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Use of Fiberglass in Boat Construction

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Learn what fiberglass is and how it revolutionized boat construction. Explore the properties of fiberglass, its benefits, and its various applications in building durable and efficient boats. Perfect for enthusiasts and professionals in the marine industry.


“If God had meant men to have fiberglass boats, He would have made fiberglass trees!” So reads a novelty plaque which I have seen on sale at several marine stores.

Fiberglass facts

Presumably they are bought and hung up by persons who just cannot get accustomed to the idea of making boats of bad-smelling synthetic stuff. A person can fall deeply in love with the natural, comfortable looks and feel of a fine wooden boat.

It seems there are many such people around. A few years ago someone in Maine started a new magazine devoted exclusively to wooden boats (THE WOODENBOAT MAGAZINE), and it has caught on surprisingly well. A number of marine museums have initiated courses in wooden boat building, to keep the art alive. Boat manufacturers themselves admit that fiberglass tends to have a bleak, cold look to it, and therefore they make liberal use of teak and mahogany trim to lend a feeling of richness and warmth.

Good friends have ended up swatting one another with dead cats in the wake of heated debates over whether fiberglass or wooden boats are best. Tsk! Tsk! Love fiberglass or hate it, it’s here to stay. That being so, one may as well understand it.

Ages ago, primitive men began to make one-piece boats by tediously hollowing out tree trunks. Provided the wood did not split along the grain, these craft had the rather nice feature of having no seams to leak. But the size of the boats which could be built by this method was limited by the size of the available trees.

So, they began to make planks by the agonizingly slow process of drawing logs back and forth over sharp rocks, first on one side and then on the other. These crude planks were fastened to the sides of log boats to make them wider and higher. Thus began the slow development of the planked boat. Several thousand years later, we had some very finely constructed craft of great beauty, and wouldn’t you think everyone would be satisfied? Well, no such luck!

There is an old proverb that says:

“The only permanent thing is change!”

And things began to change in the boating field during the twentieth century. Yachting magazines published mainly in New York began to circulate inland. Reading about the fun people were having on coastal waters made the hinterlanders envious. They cast about for ways in which they could enjoy boating. Often living far from the water, they looked to small craft transportable on car roofs and on homemade trailers.

Enter the Major Shortcoming of the planked wooden boat … it tends to leak for several days after being launched, until its planks have swelled enough to close tightly onto the seam compound. An early attempt to cope with this was the use of canoe type construction in The Evolution of Boats with Outboard Motorssmall outboard boats … the light planking was covered with canvas, stretched tight and painted smooth. Alas, this did not stand up well to the pressure and rubbing of trailer bunkers. If a small hole appeared, an outboard boat could go fast enough to let water get into it with enough pressure to start lifting the canvas off. Landing on rocky beaches chewed up the canvas too.

Construction of fiberglass boat
Transition. In front is the last wooden hull manufactured by the Penn Yan firm. Behind it is the highly polished hull mold for a new fiberglass model. The mirrorlike surface imparts its polish to the surface of the fiberglass when the resin and cloth are applied to it to build up the laminated hull

Other builders looked to the lapstrake form of construction. Since the planks overlap rather than butt edge-to-edge, they depend less on swelling. Assorted sealing compounds between the laps kept water out yet allowed some movement between the planks. One problem was that the relatively thin planks used on outboard-sized boats often split lengthwise. Another was that sand grains tracked aboard went into the bilge, and worked between the planks so as to gradually push them apart. Many builders overcame the splitting problem by using plywood for the strakes. This also solved the problem of scarcity of good planking wood. Much use was made a score of years ago of assorted synthetic sealants, claimed to remain flexible indefinitely.

But ingenuity could not overcome the problem of labor costs. Each piece of wood in a boat – and there are many in even a small one – must be handled several times to select, shape, install and finish it. Hundreds of screws, nails or other fastenings are needed to hold the parts together. Once the woodwork is done, several coats of finishing materials like primer, paint, sealer and varnish must go on, with drying time between coats and much careful sandpapering.

As far as volume production boats are concerned, the great amount of labor required to build and finish a boat is what has put an end to wooden craft. Like the stuff or not, argue the merits of wooden boats over fiberglass far into the night if you will, it is the low labor cost and adaptability to volume production which is the basic and unarguable reason why most boats today are of fiberglass.

Fiberglass boat
Fiberglass has many advantages. It produces one-piece hulls with no seams to leak. Also, rather intricate bottom shapes are as quick and easy to mold in fiberglass as are simpler shapes, for the material conforms readily to the shape of the mold
Source: unsplash.com

Thus, after thousands of years, we are back to the one-piece hull. And when you stop to think of it, that really does make sense. Such a craft does not need to soak for several days before it stops leaking. It is therefore ideal for today’s conditions, in which a great many boats are trailered and many more are stored on dockside racks by means of forklifts. As for larger craft, attack by marine borers and rotting from the inevitable dampness were for long the bane of the wooden boat owner’s existence. He is as glad to get away from them with a fiberglass hull as his trailer-owning friend is to get away from weeping leaks.

Few people have a clear idea of how fiberglass boats are made. The process begins with the construction of a “plug”. This is an exact mockup of the boat-to-be as far as external appearance goes. But it’s made of distinctly non-marine materials such as rough plywood, common lumber, foam sheeting, plaster, auto body putty and anything else the builder’s imagination suggests. The inside is just a mass of bulkheads and braces, no walk-around space at all. It’s the external surface that counts.

When the woodwork is finished, it is covered and sealed with a layer or two of fiberglass cloth. Then begins a tedious smoothing-up process. It starts with rough sandpaper to level up the fiberglass surface. Then more resin, or gel coat, or putty, or a combination of them. As work progresses, finer and finer sandpaper is used until finally the extremely fine 600 grit is used. Then wax is applied – several coats of it. The final result is a surface so doggone smooth and slippery any wandering fly that decides to land on it is apt to slip and break some legs.

Simple boats like prams and canoes are made of a single mold but larger ones have two, three or several separate molds. It all depends on how elaborate is the deck and interior. A typical large outboard boat will be assembled from a hull molding, a deck molding, a floor or cockpit molding (called a “liner”), a slop well molding and perhaps hatch and seat moldings. Sometimes separate plugs are made for hull and deck, sometimes the two are built as a unit and a plywood flange used to separate them, enabling hull and deck molds to be made separately.

Proccess of boat building
When the resin has cured and become rigid, the finished hull may be lifted out of the mold

The highly polished plug is then sprayed with “tooling gel coat,” a pigmented resin compounded to have high abrasion and heat resistance as compared to boat gel coat, which is compounded to offer high fade-resistance and scratch-resistance. The tooling gel coat is of some color that will contrast strongly with the colors likely to be used in the boats, so that later when workers are applying the hull gel coat to it, the color contrast will help them see thin spots and go over them again.

The “tooling gel coat” is the beginning of the process to lay up the mold. Upon the completion of the mold, it is “popped” and turned up like a bathtub.

Many people take it for granted that fiberglass boats are built from the inside out and that the final glossy gel coat is put on last, just as paint is applied as the finishing touch to wooden boats. But actually fiberglass boats are built from the outside in, with the exterior gel coat being the first step in layup.

After the inside of the mold itself has been polished with several coats of wax, gel coat of the desired color is sprayed onto its surface. Spraying it on properly is a critical business and requires some coaching and practice to do right. For example, if the spray gun is held too close to the work, solvent ;n the gel coat has little chance to evaporate into the air from the flying droplets. Then the gel-coat buildup on the mold will have too much solvent in it, causing “alligatoring” or other blemishes as the gel coat puckers up from the mold surface due to the solvent’s efforts to escape.

Read also: Manufacturing of Fiberglass Boats and Design Features

And, the thickness must be carefully judged. If too thin, grayish “ghosts” of the underlying laminate will show through. If too thick, the gel coat skin will be brittle and quite apt to develop innumerable hairline cracks called “crazing,” which makes owners very unhappy. The gel coat is chemically the same as laminating resin, except that it is colored with pigment or dye, is compounded to be a bit thicker so it will resist running down on vertical mold surfaces, and to have the best possible weather and abrasion resistance.

In an hour or two the gel coat will have firmed up enough so the first layer of glass fabric can be applied. The side of the gel coat against the mold will have flattened out against it and taken on the smoothness of its polish, but the reverse, or visible, side will be rough, something halfway between “orange peel” and “stipple” effect. Resin is brushed or sprayed on, the cloth laid into it, more resin brushed into the cloth and spread out with tools resembling common paint rollers but with surfaces designed to “work” the resin into the glass to best advantage, and to press out small entrapped air bubbles.

Depending on the size and speed of the boat, from three to a score or more layers of glass cloth are laid on, one after the other, until the desired thickness and hence strength and rigidity are obtained.

A variety of fabrics are available. The shiny cloth you often see in rolls at marine stores is actually used rather seldom in boatbuilding. The weaving process makes it the most expensive form of glass fabric. Much more use is made of “mat,” which resembles a coarse felt of white color. Chopped filaments of glass a few inches long are bound together with a binding agent that does not interfere with resin’s ability to wet it out. As no weaving is involved, its cost is low. It is an economical material with which to build up the thickness of a laminate and thus gain rigidity.

Much use is also made of “roving,” a cloth of very coarse weave. The strands of glass filament from which it is woven are about a quarter of an inch wide. It thus has great resistance to puncturing and tearing and is used to impart those qualities. In a typical small powerboat, the hull might be laminated of from three to five layers of mat with one layer of roving sandwiched in between the last two layers of mat.

There is also “chopped strand” fiberglass. A special spray device, looking like a Buck Rogers death-ray gun, is used for this work. Small rollers feed a continuous strand of glass filament rope between cutters which chop it up into bits one to two inches long, and blow them at the mold surface. Simultaneously the gun blows resin and a measured amount of hardener liquid. All three fly against the gel coat, flatten out and soon harden. The rope has one of its filaments colored red or blue and the density and uniformity of these bits of color among the white strands serves as a guide to judge uniformity and thickness of the spray-up.

Chopped-strand laminate rates low in puncture resistance but serves very well in moldings for decks, hatches, seats, etc., which have many surfaces and angles that take time to lay up by the hand process. Sometimes chopped strand is used for the first layer in a hull since the individual filaments flying onto the working surface do not tend to trap air bubbles between the gel coat and laminate. The roving and mat in suitable combination are used to get thickness and strength.

Making boat parts of fiberglass is easy on one hand, yet requires knowledge of the tricks of the material on the other hand. Laying pieces of mat and roving in place, dabbing resin into them with a brush and rolling out to achieve complete saturation is basically as easy as mopping a floor. Yet the resin is sensitive to temperature and the amount of hardener used. When the stuff begins to harden or “kick” as the fiberglass people say, it generates heat. If too much hardener is used for the existing room temperature, or too many layers of fabric are put on at one time, excessive heat can be generated. This can cause wrinkling of the gel coat, self-scorching of the resin and in rare cases, fire. So the work must be supervised by a person familiar with these and other peculiarities of the material.

It is common for a work crew of several persons to be under a foreman having the necessary experience. They can be quite inexperienced; all will go well if the foreman keeps watch over what is going on. Where a skilled wooden boat builder must serve a long apprenticeship before he can be trusted to do good work on an expensive boat, with fiberglass new help can be hired off the streets and put to productive work after a brief coaching in how to use the brush and roller. This is important, because there simply are not enough experienced wooden boat craftsmen around to give today’s mass-production boat manufacturing factories the labor pool they must have, and it would be entirely too slow and costly to put the required number through an apprenticeship.

One of the several things a fiberglass foreman must keep his eye on is the ratio of resin to glass in a laminate. If too little resin is used, individual glass filaments will have insufficient bonding to their neighbors. The laminate will look “dry” or “spongy” and will be weak. If rather too much resin is rolled into the fabric, the cured laminate will have a shiny, glassy look, resin cost will be uneconomically high and the laminate may be brittle. After resin is brushed and rolled into the fabric, something like a quarter to half an hour will pass before it begins to “kick,” depending on the amount of hardener used, room temperature and laminate thickness. During this period, resin can run down out of the fabric on vertical surfaces, so the person in charge must keep an eye on it during this period.

Resin is compounded to be thin enough to wet the fabric and spread through it reasonably quickly, yet thick enough to resist running. If a particular batch or low room temperature leads to running problems, the operator will add to it some powdery filler material to thicken it up.

To a noticeable extent, fiberglass boat shapes are designed to facilitate the work. There can be no reverse curves or angles that would lock the finished molding into the mold. All surfaces are therefore given enough slope or “draw” so that finished parts will lift out with little resistance. Deep grooves and sharp corners are avoided as much as possible to facilitate pressing dry fabric into such recesses in the molds. When sharply bent, each filament of glass naturally wants to straighten itself out.

If the mold has a deep groove or sharp corners in it, although the resin-saturated mat is limp and bends to fit under the pressure of brush roller, while the workers go on to another mold the glass can pull away from the mold at these places and leave air bubbles between the gel coat and laminate. The relatively weak and brittle gel coat, averaging only ,015″ in thickness, is thus unsupported by fabric under these bubbles and the first time appreciable pressure is applied the gel coat will crack and crumble. On a wooden boat, a deck toe rail might be made of three-quarter inch square wood.

Lamination of a cockpit
These workers are laminating up a cockpit liner over a special mold made to create the intricate shapes. Making large parts from a single piece of fiberglass requires much less costly hand labor than assembling many wooden parts

If this is done in a fiberglass boat, the resulting deep, sharp-cornered groove in the mold will invite air bubbles. Thus the toe rail will be made by affixing to the deck plug some wood perhaps half an inch thick and two inches wide, with beveled edges. This gives a shallower groove with less pronounced edges in the mold and so the mat goes on more readily and is less likely to spring back and form air bubbles.

Much more can be said about the peculiarities of fiberglass work. But the foregoing is enough to give a clear idea of how it differs from woodworking. Now this mention of wood brings us back to the wooden boat. If a person today decides he wants to make or have a fine wooden boat, a lot of expensive labor is going to go into it. The temptation is thus very strong to design this new boat and make it so that it can be used as a plug from which to make a fiberglass mold. It may take a month for a crew of men to frame up and plank a fine wooden hull. If they then spend a little extra time giving it a polished finish, a mold can be made from it. From this mold, many copies can be made in fiberglass.

But the real payoff comes when we get to the inside of the boat. The wooden floor, cockpit openings, lockers, shelves and other compartments call for a vast amount of slow work, cutting, fitting and assembling each small piece of wood. That is where the cost of a wooden boat starts to skyrocket, at today’s labor rates. On the other hand, once a plug and mold have been made, a fiberglass floor can incorporate as many features such as pump sumps, fish boxes and bait tanks as the designer might like. It can be made up with far less labor than is true of wood, and features like bait tanks can have rounded corners that are easy to wipe clean, without the need to make and fit fussy strips of wood molding in each one. In fact, the rounded edges of a bait well that the owner finds easy to clean, also make it easier for the shop people to laminate the fiberglass without air bubble problems.

There is growing interest in locating and painstakingly restoring antique wooden boats. We all greatly enjoy seeing them when they appear, for everyone likes the rich warmness of varnished wood and admires the craftsmanship that goes into the framework, planking and trimming. But when an unreconstructed veteran boatman points to such a craft and says,

“That’s the kind of boat for me, they really knew how to build boats then! Your modern fiberglass cheeseboxes leave me cold!,”

he is speaking without taking into account the cold economic fact that the 20-foot fiberglass boat that costs him $4 000 might well cost him $10 000, $12 000 or even more if it was made of real old-time oak, mahogany and cedar.

As a matter of fact, a factory building boats in volume today would have a very hard time obtaining enough really good wood for their output. Yet another good thing about fiberglass is, it leaves the oxygen- producing trees standing in the forests and puts to good use the petroleum by-products left over from the manufacture of gasoline. The glass is based on sand, of which there is no shortage, and the resin is based largely on petroleum. Odd thought … the Arabs have all the sand and oil they need to build a huge fleet of pleasure boats, but not much water in which to float them!

One Touch of Glass

Fresh water weighs 62 pounds per cubic foot. Fiberglass laminate weighs between 90 and 100 pounds per cubic foot, depending on the proportion of resin to glass fabric. The dismal conclusion to be drawn from this is that boats made of fiberglass will, alas, sink if swamped.

Hmmmmm! Very dangerous! Hadn’t we better ban them for the public good?

Well, the men who developed fiberglass around thirty years ago anticipated this squawk and had a ready answer. Everyone knows that steel and ferrocement boats will sink too, and so will most wooden boats that carry engines and other heavy metal equipment.

Therefore, from the very beginnings of the fiberglass boat industry, craft of this material coped with the sinking potential through the simple expedient of stuffing vacant areas with lightweight flotation material.

Simple? Well, now! Admittedly it seems that way, and many boating enthusiasts assume that it is so. But then when they find themselves face to face with some practical manifestation of the complexities of flotation, they are quite often thrown off balance.

A typical example is the chap who writes to a boating writer or magazine and says,

“I own a 16-foot Watterbubbler Hooker fiberglass boat and would like to know how much pour-in-place foam to put into it so it won’t sink when swamped?”

When the boating authority replies that there is no simple formula, no way to pull a quick answer out of a magic hat, the fellow is apt to be shocked and sometimes probably suspects the expert is giving him the run-arouond out of laziness or cussedness.

But that is not so. The subject is complicated. So much so, in fact, that even after 30 years of talking about how it should be done, top experts on flotation in government and industry still have not been able to devise the simple, reliable formula everyone would like to have.

The simplest and usually cheapest form of flotation is enclosed air spaces or tanks. Steamship’s lifeboats have used this method for a long time but before jumping to the conclusion that it must be a good method, you have to remember that such a boat spends most or all of its life suspended from davits a substantial distance above the ocean’s surface. If there are air vents in such tanks, they can be left open and in time of emergency a crewman or warning label can be relied upon to make sure they are closed when the boat is dropped into the sea.

In a small pleasure boat, air tanks are usually a horse of another color. They cannot be made of fiberglass so that they are sealed tightly and permanently, for then they would “pant” with changes in atmospheric pressure and in time develop pinhole leaks at joints. When they were used, they were usually fitted with small combined drain and vent plugs in their lower regions, to equalize air pressure and allow condensed water to drain away. The Achilles heel in this setup is that if there’s such an opening in the bottom and an unsuspected air leak higher up in one of the seams, when the boat is swamped the water can enter and gradually fill the “buoyancy space.” Not very reliable, at least not sufficiently so for mass-produced boats being sold to all kinds of people, including those who will neglect maintencance and inspections.

The better way is to place lightweight foam plastic into the boat at conventional places and in sufficient volume to keep it afloat. The various kinds of foam used for boat flotation weigh from around 50 to 62 pounds per cubic foot. One might say,

“Shucks, it’s really simple! If a boat weighs 620 pounds, just put a little over 10 cubic feet of foam into it and she’ll float!”

Comes now the first of a number of complexities, however!

Once this boat is fully immersed in water, it no longer weighs 620 pounds. In displacing some water due to the thickness of its laminate, the fiberglass in the boat develops some buoyancy; not enough to make the boat float but definitely enough to make it weigh less when surrounded by heavy water than by much lighter air ashore.

With a wire, suspend a swamped boat a few inches under water and fasten the upper end of it to the hook of a scale. With water at 62 lbs. per cubic foot and fiberglass laminate at a little over 90 lbs., it will work out that a 100 lb. skiff submerged in water will “weigh” only 33,3 lbs. In technical language, fiberglass laminate has a “specific gravity” in the region of 1,5. A boat that “weighs” only 33,3 lbs. under water can be supported by little more than one-half a cubic foot of foam. This will take up mercifully less useful space aboard than the 10 cubic feet snap judgment suggested was the proper amount.

But whoa! This isn’t the end of the story by any means!

The above figuring assumes the boat to be made entirely of fiberglass. In actual practice of course a useful boat is certain to have other materials in it. Wood in various forms and shapes is quite likely to be used for the transom core piece, keel and bottom reinforcing strips or grids, decorative trim and for the blocks embedded in the laminate to receive screws and bolts for hardware attachment. This wood, at from about 30 to 40 lbs. per cubic foot on the average, is quite buoyant, wants to float, and does add some flotation to a craft. Question: How much? Here come de slide rule men!

In any boat there is material as heavy as bronze and as light as upholstery padding. The people in charge of developing a viable flotation formula are gentlemen, so we assume they probably have not resorted to punching and shoving matches in debating how to allow for such materials in their evasive formula. But we suspect that they may have at least looked daggers at one another occasionally, as a result of disagreement over how to allow for variables like this and also such factors as how much and how fast upholstery foam will absorb water, the role of full, half-full and empty fuel tanks on submerged buoyancy and submerged stability, and, oh, you name it!

In actual practice it works out to using horse sense. Some reasonably close initial calculations are made, then the indicated amount of flotation volume is taped into a bare test hill. This is put into the water and swamped to see what will happen. A series of in-the-water tests generally leads to a practical flotation arrangement.

You begin to understand why it isn’t possible, (off the cuff) to tell a boat owner how much flotation to put aboard his craft. Even if you could tell him he needed x cubic feet of foam in such-and-such a location, he could go astray. The amount to which a batch of foam-in-place material will expand depends a lot on the air temperature in the working place. The hotter it is, the more it will expand. Thus two one- quart cans of foam mix used in an unheated garage in February will produce a substantially smaller volume of flotation than it would if used in July.

Also, it’s rather hard for a boat owner to weigh accurately his complete boat to judge how many cubic feet of foam have expanded in the places he has poured it into. Yet more, the stuff does create some pressure when it expands, and unless a person knows what it’s all about and provides adequate pressure relief holes in a confined area, the foam can press so hard as to bulge or even rupture the surrounding material.

Where best to put the flotation material? For a score of years it was common practice to put it all below the floorboards. There it was out of the way and took up no useful space in the occupied part of the cockpit. There are desirable advantages to this location. It deadens the noise of drumming over a chop. If a submerged object punctures the bottom, foam in the area will keep water from flooding in. Also, filling the bilge area with foam-in-place material adds usefully to the overall rigidity and torsional stiffness of a hull.

Fiberglass boat
Most fiberglass boats, especially the larger ones, have internal stiffening structures in the form of fiberglass grids such as that shown here. The cockpit liner will fit down and contact the top edges of these stringers, providing a strong floor and giving the hull good torsional stiffness

The fly in this tempting soup is that when all flotation is located so low in a boat, there is a powerful tendency for it to roll upside down after being swamped. With a lot of water in the cockpit, the foamed area finds itself pressed down in the sea several inches or more, depending on the size and design of the boat. The center of buoyancy is thus below the center of gravity, and the setup is like that in a weird airplane having its wing several feet below the fuselage. Just as the plane will want to roll over so that the lift is above the weight, the boat will be forced to turn over as a result of the low-riding flotation’s urgent desire to swap places with the center of gravity.

A few inches of water in the cockpit usually won’t bring this tendency into play to a serious extent. All depending on the particular boat, though, there will come a time when there’s enough weight of water there to accentuate the tendency. A slight lopsidedness in the hull’s shape that puts a bit more buoyancy on one side than on the other, an off-center weight such as the steering wheel, or a slight movement of a passenger is all that’s needed to start things moving and make the boat roll over.

When this happens most boats won’t sink, partly due to the foam flotation, but in practice it is usual for a lot of air to be trapped inside the hull as the craft rolls over. This is much like water trapped in a drinking glass held inverted and pressed down into a pan of water. Provided the bottom hasn’t been punctured, the transom drain plug is in place and there aren’t other air leaks in the sides or bottom, the hull will hold this trapped air under it for a long time and float rather high in the water. Unfortunately, the smooth bottom of a typical fiberglass hull is very difficult for people to cling to for any length of time and even if there are appreciable projections to afford a good grip, fatigue can come on rather rapidly if the water is at all chilly.

One can say that in the event of a swamping or capsize, people should make use of the life preservers all boats are required to carry by law. However, these are often thrown out of a smaller boat and drift out of reach when the craft is in a collision or flips over suddenly while going fast. Also, they are trapped in lockers or under a foredeck and become inaccessible when the boat fills with water.

Therefore, the trend has been to seek ways to move the flotation material higher inside the boats. You’ve seen more and more new boats with storage spaces under floorboard hatches. This is actually putting to use space that has been vacated by the upward migration of the flotation. Less of it below and more of it higher up, makes a big difference. More and more new boats now have “positive upright flotation” when swamped.

Again, it is not as simple as one would at first think to achieve this. One problem is to find space for flotation material up under the decks, at the cockpit sides and near the motor in the stern without using up too much space needed for other purposes. It’s common to find a bit of flotation material here, a bit there, distributed around the inside of the boat in odd-shaped and sometimes oddly-located places. But you can be assured that a lot of work went into figuring out the final arrangement.

Although they are made in the same mold, no two fiberglass boats of a particular design weigh exactly the same. Several pounds or more of variation, depending on the size and design, is the norm. The various workers in the laminating department use different roller pressures and hand movements. The amount of resin worked into the glass cloth varies depending on each worker’s “feel” for the spray gun or brush he is using and the amount of “wetting out” he judges to be desirable to fill and bind the layers of fabric. In addition, once a boat has been sold the new owners are likely to install a variety of accessories and equipment. By the time a new boat of a particular model is launched, it can weigh many pounds more, or several pounds less, than the median figure for its model.

The practical answer is to provide surplus flotation. How much more? A reasonable guideline would be to have enough so that when the boat is swamped, it will float and support its occupants with their heads and shoulders above the level of water in the cockpit. This would allow to a reasonable degree for variation in boat weight, and would permit occupants to remain inside the boat in reasonable comfort and safety while awaiting rescue.

A fine point worth knowing about is the fact that enough flotation to barely keep the boat and its motor afloat can be as bad as no flotation at all, for it gives the operator a false sense of security. Believing his boat has flotation and assuming it is of adequate amount, a skipper overtaken by a storm or venturing into a tide rip would not tend to be overly concerned about the consequences of a swamping. Then, it if actually happens and the boat shows signs of sinking under the weight of occupants clinging to it, panic can result and that’s the thing tragedies are made of.

There are many models on the market that have not been able to qualify for the coveted boating industry trade association’s certification, which includes tests for flotation. Typical are small open boats with no side decks.

Installing flotation material inside the gunwales intrudes into useful space, or the covering material needed to protect it from abrasion ups the cost out of proportion to what the public will pay for such a boat.

Installing it outboard of the gunwale can interfere with rowing, expose the material to abrasion, make the craft too wide for practical purposes (such as carrying in the back of a station wagon) and so on. They remain on the market because, after all, for thousands of years men have relied on their ability to swim, their skill at boathandling and their knowledge of the sea to cope with the risks of being afloat.

There are still a great many people around possessed of this level of seamanship and who would dare to decree that they must now accept flotation capabilities devised for the benefit of the mass of unskilled newcomers?

Different kinds of foam are used for flotation. One kind can be purchased in the form of large baulks called “logs” in the trade. These are sawn to fit into various spaces in a boat, and taped or battened in place. There is some waste in the cutting and it is usually not possible to completely fill the irregular-shaped spaces inside most boats. Polyester resin, the type most often used in boatbuilding, quickly desolves polystyrene foam so in order to build it into a fiberglass boat, epoxy resin has to be used. This adds to expense and complication in production work.

Fiberglass hull
Here a fiberglass hull is being lowered onto a deck. The white dough-like material on the underside of the deck is flotation foam. Hull and deck moldings fit together like the lid fits on a coffee can. Fiberglass cement and pop rivets hold the two parts together securely

Polyurethane foam can be put to use in different ways. Not attacked by polyester resin, it can be incorporated in the lamination work. Sometimes thin sheets of it are incorporated in the laminate, serving to thicken and stiffen it, provide insulation against “sweating” from cold outside seawater, and also give flotation. Sometimes strips or blocks of it are built in to form structural reinforcements plus flotation. Small squares of very light balsa wood, cemented to a cheesecloth-like backing and sold in rolls under the name “Balcor” (meaning “balsa core”) is used in similar fashion.

Another way of using polyurethane is to spray a layer of it onto the inner surfaces of the hull. Special and rather sophisticated spray guns are used. The method also being common for applying this type of foam insulation to large buildings. The resulting surface is irregular and homely in appearance and is usually covered with attractive vinyl upholstery or hidden behind cockpit lining panels.

Pour-in-place foam can be applied in two common ways. It can be used to fill confined, irregular spaces or can be poured in place and allowed to expand to random shape in out-of-sight places. It’s often used the first way in Basic Hull, Keel, and Rudder Shapesdouble-hull construction. A jiglike fixture is made which clamps snugly against both inside and outside surfaces of the hull to prevent foaming pressure from distorting them. The “mix” is poured in through holes made for the purpose and any surplus oozes out of them. When hard, it is trimmed off and the holes sealed with disclike covers. Foam between inner and outer shells stiffens them so that internal reinforcing strips or grids are not needed, thus saving labor.

In the other method, the mix is simply poured in place, experience teaching the worker how much to put in a given space. Look under the forward and side decks of a typical sporty-looking small runabout and you’ll see it there. It looks like bread dough.

We’ll taper off this discussion by mentioning points of general interest. One of the frustrating factors in devising a flotation formula that works well on all types of small craft is the little problem of the water’s changeable surface. Waves come in infinite size and shape. Ah, er, well, it’s a bit odd but quite illustrative of the problem to say that if we fed it into a computer, the variables would drive the computer into the nut house! As some easily-duplicated standard condition is needed, all hands have had to agree that calm water is the only choice.

You have to keep this in mind constantly! The flotation in your boat will do the job claimed for it all right on calm water. But you’re most likely to get into trouble in rough water. Naturally, no private person wants to swamp his boat out on a rough bay one day simply to find out for himself how well its flotation will work under such conditions. So throughout your seafarin’ career, you have to remember that if some day you swamp or capsize, that’s when you’re going to learn how the craft’s flotation arrangement is going to make her behave on rough water. Be ready, size up the situation and then act as coolly and reasonably as is possible.

Sometimes when puttering with their boats, owners see and feel some of the flotation foam. On pressing it, they see a little water ooze out. At once they assume the stuff has soaked up a lot of water and has become useless. The foams used for flotation in boats of reputable make are composed overwhelmingly of closed cells that can’t take in water. In between the fully-closed ones, there are usually a small percentage of broken or imperfect cells. The water gets into these in sufficient quantity to make the foam seem to be “soaked” when pressed.

Usually such a condition is found in the lower areas. The foam is not like a sponge and has not drawn water up and into its entire mass. Your flotation remains in useful quantity. If in doubt, carve out a small cube of the water-bearing foam and a similar cube from a higher, dryer area. Have your high school chemistry lab or drugstore weigh the two on a sensitive scale. If you’re convinced the foam really is full of water and objection-ably heavy, take up the matter with the boat’s builder. Be tactful, lest he start foaming at the mouth!

From time to time boat owners find themselves wondering about air bags as flotation means. In Europe, small racing sailboats do often carry such flotation. These craft used for competition are generally cared for and fussed with as are racing cars and air bags would be repaired or replaced when needed. Also, our own Coast Guard has always been very leery of any kind of inflatable lifesaving equipment for good reasons – it might fail to inflate when needed, it might dry out and prove to be porous after a number of years of idleness, it might get ripped open in an accident and be useless.

Author photo - Olga Nesvetailova
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