Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1may01) This issue reruns an important construction essay about joining plywood sheets. Next issue, 15may01, presents thoughts about "Why Build Your Own Boat?".
The 12th ANNUAL MIDWEST HOMEBUILT BOAT MESSABOUT...
... will take place on June 9 and 10 at the Gun Creek Recreation Area at Rend Lake in Southern Illinois. Take exit 77W off I-57, head towards the golf course and you will see the signs. Lots of people arrive on Friday and leave early on Sunday. The camping fee is $10 which includes the ramp fee at this Corps of Engineers facility. There is no schedule of events. We have a pitch in dinner on Saturday evening.
Frank SanMiguel in his AF3
JOINING PLYWOOD SHEETSBACKGROUND...
Almost any boat you build from plywood will require panels longer than the 8' lengths you will find at the lumberyard. For an "instant boat" the usual manner of joining the panels together to get one long panel is with a butt strap or butt plate. I've tried lots of different ways to make the joint in the 15 or so boats I've built over the years. All the methods worked. I have a feeling that the butt joints on the usual instant boat hull are not highly loaded and not too critical to overall boat strength. Since I've tried several ways and they all worked I've gotten to be pretty nebulous about the subject on my drawings. Lately I've been specifiying a butt plate as something like "Butt plate from 3/4" x 3-1/2" lumber, or equal" which doesn't tell you much. Most builders get by pretty well with just that but recently one builder asked what in the world I meant, and rightly so. So let's start the discussion with one joint I've never tried in plywood.
TRADITIONAL PLYWOOD SCARF JOINT...
Figure 1 shows the traditional plywood scarf joint. I''ve never done this in plywood although I've made a lot of scarf joints in plain lumber. The two overlapping faces are tapered and glued together such that both joined faces remain smooth. If you are building a traditional lapstrake boat from plywood you have to make the joints this way.
What's good about it? The faces are smooth both sides. Essentially you have a single piece of wood to work with after making the joint. It's quite strong, as strong as the base wood provided the taper is long enough. I've seen the taper range from 6:1 to 12:1. The shorter tapers are easier to make and probably just as good given modern glues.
What's bad about it? The tapers can be hard to make properly although thickened epoxy has made experts of most of us. Some experts have special saw rigs to cut the taper. Most use power hand planes or belt sanders, I think. When gluing the joint you must press it up against a firm surface while the glue cures, making sure nothing glues to that surface, and making sure the two pieces are secured lengthwise so the tapers don't push them apart as you apply clamping pressure.
There is one more warning for instant boat builders. Almost all instant boat designs have panel layouts which assume you will not be using a scarf joint. If you join two 8' panels with a scarf joint you will NOT end up with a 16' panel. It will be shorter by the overlap amount. That might be just enought to negate the ply panel layout.
THE PAYSON TEAL BUTT STRAP...
I think this type of joint was described in Payson's great book INSTANT BOATS. I used it on my Teal which was my first homemade boat. The side panels were 1/4" thick on my boat, the bottom 3/8". The butt straps were 3/8" plywood, 6" wide. So the effect of the joint was similar to a 12;1 scarf on the 1/4" ply sides and about 8:1 scarf on the bottom. The nails were supposed to be copper, but I think Harold might have also suggested copper rivets or short bolts for fasteners.
What is good about it? It's pretty simple to visualize and make. If the fastening is good you can make the joint and go right on building without waiting for the glue to set. You might do that with the sides, for example. Another neat thing about this plain butt strap joint is that, with a typical flat iron skiff type of assembly, the bottom panels need not be joined before assembly onto the hull. In that case you will have the hull inverted on sawhorses ready for the bottom. Then you put the first bottom piece on attaching it to the sides. Then you install the first butt strap at the end of that piece. Then you install the next bottom panel to that butt strap and the sides. And so forth until the entire bottom is planked, like laying bricks.
I might mention now that I think butt straps and plates should be well rounded at the ends to avoid trapping dirt and moisture. In boats that have taped seams I advise stopping the butt strap short of the edge of the panel so you will have room to run the tapes undisturbed. This is especially true of butt straps on the bottom. You must have a clear limber channel around the perimenter of the bottom. In that case I stop the strap about 1/2" short of the side, fill the little gap with epoxy to keep the water out, and tape over the bottom of the joint with glass tape and epoxy. Usually I don't put glass tape over the outside of the side panel joints. But butt joints in any deck should be well sealed with glass and epoxy. Here is an end view of the treatment:
What's bad about the plywood strap? I think in Maine you aren't a man until you've made a boat with clenched copper nails or rivets. Not so where I live, can't buy them any place I know of. I got by with bronze boat nails but they really aren't flexible enough for the job. They didn't look too cool. And the edges of the plywood butt straps don't look too cool either, wanting to have gaps and splinters showing. It takes a while to finish them. And, of course, you have a lump at each joint and folks will ask, "What's that?"
By the way, when I built my Toto I used the simple plywood strap method with no fasteners. Just carefully placed the ply panels over the straps which were well buttered with glue, weighed it all down with concrete blocks to provide pressure, and stayed away for a few days until I was sure the glue was totally set.
THE BIRDWATCHER BUTT STRAP...
When I built my Birdwatcer in 1988 I think I piled on more layers of plywood straps such that I wouldn't have to bend over the nails. And by that time I was more expert at finishing the edges of plywood. It worked but it was very obvious that I could have done the same thing with regular lumber and saved a bit of work.
THE LUMBER BUTT PLATE...
This is I like to advise now. Not much to it. Very quick and easy to make. Some say it looks too clunky for their tastes.
If there has been any structural problem with the above butt plate it is at the ends of a plate that joins bottom panels, ending short of the sides to allow a limber path. If the ends of the plate are not solidly glued and fastened to the bottom panels the butt plate will eventually loosen at the ends. I think that is due to the rapid change in flexibility in the system where the plate suddenly ends. A better solution might be to taper the end of the bottom butt plates starting maybe 3" in from the end of the plate and tapering down to maybe 3/8" thick at the ends. That will allow a gradual change in the flexibility and prevent a stress riser at the end of the butt plate. By the way, for any bulky butt joint care must be taken in design to see that the joints don't fall in places where the butt plate will be in the way.
LIGHT FIBERGLASS BUTT JOINTS..
Both Harold Payson and Dave Carnell presented this one to U.S. readers in the '80's but I'll bet the English inventors of taped seam boats did it earlier. Simple as can be in theory. Just a layer of fiberglass on each side of the plywood.
Dave Carnell presents lots of details at his web site. He has done scientific load tests of these joints and says the joint will be as strong as the base wood if you use one layer of fiberglass cloth in epoxy on each side of 1/4" plywood, two layers on 3/8" plywood, three layers on 1/2" plywood, and four layers on 3/4" plywood.
I used the glass butt joint on my Roar rowboat but went back to wooden butt joints later. At first it would appear that the joint is easily made by laying the ply pieces on the floor, taping one side with fiberglass, waiting to cure, flipping the panel and repeating on the other side. But I found that plywood on its own often does not want to lay flat enough to get a smooth fit, so I had to place the joint over a board and screw the pieces down flat. Next the idea of flipping the panel with only one side taped doesn't work well because that one layer of glass has little strength by itself. But it can be done carefully. Better yet is what both Payson and Carnell advise: glass both sides at once. Lay the first side of fiberglass layers wet with epoxy on a protected flat surface, lay the plywood to be joined upon it, lay the second side of fiberglass over the top of the joint, cover with plastic sheet and weigh down with concrete blocks. The plastic sheeting not only protects everything from gooey epoxy, but it should provide a smooth final finish and if you are lucky no filling or sanding required afterward.
THE PAYSON HEAVY GLASS JOINT...
Harold Payson did a little more work on the glass butt joint. I'm doing this from memory and hope I'm getting it right. To hide the build up of glass on a thicker sheet of plywood he recessed the surfaces roughly with a sanding disk in a drill. Then he added a layer of light fiberglass matt to the wood before pasting in the glass. Fiberglass matt is generally thought to provide better adhesion to wood than glass cloth although by itself it has little strength. Harold is big on using polyester resin on his boats instead of epoxy so perhaps the matt is more important for the polyester users. But you can see the advantage of the system: the final joint can be more or less invisible as the multiple layers of glass are recessed.
I'll talk about "Why Build Your Own Boat".
OZARKIAN, DRIFT BOAT, 20' X 4', 200 POUNDS EMPTY
I do a "retro" design every now and then, an old design redone with modern techniques. Max Wawrzyniak sent me a wonderful copy of the article "Ozark John Boat" by Townsend Godsby, printed in the "Sports Afield Boatbuilding Annual" in 1956. I live on the edge of the Ozark Mountains right now and have seen have seen some of the beautiful streams mentioned in the article. These are small shallow rivers, clear and beautiful, that slide and riffle over clean gravel beds at about a walking pace. In the summer it gets really hot around here and it is still a happy way to kill time to float those rivers in aluminum canoes. In popular spots you could swear you could walk across the river by stepping from canoe to canoe, your ears treated to the sounds of birds, rippling water, and laughter, all to the beat of canoes crashing into each other. The Ozark jon boat predates the aluminum canoe by quite a bit, Godsey's article quoting a builder that had been floating the river as a fishing guide in his jon boats since 1900 and even then the design was well established.
The traditional Ozark boat is in some ways better than the aluminum canoe. For one thing it carries a huge load, being 20' long, 4' wide, and having a dead flat bottom. It is a drifting barge. Godsey gives the basic use: load with fishing guide and "sport" and gear to float 125 miles from Galena to Branson, Missouri, (first home of the mythical Jed Clampett), in five days. No motor, just a paddle for the guide to steer with. At Branson it all got shipped back to Galena (just 24 miles away by land) by rail for the next trip. In the old photos the amount of gear carried is enormous. The boats have seats in the ends but the photos show the people sitting very comfortably in canvas director's chairs.
Construction in 1956 was what one could expect. You start with something you might not find at your lumberyard today - five "clean" pine boards 1" x 14" x 20' long. Three boards make the bottom with tongue and groove joints and one board makes each side, so these boats aren't too big on freeboard. I've shown wooden frames where the original boats had metal frames made from old iron wagon tires! The list of materials also included 15' of "Cow chain", whatever that is. I should add that I think a similar boat, but with pointy ends, was used in a similar way in Michigan that dragged a length of heavy chain from its stern, keeping the boat pointed reliably downstream.
Well, my Ozarkian uses standard simple plywood "instant" construction needing five sheets of 3/8" plywood. "They ain't nothin' to makin' a John boat," said Charlie the old guide. "Any hillbilly kin do it."
Plans for Ozarkian are $15 until one is built and tested.
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.
Here are the prototypes abuilding that I know of:
Caprice: Chuck Leinweber of Duckworks Magazine has finished the prototype Caprice. Here is a photo. Waiting for some shakedown cruises and a report.
Skat: I'm told the prototype Skat project is underway again and very close to being done.
Normsboat: This is an 18' sharpie being built by Cullison Smallcraft in Maryland. You should be able to check on it by clicking through to his web site at Cullison SmallCraft (archived copy, actual site no longer active). He is presenting an excellent photo essay of how to assemble a flattie.
Vamp: I received word that a prototype of the little rowboat Vamp is completed up in Utah. Hopefully photos and test report soon.
Toots: A prototype of the 10' Toots rowing skiff has been completed in New York. Waiting for an "in the water" photo and test report. Toots is not currently in the paper catalog but you can find it in "The Way Back Issue Archives" link below (*).
Family Skiff: A Family Skiff has been started in Virginia.
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED BY DATE
* Toots, in the Duckworks store plans section
Mother of All Boat Links
Messing About In Boats
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
Herb builds AF3 (archived copy)
JB Builds Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
Joe and Dan build Jon Jr.
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