Skip to Content

A Fibreglass Boat Can Get Waterlogged (How to Avoid Damage)

Fiberglass is the unsung hero of the boat building industry. It is incredibly flexible when applied and ends up being an extremely hard structural material with a lifespan measured in decades. The problem is that while it is impervious to water, the owner can be lulled into assuming that all is well inside the hull.

Indeed, fiberglass cannot absorb water and become waterlogged, however, suppose the fiberglass structure is compromised (even a missing screw). In that case, water will enter the hull, and it will become waterlogged, which will damage the flotation foam and the wooden structural components.

It is important that the condition of a boat, however large it is, be regularly assessed and where necessary, repairs are made. While fiberglass boats are less susceptible to being waterlogged than wooden vessels, it does not mean they are immune to the problem.

A Fibreglass Boat Can Get Waterlogged

There are very few craft that can be called pure fiberglass vessels.

Most are a combination of structures that work with a GRP coating to ensure structural strength.

Even the hull is not a pure fiberglass structure but is made up of a polyester resin reinforced with glass fibers, laid down in layers to form a laminate.

There are three ways a fiberglass boat can get waterlogged as follows.

  1. Water can leak into the hull area and ruin the floatation foam.
  2. Water can leak into the hull and destroy the wooden stringers and transom.
  3. Water can seep into cracks in the gel coat and separate the gel coat from the fiberglass.

Water Can Leak Into The Hull Area And Ruin The Floatation Foam

If water leaks through the hull, possibly via a damaged portion, a crack that has formed (sometimes this happens at the transom), or even a loose or missing screw, it will soak the floatation foam, causing it to become moldy and ineffective.

See also  What Does Internet Cost On a Catamaran? (The 3 Options)

A tell-tale sign the foam has become waterlogged may be that the floor is separating from the stringers and bending upwards.

The only way to remove the old moldy foam is to either cut the boat’s floor out or disconnect the upper part of the fiberglass boat away from the hull.

If you have not done this before, it is strongly recommended that you contract the work out to a professional or have an experienced boat repairer help you with the work.

Water Can Leak Into The Hull  And Destroy The Wooden Structures

Water may have penetrated the hull, which is almost guaranteed to have occurred in an older boat. It will result in the wooden stringers and other structural elements being exposed to the moisture for extended periods and, in all likelihood, have been damaged.

If you are removing the floor to change the flotation foam, it is important that you also check the wooden structures to verify their condition.

These will include the stringers, transom, and support for the ski pylon.

If you plan to do such extensive work on your boat, and you have not done this before, it is recommended that you seek help from a professional. 

Water can seep into the gel coat

Since the 1970s, boat repairers have adapted the word osmosis to describe the blistering found on GRP boat hulls.

When a GRP hull is molded, the final product consists of

  1. The first layer of gel coat is painted onto the body mold.
  2. Color markups
  3. Various layers of GRP coating are then applied over the gel coat.
See also  How to Winterize a Cabin Cruiser Boat (Steps & Methods)

During construction, the GRP laminate will have small deformities, air pockets, and microcracks within the resin and at the point where the resin and the glass fibers. 

Through the process of osmosis, water molecules diffuse through the gel coat. In certain instances, the water molecules pass completely through the GRP laminate, while others collect in the small deformities, air pockets, and microcracks within the resin.

When the GRP is laid down and cured, parts of the solvents and unreacted constituents remain during the manufacturing process. The water within the voids dissolves and reacts with these components in hydrolysis.

These unprocessed solvents and unreacted constituents absorb the water, which accelerates the water absorption rate into the laminates. The voids increase in size, and the pressure within them also increases.

If the pressure within these voids increases too much, it forms a blister. If the laminate remains untreated, it will break down, causing the process to accelerate faster and form more blisters.

While, in the beginning, it is purely a cosmetic issue, if it is left too long, the hull’s structural integrity will become compromised.

How To Repair Damage Caused Through Osmosis

While pulling the boat out onto dry land will cause the water to evaporate, the contaminates in the empty spaces will not drain away.

The optimum process to remove the blisters is as follows.

  1. Steam cleaning and thoroughly wash the hull surface.
  2. If there are isolated blisters, these can be cut off with a chisel, washed out, and filled with epoxy resin.
  3. If there are too many blisters, the gel coat can be stripped. 
See also  The Accuracy of Boat Gas Gauges (How Does It Work)

As the Gel coat is an impenetrable barrier, it is impossible to clean it off using solvents. The only product which works is a Gel Peeler. It removes a controlled gel coat/laminate thickness, leaving an even surface.

The problem with peeling the gel coat is that the small deformities, air pockets, and microcracks remain, and if they are not treated, the new gel coat will suffer from the same problem.

However, if there is a large area to be repaired, it is still preferable to treat each blister separately and fill it carefully with epoxy resin. It will fill the small pockets and help prevent the situation from reoccurring.


While fiberglass itself does not absorb water and therefore cannot rot, there are several ways for water to enter a fiberglass hull and compromise the wooden structures and the flotation foam.

As frustrating as a damaged hull is, the good news is that the defects can be repaired in almost all circumstances, and the effects of waterlogging can be rendered zero.