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How to Check a Fiberglass Boat for Rot: The Ultimate Guide

As a boat owner, one of the most important things you need to know is how to check your fiberglass boat for rot.

Rot can cause serious damage to your boat, compromising its structural integrity and potentially putting you and your passengers at risk.

In this guide, we’ll cover everything you need to know about detecting and dealing with rot in your fiberglass boat.

What Causes Rot in Fiberglass Boats?

Rot in fiberglass boats is typically caused by water intrusion. When water seeps into the core of the fiberglass, it can cause the wood or foam core to deteriorate, leading to delamination and weakening of the hull.

This can happen due to a variety of reasons, such as:

  • Damage to the gelcoat or fiberglass skin
  • Poorly sealed hardware or fittings
  • Cracks or holes in the hull
  • Exposure to moisture over time

How to Spot Signs of Rot

close up photograph of a person inspecting a boat with flashlight

One of the most common ways to check for rot is by conducting a tap test.

This involves gently tapping the surface of the fiberglass with a small hammer or coin. If you hear a solid, crisp sound, the fiberglass is likely in good condition. However, if you hear a dull thud or a hollow sound, it could indicate rot or delamination.

Other signs of rot to look out for include:

  • Soft or spongy areas on the deck or hull
  • Visible cracks, blisters, or discoloration
  • Warping or distortion of the fiberglass
  • Musty odors or visible mold growth

Checking the Transom and Floor

The transom and floor are two areas of a fiberglass boat that are particularly susceptible to rot.

To check the transom, start by visually inspecting it from both the inside and outside of the boat. Look for any signs of bowing, deformation, or cracks.

You can also use a flashlight to look for signs of rot or discoloration on the inside of the transom where there is no gelcoat.

someone with a flashlight checking a boat transom

To check the floor, you may need to remove any floor coverings or panels to access the underlying fiberglass.

Once exposed, conduct a tap test and look for any soft spots, discoloration, or signs of water damage. If you’re unsure, you can use a moisture meter to detect the presence of water in the fiberglass.

What to Do If You Find Rot

If you do find signs of rot in your fiberglass boat, it’s important to address the issue as soon as possible. Depending on the extent of the damage, you may be able to repair it yourself or you may need to enlist the help of a professional.

For minor rot or delamination, you can often repair it by removing the affected area, drying it out completely, and then filling it with a marine-grade epoxy or resin.

However, for more extensive damage, you may need to cut out and replace the entire affected section of fiberglass.

==>> Also read: Can a fiberglass boat sink?

Preventing Rot in Your Fiberglass Boat

Of course, the best way to deal with rot is to prevent it from happening in the first place. Here are some tips for keeping your fiberglass boat in top condition:

  • Regularly inspect your boat for signs of damage or water intrusion
  • Ensure all hardware and fittings are properly sealed and maintained
  • Store your boat in a dry, covered area when not in use
  • Use a high-quality marine wax or sealant to protect the gelcoat
  • Address any damage or leaks as soon as you notice them

By following these simple steps and staying vigilant for signs of rot, you can help ensure your fiberglass boat stays in great shape for years to come.


Checking your fiberglass boat for rot may not be the most exciting part of boat ownership, but it’s an essential task that can save you a lot of headaches (and money) down the road.

You can keep your boat looking and performing its best by understanding what causes rot, how to spot it, and how to prevent it.

So next time you’re out on the water, take a few minutes to give your boat a once-over and make sure everything is shipshape!

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