DUDLEY DIX YACHT DESIGN

CHOICE OF CONSTRUCTION MATERIAL

My experience with prospective clients approaching me for boat designs is that most have very definite ideas on the material to be used for the construction of a new boat. Unfortunately, some base the decision on an uneducated instinct born out of hearsay about experiences (generally bad) which others have had with various materials.

There are materials which are used for boat construction but for which I am not prepared to design. If the prospective client will not be persuaded to build in another material then I request that he rather approach a designer who does work with the required material. In general, however, a good boat can be produced from any of the normal boat construction materials provided that the design was drawn for the chosen material and proper care is taken to work within the limitations of the material when building the boat.

No material is the ideal medium for all boats, despite what some barroom experts would have us believe. Major factors in the choice include intended use, construction location, equipment and resources available and the previous experience of the workers who will be involved in the project.

The intended use of the vessel affects the material choice because that will set the performance expected, the level of maltreatment to which it will be subjected, the standard of finish required and the amount of maintenance which is acceptable. A boat intended for commercial fishing or cargo work will have very different needs from one which is to compete in the high class charter fleets.

The location of the construction site has a very large bearing on the material choice. The most important factors are whether or not the site is in a residential area and whether or not a covered area is available to protect the boat from the elements during critical periods.

All boatbuilding requires some equipment to a greater or lesser degree. Some of the more costly equipment can be bought for the project then sold to recoup most of the cost but most of the smaller items would be kept for use on future projects, boatbuilding or otherwise.

Available material resources can make or break a project. It would not make sense to build a steel boat in a remote forest area with supply and transport problems when it can be successfully done in timber. Different factors might favour the opposite choice.

The previous experience of workers should be considered as it could affect the standard of the finished boat. In contrast, no material should be discounted because of lack of experience in working with it. A short instruction course at the nearest technical school will pay dividends in the final product and ultimately reduce time and wastage.

GRP is the most common material in modern boatbuilding. Properly done, it produces sound yachts capable of safely travelling to almost any corner of the world. When a mould is available for the chosen design, the hull, deck and available modules can be produced with a minimum of effort and in a very short time. Mouldings generally include small items such as hatch covers but can be so comprehensive as to include all of the basic interior construction including all necessary hull framing.

If the amateur builder starts with professionally built GRP mouldings the biggest part of the project, the hull and deck construction, have already been taken care of. The overall construction time will be much reduced and there is more chance of the boat sailing before enthusiasm flags.

An argument often used against using GRP mouldings is that the owner would rather build his entire boat and know what is in it than risk the dubious practices of a professional builder. However, most professional builders are honest and want to maintain a good reputation so the best route is to research the market carefully before becoming committed to a purchase.

Ideally you need to talk to longstanding owners of boats from the chosen boatyard to establish what problems have been experienced and the degree of backup received when problems arose. This is particularly important if the price of the purchase appears lower than those for other boats of similar size. To buy on price only is to look for trouble because the lower price might be due to the use of inferior materials and the cutting of corners. Price comparisons must always go hand-in-hand with quality comparisons, whether looking at boats of GRP or any other material.

GRP is suitable for amateur construction but should be done under cover. Successful boats have been built out of doors but the curing process of the resins can be badly affected by exposure to the elements. Extreme care has to be taken, particularly during the laminating stages, to time the work so that curing or contamination problems do not result from dew, rain, wind or other weather conditions.

Another problem (which applies equally when fitting out a professionally built hull) is that GRP which is fully cured must be correctly prepared before any new laminating takes place or a proper bond will not be produced. Over a long period of exposure contaminants in the form of dust and a greasy film will settle on the surface and they must be completely removed and the surface roughened or the new laminate will break loose, possibly even before any sailing loads are applied to the structure.

GRP work is very smelly. It is worthwhile to check that the neighbours are not going to object to the new project and give problems with the local authority.

With regard to displacement, GRP has been used for the construction of boats across the full spectrum, from very heavy to ultra-light. The basic materials of glass reinforcement in a resin matrix can be strengthened in critical areas by the use of Kevlar or carbon fibre while major weight savings can be made by the use of composite construction. For this method of construction various core materials are available, ranging from fairly basic structural foams to exotic honeycombs of paper or aluminium, which are sandwiched between two skins of GRP to provide improved structural properties. Their use can simplify the male mould construction for the boat and reduce the amount of fairing work needed to give a good finish. Stringers are generally not needed in composite hulls so a gain in space is normally a bonus.

Steel has been the material of choice for amateur builders for a few years. It is a material which is forgiving of construction errors because they can be easily cut out and replaced. It is also forgiving in that workmanship has to be really bad before it makes the boat unsafe, not that that is a valid reason for using it as a boat construction material.

It used to be that the biggest factor against using steel was it's tendency to rust. With the modern paint systems which are available, that argument no longer applies. The big factor which remains against steel is the weight of the material, which prevents the construction of lightweight boats.

Various attempts have been made over recent years to market light to moderately light displacement steel designs but when launched they generally float much deeper than intended. One has to contend with the fact that, for equal weight, a steel hull skin must be about 20% of the thickness of that for GRP and 7% of that for timber. Steel just cannot be successfully built to those thicknesses except for very large craft for which GRP or timber thicknesses may be large.

The extra weight gives a bonus in the way of a comfortable motion at sea but applies a penulty in reduction of performance. The power, both in auxilliary propulsion and sailing rig, required by a heavy yacht is greater than that for one of light displacement so those costs are proportionately greater. In compensation, steel is second only to plywood in terms of economy among the normal materials for one-off yacht construction, up to the stage when fitting out commences.

Another negative factor due to the weight of steel is the need to manhandle the heavy sheets on and around the hull. This requires either powerful handling equipment or some spare hands. Care must be taken when working with heavy parts to ensure that they do not fall because a falling plate can easily remove a foot or even kill any unfortunate person finding themselves in the way when gravity takes control.

Fully round bilge steel construction is out of the question for all but the most able of home builders. Those of lesser ability can produce round bilge shapes but the plating is guaranteed to be unfair with much filling required, not a good practice. Most amateurs will opt for multi-chine construction for it's ease but radiused chine construction gives the best of both. The modified round bilge shape which results gives better resale value whilebeing easy to build.

For home building, steel is not a practical material to work with in a residential area. If your neighbours do not hate you before you start your boat, they almost certainly will by the time that you finish. From start to finish of the steelwork phase it is by far the noisiest material. Grinding noise (and you cannot build a quality steel boat without grinding) will turn your most loving neighbour against you. Heavy electrical welding equipment can play havoc with TV reception. Sandblasting will turn the whole area around the boat into a desert and the same applies to gardens and houses nearby if there is a wind blowing. The use of pre-primed steel will remove the sandblasting problem.

On the positive side, steel does not mind living out of doors during construction. If raw steel is used, the weathering will help to remove the millscale before the blasting and priming phase is reached. If pre-primed steel is used, regular wire brushing at weld and grinding areas followed by the application of a touch-up primer will keep the steel in good shape until the balance of the paint system can be applied.

Timber construction has fallen out of favour with the home builder in recent years, possibly due to the availability of GRP hulls. It is now uncommon to see a new timber boat launched but it remains a good construction material.

The development of the WEST (Wood Epoxy Saturation Technique) methods of boat construction have put new life into timber boatbuilding. By combining the age-old natural material of timber with specially formulated space-age resins, a light and strong engineering material is formed with predictable properties.

The WEST system can be successfully combined with various methods of construction, from strip plank to fully cold moulded, either with or without stringers. All have the common factors of trying as far as possible to produce a monocogue structure while sealing the timber to keep out moisture which would reduce it's engineering properties and cause rot.

Plywood is the closest rival to steel for speed of hull and deck construction but the shape used by most designers is limited to chine or multi-chine. A good alternative is radius chine plywood construction which closely approximates a modern round bilge hull shape using sheet plywood to the flat surfaces and laminated plywood to the radius. Plywood remains an excellent material for amateur construction, particularly when combined with epoxy resins for adhesives and surface coatings. It continues to be the most economical material for building a boat in many countries.

Nature has provided a large selection of timbers with widely differing mechanical properties. They can be chosen for light weight, strength, stiffness, rot resistance, hardness or decorative character. However, man has rather depleted the stocks of many species so they are becoming hard to obtain. This can give problems if building a timber boat by traditional methods but has minimal effect with WEST construction as the complimentary properties of the less exotic timbers and the epoxy resins do not require the use of special timbers.

Displacements with traditional timber construction methods are normally fairly high, resulting from the use of high density timbers and large scantlings. In contrast, modern methods can produce boats of very light displacement with sparkling performance.

For home building, timber does prefer to be covered over during construction. As a minimum, it can be used in the open but all unpainted surfaces should be protected by tarpaulins or they will deteriorate with subsequent painting or rot problems.

When using epoxy resins as part of the WEST system, the exposure problems are the same as for GRP so care must be taken to prevent contamination by moisture and dirt. Many resins are intolerant to surface moisture while others are not affected so it is a worthwhile precaution to use resins which will not fail if dew settles before they are surface dry.

Most neighbours will have little objection to a timber boatbuilding project, partially because most men are involved in woodwork to some extent and can relate to the project. While power tools are needed, they are relatively quiet and their use is generally of fairly short duration. Epoxy resins are smelly but are far more pleasant than polyester resins and the smell fades much more rapidly.

Aluminium is a medium seldom used by amateurs but is the material chosen for the construction of top quality professionally built custom yachts, particularly in the larger sizes when investment plays a large part in the decision. It is a material for which it is more important than most that the builder knows what he is doing or the product can be a disaster.

Of vital importance is the care which must be taken to ensure that there are no metals more noble than aluminium in electrical contact with the hull, particularly below the waterline. In addition, there are many different aluminium alloys, each with it's own properties and each more or less noble than the next. The correct choice of alloy or combination of alloys must be made or galvanic action will occur between two dissimilar alloys. Welding alloys must also be chosen to be compatible with the two alloys which they are joining.

Special aluminium alloys have long since been developed to cope with the corrosive marine atmosphere, with such success that many yachts are left unpainted above the boot stripe. Below waterline they must be painted for anti-fouling reasons but also to form a plastic protective shield to eliminate galvanic action. Mercury based anti-foulants must never be used as they will be cathodic to the aluminium which will be eroded. Copper based anti-foulants should not be used for the same reason except with extreme caution over multiple layers of barrier coat to separate the aluminium from the copper and with routine checking for breakdown of the paint system and consequent galvanic action.

The inside surfaces of the hull, particularly below waterline, should also not be left unpainted. A good paint system will give protection from the dangers of lost coins, tools, etc finding their way into the bilge water and setting up a galvanic cell to eat away the hull.

Aluminium is much softer than steel so it is more easily formed and can even be worked with woodworking machinery. On the negative side, it must be welded by one or other of the shielded gas welding processes and can, therefore, not be welded out of doors, unless under the protection of an efficient mobile enclosure. Any draft which passes over the weld area during welding will blow away the shielding gas and cause welding problems.

Weld area preparation and cleanliness are also very important as even the smallest amount of contamination from sweaty hands or dirty gloves can cause entrapment of impurities in the weld with consequent weld cracking. In general, good welding practice is of far greater importance with aluminium than it is with steel.

Aluminium repair facilities are scarce in most parts of the world which are attractive for cruising. Because aluminium is easily cut and drilled by hand, it is better to be prepared for temporary repairs by screwing or bolting aluminium or timber patches over the damage (as for a timber hull) rather than to expect to do a welded repair.

The cost per kg of aluminium is far higher than that of steel but the weight of material used in an aluminium boat is far lower than that for the equivalent steel boat. The lighter weight of aluminium allows the use of less and lighter handling equipment and it's easier working, particularly with cutting, fitting and filing, can give labour savings. The net result is that, in the smaller sizes, aluminium construction is considerably more costly than for steel. However, as the size of the boat increases so the percentage difference decreases, with aluminium being little more costly in sizes over about 18m.

For the home builder, aluminium is a quiet and odourless material which will produce a minimum of annoyance for neighbours. Grinders must not be used on aluminium and sandblasting is not required so the worst factors against building in steel are missing when working with aluminium.

I still get enquiries for ferro-cement designs so I should comment on that medium as well. As stated at the start of this article, good boats can be built from almost any material and this applies also to ferro. A few good and even exceptional yachts have been built with the medium, especially by professional yards. Unfortunately they are in the minority, vastly outnumbered by the bad to disastrous boats. It is a material not to be tackled without careful thought over the decision.

The main attraction is the low material cost of the hull and deck allowing the builder with a small budget to easily get to the point of having a hull and deck to fit out. What tends to be forgotten is that the labour content is very high and, while the labour of the amateur is free, the work tends to be repetitive and seemingly never-ending so interest flags long before the plastering stage is reached.

The plastering is critical, both in mix used and the application thereof. Many is the boat which has had gallons of expensive epoxy pumped into the voids between plaster layers applied from inside and outside and which have not met as intended. Proper curing is of equal importance as cement shrinks as it cures and, if the moisture content is not kept high during curing, it will crack and give leak problems.

The hull and deck costs are a small portion of the overall cost of the project, with the remaining costs similar for whatever material is chosen. The overall savings are small but are tiny compared with the losses which can be incurred due to the low resale value of a ferro yacht at a later date.

The important choice of hull material must finally rest with the prospective owner of any new boat. Provided that he fully understands the advantages and limitations of all the materials which are available to him, he can make a sensible decision.

For further reading, see our articles on radius chine plywood and radius chine metal construction methods.

Whichever construction material you choose, it will be necessary to have reference books available to guide you on the correct use of the material to ensure a sound boat. Many of the available books can be bought through our bookstore. Your purchase will be supplied to you by mail or courier service from USA.

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This page was updated 3 September 2002

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