Organic Carbon Moth Hulls

By Phil Stevenson, 27/3/2000

Carbon occurs in nature mostly as soft coal, which is useless for boat building and needs an expensive process to create high modulus fibres. But before it was coal it was vegetation and wood. Fortunately we can still get wood at reasonable prices, and with the right attitude and a few tricks we can create some light weight hulls which go well and cost just a few hundred dollars.

Plywood - the wonder Carbon Fibre composite:
Nature has bonded chains of hydrocarbon and cellulose to form a generally unidirectional (ie grain) composite of remarkable strength, we know this as wood. When some clever people cut this stuff into thin slices and glue three layers together with the grain crossed we get plywood. Then they offer to sell it to us at prices so cheap we can experiment all winter with new shapes if we need to.

The Good points of Plywood: It is cheap. It can be worked, cut and sanded with hand tools It can be bent into some very complex shapes. It can be easily glued and strengthened with epoxy. It can be easily finished to look great. It is light.

The Disadvantages: It is not as stiff as carbon/foam So not as good in flat panels It will soak up water if not sealed well The paint needs to be maintained to keep the ply dry

Overcoming these disadvantages:
To compensate for the lack of stiffness the plywood hull can be strengthened in several ways:

1. Make it thicker - Heavy
2. Put in stringers and frames - Heavy, tedious to do and put hard lines in the outside skin.
3. Add a carbon or kevlar skin inside - Good but doubles the cost of the plywood
4. Add a carbon skin on the outside - Good but doubles the cost and you have to sand it smooth to get a finish.
5. Design a curved shape - Better than flat sections, but still not stiff enough.
6. Install styrene foam frames at regular intervals -Provides remarkable stiffness at very little cost and not much effort.
7. Give it two good coats of epoxy inside and out - Virtually eliminates water damage except for serious damage.

How Does it Work The Stevo clan moth factory has just completed our third hull in 15 months.

Number 1 (Splinter) was an Ian Ward type ply box with rounded glass chines (via 75mm drain pipe). Ian specifies stringers and frames inside a 2mm ply shell. Expecting the problems identified above we chose to use 3mm ply with foam frames. This was a bad move as the 3mm ply was too heavy. The boat is very strong and survived the serial capsize stage of our narrow moth sailing. We learnt to use thinner ply and lots of foam.

Number 2 (Anachronism) is as skinny as anyone approaching 50 years should sensibly try to sail. It looks narrow enough to scare Laser sailors, but with about 500mm waterline is very stable by moth standards. The entire hull and foredeck is wrapped from one piece of ply, comprising a whole and two half sheets spliced together.

There are three darts in the sheet, joined by carbon tapes inside and out. The first starts just below the mast step and runs down the keel to the bow, up the bow, and back along the centre of the foredeck to the mast post. The transverse sections forward of the mast are egg shaped.

The other two darts form chines from the centre of the V shaped transom, forward a little over a metre. At the front they are 600mm apart, measured around the ply. At the front of the chines the hull section is almost semicircular.

Forward of the chines up to the mast, the ply runs gunwale to gunwale with no joins.

It is possible to build this without any jig. You need to know the exact dart cuts (I have some data or fiddle around and work them out). Tape the seams together with masking tape before applying the internal carbon tapes. It also helps to have the gunwales spread out wide while you apply the external tapes. This helps to hold the hull in a round rather than Veed shape when the gunwales are pulled into the correct beam of about 500 - 600.

The additional structure inside the hull comprises:
Two gunwales 20mm x 32mm WRCedar,
The fin case, and associated supports,
A ply frame at the back of the foredeck,
Something to secure the wings and forestay fitting to,
The transom with a piece of timber for the rudder fitting,
AND lots of 75mm thick polystyrene foam frames at 400mm spacing.

The cockpit / deck is also 2mm ply but is strengthened with a layer of kevlar on the underside.

This boat now has had a full season of weekend racing without damage and is remarkably stiff. It is also quite quick when I sail it well enough. The painted hull weighs 13kg.

Number three hull is a replacement for Splinter. Andrew being the younger, more nimble Stevo, it is fashionably narrow. It is built much the same as Number 2 except that the chines are longer and are only 300mm apart at the front. This results in the back looking a bit Hungry Tigerish, while the front half is still rounded. It looks very sleek. This one weighs about 12kg because we are learning where to leave things out.

We will see how it goes at Marmong.

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