Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(1May99) This issue will show some different plywood butt straps. Next issue, 15May99, will show how to figure the centers of sail areas.


Craig O'Donnell at Cheap Pages has one of the best boating sites on the internet. He has recently added some more goodies. For example he has the complete online copy of H. Warington Smyth's MAST AND SAIL IN EUROPE AND ASIA originally published in 1906. Smyth travelled the world drawing and writing about the boats he saw at a time when England ruled the waves. And when men were allowed to write about boats like Smyth wrote about the Nordland Cod boat ---"The most feminine of boats, she demands a real man for her helmsman, who knows his own mind and has a strong hand to effect his purpose. Capricious, quick, seemingly, to betray her charge, she yet loves to be ruled strongly. It is only this she wants; and once she finds that she has her master, she will take him through the wildest winter night in safety, yet not without throwing more water than is either seemly or safe." We don't write 'em like we used to!


The 10th annual Midwest Messabout will take place at the Gun Creek Recreation Area at Rend Lake in Southern Illinois on June 11,12, and 13. The usual routine is that some folks come on Friday and leave early on Sunday. To get to Gun Creek take Interstate 57 to exit 77 West, head towards the golf course on the south side of the road and follow the signs. This is a Corps of Engineers rec area with a $2 launch fee. Camping is 8$ a night (I think) and that will cover the launch fee. There is no schedule of events - we just wing it. Email me at michalak@apci.net if you have questions.




Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



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.


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 panels 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.


Payson 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's 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:

joint end view

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 lay 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.


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.


lumber butt plate

This is what 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 fail in places where the butt plate will be in the way.


light glass joint

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.


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 show how to find the center of a sail plan's area.





Oracle might have been called Roar3. It's a bit longer and lower than Roar2 so it looks sleeker. In truth it is closer in shape to Toto than to Roar2. I'm pretty sure it will be a visual imrovement over Roar2. Hard to say if it will be a practical improvement.

There are a couple of things I'd like to point out. First involves the way I drew the lines of Oracle. I make card models of my designs which help me to check the panel expansions and allow me to get a 3D check of the shape. The models are done in the same scale as the drawing, in this case 1/8 size and the model is fairly large. So I know that Oracle has nice curves all over. But if you look at the end view in the lines drawing you will see all straight lines!


In a way this is a throwback to the original "instant" boats which had hull panels made from straight saw cuts.But it's easy for a designer to make the jump to more flexible shapes, that is hull shapes which can look like about anything you might imagine made from plywood. Back when I started drawing boats I noticed that some of the nicest shapes in 3D had end views made totally of straight line segments. Don't ask me why. Not only does it give a nice shape, but the designer's job can be simplified. For example with Oracle I might start by drawing the top view. Then I might sketch in the side view needing nothing more than an idea of what the stem, midsection, and stern will look like. Next I draw the end view which will include those three elements all connected by straight lines as you see in the end view above. In normal mechanical drawing it only takes two views to totally define an object. So without even drawing the side view of the boat, the shape is defined. It only remains to go back and figure it out. Of course, if you don't like the resulting side view, you will tinker with it. Then you will have to go back and adjust the end view and the top view, etc., etc.. Happy is the designer who hits it right the first time through.

The second thing I'd like to point out is the locations shown for the oarlocks. I almost always show the oarlock sockets mounted outside the hull on a good pulling boat. That allows better spread of the locks and thus slightly longer oars. It allows you to dip the oars without rubbing them on the wales. The only disadvantage I see is that if you use a boat like this to come along side another boat, you will present to that boat the only metal fittings you have on your craft. So a true tender should have the oarlock sockets mounted inside the wales where they won't scratch anything. Also, any dimensions I show for lock locations are only suggestions. You really should fine tune the socket locations to keep the boat trimmed level at your favorite rowing location. For that matter, I never show seats in my boats having found that a boat with a flat bottom plank like Oracle's will accept a rowing seat/ditty box that allows for endless adjustments and a totally open interior. And once you've found that best location, fasten some cleats to the boat where your feet like to be to act as foot braces.

Oracle is built from the same pile of stuff as Roar2. Four sheets of plywood with taped seams. No jigs, no lofting.

Plans for Oracle are $12 until one is built and tested.


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.

Usually when a design from the Catalog of Prototypes starts getting built and is close to launch I pull it from the catalog and replace it with another prototype. So that boat often goes into limbo until the builder finishes and sends a test report and a photo.

Here are the prototypes abuilding that I know of:

IMB: I heard recently from the Texas IMB builder who said, "I want to let you know that IMB is alive and looking good. She is now right side up and sitting on her cradle. Soon this boat and carrier will be mounted on the small trailer I have ready......" Then I got this photo from Roger Harlow by way of Tim Webber.

imb under construction

Fatcat2: There is an old timer (80 years +) in Minnesota who has completed the hull of a Fatcat2. Fatcat2 is a simple 15' x 6' catboat, gaff rigged and multichined. I think the boat is totally done now.

Mixer2: Mixer2 is more or less the original Mixer with a rough water bow like Toto's. 12' x 3'9" and about 90 pounds. Has a sail rig. I got a call from a Colorado builder who wanter to learn to sail and apparently has the boat well along. He also built last year a Smoar which is a 12' version of Roar2 and was very satisfied with it. As I talked with him on the phone I got down on my knees and begged for photos.

Frolic2: I recently heard of two of these going together. One is in Colorado and seems to be on the back burner for now. The other is in Oregon and is on the front burner. He started it in January and is about half done, hoping for an early summer launch.

RB42: This is an 18' rowboat meant for two. It's never been in any of my catalogs but the Canadian builder has it well underway. Imagine the Oracle shown above stretched to 18' and you have it.




Mother of All Boat Links

Cheap Pages

Messing About In Boats

Duckworks Magazine

Backyard Boats

The Boatbuilding Community

Kilburn's Sturdee Dory

Bruce Builds Roar

Dave Carnell

Herb builds AF3 (archived copy)

JB Builds Sportdory

Hullforms Download (archived copy)

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