By Christopher Murman
During my years as a professional shipwright, I have seen many amateur boat builders – as well as some professionals – cut corners and skimp during the hull construction phase. However, when the vessel nears completion, the owner would invariably spend large sums of money on electronic gadgets and expensive interior fittings. To my way of thinking this is the wrong way around, as there is no substitute for a stoutly constructed hull. It is in the hull construction phase that no expense should be spared. The addition of electronic instruments and interior fittings can always be postponed until finances permit.
Each construction method has its own advantages and disadvantages, some of which are not always obvious. There is no perfect construction material. Some materials and methods of construction are better suited for a particular type of vessel, as we’ll discuss.
Boat building began with timber and over the years many different ways of utilising this most versatile of materials have been developed. Many believe that a timber boat automatically means higher maintenance, but this isn’t necessarily so. A very well constructed timber vessel will last many years with minimal maintenance. However, this can only be achieved if the builder thinks through all of the issues before and during the construction process. Once the vessel has been completed, it is often far too late to fix any inherent maintenance issues. The most common methods of timber construction are outlined here.
Large transverse frames and other structural members (either fabricated or laminated) support heavy longitudinal planks. A caulking material – either cotton or oakum – is driven into the seams to ensure water-tightness. Plank and hull fastenings are usually made of copper or bronze.
This traditional method of construction tends to ‘move’ or ‘work’ with the surrounding environment, the main causes being temperature, moisture, stress from heavy seas and, in the case of sailing vessels, the loads imposed while under sail. Thus carvel-built vessels should be finished with traditional oil-based paints to accommodate this movement. While oil-based paints can produce a fine finish, they tend to only last for a year or two, so maintenance can be higher than other forms of construction.
This form of construction is best suited to heavier displacement vessels, where the additional weight is of less consequence.
• Damage to the structure is easy to identify.
• Maintenance is frequently higher.
Double Diagonal or Multi-diagonal Construction
Planks between 150mm to 230mm in width and up to approximately 9.5mm thickness (depending on the length of the vessel) are laid at 45 degrees to the centreline. The subsequent layer is laid at 90 degrees to the previous layer. Sometimes a fore and aft layer is added to the outside. Each layer of planking is glued and fastened to the previous layer. The hull planking is supported by light, closely-spaced, fore and aft stringers and widely-spaced laminated frames, all of which are fastened into a heavy laminated backbone.
The completed hull should be fibreglassed using an epoxy system to seal the exterior. The internal structure should be saturated with an epoxy-based timber preservative to completely seal the timber from any water penetration. The completed hull will be very rigid and should be finished with a two-pot polyurethane or epoxy paint.
• Planks can be steamed to fit complex shapes.
• Can be difficult to repair.
Heavy laminated transverse frames are used to support a double layer of heavy fore and aft planking. The second layer of planking is positioned so that the edge of each plank lands in the middle of the first layer of planking, thus covering the join. The outer layer is glued to the first layer and screwed into the frames. The edges of the outer planking are cut with a ‘V’ shape to allow wooden splines (long strips of wood) to be glued into the seams.
The completed hull is fibreglassed using an epoxy system. The internal structure should also be saturated with an epoxy-based timber preservative to completely seal the timber from any water penetration. The completed hull is very rigid and should be finished using a two-pot polyurethane or epoxy paint.
Sometimes described as a modernisation of the traditional carvel method, this approach is best suited to heavier displacement vessels where the additional weight is of less consequence.
• Produces a strong hull.
• The hull structure is heavy.
GRP (Glass Reinforced Plastics) and FRP (Fibre Reinforced Plastics) construction methods have developed into a wide array of variations. Each has its own characteristics and applications.
GRP construction initially started using polyester resins and a small selection of fibreglass cloths, resulting in what is referred to as solid construction (ie: Fibreglass and resin, with no core). Resins have been subject to rapid development and can now be classed into three main types: Polyester; vinyl-ester and epoxy. Each of these also has special blends for specific purposes. Correspondingly, cloths have also developed into a wide selection, with the inclusion of some more exotic materials (eg: Carbon fibre and Kevlar), each with their own specific applications. FRP refers to carbon fibre and Kevlar reinforced construction.
Initially, core materials for composite construction (sometimes called sandwich construction) started with end grain balsa, but core materials now also include a wide variety of closed-cell foam materials and honeycomb cores, each with specific properties and applications. End grain balsa is still being used and is still one of the best composite core materials. Solid construction is still widely used, however, more and more boat builders are utilising composite construction methods because of their inherent stiffness and reduced weight.
Fibreglass moulding methods use either a male or a female mould. A female mould is the reverse shape of the required hull or deck shape. Production vessels typically use female moulding techniques. The finished vessel typically has a shiny white external surface straight out of the mould. The surface should be polished at least once a year to retain a protective coating. A male mould is the same shape as the finished hull or deck, but slightly smaller in size. After a hull or deck has been laid up over the male mould, the outer surface must be sanded smooth before being coated with a two-pot polyurethane or epoxy paint.
The latest developments in FRP construction are reducing the labour content, whilst also improving the structural properties of the completed laminate. This facilitates lower construction costs and higher quality finished products. These developments also improve the working environment during construction, as well as releasing fewer toxins into the environment.
• Can result in a very light hull and deck.
This construction method is the strongest, however the completed hull tends to be heavy, especially when compared to other construction materials. Then again, the additional weight is of less importance in heavy displacement vessels.
Different types of steel are available and some are remarkably resistant to rust, such as Austen. Austen is also stronger than mild steel, thus lighter components can be employed with a corresponding weight saving, although Austen is harder to work.
The completed hull should be finished with the epoxy-based painting systems that have been specifically developed for steel vessels. Commercial pressures have driven the development of these advanced paints and pleasure boat owners can now enjoy the great advantage they offer.
The completed hull tends to be very noisy and temperature-sensitive. Insulation material can be employed to reduce noise and increase comfort below deck. However, it is very important to ensure the insulation material is installed correctly in order to prevent moisture being trapped, otherwise rusting and other forms of deterioration are likely to occur.
• Produces a very strong hull.
• Difficult to develop into complex hull shapes.
Aluminium construction has many of the characteristics of steel construction, but results in a lighter hull. However, in practice, aluminium hulls are more susceptible to damage. It is paramount that the correct grade of aluminium plate is selected, as the marine environment can be very hostile to aluminium boats.
Aluminium hulls also tend to be very noisy and cold or hot (depending on the local environment), thus hull insulation is recommended. As with steel hulls, the correct installation of the insulation is paramount.
• Produces a strong, light hull.
• Very susceptible to attack from electrolysis, galvanic reaction
and stray currents.
It is well worth the time and effort to look carefully into the type of material and method employed to build your boat, as it could well mean the difference between enjoying time spent with your pride and joy or getting your feet – and everything else a bit wet.