Sure, hullside windows let beatific visions of natural light into staterooms. But what happens if you break one?

Let There Be Light

Glass or plastic hull windows can’t be beat for letting natural light into staterooms.
But what if you break one?

While taking a break from writing this column, I poured myself a tumbler of lemonade, chugged it in one gulp and rinsed the glass. But I was in a rush to get back to work, so my handoff to the draining board went awry. En route, the tumbler took a tumble onto the kitchen floor, exploding into a million diamond-sized shards and making a bang loud enough to make me wonder whether I’d been inadvertently drinking from a hand grenade. Sure enough, turns out the tumbler was made of tempered glass, a safety measure for ham-handed drinkers like myself to keep them from slashing a wrist on jagged shards. This is the first glass I’ve dropped in years; funny how it happened while I was writing about hull windows, many of which are made of tempered glass. The universe works in mysterious ways.

In the past decade or so, hull windows have become common in mid-sized powerboats and superyachts alike. Big yachts, especially those with full-beam staterooms under the salon, depend on hull windows to bring light into the accommodations, while providing a picture-window effect as well. Unlike portlights and opening ports, which are much smaller and usually mounted higher, hull windows often live close enough to the waterline that failure at sea could cause a serious problem. Consequently, conscientious builders choose materials for the windows and install them carefully, in keeping with ABYC (American Boat and Yacht Council), ISO and other rules.

Hull windows may be made of glass or plastic, so I figured I should finish my unintended experiment: I pulled a Strahl goblet off the shelf, chugged some more lemonade and dropped it on the floor. It bounced, coming up with nary a scratch in its polycarbonate body. And that’s it in a nutshell: If you want hull windows that don’t break, make ‘em from polycarbonate, nice and thick. Case closed, in my opinion.

Most people, however, disagree. There’s more to hull windows—to all marine glass, in fact—than just picking the strongest material. Yes, hull windows are big enough to let a lot of ocean aboard if they break, so the strength and durability of the glazing is important. But folks lazing in a master stateroom on a hot summer day, enjoying the view in air-conditioned comfort, aren’t thinking about rough water; they want to see clearly, so the windows have to be, to coin a phrase, “as clear as glass.” And nothing but glass is that clear.

Not only do hullside windows bring lots of light aboard these days, they also add style to a boat’s profile.

Not only do hullside windows bring lots of light aboard these days, they also add style to a boat’s profile.

Glazing Isn’t All Glass

Most hull windows are made of tempered glass, although they don’t necessarily have to be. The ABYC addresses glazing materials and practices in their short and simple Standard H-3. It’s not as comprehensive as most standards, and the ABYC engineer I spoke with said the standard would likely be expanded when it comes up for review in 2019. Presently, though, they recommend tempered glass, laminated glass, rigid plastic (polycarbonate, e.g., Lexan) and glass-plastic (polymethyl methacrylate—that’s acrylic, e.g, Plexiglas) for glazing windshields, windows, portlights, deck lights and doors and partitions. Windshield glazing must have light transmission of 70 percent or more; other applications need not be so clear. Hull windows are usually tinted for privacy.

Tempered glass starts with “float glass,” which is made by flowing molten glass onto a bath of molten tin; the lighter glass floats on the tin to form flat glass with two perfect surfaces. Most flat glass is produced this way via what’s called the Pilkington method. If required, the float glass can be tinted or curved, and is then toughened with heat. More to the point, the glass is fired to about 1,200 degrees, then hit with a strong flow of cold air. The glass surface cools faster than its center, which creates compression and adds strength at the surface. Glass can also be chemically toughened by bathing it in molten potassium salt; larger potassium ions replace some of the smaller sodium ions in the glass surface, creating compression that strengthens the glass.

Tempered glass is four to five times stronger than untempered float glass and can take a good whack before breaking. But when it does break, the internal stresses from the tempering cause it to shatter into a million small, relatively harmless, pieces—just like my drinking glass. That’s why they call it safety glass, and why tempered glass is used in eyeglasses, shower doors and other applications where human safety is paramount. Tempered glass is easy to spot: It shows a pattern when you look at it through polarized lenses.

Laminated glass is also ABYC-approved for marine glazing. It has a core layer of, usually, polyvinyl butyral (PVB) plastic sandwiched between inner and outer layers of glass. Laminated glass is designed to resist the hazards of impact, and broken pieces cling to the PVB, so they’re not too dangerous. This is just right for car windshields and bulletproof glass (bulletproof glass has a polycarbonate core layer). That’s why the tempered-glass side window of your car shatters when you leave your laptop on the seat and some lowlife steals it, but when a stone hits your laminated windshield the whole thing doesn’t disintegrate and land in your lap.

Laminated glass can be made with float glass or, even better, tempered glass. Combining an impact-resistant core with tempered glass makes a nice window. And some laminated glass is even tougher: If you’re a fat enough cat to own a megayacht, you’re probably fat enough to attract bad guys. So maybe you want your hull windows made of laminated glass that one manufacturer, Glasshape, calls “pirate-resistant.” It has a polycarbonate core. And if you think you might be irresistible to pirates, maybe you should look into upgrading your deckhouse glazing, too.

This fully-sealed hullside window is fitted with a waterproof opening port for ventilation.

This fully-sealed hullside window is fitted with a waterproof opening port for ventilation.

Care and Repair

Carver Yachts uses tempered glass for all the windows in their boats, so I called their customer service department for advice on care and repair of hull windows. There’s no secret to maintenance, according to the technician I spoke with. Take care of them like you do your car’s windshield: Wash with plenty of water to get the salt off. Keep an eye on the outer caulking, usually black silicone. It’s mostly cosmetic—windows are typically held in place by super-high-bond adhesive—but prevents water and dried salt from getting between the glass and the surrounding fiberglass.

Here’s a tip the guy handed me: Clean your tempered-glass windows sparingly, and always use a mild, non-abrasive cleaner for the job. Tempered glass relies on its surfaces for strength: Remember, the tempering process puts it in compression, which is where the strength comes from. But the compressed layers are very thin, and enthusiastic cleaning can erode and, eventually, weaken them. Typically, the insides of hull windows are cleaned way more often than the outsides, so it’s the inside surfaces that get weaker. If something—a whopping big wave, for instance—presses against a weakened window, the force from outside can create tension on the inside surface and it can fail.

In most applications, hull windows are glued into recesses in the hull, secured to molded-in ledges much like window glass ashore is fixed into a sash. When combined with a sturdy rubrail to absorb docking miscalculations, this arrangement protects the windows from most impacts. If a window gets scratched, however, it can often be repaired in much the same way auto-glass shops fix chips in a car windshield, said Carver’s tech. But, he added, the polishing can leave distortion in the glass. Moreover, a deep scratch can cause spider cracks, which means a new window.

According to Rob Andrews, a carpenter in the crafts department at Saunders Yachtworks in Gulf Shores, Alabama, you don’t always need an impact to break a hull window: Hard running in big seas can make the hull flex more than the glass can absorb, and the window cracks. Usually the replacement job is straightforward, Andrews said, and can be done from the outside, so the associated stateroom doesn’t have to be torn apart. You simply remove the old window, or what’s left of it, make a template and have a new one made, or order one from the boatbuilder. Then, in either case, you merely glue the new one in. But, Andrews added, sometimes the replacement process damages the gelcoat around the window, or other repairs become necessary.

The only fly in the adhesive here is how long it takes to get a replacement window. Hull windows are usually curved and they have to be tempered, so making a new one isn’t a job for the local glass shop. The new window can take weeks to arrive. So, to compensate, the Saunders crew makes a temporary “window” out of plywood and seals it in place with epoxy as a short-term fix, so at least the boat can still be used. The view from the stateroom isn’t especially good, though.

So what, in my humble opinion, is the answer? For folks who must have hull windows made of glass, I think impact-resistant, tempered laminates are the ideal choice. Glasshape’s DuraShield Marine laminated glass, for example, combines tempered glass with polycarbonate and will remain watertight even when the glass is broken. The glass protects the polycarbonate, resisting scratches and fading, and the polycarbonate provides ultimate strength and hangs onto the pieces should the glass break. It’s the best of both worlds, really; it’s just too bad DuraShield doesn’t make drinking glasses. ρ

What About Plastic?

Plain tempered glass is great for looking through, but unless it’s laminated, it’s too susceptible to damage for my liking, especially for hull windows in a yacht that’s designed to, as they say, “go to sea.” Even minor damage can occasionally lead to shattering, as has happened in high-rise buildings in Chicago, Toronto and Austin. (Experts opine these events resulted from edge damage to windows that went unnoticed during installation.) Tempered glass, however, is used for hull windows in thousands of yachts these days, and unless I’m missing something, not many are going to the bottom from glass failure. Nevertheless, I’d choose stronger polycarbonate for my hull windows and deal with its disadvantages.

Polycarbonate, of course, is nearly indestructible and many times more impact-resistant than tempered glass—anywhere from 20 to 50 times as resistant, depending on whose numbers you use. You can beat it with a sledgehammer and it won’t break. It’s half the weight of glass of comparable thickness, which makes it easier to install and puts less stress on the adhesive, and maybe saves a bit of fuel, too, if you have lots of windows on board. It can be cold-formed—many hullside windows are curved—and is resistant to cracking. Only thing is, it’s not as clear as glass, it’s much easier to scratch and it can turn yellow after years under UV rays.

But on the other hand, polycarbonate is less expensive and can be worked with normal tools, so replacement is easier if the window gets too yellow for your taste. I lived in crew’s quarters under a polycarbonate hatch on a sailboat for four years, and I never saw any yellowing—so in my book, yellowing’s not a concern. I never managed to scratch the thing, either, even though I dropped stuff on it with some frequency.

What about acrylic? It’s not as strong as polycarbonate, but it’s still stronger than tempered glass. It’s clearer, too, almost as clear as glass, more abrasion-resistant and stands up better under UV light. Moreover, small scratches and abrasions can be polished out. And while it’s stiffer than polycarbonate and therefore more apt to crack or chip, it can be a good choice overall for ports in the hulls of smaller boats not meant to go to sea. Plenty of deck hatches are made with acrylic, too, and they seem to hold up fine.

This article originally appeared in the March 2019 issue of Power & Motoryacht magazine.

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