The Grainger MTB920 is a composite round bilge light weight racer cruiser design, and in my view he is one of the best multihull designers on the planet. I had been a mono hull sailor since in young boy, but later had the desire for both windward performance plus extreme speeds constantly, that can only be achieved by a mono hull for a few seconds as the hull planes down a swell or wave. Sure monohulls can reach incredible speeds, but not at the same numbers constantly achieved with a multihull on reasonably flat waters.
Well designed modern trimaran achieve both windward performance and extreme speed, so a trimaran was my choice for this project.
I decided to build the trimaran using duracore as opposed to foam core or western red cedar strip plank construction and the following article is my reason for choosing Duracore over a couple of other core materials and I sincerely hope I have explained in simple layman’s language the theory behind my decision. A jig saw, skill saw and battery powered screw driver were the main tools used to construct the hull core. Let’s get into the core material. Foam cores require a more complex mould to support the foam, and some foam does not offer anywhere the same strength and durability as wood fibres cores do. The foam cores that are good are extremely expensive and out of reach of most amateur yacht builders. I would have liked to built a boat using a foam core, but was not prepared to spend the extra hours and expense in building a more complex mould require to support the foam layup, plus the added expense of a good high quality foam.
Western red cedar appealed to me as it is affordable and provides the amateur builder a method to produce a strong strip planked round bilge hull with a simple mould, male or female and very simple tools. Fibre glass is then laid up over the planked up hull inside and out with the glass fibres orientated to the designers plans. I was very close to selecting this method of construction.
Duracore is a composite in itself. The material I used was made of Balsa which is 10mm thick end grain balsa bonded in between two thin 1.5mm veneers of good quality marine hardwood. The end grain balsa provides immense impact resistance, as the grain runs across the thickness direction of the plank. Imagine placing a finger each side of the plank with the hard wood veneers removed and the ends of the balsa fibres are pricking into the ends of your fingers, because this is the direction that the end grains of the balsa are laid up and bonded in between the hardwood veneers. This construction provides very high impact resistance, while being a very light material for its strength. The hardwood veneer fibres run the length of the plank, and therefore the length of the hull.
These fibres are also very strong in both tensile and compressive strength and unlike synthetic fibres they are the most durable fibres and will not fatigue. In my opinion foams do not provide anywhere near the toughness, fatigue, tensile and compressive properties that Duracore. Some foam core hulls have resulted in the foam shearing away from the glass laminates as the high load stresses transferred from say the outer skin through the foam core to the inner skin have resulted in the foam failing to hold the skins together in position, like a web holds the flanges of a steel I beam.
Imagine steel beam supporting the load of a heavy concrete tile house roof. The top flange is trying to compress or grow shorter as the beam is resisting bending and the bottom flange is under tension and trying to resist stretching under tension. Now the web which is the part in between the top flange and the bottom flange is like the core material in the hull of our yacht. It its job is to make sure that the top flange and the bottom flange stay the same distance from each other, as if it cannot and the distance starts to get smaller then the resulting loads that the top and bottom flanges will increase, the web buckles more and so on until either a total failure of the webs or the flanges occur. In our case the core or the laminates fail. Foam as the web holding the fibre glass top and bottom flanges or outer and inner skins together is more flexible and with moderate compression resistance allows the skins can move around in relationship to each other, and it is this action that will cause some foam products to fail, turning to powder where it was bonded to the skins.
Duracore uses hardwood veneers as the top flange and bottom of a beam, and the balsa forms the web holding the hardwood skins apart and in position much better due to Balsa’s superior compressive properties. Now with a specific designed layup of glass, or Kevlar fibres etc inside and out you have an extremely very strong hull, and it is one that the amateur builder is able to produce with basic tools and mould techniques. One major benefit as I learnt through experience, is if water should get into the core through a leaking fitting, then the moisture follows the grain of the balsa, only 10mm of grain meaning that only a very small area will at worst suffer from rot. This can be easily repaired, by removing the damaged area and restoring to new. There is a great method of fitting fixtures to hulls that prevents moisture getting into the core or the boat which I will cover in more detail in a later article.