Introduction


Piece of Trivia for calculating the bouyancy of your hull -------
Each in³ of hull volume displacement provides 16.378 grams, or 0.578 oz., or 0.00361 lbs. of bouyancy. Another way of saying this is that it takes about 30 in³ of hull displacement to support each pound of weight.

Boat control primer - need Acrobat reader to view

The EE labs isy trying out a new and exciting twist on the robotics labs. For this reason, I have been playing around with simple and cheap ways of desigining and making boat hulls. Although one can simply buy boat models at hobby shops, a disadvantage is that they are expensive, designed for speed (insufficient buoyancy to carry extra batteries, microcontrollers etc.), and since they have cheap flexible molded plastic hulls, it is difficult to mount components. Also, if you drill too many holes in the hull, you must plug them up. After a while, the hull is no longer usable.

To this end, last weekend I put some thought into it. I think I have come up with a simple feasible method that allows you to design hulls to suit your needs, and to fabricate them rather easily. Once I made my templates, I was able to make each hull in about 15 minutes! The first hull design I have worked on is a catamaran. The catamaran provides a large stable platform of wood, ... easy to mount battery holders, microcontrollers etc. onto. My first thought was to carve hulls out of Balsa wood blocks, until I saw that a reasonably sized block of balsa was over $11 at the local hobby shop! Thus I opted for styrofoam, which is virtually free!

The hull blank was made by cutting up a 3/4" thick sheet of styrofoam (availible @ Builder's Square, or any lumber yard), and stacking (3) layers (using carpet tape - which is adhesive on both sides - to attach layers to form a thicker hull blank). Next I designed some templates that defined the shapes for the hull bottom and hull top. The patterns were generated by a "quick basic" program (see below) that allows you to design a wide variety of hull designs by specifying six simple "control points". A typical top hull is shown below.



The parameters that must be input are: L = Overall length of top of hull in inches. L₁ = Location, from bow (in inches) of the beam. B = Half/Width of beam (inches). S = Half/Width of stern (inches). Alpha = Bow angle, in degrees. Beta = Stern angle, in degrees. If you hit enter for each prompt, the program will take defaults that will design a "catamaran" hull. By varying these parameters, a wide variety of hulls can be designed.

Hull Design
Designing the hulls is relatively easy. You can choose from a fat stocky hull (good buoyancy/capable of carrying heavy loads), or narrow sleek hulls (fast). Since speed won't be a criteria for the robotic competitions, we're more interested in buoyancy, stability and maneuverability. With my technique, one designs the hull top and hull bottom. The quick basic program can then generate (in PostScript) a paper template for the hull designs. If you don't have access to a postscript compatible printer, you can optionally just have the quick basic program print out the list of "half-beam" widths, and lay out the templates yourself using a ruler and pencil. Wood templates are then cut out of 1/4" hardwood (such as 1/4" thick poplar boards, availible at Builder's Square). I placed my paper templates on top of the wood and used a hammer and nail to mark points into the wood along the circumfrence of the hull -- then later "connected the dots" using a ruler and a pencil. I then cut out the wood template using a coping saw. You then drill two bolt holes in the boards, along their center lines. Holes are also drilled in the styrofoam, and the styrofoam blank, top hull template and bottom hull template are joined via two bolts. One can then cut out the hull using a "hot wire cutters". At the time, I didn't have a hot wire cutters, so I used a sharp serrated "steak knife" to cut it out. More recently, I have fahioned a cheap hot-wire cutters (see below) out of a guitar string and a wood clamp (see below). This makes a much nicer, neater and fast job of things. Also shown in this picture are other useful modelling tools, such as a vise, keyhole saw, a coping saw etc..


Homemade Hot-wire cutters and other useful tools


Shown below are two "Acrobat Reader" pdf files of the actual templates I used for the catamaran hulls. Since my hulls were 14 inches long, I had to print the hull template out on two (8-1/2 x 11 inch) sheets of paper, and tape two sheets of paper together. (The quick basic program will automatically split to two separate template files if your specified hull length is greater than 9 inches.). Note that I used acrobat pdf files since they will print out in actual dimensions (whereas , e.g. a bitmap printed using PaintBrush, often gets rescaled on printout).

NOTE: YOU MUST HAVE ADOBE ACROBAT READER TO VIEW THESE .pdf FILES
boat.pdf - Front Top Hull Template
boat2.pdf - Rear Top Hull Template


To facilitate understanding, a short video will be made available to lab students which will show how easy it is to design and construct a catamaran boat.

Quick Basic Source Code for Boat Design

Note: The following DOS executable file is "zipped"(use WinZip or pkunzip to extract)
DOS Executable Code for Boat Design

Note: The following ".pdf" file requires acrobat reader to view:
Step-by-Step instructions for making styrofoam hulls - in Acrobat Reader *.pdf format



Obviously, the next big "mechanical" concern is propulsion and steering of the boat. Actually, in some ways this is probably easier than the robotic cars. The boats are lightweight and move thru the water with little effort, so small motors can be used. Elements such as rudders etc. can easily be fashioned from Balsa wood (a hobby knife and some sandpaper are all that are needed). Balso wood is cheap and easily glued, cut, formed by sanding etc.. Of course, any submerged balso parts need to be waterproofed by painting them with dope or epoxy etc.. What is neat is that propulsion can take many forms such as:

A prop and rudder
An "air" prop (like an airboat uses)
Paddle wheels (like the old Mississippi steamboats)!
Little toy water pumps and clear plastic tubing to make the equivalent of "inboard" engines.
Many more possibilities!

Steering can either be done using a dedicated rudder, steering the air fan, or by using separate motors, e.g. to (differentially) drive two opposing side paddlewheels (rudderless). In a catamaran design, it seems logical to use two motors(one for each hull) driven differentially for steering.
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