Fiberglass strength for ocean sailboats.

A review of lamination methods in the boating industry.


A candid look at high-strength fiberglass construction.

   Because many people understand that fiberglass is a strong building material for boat hulls (and it is in comparison with other materials), they tend to drop the subject at that. However, it is fact that there are dramatic differences in the strengths of different glass laminations. Only when sailors plan to subject their boats to the heavy pounding of the open sea, with family or friends aboard, does the subject receive further investigation.

   When we started in the business of making ocean sailboats, we visited a number of repair yards in the area. (We learned as much in these visits as we did by reading countless volumes of data.) After looking at scores of jagged holes and cuts in boat hulls, and listening to the repair veterans, we had graphic examples that the lay-up schedules on fiberglass laminates were of critical importance - and available engineering data deserved careful study.*

   Summarizing our research, we have found that:

  1. Thickness of the hull usually is not a good measure of boat strength.

  2. Woven-roving laminates are much stronger than laminates using mat-roving combinations.

   This brochure will detail for the reader our findings on high-strength fiberglass hull construction.


Jerry Husted President, Blue Water Boats

* (The predominant authority in the U.S. fiberglass industry is GIBBS & COX, noted New York engineering firm, who has shown graphically the differences in fiberglass strength. Some of the information in this brochure is based upon their findings. We'd like to give thanks to them (and to Owens Corning Fiberglass, Inc. - who sponsored Gibbs & Cox's work) for allowing us to reprint their graphs on fiberglass strength.)

Why you need an ocean boat that is strong.

   Let's talk about boats sailing on the ocean - as opposed to those sailing in protected inland waters. There are very different kinds of stresses on the ocean boat. Here are the main ones:

Driving hard into the wind.

   This often occurs for a week or more at a time. The hull plunges down off a crest and hits the trough, bringing everything to a shuddering halt. Spars and rigging have a tendency to continue their arc and put immense strains on their hull connection points.

   The forward sections of the bat are continuously pummeling their way through walls of hard water. (Imagine a giant medicine ball being continually clammed into your boat's hull sides. If the boat were a living thing, it would have the wind knocked out of it many times!)

   There are many records that document how this punishment begins to crumble the resin out of the laminate in the forward sections of the hull, and how it can loosen the bulkheads from the hull. This is why marine surveyors always look carefully at the bulkhead-to-hull joins in the forward section of a boat under survey.


   No one expects his boat will go aground! But if a boat truly does make passages, any sailor has to be concerned with the possibility. Even the most experienced seaman admits to the strong possibility of grounding. For example, there is a reef around every island in the Pacific. Enough miles of travel, combined with unpredictable currents and poor visibility, makes grounding a high risk factor.

   Some groundings can be fatal to a boat. Others cause only minor damage. The boat builder has a lot to do with the final outcome.

Collision damage.

   We're not necessarily suggesting being run down by ships. We're simply talking about coming into docks harder than usual; or having a fishing boat raft up to you and give your midships a terrific squeeze; or being anchored off an island where the swells have worked your boat loose from its anchorage and backed it onto the rocks for a time. (In this case, the rudder is forced to act as a bumper for a multi-ton boat without breaking up immediately.)

   The ocean boat must have more potential for punishment than its inshore relatives.

Does thickness mean strength?

   Everyone knows that the thicker a piece of wood or steel, the stronger it is. The same DOES NOT apply to fiberglass laminates. Thickness in glass laminates can be easily built up by using lower strength material (often with less labor costs as well). For example, a 1/4" thick glass hull can be many times stronger than a 1/2" thick hull that used different materials.

   Several basic truths about fiberglass hull strength:

  1. Strength in fiberglass laminate comes primarily from WOVEN ROVING - bundles of continuous glass strands, loosely woven together.

  2. RESIN, which is used to saturate and bond the glass together, has very little inherent strength by itself. This is easily demonstrated by dropping a chunk of hardened resin on the sidewalk and seeing it shatter into tiny pieces.

  3. Glass strands and resin joined in a laminate section is measured by engineers as a percentage of glass (by weight) to the total. For example a laminate can vary from 35% glass (65% resin) in a normal boat hull to 50% glass (and 50% resin) in a high-strength hull.

    Some hulls arriving on the market actually test out to a low of 25% glass - 75% resin!

  4. The only way to achieve a 50% glass-to-resin ratio in a hull is to use woven-roving exclusively - and to squeeze out excess resin after saturation.

   When examining damage to fiberglass hulls, there is a definite pattern of glass content to strength. The 35% glass-content hulls will generally shatter out and leave a large jagged hole upon impact. The 50% hulls (there aren't many made that strong) may have their resin shattered leaving a whitish blotch in the hull, but generally are left intact with no hole from the impact!

Types of fiberglass construction.

   There are three types of fiberglass material usually used in boat construction:

Chopped strand mat. (low strength)

   1 1/2" hairlike, fine strands, laid in random pattern. Generally mat is laid in the boat from a roll, or blown in with a chopper gun. Its purpose is to bond other layers of cloth or roving to beginning surfaces. It acts like a blotter and soaks up lots of resin. Its ability to build up thickness (don't confuse thickness with strength) delights some boat advertisers who loudly proclaim: "Our hull is xx inches thick!" Chopped strand mat has a purpose but it certainly is not high strength!

   Don't be misled by some boat advertisers who are now calling chopped strand mat "strand roving" in an attempt to capitalize on the strength implied in the roving type of glass.

Glass cloth. (medium strength)

   This is a continuous strand of glass, woven in a cloth pattern. It is thin and light. It belongs in lightweight strength applications, like canoes, kayaks, etc. It is also used as a layer of glass next to the gel coat - for it cosmetic value and smoothness.

Woven Roving. (high strength)

   Heavy bundles of continuous glass strands - loosely woven, similar to a heavy wool sweater. A bundle of these strands can be tugged and pulled and will demonstrate extremely high tensile strength. Woven roving is often compared to putting layers of steel reinforcing rods in cement.

Hulls laid with "Mat and Roving"

   Most boat builders alternate one layer of "mat" with one layer of "roving". The mat's purpose is to conform to the rough surface of the previous layer and fill in the rough surface of the new layer of roving. It provides a good bond, meaning that it sticks the two layers together well.

   Additionally, the technique of laying this type of laminate allows workers with long handed rollers (like paint rollers) to roll out the air bubbles trapped inside the wet laminate. This is the technique of "hand laying" fiberglass - as opposed to "blowing" the chopped strand mat in with a chopper gun. This method is commonly used in production boats.

   This illustration shows how the two types of fiberglass and resin are combined. Notice how the mat acts as a blotter and creates pockets of resin between the roving layers.

   Picture what would happen if the resin shattered out on impact. Since the layers of roving are not compact, jagged holes may be left in the glass.

Hulls laid with woven roving only.

   This stronger technique uses several layers of wet saturated roving (three at a time is ideal) that are squeegeed with a rubber squeegee.

   The squeegee does three things:

  1. First it squeezes air bubbles out.

  2. Second, it compresses the strands of glass down into one another (like ironing the wrinkles out of cloth) to eliminate pockets of pure resin. It also locks the strands together and compacts them, providing a bond equal to that of mat.

  3. Third, it squeezes out all of excess resin, leaving a laminate rich in glass content (as opposed to one being rich in resin content).

   The combining of layer upon layer of roving is shown below. The squeegee eliminates excess resin. As a result, the laminates are tightly bonded and much more compact than the mat & roving sample.

   On severe impact, the resin may shatter and leave a white spot, but the strands of glass still hold together because of extraordinary tensile strength.

   Why all boat builders don't use woven roving exclusively:

  1. The woven roving method takes more skill.

  2. It requires a great deal more labor.

  3. Many boat builders have played down the importance of fiberglass strength in favor of thickness. The public is often not aware of the difference.

The Gibbs & Cox study.

The predominant authority in the U.S. fiberglass industry is GIBBS & COX, noted New York engineering firm, who has shown graphically the differences in fiberglass strength.


   The tensile strength of the roving laminates is stronger than the mat-roving laminates. A greater tensile strength gives the glass laminate the ability to hold together under enormous strain and remain intact. Mat-roving laminates with less tensile strength would tend to shatter and leave a gaping hole on impact.


   The flexural strength is the resistance to bending motion. Insufficient flexural strength results in "oil-canning", or the hullsides flexing in and out.


   When an ocean wave hits the side of a hull it tends to force the hull inward. The hull must be strong enough to minimize, resist, or endure this force through violent and prolonged conditions. Also, When the hull flexes, the inside skin exerts a force that resists bending.


   Although the gain in compressive strength is not as dramatic as the tensile and flexural strength, it is still important. Notice from the graph that the roving laminates are approximately 18% stronger than the mat-roving laminates.

Blue Water Boats construction methods.

   At Blue Water Boats our mission is building a high-strength hull to withstand the punishment of ocean sailing. Our position in the market place is assured by concentrating on structural integrity.

   We feel that an explanation of fiberglass strength to those who plan on ocean sailing is vital. (For inshore sailors, the strength of the boat is of somewhat less importance.) Many boat manufacturers dismiss lamination construction in their brochures. In ocean sailboats this omission is unforgivable! Beauty and performance are important, but strength should be the primary consideration for safety at sea.

   We hope that this brochure has given you a better insight into what makes a fiberglass boat strong. If you are interested in other aspects of boat strength (bulkhead construction, installation of ballast, etc.) please write for our FREE information.