The Hull

The hull(s) of a boat is more complex than any other part of the boat. It's purpose and offset by a balance of interior space, wetted surface, stability, seakindlyness, windage, deck space, strength and looks.

Boats can have anywhere from one to three hulls, these are known as mono-hulls, catamarans and trimarans. Multi-hulls have gained favor in the racing world due to superior stability and reduced weight since they do not require weighted keels. Mono-hulls are still more common but the number of catamarans is increasing year by year for cruising and casual sailing. Trimarans are still rare outside the racing circuit because of poor interior space while catamarans generally have more space than mono-hulls although they can carry less weight without affecting performance.

Hulls have been crafted from just about any structural material used by man, there are even cases of paper hulls. Boat-building materials can be split into five main categories.

	Composite (Fiberglass,Epoxy,Carbon Fiber,Kevlar)
	Metal (Steel,Aluminium,Cupro-Nickel,Titanium)
	Ceramic (Concrete)
	Organic (Wood)
	Cored (Wood+Fiberglass,Wood+Epoxy)

Each one of these construction materials has distinct advantages and disadvantages. The selection of a material should be based on solid requirements and analyses of material properties.

Your preference depends on your priorities, if cost is all that matters you can build a Ferro-cement (concrete) boat for less money than anything else. If you want high performance and cost is no issue go for a carbon fiber trimaran. If you want a boat which will never corrode and have very deep pockets go for a Titanium hull, if you never want to worry about anti-fouling a Cupro-Nickel boat is excellent. If wood is available cheap in your area and you are a skilled carpenter a wooden boat might be your thing.

In our case we were looking for a blue-water cruiser which had decent performance, easily custom built and fairly cheap. We also had good knowledge of steel work and excellent availability/cost. For most people buying a new/used boat fiberglass will be preferred because it lends itself very well to mass-production. Because fiberglass requires molds to be constructed prior to constructing the shell I can't consider it a good material for a custom built boat unless it is used with a core material or in strip plank construction. Core materials such as balsa or plywood also increase strength and stiffness but can cause maintenance problems if they are not properly sealed resin/epoxy.

Building Material Comparison

For a good article on the choice of materials see: www.dixdesign.com/material.htm

* Availabilty can vary depending on your locality, these apply for western cities.

** The impact resistance depends heavility on the weave and can also be changed by using kevlar which is very impact resistant.

Paint Systems and Surface preparations

Marine coatings are among the most demanding and as such are expensive, hard to apply and require strict adherence to manufacturer specifications and surface preparation. A good modern paint system on a properly prepared surface will last 10-15 years with minor touch ups to damaged areas, re-coating outsides with full grit/shot blasting will need to be done every 20-25 years.

Before discussing the paint itself we need to look at surface preparation which is vital, first off there is no substitute to abrasive blasting, using pre-primed steel or hand cleaning can simply not produce the same quality surface as a good blasting can. There are two factors to consider, cleanliness and surface profile (roughness), first the surface must be clean of all rust, mill scale or old paints and absolutely free of contaminants such as salt or dust as these can be hygroscopic and cause water ingress via osmosis. The standard most often demanded is SA 2.5 which is white metal but allows for areas with a slightly darker tint, SA 3 is preferred which specifies uniform white metal. The second factor is surface roughness to ensure proper paint adhesion, here the standard most often encountered are G2-3 from ISO 8503, ISO 8503 is a visual standard and conformance is determined by using grit (G) and shot (S) comparator plates by look and feel. The most popular and compatible US standard would be ASTM D 4417. What this really means is a surface with an average maximum variance of 45-75 micron between peaks and valleys, the predominant manufacturers of comparator plates are Elcometer and Rubert. It's not a trivial task prepare a surface to G3/SA3, depending on your locale and choice of blast media it might also get costly. SA 2.5/3 can be attained using most blast media assuming the blast media is dry, free of contaminants such as salt but the surface profile is harder and requires a more specific blast media. A grit size of G50/G40 (0.2-0.85mm) or alternatively a shot size of S230/S280 (0.85-1.2mm) should be suitable but they also need to be of sufficient hardness and ideally with sharp edges.

Now I must mention that blasting with silica sand is a bad idea because breathing small silica particles leads to a disease called silicosis from which you can die. Since silica is off limits blasting operators are left with two choices, use a locally available media which is cheap and does not need to be recycled or buy more expensive media which needs to be used in an enclosed space but can then be filtered and recycled. Using a local media is fine assuming there is little or no grit < 0.2mm or > 1 mm, it's salt and mineral free and has rough edges and is Mohs 6 hardness or higher. If the operator is recycling he is most likely using either steel shot or garnet sand which should give excellent results assuming the right particle size and that the finer broken down particles are filtered before being re-used.

Now as if that's not enough to deal with for blasting we also need to make sure that air humidity is low and blasted surfaces should be primed within at least 4 hours of being blasted. Primer should be brush painted onto corners, stringers and other "hard to reach" areas.

Now that surface preparation is out of the way we can start talking paint systems, filler and finishes.

Galvanic effects need to be kept in mind, never use copper or iron oxide based paints on aluminum hulls.

Zinc rich primers should only be used above the waterline and on insides, etching primers are frequently used for aluminum.

Essentially all professional metal coating systems consist of the same components, a primer, an undercoat, an overcoat and anti-fouling, to make things even simpler I would only consider epoxy based primers, epoxy or polyurethane undercoats and polyurethane overcoat with all being plural component paints. The primer is designed to bind to the metal, more like a mechanical grip than a glue type binding, it can also contain an anode material like zinc or aluminum which will provide galvanic protection even if the paint above gets scratched but primers containing anode materials should only be used above waterline. In the case of aluminum the primer can be of the etching type in which case it will contain an acid which will chemically roughen the aluminum and break down the oxide barrier on the surface. The undercoat is the workhorse of the paint system, it provides the majority of the thickness, the hardness and fills up the surface profile after blasting. Often the undercoat can be the same material as a non-anode primer, most times it will be epoxy based but some systems use polyurethane undercoats. If the boat is to be faired using a fairing compound this should be done before the undercoat is applied but after the primer, fairing compounds should be epoxy with 0% shrinkage (100% Dry material by volume). Next up is either an overcoat or anti-fouling depending on if the area is below the waterline or above. Above the waterline a high gloss polyurethane coat is used to protect the epoxy from ultraviolet light, oils, contaminants and provide abrasion resistance. On decks a non-slip surface might be desired, there are four methods to achieve this, a special polyurethane paint containing particles which form on the surface, a regular polyurethane paint where the particles are spread into the wet paint before it dries, silicone non-slip dust mixed into the paint and lastly pre-made surfaces can be glued on. Below the waterline there would typically be a boundary layer (primer) since most anti-fouling paints adhere poorly to epoxy primers, this is applied over the undercoat followed by the anti-fouling, be sure to launch the boat before the time specified with the anti-fouling or risk the anti-fouling becoming useless and having to apply another coat.

I would recommend treating the insides just like you would topsides or cabin, the fact is that most steel boats corrode from the inside out and re-painting or even worse blasting the inside is exponentially harder than the outside. Because there is little or no ultraviolet light inside the boat some manufacturers will only use a primer and undercoat inside and leave it at that, I would recommend a zinc primer, full undercoat and a polyurethane overcoat for the best protection possible, this is even more important if spray-in foam is to be used.

Included is a spreadsheet to calculate paint usage and show film thicknesses, it contains examples of paint systems from Hempel, Jotunn and International Marine.

Paint Systems

PaintSystems.ods