Boatbuilding Materials

In this article, we're taking a brief look at the various boatbuilding methods and materials. We're trying to cover each, giving the pros and cons and some opinions based on our experience. At the end there is a suggested reading list for those who want to delve further into the subjects and would like to have a starting point for their reading. Since the latest boatbuilding technology is always a moving target, some of this material will eventually be out of date. However, we like to think of our work as being mainly with "low-tech" materials and technology. We're certainly acquainted with the hi-tech world and use it for some of our work. But, use of hi-tech would not be appropriate for most of our designs, not only for the expense, but also for the practicality of living with it and the costs of repair.

ALUMINUM CONSTRUCTION

Over the last several decades aluminum has gained in usage and acceptability. A significant portion of the custom building of larger yachts is done in aluminum, and some builders produce stock aluminum boats. This is due to the high strength to weight ratio of aluminum being attractive where speed is a prime consideration.

Great care in building is required to be sure that there are no abrupt areas in the structure. Much of this is taken into account in designing the framing and structure, but the plans must be carefully followed by the builder.

The builder must have specially trained welders to do a proper job of putting the materials together. Most of the work of cutting and drilling can be done with wood working tools.

The owner or operator must also exercise great care and caution to be sure that the boat's galvanic corrosion protection is maintained properly and that the electrical system remains isolated from the structure.

The 5000 series of aluminum alloys are normally used in marine work. You should check with your supplier though and be sure the material you have chosen is appropriate for use in a saltwater, marine environment. The 5000 series alloys have good properties for this work and can often be left unpainted with the surface forming an oxide that protects itself from further deterioration. Of course there is no getting away from the need for antifouling paint. In this, there are specially formulated paints for use on aluminum to protect the aluminum from deterioration and slow the growth of barnacles.

If the boat is left unpainted above the waterline there will certainly be savings in maintenance costs on the coatings. However, most people paint yachts for esthetic reasons and then the coatings themselves must be looked after. We've seen white painted topsides pick up enough yellowing at the bow in about five years that a cosmetic recoating is done. The basic paint was still in good order, so the linear polyurethanes do work well.

Aluminum is a good thermal conductor. Thus all living spaces need to be well insulated to keep them dry and at comfortable temperatures. Since aluminum is quite a rigid material, care is needed to be sure that the machinery noises are well insulated and isolated from the living spaces. This means doing a good job on the isolation mounts for the machinery and sealing off any openings that can permit the transmission of airborne noise.

The majority of our past work with aluminum has been with larger sailboats that were operated with small crews. The lighter weight of the aluminum structure let them operate with smaller crews since the rigs were smaller than if the boats were built of steel. The trade-off is that the aluminum boats require more sophisticated repair facilities when and if they need work done. In powerboat design work, we would only recommend using aluminum on special use projects. For most of our cruising boats, it is hard to justify the extra expense and level of care required, both in building and in living with the boat.

We expect to see more use of aluminum in long-distance cruisers where they want to trade-off weight savings for additional fuel capacity. Or for reduced power requirement for the lower displacement. This is one area where it does make sense.

WHAT ABOUT FERRO-CEMENT?

What Is Ferro-Cement? For those who aren't familiar with Ferro Cement, it's a method of building in which the shell of the boat is built of a steel wire mesh and rod matrix, which is then permeated with a concrete mixture. The mortar is cured carefully to a minimum of 8,000 psi (to meet our specs). The resulting structure is a strong and rugged shell that will not burn, is not affected by ice, is resistant to chemical attack, and has a mortar that actually strengthens with age.

How Did It Get Started? The first ferro-cement boats were built in the middle 1800's and are still in a museum in Europe. World War I saw the building of large concrete ships, some of which I've seen still afloat as breakwaters. During World War II, some experimental vessels were built which proved the material on some good-sized working vessels and later on some yachts.

The building craze that began in the late 1960's was fueled mainly by promoters who spent more effort in selling franchises for their books and plans than in improving the technology. The few builders who built good quality boats often got overlooked with the vast numbers of amateur built boats sprouting up everywhere.

I had learned how to do it the right way while working for a licensee of the English Windboats Ltd. firm, who were the only ones building Lloyd's approved structures.

Following this experience, I did some additional work in the testing labs to develop a range of shell layups that would span the range of 12' to 90' designs we worked on. This knowledge gave us the technology to design much lighter and stronger boats than were commonly being done. It was an uphill battle to get people to recognize that chicken wire was not the best choice, though in the end almost everyone ended up using the same square welded mesh that we did.

PRACTICAL FERRO-CEMENT BOATBUILDING

After a bit of this development and designing work, I teamed up and co-authored a book on the subject, titled Practical Ferro-Cement Boatbuilding. It was quite well received and widely distributed, going through 11,000 copies in four separate editions. This book covered the ideas, tools and techniques that we had developed to build lighter and better ferro-cement boats.

This book has been out-of-print for a long time now, though I still am able to occasionally pick up a copy in a used bookstore. Thus, we have a very few copies that we keep for those who are determined, usually against our advice, to build in ferro-cement.

What Ever Happened To Ferro-Cement? Too many people who read Samson's ads saying they could build the hull and deck of a 45-footer for $2,000 didn't realize that was only the beginning of the money they would have to spend to make a complete boat. It usually took ten times that much to fully outfit the boat in those days.

Also, their publications stressed how easy and quick they were to build. Too many of the resulting boats looked like the builders took that advice literally. They were - and still are, for how do you dispose of cement? - an eyesore, enjoying very low, if any, resale value. Their being readily identifiable as ferro-cement has given a bad name to all ferro-cement boats. The good ones were always mistaken for wooden or custom fiberglass boats, and thus no credit was given to the medium of ferro-cement.

So, although it is still a viable way to build a tough and long-lasting boat, I have not been able to give it an unqualified recommendation for some time. It is very frustrating for me and much more so for the owners of the good boats to find that they can't get the same return on their invested time and money than if they had built in wood or fiberglass.

Do You Still Sell Plans For Ferro-Cement? Yes, but only when we've given the caution above to the prospective buyers. Most of our few remaining ferro-cement sales are now going overseas where there is still a bit more viable market for the finished boats.

What About Buying A Used Ferro-Cement Boat? With the negative attitudes prevailing about ferro-cement boats, their prices are usually quite low. The only problem in buying one is the difficulty in doing a proper survey. This is a two-part problem with the lack of many experienced surveyors and the difficulty of knowing what few clues to look for in the survey. If the seller has photos documenting the whole of the construction of the armature and the plastering, this is a big help. It they did mortar sampling and testing to assure the correct mortar strength this helps to prove the quality of the initial construction.

From there, it is a matter of looking at the level of finish and fairness of the structure, the quality of the detailing of how things are attached to the structure, and if there are any visible clues to maintenance work that has been overlooked. If you can find a good one that can be used as is and without any major investment in finishing it or adding equipment, then you might have some hope of reselling it later without loosing all your investment.

FIBERGLASS

Over the last several decades, fiberglass has become a sort of de facto standard for boatbuilding materials. Those of us old enough to remember its introduction recall when it was a novelty and no one knew how long it would last.

The most efficient use of fiberglass is building sister ships from a mold. There are a high number of man-hours required to get the plug, or form for the mold, truly faired and ready to lay-up the mold. Once this mold is made, the number of man-hours to build a sistership is greatly reduced. Thus the best use of fiberglass is not in building one-off boats, for in building a one-off boat, it is hard to justify the expenditure of this amount of time, particularly if the boat is to be competitively priced.

Certainly the homebuilder can think of discounting the value of the time spent in fairing work, but the work is not pleasant and often tends to get short-changed, to the detriment of the future resale value.

A one-off building technique that has been used with 'glass is the building of a large smooth table, using plastic sheet laminates (like Formica®) for the surface. On this are laid up large, flat, 'glass panels. These are then used to "plank" a hull, deck or house that has developed surfaces. The edges are trimmed and taped together and the outer gelcoated surface is repaired to make it all look alike. This method can give significant savings in labor when compared to having to manually fair all the exterior surfaces.

There are a number of core materials used with fiberglass. These add extra stiffness and rigidity by separating the inner and outer skins like the web of an I-beam separates the flanges for strength. They also serve to give insulation. Since the core is by definition sandwiched between two layers of fiberglass, it must be something that will be durable and not be subject to deterioration. Wooden cores, like end-grain balsa or plywood, can sometimes experience deterioration through water getting into them through breaks in the skin or careless installation of gear or just the porosity of the laminate. If water does get into a wooden core, it can at best waterlog it and at worst have it all rot out and be very difficult and expensive to replace.

Our preferred core materials are the closed-cell PVC foams, like Airex®, Core-cell®, Divinycell®, Klegecell® or Termanto®, which are not subject to rotting if any water happens to get into the core. They also are good thermal insulation and can make for a surprising difference in eliminating condensation in lockers and living spaces. The techniques for installing the foam, whether in a mold or on a one-off, are well established and the material suppliers can give good guidance on how-to questions.

We have only specified closed-cell foams for core materials. These should not be confused with the cheaper foams that crumble and crush under ordinary use. Their closed-cell formulation means that water cannot migrate from one part of the foam to another and they are not subject to rotting.

The chemicals used in the resins and cleanup work are often unpleasant and need special care in handling. Some have explosive and/or toxic fumes, but there have been a lot of improvements of late and there are many more alternatives now.

The origin of the use of fiberglass in building boats is not officially known, and I would seriously discount the stories about it being an attempt to dispose of overstocks of the foil-backed, household insulation materials....

BOATBUILDING IN STEEL

Why Steel? Coastal cruising is often harder on boats than offshore voyaging. This is because - as anyone who's really been cruising knows - along the coast is where boats go aground and generally bump into things. To survive this gracefully, the boat must be built of a rugged material.

Steel is an excellent material for a coastal cruiser. It is rugged. And it is easily repaired almost anywhere. Too many of the modern cruising boats are built of high tech materials, and are not repaired easily in areas with only low tech facilities. Low tech repair services are what is usually found in the out of the way places that are interesting to cruise into and where the boat might have an adventure leading to needing a repair.

Steel is typically the heaviest material used to build cruising boats. This can be used to advantage by the designer in some types of boats. The added mass of the steel can be used to design a boat that gives the comfortable motion of a much larger boat. For anyone spending periods of time coastwise cruising, most of the time ends up spent not underway, but at anchor or tied up somewhere. At these times, the higher the stability the better. The way to do this with steel boats is to keep the waterplane wide with beamy and relatively shallow boats.

The additional mass of steel over other materials means the weight of one crewmember is a smaller proportion of the total displacement. Thus, moving around on the boat has less effect on her heeling and trim and thus makes for a more comfortable boat.

Although steel is probably the heaviest material, it can also be one of the most affordable, if the design is done carefully. This is because steel can be used in large pieces and assembled quickly, particularly if the design was done for developed surface construction.

Steel boatbuilding can be done by amateurs if they take the time to learn welding sequencing and take care about not distorting the plating. Having a design that is thought out for ease of assembly helps make for successful construction, whether by an amateur or professional.

If the designer uses more flatbar and cut plate framing instead of flanged shapes for framing, it will be easier for the builder to do a good job of sandblasting and painting. (This requires careful design to make it successful.) Then, using good quality finishes, with lots of zinc in the primer coats, should give good protection. Some of our boats have been built upside-down, with the sandblasting done before rolling over, so the sand will fall out easily. This is a time-saver for the builder and is reflected in the final costs, whether these are counted in hours or dollars.

Sometimes we consider using aluminum houses on steel hulls for some weight savings. In the past, we used to join aluminum superstructures to steel hulls by stainless steel bolts in insulated sleeves, having put neoprene gasketing in between the steel and aluminum. Now, we use the explosively bonded strips of steel and aluminum, which permits welding both metals to their respective sides of the strip.

However, our usual practice is to design the whole vessel in one metal. Usually the deckhouse has lots of holes in it for the windows and hatches, and thus it may not be a major weight item. If the allowance for it is made in the design phase, there is no reason to go to the added expense and complication of putting an aluminum deckhouse on a steel hull.

We've been designing our structures lately to make building and outfitting as easy as we can. Sometimes this makes for a little more steel weight, but the end result is a boat that goes together quicker and saves the builder and owner money on the first cost. Some of the things we do in this vein are making the engine beds and engine room floors all one height, so the plating or sole just lays on them, arranging for as many duplicate pieces as we can in the structure, and laying out design of the hull form and deckhouse to make maximum use of standard size plates and framing member lengths.

Insulation: Our first choice for insulation is for sprayed in place foam. This is put on over the zinc-rich primer, and eliminates any pockets in which condensation might occur. Condensation that occurs behind blocks of foam stuck on the plating is the primary candidate for rusting out the steel.

If it's not possible to have sprayed on foam, then an air space all around the plating, ventilated the way a traditional wooden boat should be done is the second choice. If this is done, I would also put a fan in the ventilation system, to slightly pressurize the bilge spaces, forcing air flow through the whole structure. I've been aboard a Dutch built 45' ketch that was over 20 years old that was built with this sort of ventilation, and she was in excellent condition. True, she needed a lot more energy expended to heat her in cool weather, but you could look at the shell plating all over, readily ascertaining it's condition.

How Long Will It Last? Life expectancy on a steel boat can be anywhere from less than a decade to several generations. The variable in this is how well the boat is designed, built, and most importantly, how it is looked after on a regular basis. Any indications of oxidation, like visible rust bleeds and pinhole rust spotting, calls for immediate action to stop the flow of oxygen to the steel and to re-protect the surface.

WHY NOT WOOD?

Wood is usually my first choice in boatbuilding methods. All three of the boats that I've lived aboard, to date, have been of traditional wooden construction. They were well built and have been well maintained over the decades. As a result, they both are looking quite well, in spite of being built in 1926, 1973, and 1926 respectively. WoodenBoat magazine (issue #100) did a survey a while ago and found that boatyard operators' records indicated that cold-molded wooden boats were the best investment when all costs of ownership were considered. The warmth and charm of living with a wood boat is hard to beat, even when putting a price on it.

CARVEL PLANKED

Carvel planking is smooth planking applied over frames that are usually steam bent in place. Sometimes there re sawn frames in some working boats or heavier cruising boats. All of my floating homes were built of plank-on-frame construction. The major advantage of this method is the ease of replacing any particular element of the structure, if need be, at a later date.

Carvel construction, to make it weather and watertight for comfort in living aboard, requires building of top quality materials. If the materials are not well chosen and well seasoned, there could be considerable movement in the structure as the wood shrinks and swells with moisture and humidity changes. This can lead to leaks, the most annoying of which happen through the cabins or deck - say, right over your bunk. If the right materials are chosen, the structure will be stable, if it is well designed and constructed. If not, there will be the frustration and possible peril of the boat leaking.

A carvel planked boat often has an inner layer of planking inside the frames, called the ceiling. (The underside of the housetop or deck is the "overhead".) This ceiling provides an air passage from the bilges up to the deck. The free flow of air through here keeps the boat from growing mold and rotting out. I often advise people who are going to leave their boats for a while to install a solar powered ventilator to pressurize the bilges to force the airflow. Professional boatbuilders often remark that carvel construction is the quickest for them to do, which can make it the most economical to purchase.

LAPSTRAKE OR CLINKER

The planking method in which the bottom edge of one plank overlaps the top edge of the next one is called either lapstrake or clinker construction. This is most often seen in small craft or dinghies, where the lightness and stiffness gained in the overlapping makes for a sturdy but light boat. It has been used for some quite large boats too, like the Viking Longships.

The skills required for this method are similar to that for carvel except the close fits have to be made on the overlaps instead of the seams that butt to each other. A popular variation on this construction now is for epoxy gluing plywood planks, which can be done in conjunction with very little inside framing. Our own 11' Oregon Peapod dinghy was built by this technique and is light and sturdy.

STRIP PLANKING

Strip planking usually is done with planks that are almost square in cross section. They are nailed and glued together to form a rounded shape over bulkheads and mold frames. If the spans are long enough, some additional bent or sawn frames may be fitted.

Strip planking will typically support itself over longer spans than carvel planking. However, it still needs something to give it strength across the grain such as the bent frames in a carvel planked structure. One approach that has been used is to rely on heavy cloth and resin sheathing, with the strips effectively becoming a core or spacer to hold the cloth skins apart. The downside to this is that some of the poorly applied coverings sheared off the cores from lack of bonding and the differing expansion or elasticity rates of the materials. Some of this can be forestalled by putting mechanical fasteners (staples, nails, or screws) through the skins into the core.

A better alternative is to cold-mold some veneers over the strips, applying them at say opposing sixty degree angles to the strips. These would have sufficient material to take the place of the ribs and help hold the strips together. This technique would usually be my recommendation for someone building their first round-bilged boat. It allows for easy handling of smaller pieces and can be done in stages.

COLD-MOLDING

Cold-molding refers to the cold bending, as opposed to hot or steam bending, and laminating of smaller pieces and layers of wood to form the shape of the structure. As currently practiced, this is usually done with epoxy glues and sealants to stabilize the moisture content of the wood and prevent shrinking and swelling. Combined with linear polyurethane paints this produces a structure that often needs less maintenance than fiberglass, since the gelcoat needs buffing and waxing and eventually needs painting.

The New Zealand method of cold-molding is to set up bulkheads and temporary mold frames. Over these are bent on substantial longitudinal framing, which is let into the bulkheads and permanently fastened to the bulkheads. Then, a minimum of three layers of planking are applied on opposing forty-five degree diagonals. These are glued and fastened to the longitudinals and the bulkheads.

The combination of strip-planking with cold-molded layers over it is more often used when weight savings are not a critical issue. This results in a smooth interior, which is often easier to maintain, and takes less work in making sure that the pieces added during outfitting do fit well. It also means that there are no collections of bilge water on the frames since it can freely flow to the lowest point on the hull. Adding limbers to the non-watertight bulkheads and floors will facilitate using a single pickup point.

PLYWOOD

The popularity of good epoxy glues and sealers has given plywood boatbuilding a new life. With these, the plywood can be protected from water getting into the cores and setting up rot or deterioration there. The basic principle to remember is that whenever a hole is drilled in the ply be sure that it is done a little bit over size and all the exposed edges of the ply are sealed with epoxy before installing the new piece of hardware.

The quickest wooden boatbuilding is usually using plywood in sheet form over a developed surface form. (A developed surface is one on which the plywood will lay without any distortion, such as on a cone or cylinder.) We're using our FastShip computer design software to speed up the work of developing the surfaces and "unwrapping" them to check the shape and size of the panels. From this, we can be sure that we've made them in a manner that will be an economical use of the materials.

Like most strip and cold-molded boats, plywood is usually sheathed with a layer of cloth to take the local impact loads and abrasion. This sheathing is not structural on most boats, but rather a protective skin to keep harm from coming to the wood underneath.