Fiberglass Core Repairs, Part One

If you are considering buying an older fiberglass boat that your marine surveyor has indicated has water in the hull, don’t be put off– this is fairly common and can be easily remedied. Boat hulls and decks built with fiberglass skins sandwiched around a core are lighter and typically stronger than boats built with the same thickness of solid fiberglass.  For this reason, sandwich core construction is a popular method of building fiberglass boats. However, water intrusion can rot wood cores or delaminate the sandwich structure and weaken the hull, posing a potential problem with this type of boat. Some builders have compromised by building the section of the hull below the water line with solid glass, while taking advantage of sandwich core building techniques above the water line and in decks to minimize the problem. In some boats, such as race boats where weight and strength are still paramount, sandwich core construction prevails throughout.

Foam Core is serated on the left to conform to curves while on the right is the material on the backside that holds the sheet together. End Grain Balsa core also comes in flexible sheets

Core material can range from different types of foam to lightweight processed woods, such as end-grain balsa or plywood, and even in some applications, aluminum honeycomb.  In this blog, I’ll explain how core damage can happen and in future blogs I’ll explain how to repair minor core damage with the more common core materials, foam or wood.

Water can enter the core around through-hull fittings like speedos and depth transducers and more typically through deck fittings like chain-plates, winches, and stanchions–basically, anyplace you drill a hole in order to mount hardware.  Builders have different techniques for sealing cores around hardware, but none of them can prevent damage to the core from collision or accident after the boat is built. Even when bedded with caulk, fittings for anchor windlasses and such tend to rock under load; the caulk breaks down and eventually water gets in.  This water intrusion is what marine surveyors are looking for when they use moisture meters.

With popular lightweight wood cores such as balsa, once the water gets in, the biggest problem is rot.  The second problem is freezing and the separation of the laminate sandwich. Either problem will ultimately cause the loss of structural integrity.  Even with foam cores, such as Klegecell or Airex, plywood inserts are often used under high-load areas like winches for increased compression strength, leading to the same wood/rot delam problems.  Of course, if your boat’s core is wicking up water, the boat is far heavier than it should be, even before any structural breakdown happens.

Cutting away the fiberglass skin exposes rotten balsa core. Note all the test holes drilled to determine the extent of the problem

The simple solution to core breakdown is to open up the affected area, remove any water, rotted or delaminated core, and epoxy in a replacement core that re-establishes the laminate strength and structure.  The more advanced solution is to do it in such a way that the problem does not return by also sealing the core from future water intrusion. For more on these procedures, see Rotten to the Core: Fiberglass Core Repair, Part 2.


  1. Boat Wax says:

    Really informative article! It’s really tricky working with fiberglass, but reading about it like this and understanding it makes tasks like this not as daunting. Great post!

  2. Question please: I am considering using a Airmar P79 in hull transducer over 1/2″ Airex foam core and 1/4″ laminate = 3/4″. I have heard that Garmin says no to the idea of shoot thru transducers and any kind of core hull. Can you please comment on this? Thanks you, Charlie

    • Doug Logan says:

      Hi Charlie. Core material in a hull sandwich can distort the acoustic signals leaving and returning to the transducer, producing garbled or inaccurate readings. Your best bet will probably be to remove the coring around where the transducer will go and replace it with solid fiberglass or hard epoxy, adhered completely to the outer hull skin. This will provide both reinforcement and clearer sound signals. Obviously, you’ll need to be very careful not to damage that outside skin, and make sure to seal the Airex at the edges of the cut-out. Here’s an Airmar PDF that explains things more clearly. See page four. Good luck with the project.

  3. Susan Yazdzik says:

    I’m looking at buying a 2001 sea ray sedan and I’m getting a lot of bad reports with the hull being made with Balsa Wood, and it turning into rot. Do you know if there are a lot of 2001 and prior that have rotted out or does it depend on the boat itself?

  4. WSI says:

    Great article on fibreglass core repair. This can be a slightly complex process so it’s good to see an article that explains the structure clearly. You may be interested to read this article on replacing stringers:

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