Plastic Fantastic  

by Rod Cantrill - Brisbane, Australia

“Dad! Dad! Dad! Can we build a boat?”, my three sons, aged 4, 7 & 10, asked me.

“Why not?”, I replied, “all we need to do is cut this bit of foam to the right shape and then we can stick a skewer in it for a mast. Then we can ……”.

“No, Dad! We want a big boat to ride in!!"

Stunned silence from me as I digested this.

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Dad! We want a big boat to ride in!!

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“What the ....? A real boat? How am I going to make a real boat? How much is that going to cost?”, were the first thoughts that ran through my mind.

The following is a brief summary of how I constructed an inexpensive boat using plastic sheet, in particular the methods for working with the sheet. Total cost of this boat was AUD$25, however I got the sheet for free. I would expect that you could purchase enough sheet to build a boat of similar size for an additional AUD$80.

Design

Gregg Carlson’s hull design software was invaluable in determining the waterline of the hull with a load, and to create the undeveloped panel shapes of the hull.

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A screen shot of the hulls file. (click HERE to download the file)

I used the co-ordinate tables that Gregg’s program can generate to mark out the perimeter points for each panel, and then used a thin strip of timber to create a ‘curve of best fit’ between these points.

At this point I was not putting too much thought into the design, nor did I take a lot of care when I was cutting out the panels, as I thought that the material I was going to use would not be successful …… in hindsight I wish I had!

Material

I decided to experiment fabricating a hull using ABS (Acrylonitrile Butadiene Styrene) sheet. The main reason for this decision was that the company I work for manufactures the sheet in 2.4mm thickness for use in our finished products, and I was able to obtain sheets that had been rejected for minor surface blemishes at no cost.

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I was able to obtain sheets that had been rejected for minor surface blemishes at no cost.

I had used the sheet on many previous occasions to prototype components as it is extremely easy to cut & shape, and fast, strong bonds can be made using acetone to chemically weld the sheet together.

ABS is a plastic that has very good impact resistance, excellent resistance to water & salts, and has good gloss, all of which mean that the completed hull does not require painting.

ABS does not have good resistance to UV light, so if the hull is going to be stored outside for extended periods it would be advisable to give it a coat of paint (use a plastic primer first) to protect it from the sunlight.

The density of ABS is approx 1.05grams per cubic centimetre, so it is denser than plywood and will not float, however this is offset by the fact that you can use thinner gauges and still keep the hull reasonably light.

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ABS is a plastic that has very good impact resistance, excellent resistance to water & salts, and has good gloss.

The ABS sheet is not very stiff… if you pick up a 4’x8’x1/8” sheet by the middle, the ends will be just about hanging vertically. This is advantageous when twisting hull panels into shape, however I was dubious about the rigidity of a finished hull. This proved not to be a problem when the panels of the hull were bonded, as the curvature in the panels created integral stiffness in the hull.

Sources of ABS sheet can be located by looking up ‘Thermoformers’ (also known as Vacuum Formers) or ‘Plastic Extruders’ in the Yellow Pages. With a bit of haggling you could most likely get some sheet for a reasonable price. If they have some sheet lying around that they have rejected for minor surface blemishes, then you could convince them to sell it to you at their material cost price. They would otherwise have to granulate it to re-use it (at extra cost to them) or sell it to a recycling company for a fraction of what they paid for it.

Based on material-only-cost you should be able to get a 2400mm x 1200mm x 3mm sheet for $20 (Australian dollars). For comparison, a 2400mmx 1200mm x 4mm marine ply sheet costs around AUD$77.

Cutting

The sheet does not have to be cut with a saw. In fact the use of a high speed power saw, such as a jigsaw, tends to generate too much frictional heat resulting in the molten material in the kerf joining back together behind the blade. By the time you get to the end of your cut, the two halves have welded themselves back together!!!

The simplest way to cut the sheet is to firmly score the sheet with a sharp hobby knife and then snap out the shape by bending along the scored line. This method is extremely quick, an 8 foot long panel for the boat I built typically took 5 to 10 minutes to score and snap.

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The simplest way to cut the sheet is to firmly score the sheet with a sharp hobby knife and then snap out the shape by bending along the scored line.

The edge of the knife blade can be dragged along the cut edge to remove any burrs.

An open style rasp like the ones used to shape car-body filler, is useful to perform any cleaning up of the edges and also to shape thicker components that you have created when laminating sheets together.

Bonding

Acetone is ideal to bond ABS to itself. An eyedropper (preferably made from polypropylene which is not affected by acetone) is used to drip acetone onto the join. The acetone wicks into the joint and partially dissolves the ABS that it comes into contact with. The dissolved ABS will bond firmly together as the acetone evaporates.

Fillet style welds can be created by dissolving small offcuts of ABS sheet in a sealed glass jar (make sure the jar has a metal lid) of acetone overnight. The resulting viscous liquid can be applied to an internal joint to create a fillet. The acetone component in the liquid partially dissolves the parent material, bonding it to the fillet, and then evaporates away over the next 24 hours (Fillets are generally ‘touch dry’ and have some integrity within an hour or so).

Laminating can be used to create thicker panels for items like rudders. When laminating it is preferable to cut the shapes from the sheet before laminating them together, as a 10 or 12mm thick laminated piece of ABS is difficult to cut.

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Laminating can be used to create thicker panels for items like rudders.

When your quantity of cut shapes are ready, use the eyedropper to cover one side of a shape with acetone. Place the next shape on top and repeat the process until the desired thickness is achieved. A couple of house bricks atop the whole thing ensures that you don’t get any small gaps between the laminations.

Construction

The cut hull panels were assembled using a similar method to ‘stitch & glue’, except in this case I used plastic packaging tape to hold the panels together. A few areas, especially around the bow where the curves are tight, were stitched with copper wire as the tape did not have sufficient adhesion where the bending forces were high.

The inside of the joints were filleted as described earlier, and a 20mm wide strip of thinner gauge ABS (1.1mm in this case) was bonded to the outside of the joint to improve the joint strength. The strips were held in place with tape and acetone was wicked into the joint to bond the strip to the hull. Once this was secure the tape was removed and thinned-down filleting liquid was applied with an eye dropper to the edge on the strips to seal them and improve their adhesion to the hull.

At this point the hull was still an empty shell, however it was already exhibiting a high degree of rigidity. When held at each end and a twisting force applied, the hull did not bend or warp. The hull was placed in a mate’s swimming pool and was of sufficient strength to support my weight (a not inconsiderable 100kg).

From here on I had no further plans as I did not expect the hull to be as strong as it was, so I made the rest up as I went along.

I added a frame near the bow, and ribs along the sides and transom to increase strength and to assist in creating watertight compartments.

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I added a frame near the bow, and ribs along the sides and transom to increase strength and to assist in creating watertight compartments.

As this boat was for my kids, I decided to make the centreboard deploy by swinging down and forwards. This means that if a submerged object is struck, or they forget to raise the board when beaching, the board will simply swing up out of the way. A centreboard box was created by lamination and bonded to the hull. The hull was cut from underneath to expose the inside of the centreboard box. The centreboard was also made by lamination.

Ribs were added to the bottom of the hull to allow a floor panel to be bonded over the top of them. The corners of each rib plate had a 5mm x 5mm chamfer to allow any water that may find its way inside to flow to the keel area, and then flow back to the transom where a bung is fitted.

Expanded polystyrene foam blocks were cut to size and fitted into the spaces between the ribs to create about 40 litres of positive buoyancy. The bottom edges of the foam were also chamfered to allow water to pass.

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Expanded polystyrene foam blocks were cut to size and fitted into the spaces between the ribs.

A bead of filleting liquid was applied to the top of each rib, and then the floor panels place on top of the ribs. Weights were placed on the floor to ensure a good joint.

The two square holes beside the centreboard box allow access to the centreboard pivot pin

The frame near the bow was sealed off with a piece of sheet to make a buoyancy tank. The ribs on the sides and transom were also panelled over to create further buoyancy tanks.

Rudder cheeks and the rudder blade were made by lamination. The rudder pivot blocks on the hull were also made by lamination and then a strip of sheet was heated with a cigarette lighter, bent to shape, and bonded over the pivot blocks to improve their adhesion to the hull. The rudder blade pivots up and backwards for the same reasons described regarding the centreboard.

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This pulley was attached to the top of the rudder cheeks for the sheet.

All the pivots for rudder and centreboard are made from pieces of 4mm stainless rod.

Another strip of sheet was heated, bent, drilled and bonded to the bow to create an eye for attaching a 40 foot ‘leash’ so that the kids can learn to sail without inadvertently ending up in the Pacific Ocean.

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An eye was made so that the kids can learn to sail without inadvertently ending up in the Pacific Ocean.

The mast and boom were a donation from a mate. The mast a telescoping windsurfer mast made from aluminium tube. This works out well when used with the windsurfer boom as I can locate the boom high enough on the mast so that it doesn’t hit my kids on the head, but the sail can still extend down between the two legs of the boom.

The mast is held in place by a block in the bottom of the hull, and another on the underside of the deck. Both of these were made by cutting out 60mm square pieces of sheet and using a holesaw to cut a suitable diameter hole in the centre of each piece. When using the hole saw I had water trickling onto the workpiece to prevent the saw melting into the ABS sheet. These pieces were laminated together to form the blocks. The mast simply slips into the blocks and does not use any stays.

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The mast is held in place by a block in the bottom of the hull, and another on the underside of the deck.

Ribs were added in the area of the blocks to increase the strength in this area.

The sail was fabricated from polytarp, and is hauled in via a pulley, which I also fabricated from ABS, which is tied to the top of the rudder cheeks.

The pulley wheel was made by holesawing several pieces of sheet, laminating the resulting discs together, putting a bolt through the central hole and tightening a nut onto the other end. When this was dry, the end of the bolt was secured in a drill chuck, the drill was clamped to my workbench and powered up, and then judicious application of a ½” round file created the groove in the pulley wheel ….. sort of a poor man’s lathe.

The pulley body was made by lamination.

The two photos below were taken on our first test. On the day there was only a light breeze of maybe 4 or 5 km/h, however this was still enough to push this little boat around the lake. The weight of the hull without mast and sail is around 28kg.

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When the boat was first tested the centreboard would not drop down under gravity. A pull cord (I used 4mm lawnmower starter cord) was added which runs down through the top of the centreboard box and attaches tangentially to the top of the centreboard. Pulling this cord lowers the centreboard.

Addendum:

Chuck, final three photos.

First one shows the ribbing I added around the centreboard box. What I'm trying to do here is put a floor over the top of the centreboard box to allow the operator to sit in this area comfortably.

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The ribbing I added around the centreboard box

Second photo shows the application of the ABS/Acetone using an eyedropper to create a "fillet weld" on the new floor ribs.

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The application of the ABS/Acetone using an eyedropper to create a "fillet weld" on the new floor ribs.

Third photo shows the new floor in place. Towards the bow you can see the paddle made from a telescoping mop handle with a laminated ABS blade. This stows in the small area in front of the mast keel step (in the first photo you can see the handle in its stowed location). The cord protruding from the top of the centreboard box is the pullcord that lowers the centreboard.

The new floor in place.

Well, thats about it! Probably the only thing left to do is put a dart in the trailing edge of sail as it seems to spill a lot of wind.

I've already started on a hull design for a 14' ABS hull to use on Moreton Bay. Hopefully I can put the lessons I have learnt on to good use on the new hull.

Thanks for 'listening' to me and giving me the opportunity to share this with others.

More Resources:

SAILS

EPOXY

GEAR