A GRP yacht hull may be strong and durable, but it’s also, unfortunately, relatively heavy. To overcome this problem, naval architects and yards will laminate much lighter core materials between the GRP layers to reduce the boat’s weight and increase its stiffness.

Older-style balsa wood cores suffer from rot and structural failure once moisture has begun to make its way along its grains. Modern balsa materials are, thankfully, a lot more resilient to moisture transmission while offering even greater compression strength and impact resistance. Newer synthetic foam cores are lighter still and a lot more moisture resistant.

But while they will never rot, as such, they can still delaminate from the GRP layers because their bonding properties are a lot weaker than balsa’s. Any water getting into a foam laminate and freezing has the potential to reduce the foam core to little more than an ugly ineffective mush.

As a marine surveyor, the majority of structural issues I tend to come across on boats are usually common and predictable degradations that can be easily spotted visually. However, trying to discover what might be lurking within a yacht’s hull and deck GRP layers and cores can be a lot more difficult to assess without taking discreet, but intrusive, core samples.

A thorough assessment of a yacht’s hull or deck core will, therefore, almost always have to be an inspection from the inside of the craft. The locations of the laid-in core within the hull will usually be fairly obvious, especially when you’re looking inside anchor lockers, aft cockpit lockers or hull voids. However, access to the underside of the working deck will, in many cases, require the unwelcome task of having to remove fitted cabin linings or cabinets.

Check your hull and deck fittings to ensure they are sealed properly – freezing winters can quickly compromise a yacht’s laminate core. Photo: Ben Sutcliffe Davies

Skin-fitting failures

For the last 40 years the majority of boat builders have usually ‘relieved’ (cut back) the core in a hull all the way to the external hull laminate when they are attaching skin fittings. When it comes to decks, they usually add extra suitably sized plywood blocks onto all of the areas where additional reinforcement will be required, while continuing with the usual balsa or foam core for the rest of the deck’s non-loading areas.

Some Asian-built craft have even been found to have had multitudes of small plywood off-cuts laid-in to provide core strength. This will work – but only up to a point. It’s important to be aware that as fittings age and sealants break down, moisture will work its way into the core via these vulnerable areas, creating, in some cases, very costly problems.

On most craft moisture ingress through underwater skin fittings or deck fittings is nothing more than an operational inconvenience. But on a yacht that doesn’t have a good core set in place to provide the required reinforcement to strengthen its hull and decks (to protect it from you lumping over and back across its deck) things can become a complete disaster very quickly. And, trust me, there are a lot of craft just like that out there.

Balsa core laminated into the GRP hull of a Jeanneau Sun Fast 2000. Photo: Graham Snook / Yachting Monthly

DIY dangers

Sadly, over the years, I’ve discovered teak decks where there were no obvious screws or dowel caps present but where, instead, small grub screws were put into the ‘lands’ (joints) by the boatbuilder to keep the strips laid down while the bedding mastic was curing – something that will only be revealed when the caulking starts to fail or wear out.

Other unfortunate issues crop up when someone has decided to drill through a boat’s topsides to fit a discharge for an item such as a heater or extra bilge pump, or drilled through the deck when fitting a few extra blocks or clam cleats for lines leading back from the mast to the cockpit.

Classic examples are where holes have been made to add extra spring cleats to a boat’s midships or to pass solar panel wiring through the deck. These actions will always compromise the core. After just a few years those innocent little holes through the deck sandwich will start to absorb a lot of moisture.

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Any damp in any section of a core is never good. Moisture making its way into balsa will, in just a few seasons, turn it into little more than papier-mâché! And, all too often, it will only be discovered when a pre-purchase survey is being carried out, through hammer tapping or when the hull or deck has been observed starting to flex.

A damp core will, in some cases, have been caused by someone fitting fastenings internally in a location where water has a tendency to gather. The most memorable example I’ve come across was in the bilges of a yacht where the heating conduit pipe work had been run within the heads shower arrangements!

Check the condition of plywood used as backing for your mast step. Photo: David Harding / SailingScenes.com

Foam and plywood

Before you start thinking that foam or plywood cores are any better than balsa cores, let me gently disappoint you. Generally. moisture in a foam core will not break down the foam’s actual structure, but freezing water near the foam will slowly degrade the bond between the materials used and, in time, compromise the strength of the hull and deck strength.

I’m sure most of us will, over the years, have had issues with high moisture content building up in plywood used as backing for deck winches, cleats, genoa tracks or mast steps.

Compared to balsa it’s not such a rapid failure, but it can lead to quite significant reductions in structural strength. It’s vital, therefore, that boat owners understand that regular checks to monitor and prevent damage to your boat’s structural core is a lot better than the expensive headaches that can result.

One example I came across recently was a real surprise to me. The stanchion bases on the yacht had the commonly found loops provided to secure life lines, etc. However, I found that one of the bases was loose and, with just a little wiggle, the entire fitting came off in my hand! Both of the stainless steel fastenings had completely dissolved due to crevice corrosion, where water had found its way in and around the fastenings due to a lack of sealant.

Core damage revealed after the decking was removed

Sealing surfaces

So, what can we do to prevent core disasters happening, and how do we assess the integrity of our decks? Well, first of all, if you have a serious urge to fit anything through your deck that will have to take any kind of load, it would be prudent to remove an area of the hull or deck core, sealing the exposed edges properly to stop moisture ingress, and then ensuring that a well saturated epoxy coated plywood packer is fitted exactly where the extra strength will be required. Sometimes it will also be sensible to add a suitable stainless-steel plate.

The fastenings need a good sealant and it’s important to seal the hole in the packer you create for the fastenings. Use appropriately sized backing washers as well. It may sound strange but on some boats I have surveyed with a balsa core I’ve frequently found equipment like boat hook clips, spinnaker snubbers, anchor chocks, and wiring plugs around the mast, just screwed in with no mastic applied.

I remember back to how my father loved his old mastic sealant. It came in tubes, like toothpaste, with a massive key to help roll the tube up! It never really went off, but it was the best long-term means of sealing off surfaces while still allowing a bit of movement.

Stanchion bases are especially vulnerable to water ingress, so make sure there is no movement at the base

Vital GRP hull checks

Hulls Make sure there are no skin fittings drilled through a core. A light hammer and moisture meter will quickly reveal what could end up being significant and costly core issues. Sadly, I’ve seen well-built craft where fittings have been drilled through both foam and balsa cores, proving very costly for the owners to properly repair.

Vital GRP deck checks

Carefully check all deck fittings, including pulpit and pushpit feet, for any movement, and look for any fittings with poorly sealed screws or bolts because overloading will let moisture into the deck substructure. And check around baby stay and life line harness points on deck.

Then there’s the mast step: How is it supported? What is it sitting on? And how well are the deck sockets sealed around the mast shoe? Also, is there any downward compression developing around the mast step? Any pooling of water will be an important clue!

Poorly sealed screws will destroy the core of a teak deck

Vital teak deck checks

If you have teak decks fitted, how are they secured? I recently surveyed a 15-year-old production craft that had terrible issues with small set screws that had allowed moisture to leak into the balsa core. The whole deck was a write-off at a cost of around £30,000. I’ve also seen several craft with ply blocks that have been incredibly costly to deal with. Initial assessments can be made very simply by just walking around the deck to see if it is flexing more than you think is normal.

Vital chain plate checks

Chain plate fittings: There are many ways that shrouds are connected and anchored these days. Some have basic ‘D’ bolts fitted though the outer deck margins with bolts held internally by the hull and deck joint. Others can be hidden away behind side deck underside soft linings which, if you are lucky, can be pulled back carefully to expose the fastenings. Others, where plywood is secured as part of the structure, give you little hope. That’s where a good moisture meter help.

I often use a moisture meter on the underside of cabin linings and around plywood fit-outs where deck fastenings are located. It’s quite sad when, for instance, you find out that every screw used for the spray hood hooks and blocks is leaking!

This tiny Infiray P2 Pro thermal sensor plugs into your smartphone and costs around £200 online

Essential equipment

A wonderful piece of kit I use regularly is a thermal imaging camera, which will show up any damp very quickly. I know they can be expensive but they have come down in price dramatically in recent years. I’d say they are worth every penny.

If you are a serious cruising sailor a thermal imaging camera will also be great for checking that your engine is running at the right temperatures. Also, if you take a photo with your phone’s camera with the flash on, you will very often be able to see any damp or discoloration to the wood finish under the lacquer!

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