Boat Cannon Props

Our kids are in marching band, and this year the band director wanted a boat prop to move across the field during the performance.  I volunteered to make some of the boat accessories to spruce up the overall appearance.  In this instructable I show how I made a line of cannons for the side of the ship.

The basic structure consists of a central cardboard tube, a laser cut cardboard frame, and a cloth mache outer layer.

I made this at TechShop  www.techshop.ws

Vector drawing software (I used CorelDRAW)
Laser Cutter (my TechShop has a Trotec Speedy 300)
18" long 4" diameter cardboard tube (varies with size of cannon)
16"x16" corrugated cardboard (varies with size of cannon)
Hot glue gun
Expanding foam (such as GreatStuff)
Masking tape
Thin cloth (such as an old bedsheet)
Glue (white or wood)
Black paint
Spray polyurethane (for limited outdoor use)

This instructable assumes some basic familiarity with CorelDRAW and a laser cutter.

There are approximately 7.8 gazillion cannon profiles, so I sketched out a few options.  I started with a scale drawing of the central cardboard tube and drew profiles until I thought I had a good basic shape for our ship.  Then I made some basic measurements so that I could reproduce it in CorelDRAW.

Start with the two straight lines (back of the cannon and the edge that mates with the tube).  Draw the rough profile using the B-Spline tool.  Then use the Shape tool to fine tune the profile by directly editing each point on the curve to reasonably match the dimensions of the hand drawing.  Eight of these pieces will be placed around the circumference of the tube to make the basic shape.

The profile pieces will be aligned and held together by three rings placed over the tube, so add three rectangular cut-outs distributed along the length of the profile.  Make them approximately half the height of the profile at each location.  Size the width of the cut-out to match the thickness of your cardboard (mine was 0.15" thick).

To both make the cutting process a little faster and the cardboard use a little more efficient, make seven copies of the original profile and pack them in close to each other.  This arrangement fits on one side of a medium U-Haul box.

Using the Ellipse tool make three rings.  The inner diameter of each should be slightly larger than the cardboard tube.  The outer diameter of each should be slightly smaller than the height of the profile at each of the corresponding cut-outs in the profile piece.

Add eight equally distributed cut-outs to each of the rings along the outer circumference.  These should match the width of the cardboard that you are using.  The depth should be equal to the width of the ring minus the height of the corresponding cut-out in the profile piece.

Cardboard cuts very quickly and easily on the laser cutter.  I found that 100% power and 3% speed worked well.

Slide the three rings over the tube and to their approximate location.  Use a profile piece to get them into their final orientation (both location on the tube and correct rotation).  Add the remaining seven profile pieces to complete the frame.  Hot glue a handful of the joints to keep the pieces from shifting around during the next stages of assembly.

To make the ends of the cannons more robust (since they are most apt to get bumped and knocked around), spray expanding foam into the end cavities of the frame.  Make sure you use enough to extend past the profile pieces.  Once fully hardened, cut or sand off the excess foam to match the rounded end of the cannon.

Next wrap the frame in masking tape.  This will help support the cloth mache as it dries (which would normally droop quite a bit if not well supported.

Cut the sheet into smaller strips (smaller sizes generally work better for sharper curves, larger pieces for long sections).  Pour the glue into a bowl, then add a small amount of water to thin it out a bit.  If you don't thin the glue it won't easily soak into the strips of cloth.  I don't precisely measure the ratio, I just add a little water and stir it well until the glue feels a little more workable.  You definitely don't want the mixture to approach that of water or the cloth won't dry rigid enough.

Dip a strip of cloth into the glue then run it through your fingers to wipe off the excess glue from the cloth.  The cloth should be soaked through with the glue.  Apply it to the taped frame making sure all of the edges are pressed down.  Add another piece making sure you overlap the first piece.  Keep adding pieces until the entire frame is covered.  Set aside and let dry.

Once the cannon has dried, trim any sharp cloth edges from the back edge of the cannon.  I chose to paint the cannon a plain black for a traditional look.  I didn't worry about adding any detailed shading (perhaps a sponged dark gray for effect) because these would not be seen up close in the show.  Once the black paint dries, use a spray based polyurethane to seal the cannons.  While this won't do a great job protecting them from a downpour, the band only plays in light rain or mist so all that is required is some basic protection.

We needed eight cannons for our ship, so I actually worked on 10 or so in parallel (in case we needed spares).  Here you can see them in various stages of construction.  While the laser cut cardboard approach may seem like overkill, it made it really easy to make the cannons uniform in size (which was important when making so many of them).  Happy sailing!