Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(15October 2015) This issue will show the making of a hull shape computer model. The 1 November issue should be about the Port Aransas, Texas, boat show.



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.


Tom David built a large flat bottomed skiff, not my design, and later added this bottom feature. He wrote, "I had some disturbing bottom flex, so I took it out and retrofitted a 16" wide by 1 3/4" inch keel to stiffen it. It stopped the flexing, but the big difference is in sound and pounding. Really made a significant change". He might try it on his brother"s future AF4.



Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



Hullforms Model

I WROTE THIS 16 YEARS AGO! I am not sure if the free Hullforms program I used is still around, except, I hope, in my older computers. But try this link, seems to work... Hullforms Download (archived copy). I'm using the oldest version in this issue, Hullforms 6S, if you care to follow along.


Here is a figure showing AF2 with my guess at the weights of the major components and a first shot at their locations. Last issue I found the first best guess weights and cg's for this boat for several different loading conditions. This issue will show how to make a computer model of the AF2 hull using the free Hullforms modeling program to enable us to get stability information using the different weights and cg"s next issue.



...because I'm not an expert at it. I've never really read the instructions which are in the zipfile that you download. I've decided to present what I know and how I do it because I've found it to be so useful. Hullforms 6s was written back before Windows became dominant. It runs in DOS and is very compact and effecient compared to more modern stuff. It has no whistles or bells but I've never noticed it trying to deceive or lie to me. It occurred to me that I'm breaking all sorts of laws by presenting this but I hope not. If so, explain it to me and I'll remove the essay.

Also I'll say that to use this program you really need to design the boat first. It's not the sort of thing where you drag lines around to get a pretty picture of a boat. What it is very good at is crunching the numbers for a design that you have already drawn.


...download the program, get into DOS, unzip it, and start the program "Hullform". Then select "File". If you select the "Open" entry you should find a bunch of sample boats put there by Peter Rye. Poke around there a bit to see what the program can do.

Next select "New" to start a new design. Next you will get a screen that allows you to input the basic dimensions of the boat. Here is that screen for the AF2:

I've replaced the default values with the numbers for the AF2 here. To explain, the hull dimensions I'm using are all in "feet" although you will find you can reconfigure the program for metric. Also I'm using a convention where the z axis is positive when down. You can reconfigure that also if you wish.

Anyway, I've input the length at 20', the breadth at 5', the bow height at 3' and the stern height at 2' and stern width of 3.3'. The program also asks for the rake of the bow. I've input that as .6' and that is how much the stem rakes aft from the top to bottom. You must also input the number of hull lines you will be using. For AF2 I've selected 3, one for the keel, for the chine, and for the deck/coaming. (Since Af2 is a true flat bottomed boat I think the program would also work with 2 lines, just the chine and the deck/coaming.) For the rest of the input values I've kept the default values. I set the value for the waterline at 0.0 but it can be changed very easily later in the program.

One note about the black blotches that appear on the screen. The program on my screen blanks out the item you have selected. The items blanked out in the above screen refer to the cross section being a hard chined boat, as opposed to a rounded cornered boat. And that the keel line will be used as a reference line, not the waterline.

Next select "view" and then "general orthogonal" and you will get a view of things so far. Like this:

Not exactly what we had in mind! But from here on we will correct and massage it to shape with the Edit function. So select Edit.

To begin with we must correct the stem. The convention for the program is to put the rake forward of the zero station and I always design my boats with the rake behind the zero station. In other words my boat is really 20' long while the Hullforms convention puts the .6' rake ahead of the zero station for a total length of 20.6'. It's easy to fix. Select Edit and then Edit a Station, then select Station 0 and stay with "graphical edit". You will see a graphical view of the side view of the stem. There are three points there that correspond to the stem values of the three hull lines. Drag the 1 point aft .6' so it is on the zero station. Then drag the 3 point which represents the keel line to the design value of .6' aft of zero, and up to a z value of -.5" (remebering that "up" is a negative value in this system). Then drag the 2 point to be right on top of the the 3 value. The stem now represents the actual design.

Everytime you change something on the model check the View to make sure it did what you intended. If it did then save it back under File. The saved files are quite small, maybe 5k.

If you check the View now you will see not much has changed, only the stem moved aft making the boat a bit shorter. Next we have to input the dimensions of our AF2 one station at a time (which is why you need to design the boat first to the point of having a table of offsets). Select Edit, then New Section. It will ask you where to place the new section (in feet in this sample) at the same time showing you where the old sections are. For AF2 the next section will be at 2' so that value is entered. The program pauses a bit with a notice to select "Edit a section" to input values for the new section. Select Edit a Section. It will ask you which section to edit and you will tell it to select the new section (now called Section 1) and you need to tell it also to use a "text" edit instead of a "graphic" edit. That done it will show you this screen:

Here I've already input the offset dimensions for the AF2 station 2. You can check the location of the station in the upper right and you can click on that value and change it if you wish, moving the station fore and aft on the model. I've input the half breadths (lateral offsets), in feet, for the three lines. The half breadths for the keel will always be 0.0 in this case since it is essentially the boat centerline. The heights (vertical offsets) are shown in the next column and are also dimensioned in feet and in this convention are negative since up is negative. Since this is a flat bottomed boat the z value for the keel is always the same as for the chine. Now the model looks like this:

View it and save it and move to the next section. Eventually you will come to the main cross section which the program in its wisdom has placed at X=9.1. In reality it is at 10.0. To set it right, select Edit, then Edit a Section and choose that section. Correct it's X dimention to the desired value and adjust the lateral and vertical offsets as you treat it now as a normal section. Here is what the model looks like at this point:

While you are at it you will have to correct the transom in the same way. I should say here that I know of no way to give a boat a raked transom in Hullforms.

Then continue with the rest of the sections until you are done. The model then looks like this:

As you were working on the model you will have noticed that the program has connected all your sections with short straight line segments. This is a great advantage of this program in that you can place sections anywhere and produce abrupt changes in the lines, something the free "fairing" programs won't do. So with this program you can model the effects of abrupt changes in decklines.

And for the AF2 studies the ability to model abrupt changes allows us to model in the effects of a flooded cockpit, something we'll check out next issue. At some point AF2 will heel to a point where its cockpit will flood, and yet the sealed cabin and stern locker still provide buoyancy. When that happens we can adjust the AF2 model like this:

What I've done here is add new sections at the ends of the open cockpit. (Actually the new sections are spaced .1' from the original ones so the program won't gripe.) Then I've adjusted the "coaming" line down to just above the bottom. Now when the program figures its hydrostatics it will think the volumes of the cockpit are quite small - it's flooded! I've saved this version as a separate file.


We'll use these models plus the cg and weights data from last issue to predict stability for AF2.




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.

We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...

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 to the point the builder can sit and relax in it and imagine boating. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....

The first Jukebox3 is on the (cold) water. The mast is a bit too short - always make your mast too long. A bit more testing will be nice...

A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) Double layer bottom on and glassed, hull returned to upright. He is decking it now...






1nov14, Chine Runners, Piccup Pram

15nov14, Lugsail Rigging, Caprice

1dec14, Sail Area Math, Ladybug

15dec14, Poly Laminates, Sportdory

1jan15, Sharpie Spritsail, OliveOyl

15jan15, Knockdown Recovery, Dockbox

1feb15, Mike Monies, Laguna

15feb15, Cartopping, IMB

1mar15, WeeVee Lessons, Vole

15mar15, Bulkhead Bevels, Frolic2

1apr15, Capsize Lessons, Riverrunner

15apr15, Hollow Spars, Slam Dink

1may15, Boat Costs, Blobster

15may15, Small Boat Rudders, Roar2

1jun15, Emergency Flotation, RB42

15jun15, Thailand Mixer Cruise, Mixer

1jul15, Rend Lake 2015, Musicbox3

15jul15, Box Boat Stability, Mikesboat

1aug15, Taped Joints, Cormorant

15aug15, Plywood Butt Joints, Paulsboat

1sep15, Navigator Cabins, Vireo

15sep15, Boxboat Stability 2, Philsboat

1oct15, Center of Gravity, Larsboat


Mother of All Boat Links

Cheap Pages

Duckworks Magazine

The Boatbuilding Community

Kilburn's Power Skiff

Bruce Builds Roar

Dave Carnell

Rich builds AF2

JB Builds AF4

JB Builds Sportdory

Hullform Download

Puddle Duck Website

Brian builds Roar2

Herb builds AF3

Herb builds RB42

Barry Builds Toto

Table of Contents