacher" which became IMB. I'm featuring the IMB below in the design section if you want to read more details but in general it was supposed to be a self righting mini (13' x 5') cruiser that could be pulled behind a small car.
To keep the weight way down I made it a Birdwatcher boat with a cabin style invented by Phil Bolger in the mid '80's. Everyone rides inside a watertight cabin looking out through Plexiglass windows. Their weight is kept low and thus becomes the boat's ballast. That plus the high cabin sides might make the boat self righting to 90 degrees of roll, I hoped.
But we were finding it was not all that foolproof. Scram Pram, which is a 16' x 6' version of IMB, was tested to capsize twice (once accidentally, once on purpose) and found to lie stable on its side when knocked down to 90 degrees. That was not much of a worry since Scram is supposed to have 300 pounds of water ballast and when that ballast was tried on the prototype, the boat quickly self rigthed from 90 degrees. There was a feeling that much less ballast was needed. But the fact was that with one large crewmember inside acting as ballast, the Scram would not self right without the water ballast.
So I was concerned about the smaller IMB.
I was very fortunate to be able to attend the Lake Conroe (Houston) messabout. Among all the attractions were two IMB's! They are the only ones I know of. The first was built by Gerry Scott and the second by Bob Williams.
I got to sail in both, although the "sailing conditions" were light to say the least.
At this messabout there is a tradition of turning over someone's boat! That is where the Scram tests were done a couple of years back. Could we please turn over an IMB?
Gerry Scott was a great sport about it.
Here is what we did. The IMB was beached and we removed the trolling motor and battery and all the loose things. The boat was walked into waist deep water, parallel to the shore, and human anchors placed at bow and stern. The halyard was secured to the mast and its free end given to me close to the shore. So I could pull the boat over with the halyard while the good folks held the boat in place.
The sail was stowed to keep tangles to a minimum but the sail and boom can have an effect on the result. I would expect the boat would be slightly less stable due to the weight of the sail and boom up high. Once the boat is knocked down the boom and sail can help float the boat and keep the boat from going turtle. In fact that effect is really strong with a lug sail who's yard acts like a float out on the top of the mast. Then when it comes time to right the boat it is very very important to totally release the sheet because if water pools in the sail as you try to raise it off the water the weight of that water can make the boat much harder to right.
Remember that the skipper's weight is ballast in a boat like this. Gerry was skeptical about riding inside through the test so he watched from the outside.
I pulled it over slowly. I can't recall a point where the righting moment peaked but I would expect it to be at maybe 30 degrees or so. When the windows start to into the water the empty IMB still has a lot of righting ability (Gerry says he has sailed his with the windows touching the water). Somewhere about 80 degrees of heel the righting moment, as felt by tension in the halyard used to pull the boat over, went to zero and she wanted to go the rest of the way onto her side. I eased it down and let the mast into the water at which point the roll stopped and she was stable on her side.
Now time to right the boat. I held the halyard slack as Tim Weber attempted to push down on the skeg and see how much effort was needed to right the boat. Up she popped quickly. Tim said it was pressure of one hand that did it, maybe 15 pounds. Once it got past the 80 degree point it righted by itself and I did my best to ease it upward since these things can roll upright very quickly indeed.
Next Gerry, great sport that he is, said we should repeat the test with himself inside to check on the human ballast effect (adding that a seat belt would be nice). So he got inside and braced himself hanging on to the mast.
Repeating the test was quick and simple although the effort to haul it over was quite a bit greater with Gerry's ballast inside. We pulled her over slowly to make sure Gerry had time to brace himself as required. You might image what being inside would be like, and I'm thinking of that old Fred Astaire film where he dances in a rotating room the final affect being that he goes from dancing on the floor then on to the walls and then for a stint at dancing upside down on the ceiling. Anyway, here is the scene close to 90 degrees with Gerry smiling and waving to the crowd, something he might not do in a real capsize.
Unlike the Scram, the smaller IMB had positive righting all the way to 90 degrees. There was tension on the halyard the entire range. When it came time right the boat no external push was needed, Gerry's weight and the boat's geometry did it all.
Well, that made me feel better. I'm not sure why the IMB would self right from 90 degrees when the larger Scram wouldn't except to say that on the smaller boat the crew's weight might be a larger percentage of the total making it in effect "more ballasted". On the other hand, the Scram has higher sides - so go figure. But one thing is for sure. You can't really take self righting for granted. You really should test. Changes in construction can upset the equation too.
Unlike the AF3, the Birdwatcher cabin on IMB not only brings the boat back upright without anyone going for a swim, the boat when upright is ready to go again. Nothing has flooded and she is dry inside.
But as with any boat it is worth repeating that the knockdown, even a controlled one, is somewhat violent and anything that isn't secure will go flying around. So if you sail in knockdown conditions everything needs to be strapped down. More easily said than done and don't look to me as an example.
We take another look at estimating power..
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.
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 I heard about through the grapevine.
We have a Philsboat going together in California. You can see the interior room in this 15' boat:
And here is another Philsboat in northern Illinois:
HOLY COW! A Jukebox2 takes shape in Minnesota. Unheated shop means no work during the winter. Check out that building rig!
And the Vole in New York. Going very quickly but most likely there will be little more done during the cold winter.
AN INDEX OF PAST ISSUES
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)
Plyboats Demo Download (archived copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds Toto
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