The two metals used for hull construction are steel and aluminium. These are both very strong materials and will last a long time as long as they are cared for, which primarily means protecting steel boats from rust and aluminium boats from electrolytic action.
Metal boats have a long history dating back 200 years and of course most modern ships are made from steel. It is only in comparatively recent times that steel and aluminium have been used in leisure boat construction. Steel and aluminium are both strong materials and are popular with blue water cruising boats designed for living aboard. Each material has its advantages, but as a rule of thumb aluminium will be favoured by racing skippers as it is much lighter. Steel on the other hand may be heavier but it is a cheaper material, so for those not so concerned about top speed performance, this may be their choice.
Both steel and aluminium boats can be built by competent amateurs. This means that someone with good welding skills can save a lot of cost by buying a boat in kit form and constructing it themselves.
Steel hull construction
Steel hulls are constructed of a frame with steel plates welded to it. As soon as steel is exposed to the air it begins to rust, where the steel reacts with oxygen to form iron oxide. If seawater is added into the mix then the process accelerates rapidly. Steel boats built before the introduction of epoxy coating systems in the 1980s are more prone to rust and their hulls need to be closely monitored and re-painted frequently. Those with epoxy paint systems tend to be far more resilient, as long as the paintwork is not damaged. If it is, rust will very soon begin to appear.
Aluminium hull construction
Aluminium hulls are constructed in a similar way to steel hulls, but the two metals are quite different from each other. If you leave aluminium exposed to the air it will not rust, which gives it a big advantage over steel. However, aluminium suffers from electrolysis, which is the form of corrosion caused by different metals coming into contact with each other. The tell-tale sign, the equivalent to rust with steel, is a white powder which literally crumbles the aluminium away as you touch it. The two main preventative measures taken by builders of aluminium boats are firstly to ensure that different metals, such as aluminium and stainless steel, are very well insulated from each other and secondly through the use of sacrificial anodes.
If you are interested in purchasing a metal hulled boat or are already an owner of one, then the metal hull inspection checklist below will hopefully be of use to you.
Metal hull inspection
- Check for any distortion in the hull that might have been caused by an impact or collision.
- Check the outside of steel hulls for any signs of rust. If the rust is only on the surface it can be rubbed down and treated. If the surface is pitted it indicates a deeper level of corrosion and this will need to be taken back to the bare steel and treated.
- Check for corrosion around the stern gear and through hull fittings, which could be caused by electrolytic action. Pay particular attention to where the heads through hulls are located as this is a known problem area.
- Check that the paintwork is in good condition and the surfaces are all sound. Look for any signs of the paint bubbling which could indicate rust is about to break through.
- Check all the anodes and replace if necessary.
- Check the inside of the hull for any signs of corrosion, which will most likely entail reaching into inaccessible places using lights, mirrors and cameras. For a thorough hull inspection it helps to know where specific areas of vulnerability are. These include beneath fresh water, black water and fuel tanks. This is not necessarily due to the tanks leaking, but because of the condensation build up in these areas that can lead to pools of water forming. Another obvious place to inspect is the hull areas beneath where the shower or heads and galley are located. Seepage of water can work its way to these inaccessible areas, collecting along the frames with nowhere to go.
- Look inside all the bilge areas and make sure they are dry. If water is allowed to sit in the bilges of a steel boat it will soon cause problems.
- Check along the welding that joins the frame to the plating, as this is a common place for rust to start inside the hull.
- Look carefully at the edges of any timber that is fastened to the steel. Check for signs of rust, as timber is notorious for taking up moisture which works its way between the timber and the steel, which then rusts.
