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Preventative Maintenance
Why boats sink at the dock

When a boat sinks at the dock, the question most likely to be asked is: “What happened to the bilge pump?” That’s the wrong question, however. By dutifully emptying the bilge periodically, a bilge pump can actually hide a problem--until the pump clogs or the battery goes dead. Water, not bilge pumps, sinks boats. The correct question should be: Where did the water come from?


  • In 50% of dockside sinkings, water found its way into the bilge through leaks at underwater fittings. The majority of the leaks are at stuffing boxes, followed by outdrive or shift bellows, failed hoses or hose clamps, sea strainers, and drain plugs.
  • There were sinkings from air conditioning fittings, gate valves, transducers, mounting bolts, and mufflers. Boats went to the bottom as a result of a leaking speedometer impeller. It is certainly possible that more than one fitting had been leaking.
  • It is also interesting to note that the finger was pointed at fittings above the waterline in 9% of the sinking claims. (Question: How can a fitting that
    is above the waterline sink a boat? Answer: Fittings that are above the waterline aren't always above the waterline.) More on this later.
life saver
  • Water from the sky,
    either rain or snow/sleet, accounts for 32% of
    sinking claims. Everybody has seen a rowboat or
    two awash, so this shouldn't be a surprise. What may be startling is that all of the claims involved boats with self-bailing cockpits that
    should have shed the
    water overboard.
  • Boats that sank after getting caught under a dock or banging against a piling accounted for 8% of sinkings. This number does not include boats that sank during hurricanes, or the number would have been much higher.

Visiting your boat
The First Line of Defense Against a Dockside Sinking
boats sinks at the dockIf you need a reason to visit your boat more often, consider that the cost of repairing a boat that has been underwater, even briefly, is usually about 40% of its value. Besides having to pay the deductible, the skipper typically loses the use of the boat for several weeks while it is being repaired.

At least twice a season, inspect any fittings above or below the waterline that could be letting water into the boat. All too often, skippers rely on bilge pumps to bail them out when they can’t visit their boats. The pump fails and the boat sinks. If you can’t visit your boat regularly, consider using a buddy system with other boat owners to watch each other’s boats. Another alternative is to ask your marina manager to keep an eye on the boat.
Many marinas offer routine inspections, but usually at an extra cost.



  • Damaged Outdrive Boots - Boats frequently sink because the rubber boots on the outdrive deteriorated. According to experts, outdrive boots should be examined two or three times a year. Rubber that looks dried out and cracked (cracks
    are most likely to appear in the creases) needs replacing.If possible, store the outdrive down, which eliminates most creases and prolongs the life of the rubber. Finally, for whatever reason, muskrats and other water-swimming vermin like to chew on outdrive boots. "RO-PEL" a malodorous commercial product, is an effective deterrent
    (One source: http://www.pestproducts.com)
  • Damaged Mufflers - Backfiring can blow a hole in
    a plastic muffler. Corrosion can eat a hole in an metal muffler. Both the muffler and the exhaust hose should be inspected carefully.
  • Dockside Freshwater Hookups - Many boats sink because of problems in the boats' dockside freshwater systems. Water may enter through a broken fitting in the boat's hot water heater. Many sink after a hose burst (the freshwater system hadn't been properly winterized). The first line of defense against this sort of sinking is to turn off the water at the dock whenever you'll be away from the boat for more than a few hours.

    (There are also devices available at hardware stores that can be preset to shut off the water supply automatically.) Hoses and clamps throughout the system should be inspected periodically. While you're checking, make sure there's a pressure-reducer valve and only reinforced hose (look for the criss-cross pattern if the hose is made of clear PVC) is used, which accommodates the greatly increased pressure of a city water system.
  • Through-hull Fittings - As a general rule, a boat whose gunwale is close to the water (low freeboard) has a greater chance of sinking accidentally. A ski boat, for example, is more likely to be overcome by rainwater, a slow leak, or a following sea than a cruiser whose impressive hull towers far above the water.

    But a boat is often much "closer" to the water than its freeboard would indicate. A cracked thru-hull at the boot stripe or a cutout at the transom for an outboard motor well that isn't protected by a splash guard means that, as a practical matter, the boat has to sink only an inch or two before it floods and heads to the bottom. Inspect fittings and hoses above the waterline with the same critical eye that you used on fittings down in the bilge. Double-clamp the thru-hulls and consider adding an anti-siphon loop or check valve to any that are within 8" to 12" of the waterline.
  • Scuppers and drains - Even aboard boats with cabins and self-draining cockpits, it isn't unusual
    to have a leak or two at hatches, ports, chain plates, etc. Caulking these leaks keeps water out of the bilge and also may prevent costly structural repairs later. Open boats and boats with especially low freeboard should be hauled for the winter in colder climates, as they are prone to being shoved underwater by snow and ice.

    When scuppers are clogged with leaves or debris, water backs up and has a tendency to find a way into the bilge. Two other sinkings occurred because scuppers were cracked or broken scuppers and water leaked into the bilge.
  • Seacocks & Valves - According to voluntary industry standards, seacocks or gate valves,
    which can be closed in an emergency or when the skipper is away from the boat for extended periods, must be used at all thru-hulls below the heeled waterline. The valves and fittings must be made of bronze or Marelon®, which are not likely to break when struck accidentally with a foot or anchor. (RC Marine's Marelon® seacocks are the only plastic seacocks that meet the requirements of Underwriters Laboratories.)

    Seacocks are widely regarded as being more reliable than gate valves. In an emergency, a quick glance at a seacock will tell you whether it is open or closed. With a gate valve, you can't tell. Gate vales also have a reputation for failing internally because the different metals-steel inside, bronze outside-aren't compatible. Look for a pinkish color on the bronze, which indicates corrosion.

    Other thru-hulls that need inspecting periodically are transducers and raw-water intake strainers. Ice can bend a strainer that isn't winterized properly. You should either drain the bowl or fill it with antifreeze. Even if the seacock has been closed for the winter, water can enter the boat when the seacock is opened in the spring.

    Removable transducers and impellers must be locked in place securely or they can work loose and sink the boat.

    Boats sink when hoses slip off the seacocks' nipples. Hoses connected to the fittings must be double-clamped with stainless steel clamps. Rusted clamps should be replaced.

    Boats sink because a hose split. Hoses at thru-hulls should be the reinforced type, which is usually a heavy black hose. Lighter, unreinforced PVC hoses can (and do) rupture and crack. Check the entire length of the hose, as excessive heat from the engine or chemicals (bilge cleaners, battery acid, etc.) can cause isolated failures. Replace any hoses that are suspect.
  • Keep the boat away from the dock - Boats sink because they either get caught under the dock or bang against the dock. Bow, stern, and spring lines should be arranged to keep the boat in the center of its slip.

    Fenders and fenderboards can be used to cushion minor bumps but they will not overcome a poor docking arrangement. Double up on lines and use chafe guards if the boat is in an exposed location.

    Plastic thru-hulls turn brittle and eventually crack form ultraviolet (UV) sunlight. Failures usually occur inside the thru-hull opening. If the thru-hull is only an inch or two above the waterline, rainwater or snow accumulations can force it underwater and sink the boat.

    A plastic thru-hull that was an inch or two above
    the waterline cracked and the weight of the snow lowered the damaged fitting to just below the surface. The boat gradually filled with water and sank.

    This shift bellows cable was not found until after the boat had sunk.