- The Best Source for Boat Buying Information

Attaching Hardware to Your Boat

Taking the quick and easy way out by just ramming in a few screws through a cored structure can and does cause thousands of dollars of unexpected damage. You either take the time to do it right, or pay the price when it comes time to sell your boat.

While we've been warning boat owners for years about the dangers of making holes in cored decks and other boat structures, it's become clear that large numbers of boat owners simply aren't aware of these potentially very costly self-inflicted wounds.


In the last month or so, we've run across no less than six larger boats where deck and superstructure cores were badly damaged as a result of water intrusion into the cores. The water intrusion came about as the result of the indiscriminate attachment of hardware -- anything from canvass snap fasteners to antenna mounts, windlass foot switches and tender cradles -- while failing to properly bed the fasteners to prevent water leakage. In other cases, hardware was removed and holes left wide open, and in many cases all the owner did was to smear some putty over the hole with no more concern than a person filling a hole in the living room wall with toothpaste.

In all of these cases, major damage to the boats occurred to the tune of thousands of dollars each. In one astonishing case, a ten year old 50 foot sport fisherman was effectively destroyed, damaged beyond the possibility of economic repair. The fore deck, house sides, bridge deck and aft cockpit deck were all rotted out and utterly beyond repairing. This was undoubtedly the worst case we'd ever seen, and yet the ruination of the boat was entirely the fault of its owner who had made dozens of holes throughout its cored structures.

Apparently many boat owners have not made the connection that making holes in decks and other boat structures is no different than drilling a hole in the roof of their house. They are unaware that putting a screw into a deck has exactly the same result as doing the same thing with a building roof: it is going to leak for certain.


While the risks to balsa cores are widely known, many people think that closed cell foam cores are impervious to water.  We assure you that they are not. They are subject to the very same problems that cause blisters on boat bottoms. While water itself does not directly affect most foams, water does react with the plastic resins and bonding agents used to adhere the foams to the laminate skins. Just as with bottom blisters, styrene precipitates out of the polyester matrix.

Styrene is a solvent, and it will soften or even dissolve foam. So once water gets into the core, this chemical reaction then goes to work on the core, softening it to the point where the deck gets spongy and eventually the foam separates from the fiberglass. The end result is exactly the same as a rotted balsa core. The core turns to mush.

To prove the point, we have taken fluid removed from bottom blisters and applied to various core materials. And guess what? Yep, the core dissolves in the blister fluid.

Okay, so now that you know this, you can no longer tell yourself, "Hey, my decks are foam cored, no problem. It's the latest and greatest space age material." Maybe so, but it is extremely unlikely that your superstructure is made with vinylester or epoxy resin, meaning that it is orthopthalic resin, which means that it's subject to the very same problems as all boat hulls made with this plastic. It is unstable when in long term contact with water.

deck screws

Typical result of screwing hardware to a deck with no bedding. For a windlass  foot switch, this amateur installation,  done by a boat dealer, is going to cost about $5,000 to repair since the core is water saturated and delaminated.

When it comes to water leakage, it seems that many people do not understand what is known as the capillary effect, the uncanny ability of water to pass through micro-fine spaces between two objects -- like a screw and deck, or window frame and house side. But the fact is that very small fissures and openings can transmit very large amounts of water because the capillary effect functions like a natural pump. This results in more than just leaks. Rather the capillary effect has the ability to generate a flow of water far greater than the usual gravity effect. In other words, where it may look as if a screw, by means of the screw pressure generated, should seal itself, actually can result in an accelerated leak. As you can see in the photo above, tight screws did nothing to keep the water from getting under it.

To make the matters worse, many boat builders, boat yards, dealers and canvass installers do not themselves understand how they are causing  serious damage to boats by cavalierly drilling holes and running in screws. Part of the reason why is that it takes years for the damage to manifest itself.

Snap fasteners? You mean those little snaps that hold my enclosures and covers on could be causing me a problem? Yep, that's exactly what I mean. Every single snap that is installed into a cored structure is likely to be allowing water into the core. One recent example turned up a 31' Tiara in which the entire deck and cabin trunk core was filled with water, so much so that water was running out from under the snap fasters, leaving nice trails of green slime. This happened because someone installed snap fasteners all over the cabin top to secure sunbathing cushions.

core holes

The reason for the extensive delamination of the house side of this yacht became obvious after the laminate was peeled away. Notice all the plugged holes. Water got into the core and caused extensive blistering.

For that rather insignificant pleasure, the boat owner had effectively destroyed his boat. Since no one is going to buy a boat like that, the boat ends up in a fire sale as a handy man special.

You are now asking yourself, "But how the hell is anyone supposed to attach covers and enclosures if you can't just screw these things into the boat?"

That's a very good question, indeed. In the past, this wasn't a big problem before builders started going hog wild coring every structure on the boat in their ill-advised attempts to save a few bucks and make boats cheaper. Back in the good ole days of solid fiberglass, it didn't much matter. But now it is a very big problem, one for which the effects and damage don't begin to show up for years  -- like when you go to sell the boat and the surveyor discovers the problem.

Lately, the job of the surveyor has come to resemble that of a physician who has to inform his patient, "Sorry, sir, but you are dying of cancer."

core holes

After a little exploratory surgery, the reason for the deteriorated core becomes painfully obvious. Note the water weeping out of these filled screw holes.

Installing snap fasteners is less of a problem when done in places like the tops of flying bridge coamings and other areas where the structure is not cored. But to install them on flat surfaces like decks and cored house sides and tops is an invitation to disaster. Unless the boat designer has taken this problem into consideration, and has created an area of only solid fiberglass into which the fasteners can be safely put, then there is NO solution for the problem.

Can Screws Be Sealed?

Think about it, if a piece of hardware is under load, then something is always pulling at the fastener, attempting to loosen it. And, of course, screws into fiberglass have notoriously little holding power. It is easy to rip them out. Just look at how easy all those snap fasteners pull out. Look at any boat and see how many of them have already come out.

So, yes, you could use some caulk under the fasteners, but that isn't going to help much. What does help is to through bolt all stress loaded hardware. That includes everything from antenna mounts to hand railings to rod holders.

deck cleat

The right way to do it. Note that there is no core around this hardware mount, and the 5200 bedding squeezing out from under the back up plate and around all the bolts. There is also not a trace of water leakage because it was done right.

What Bedding to Use

I hear it over and over again: "I don't want to use 3M 5200 because it's messy and nearly impossible to get off." Sorry folks, but that is precisely why you should use it. 5200 is an adhesive: silicone and polysulphide are not good adhesives, which is why they don't work well. When mounting hardware, in my opinion, 5200 is the ONLY thing to be used that is highly effective.

Why Stainless Steel Rusts

Ever wonder why you see all those rust stains around screw heads and the mounting surfaces of hardware? You most often see this around rail stanchion bases. It's because there is water in the screw hole or under the hardware. This causes closed cell corrosion which will rust even the best stainless steel. In fact, some metallurgists say that it is the highest grades of stainless that are  the most vulnerable to closed cell corrosion. What many blame as low grade stainless is often simply the result of failure to bed the hardware properly. After all, it's called stainless steel not Stainproof steel.

When you see rust showing around fasteners or hardware bases, you can be sure that if it is a cored structure to which it is attached, there is water going into that core. The rusty hardware is waiving a red flag at you saying, "Hey Mr. Boatowner, there's water going in through these screw holes."

Doing it Right

How to install hardware on a cored deck is easy in theory but hard in practice because of the accessibility problem to the underside of the deck or whatever you're attaching to. My advice is that  hardware should always be bolted, and never screwed, even if it's not load bearing hardware.

To do it safely, all you have to do is use a 2" hole cutter and remove the coring from the underside of the deck at the points where the bolts are to be installed. Then seal the exposed edges of the core with epoxy paste ( 2 part epoxy glue will work fine. Now you can drill your holes and mount the hardware with 5200 bedding and large washers on the underside (with 5200 under those, too) and presto! Now there is no chance of water ever getting into the core, plus the attachment is not going to leak or ever come loose.

So why not just bolt through the core? Because when you draw the nuts tight, this will crush the cored laminate, the part will come loose, and it will leak like a hole in the bottom of the Titanic.

Yes, it takes quite a bit more time to do it right. But if you figure the cost to repair serious core damage, say $3,000, then by whatever extra time you spend doing it right, you can figure that you've probably just saved about $1,000 per hour by that bit of extra time.

Just remember that when the time comes to sell your boat, that is the time these little chickens come home to roost. It's just a matter of pay now, or pay later -- with interest.

Posted August 14, 2000

David Pascoe Power Boat Books

Buyers' Guide to Outboard Boats Surveying Fiberglass Power Boats (2E)

David Pascoe Power Boat Books Visit for his power boat books

David Pascoe - Biography

David Pascoe is a second generation marine surveyor in his family who began his surveying career at age 16 as an apprentice in 1965 as the era of wooden boats was drawing to a close.

Certified by the National Association of Marine Surveyors in 1972, he has conducted over 5,000 pre purchase surveys in addition to having conducted hundreds of boating accident investigations, including fires, sinkings, hull failures and machinery failure analysis.

Over forty years of knowledge and experience are brought to bear in following books. David Pascoe is the author of:

In addition to readers in the United States, boaters and boat industry professionals worldwide from nearly 80 countries have purchased David Pascoe's books, since introduction of his first book in 2001.

In 2012, David Pascoe has retired from marine surveying business at age 65.

On November 23rd, 2018, David Pascoe has passed away at age 71.

Biography - Long version


To Page Top