How can “gently falling snow” sink hundreds of boats?
The snow season started early last year when Sandy brought as much as three feet of snow to the mountains of Maryland and West Virginia, with lighter snows in Virginia, North Carolina, Tennessee, Kentucky, and Ohio. But that was just the beginning. Strong winter storms right before and right after Christmas affected almost the entire country, snarling holiday traffic and backing up airports from coast to coast. Boats don’t normally come to mind when the forecast turns to blizzards, but if your area has experienced heavy snow, especially if it doesn’t normally, go check on your boat if you haven’t already.
Oklahoma doesn’t usually even make the list when talking about areas with high snowfall. But in February of 2011, a major snowstorm dumped nearly two feet of snow in the Grand Lake area, near Tulsa, where about 5,000 boats are kept. When the snow finally stopped, roads were impassable and no one knew how the boats and marinas had fared. The biggest area of concern was the many covered floating docks that dot the lake.
When helicopters finally flew in to the area, it was a grim sight. Many of the metal roofs had caved in, but the worst of the damage would not be seen until investigators were able to access the docks. Tom Benton, a Society of Accredited Marine Surveyors (SAMS) surveyor from Tulsa, was sent by BoatUS to assess the situation. His report was bleak: 14 marinas, along with countless private docks, were damaged or destroyed by the snow and over 600 boats were damaged, many of which were sunk. As the snow load increased on the roofs, the floats couldn’t support the weight and eventually, the snow-covered roofs settled onto the boats, forcing many of them underwater. Tom noted that in some cases, private dock owners, whose structures seemed to fare better, didn’t notice damage until much later, when they began to de-winterize their boats. Even though the roofs appeared sound, the weight had forced the docks — and the boats — under water just enough to cause back-siphoning through scuppers and exhaust systems, ruining the engines.
The snowstorm on Grand Lake was not the first, nor will it be the last of its kind. Seaworthy wrote about a similar storm in 1996, in Washington state. At one marina, 24 out of 26 roofs collapsed onto the 400 boats underneath. Of those boats, 100 were sunk and 200 were damaged but still afloat. At another marina, two more roofs collapsed, destroying 244 empty slips and sinking 37 boats.
The weight of snow varies greatly. A dry, fluffy snow might weigh a little over five pounds per cubic foot while a wet snow can weight 15 pounds or more. Unfortunately, warmer areas tend to have the wet snows. If buildings ashore rarely have problems with snow loads, why have the very same loads been such a menace to floating docks? Seaworthy talked to Shannon Kinsella, a structural engineer with Reid, Middleton, Inc. who said codes for marina structures include snow loads only for the roofs, not for the floats. Years ago, in the Washington storm, the roofs were built to hold about 25 pounds per square foot (psf) of snow load, but the flotation that the structure sat on was only designed to hold 10 psf, which was a recipe for disaster. These days, most larger marina roofs are built to withstand 30 psf of snow load. As for the floats, engineers do their best to design floats that will stand up to anticipated loads but are limited by things like the width of finger piers and even water depth. Other engineering remedies include designing steeper roofs made with slippery materials that may shed at least some of the snow.
So what’s a boat owner to do? As with any boat, the safest winter storage is ashore; boats don’t sink on land. If you choose to leave your boat afloat, monitor winter storms as you would hurricanes and, if practicable, prearrange to have your boat hauled at a nearby marina if a severe snowstorm threatens your area. As a last resort, move the boat from under any roof that seems questionable. Tom Benton observed that very few of the boats left outside the covered dock areas at the lake sank from excess snow. A well-fitting cover can direct snow overboard. Timely visits to your boat can prevent snow buildup on covers and in scuppers.
Don’t let that gently falling snow fool you.