Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(15October13) This issue will continue a discussion about modifying boats from the given plans. The 1November issue will continue the topic.



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....


...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.


Chuck Pierce sent this great photo along with the note, "My friend Pehr Jansson sent this to me-he took this as we were preparing to leave Army Hole on this years' Texas 200. From front to back, Pehr's PU Pram, my Mayfly 14, Stan Roberts' Family Skiff, and Lee Morgans' Mayfly 16."



Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



Modifying Boats 2


One of the most common and useful changes to a hull is to make it longer without changing the beam or depth. Going longer clearly increases the load ability of the hull. And in theory it increases the "hull speed" of the boat which is related to the waves the hull makes. Below planing speeds at least, longer is faster. I suppose it might not always be true due to weights of the lengthenings but you could end up with a hull that has both more capactiy than the original, and more speed to boot with the same old engine! Bolger points out in one of his books that this doesn't apply really to boats that plane - in that case the weight is the deciding factor on speed and longer boats are heavier.


In olden days a set of boat plans would provide you with the "lines", dimensions of section lines or frames or forms and the spacing of those and the shapes of bows and sterns. Often little else was provided and not needed (or wanted) as there were plenty of experienced builders who all had their own way of building a boat. All they wanted was the basic shape - no real building details were needed. I think of it like house building where the basic house is drawn but details like stud placement are not because those will be "standard". Things like bevels would not be shown. Those might be determined in "lofting", which is drawing the lines to full size on the shop floor somehow, or maybe they could be determined by an experienced building more or less on the fly as the boat is being assembled.

So let's say our builder has drawings for a 12' boat which show the section lines every 24". But the customer wants the same thing in 16'. So when the builder lofts the drawings he simply increases the spacing of the sections from 24" to 32". The shape of the bow and stern remain the same. The boat is longer but not leaner or deeper. The builder then makes up his forms to match the section lines and installs them at 32" spacing by attaching them firmly to the shop floor somehow. Then he starts framing and planking although he has to figure out bevels as he goes. All hand fitting of course but with good wood, good tools, and a good experienced man (remember that builders made boats this way every day all their lives and would get very good at it - unlike us who might try this once or twice in a lifetime) it went quickly.

And it might get a lot quicker with added boats since the builder won't throw those basic forms made from the section lines away. When the next customer comes in and wants the same thing in 18', the forms quickly come out and are set up at 36" spacing and away we go - no need to loft again.


OK, so right away you begin to see the advantages of building an "instant" boat. Basically the designer has done all the lines and lofting for you and determined the shape of all the major parts that make up the hull. So when you starting laying out those parts on plywood full size you aren't really "lofting" like the old builders and don't really need that large space needed for full size lofting. When you have all the pieces drawn and cut out you can assemble them anywhere that is large enough - it doesn't need to be especially flat. Actually many instant boats are just assembled on two sawhorses in the back yard somewhere.

I think when I got the boat bug around 1980 what really put me off about traditional building was the need to attach the forms and such solidly to the concrete garage floor. Or the need of a sturdy stiff strongback and support structure. At the time I was thinking about a light dory and it was clear that the investment in lumber for the forms and strongback would exceed that of the boat's wood. That is fine if you are going to make lots of copies but that is not the case for the usual builder. So I discovered Payson's and Bolger's books just in time, although other designers like William Jackson had used "instant" construction before and indeed, the method was used by flat iron skiff builders for generations.

NOW, let's say you have 12' instant flat iron skiff plans in front of you and the sides are drawn with dimensions every 24" to give you the shape of the panel. And you really want a 16' boat. Remember that a boat that is 12' long overall will have side panels longer than 12' because it will take a curve on a way from bow to stern (usually, not true for Puddle Ducks and such with straight sides). Can you simply redraw the sides spacing the dimensions at a proportion of 16/12 to get the longer boat?

In real life I would say "yes", it will work. It is not exactly correct but in real life you won't tell any difference. This is especially true of a simple flat iron skiff since its bottom will not be precut to shape, the plywood is simply flopped atop the assembly and cut to shape.

Note that the shapes of the forms and bulkheads will not change but the edge bevels will as will the stem and stern pieces if they are raked. So you will have to figure as you go. I'm pretty certain this method has been done lots of times. And, by the way, if you are building with taped seams then you worry not about bevels.


Now let's say you have an set of instant boat plans for a 12' multichine hull and want it in 16'. Can you simply expand all the side, bilge and bottom panels by a factor of 16/12 and get it all to fit? Well I say, "Maybe but I don't really know." With the simple flattie it will work because only the sides have been stretched and the bottom simply cut to fit. The actual hull shape won't be a pure stretch of the 12' design but I think the shape error will be small and won't be noticeable. What happens is that if you stretch the dimension spacing at the boat's center by 8" the finished boat will get longer there by 8" because the sides are there parallel to the boat's length. But near the stern or stem, where the panels curve inward, the same 8" added spacing won't give you the same 8" added boat length. For example if the panel curves inward at 20 degrees at the stem (sort of typical) a 24" panel gives only 22.5" of boat length. The 16' boat curves are less severe than the 12's. So the 12's bow angle is 20 degrees and the 16's bow angle is 15 degrees. That panel stretched to 32" will give boat length of 30.9". Like this...

What you were thinking about was a hull length stretch there of 16/12 x 22.5" = 30". So it is there about .9" longer than expected because of the difference in bow angles (because we are just making it longer and not wider). There are twelve sets of side panel dimensions in this example but the above would be the worse case. I would expect a boat streched this way will end up a few inches longer than the expected 16'.

But....if you were to expand all the panels of a multichine this way there is another worry. The sides, bilges and bottom don't usually bend to the same curve and so the curve errors that happen on the sides, bilges and bottom might all be different and you may get significant gaps in the assembly.

Given the example of the sides, I would say that stretching a multichine this way will work with some but not all the time. But it could still be done if you work this way. Don't cut the bilges and bottom to line yet (always good advice even when you aren't stretching). Start the assembly as usual with sides attached to the forms and bulkheads, stem and transom. Now the shape is really defined and you need only fill in the holes left for bottom and sides. "Survey" the shape of the bottom either by making a pattern or just flop the oversized bottom panel into place and trim to fit and install. Then the same with oversized bilge panels. That is how I would do it. Make sure the initial assembly is straight and sturdy as you push and shove bottom and bilge panels into position. It's sort of like sculpting, more like old time building. By the way my Toto was "designed" this way since back then I didn't know how to do the twisted bilge panel expansions with a computer.


But myself I would not do that. The slam dunk easiest and best way to stretch a boat is to use a simple "plug" in the center. At the boat's widest beam the panels should all be parallel to each other. Just imagine cutting your 12' boat apart at the widest beam and inserting a straight 4' piece between, joining all three pieces together and off you go. The bevels of the bow and stern section have not changed at all. The extra length has been added to the boat's greatest cross section where it will give the greatest added buoyancy and capacity and passenger room.

I think large steel ships are routinely lengthened this way - cut them in two in the middle, spread the two ends apart as required, and weld in plates to rejoin. I am ignoring the huge amount of interior work needed and even the homebuilt boat may need some added bits to stiffen. I guess I forgot to mention that the longer boat will have greater stresses in the middle. I don't think that is much of an issue with the typical plywood homebuilt - I've never heard of one coming apart in the middle, but it would be an issue with stretching out a tanker which sometimes do break in half.

But, you say it won't be sleek. It will look slab sided, just like big ships are all slabs with pointy ends added. For some reason it is not as bad as you might expect. I've drawn a few boat with constant sections in the middle, just with the idea that someone will want to lengthen/shorten without much use of the moaning chair. I did it long ago when I stretched Toto...

with a 30" plug to get Larsboat...

To make serious room for a second paddler. The 30" straight plug just isn't noticable.

I hope to continue this next issue to cover in particular the nasty surprises you can get when you stretch a hull without regard to sailing rig.




Jonsboat is just a jonboat. But where I live that says a lot because most of the boats around here are jonboats and for a good reason. These things will float on dew if the motor is up. This one shows 640 pounds displacement with only 3" of draft. That should float the hull and a small motor and two men. The shape of the hull encourages fast speeds in smooth water and I'd say this one will plane with 10 hp at that weight, although "planing" is often in the eye of the beholder. I'd use a 9.9 hp motor on one of these myself to allow use on the many beautiful small lakes we have here that are wisely limited to 10 hp. The prototype was built by Greg Rinaca of Coldspring, Texas and his boat is shown above when first launched with a trolling motor. But here is another one finished about the same time by Chuck Leinweber of Harper, Texas:


In the photo of Chuck's boat you can see the wide open center that I prefer in my own personal boats. To keep the wide open boat structurally stiff I boxed in the bow, used a wide wale, and braced the aft corners.

I usually study the shapes of commercial welded aluminum jonboats. It's surprising to see the little touches the builders have worked into such a simple idea. I guess they make these things by the thousands and it is worth while to study the details. Anyway, Jonsboat is a plywood copy of a livery boat I saw turned upside down for the winter. What struck me about that hull was that its bottom was constant width from stem to stern even though the sides had flare and curvature. When I got home I figured out they did it and copied it. I don't know if it gives a superior shape in any way but the bottom of this boat is planked with two constant width sheets of plywood.


Greg Rinaca put a new 18 hp Nissan two cycle engine on his boat, Here is a photo of it:


The installation presented a few interesting thoughts. First I've been telling everyone to stick with 10 hp although it's well known that I'm a big chicken about these things. Greg reported no problems and a top speed of 26 mph. I think the Coast Guard would limit a hull like this to about 25 hp, the main factors being the length, width, flat bottom, and steering location. Second, if you look closely at the transom of Greg's boat you will see that he has built up the transom in the motor mount area about 2". When I designed Jonsboat I really didn't know much about motors except that there were short and long shaft motors. I thought the short ones needed 15" of transom depth and didn't really know about the long shafts. Jonsboat has a natural depth of about 17" so I left the transom on the drawing at 17" and did some hand waving in the drawing notes about scooping out or building up the transom to match the requirements of your motor.

I think the upshot of it all is that short shaft motors need 15" from the top of the mount to the bottom of the hull and long shaft motors need 20". There was a lot of discussion about where the "cavitation" plate, which is the small flat plate right above the propellor, should fall with respect to the hull. I asked some expert mechanics at a local boat dealer and they all swore on a stack of tech manuals that a high powered boat will not steer safely if the cavitation plate is below the bottom of the hull, the correct location being about 1/2" to 1" above the bottom. But Greg had the Nissan manual and it said the correct position is about 1" BELOW the bottom. Kilburn Adams has a new Yamaha and its manual says the same thing. So I guess small motors are different from big ones in that respect.

But it seems to be not all that critical, at least for the small motors. Greg ran his Jonsboat with the 18 hp Nissan with the original 17" transom for a while and measured the top speed as 26 mph. Then he raised the transom over 2" and got the same top speed!

There is nothing to building Jonsboat. There five sheets of plywood and I'm suggesting 1/2" for the bottom and 1/4" for everything else. It's all stuck together with glue and nails using no lofting or jigs. I always suggest glassing the chines for abrasion resistance but I've never glassed more than that on my own boats and haven't regretted it. The cost, mess, and added labor of glassing the hull that is out of the water is enormous. My pocketbook and patience won't stand it. Glassing the chines and bottom is a bit different because it won't show and fussy finishing is not required.

Plans for Jonsboat are $25.


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.

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/ ....







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