80% of Boat Sinkings Happen at the Dock - 12/05/2007

This “state-of-the-art”, brand new, never-been-used fire boat sank at the dock in its marina in the James River. When found at 7 AM, the bow was pointed straight up in the air.

Robert A. Adriance, in his book “Seaworthy” published by International Marine, says that –

  • 50% of the boats that sink at the dock do so because a fitting below the waterline failed, as happened in the case of the new Chaparral in another story in this issue.

  • Rain and snow account for another 32% of the cases filed with Boat/US’s insurance division,

  • 9% caused by a failed fitting above the waterline,

  • 8% by poor docking “arrangement” (for example, a boat being caught under a dock by a rising tide.)


Those four categories of failure account for 99% of all sinkings at the dock, according to Adriance.

All of these causes can be easily avoided by some simple vigilance. Here’s what to look for—

  1. Every hole in the hull below the waterline must be suspect.

    a) Systematically inspect every below waterline thru-hull to make sure it is properly sealed and that all hoses are double clamped and that the clamps are tight (but not too tight) and in good repair.


    These are the type of hose clamps that are typically used in the marine industry and they are NOT recommended by BoatTEST.com because they are vulnerable to saltwater corrosion and easily broken. Note how little material actually holds the band together on either side of the cut outs for the worm gear.

    b) Check all hoses from thru-hull fittings below the waterline. With age rubber hoses become brittle and sooner or later will split. Hoses for engine cooling are particularly vulnerable.

    c) Check stuffing boxes for shafts and rudders. There should not be more than a few drips per minute from shafts, even less on rudder stocks.


    Note double hose clamps on each end of the of the rubber tube and that everything is in good repair.


    Note rust on hose clamps. Imagine how the undersides look where the holes are in the band for the worm gear. When in doubt, take it out.

    d) Check all rubber “bellows” or gaskets in the transom where outboard or stern drive hoses or cables penetrate the hull. In any case these should be above the waterline, but often they are very close to the waterline and can be easily submerged for a variety of reasons. If you have an old OMC stern drive with a rubber boot around the drive unit and sealed to the hull you are particularly vulnerable. Rubber degrades from UV, temperature changes and time.


    Here is an old OMC stern drive lower unit. Note that half of the rubber boot is below the waterline. OMC designed their system this way to get around Volvo Penta’s stern drive patents, and did no one a favor in the process.

    e) Check all drain plugs, thru-hulls for speedos, depth sounders, etc. Check the “O” rings and lubricate them as directed. Make sure plastic fittings are snug and serviceable.

  2. All thru-hulls below the waterline should have ball valve seacocks.

    a) The primary reasons you must have marine grade ball valves for all thru-hull fittings below the waterline is that they can be quickly shut off, they have a positive metal close, and they are stronger than gate valves. Gate valves are almost non-existent on boats these days. If you have an old boat with gate valves, immediately call your boat yard to replace them with ball valves.


    This is a gate valve. If you have them, we urge you to replace them with ball valves.


    This is a retrofit installation with a typical marine ball valve seacock.

    b) Even though you have ball valve seacocks you still must cycle them (turn on and off) every few weeks to make sure that they are not stuck open. If they are stuck, simply tap the handle with a mallet. Make sure that all members of your regular crew know where all seacocks are and can turn them off.

  3. Cockpit scuppers and drains.

    a) Rain and snow sink 32% of the boats at the dock. The first thing you should do is become aware of whether or not you have a self-draining cockpit. If you do not, obviously rain water can eventually sink your boat. Typically, runabouts without self draining cockpits have covers to keep the rain out. But even with a cover you must periodically check your boat because covers are notorious for leaking and even if you have an automatic bilge pump, eventually it will lose its charge.

    b) If you have a self-draining cockpit you should determine how far your cockpit sole is above the waterline. The amount of buoyancy your boat has is important to know so that you can act accordingly. For example, if you have a 25 foot boat with a large cockpit and the cockpit sole is only an inch or two above the waterline, then you do not have much reserve buoyancy. If your scuppers become clogged with leaves, snow or other material it won’t be long before the water backs up and finds a way into your bilge.


    It won’t take much to clog these cockpit drains.

    c) Examine the size of your cockpit scuppers and drains and ask yourself how easy it would be for them to become clogged. If it appears easy, then take action.

    d) If the scuppers do not lead directly overboard, then inspect the hoses and fittings below deck that lead overboard. Make sure all are in proper repair, as a split or detached hose here can sink your boat with rain water.

  4. Thru-hulls ABOVE the waterline.

    a) According to Adriance, 9% of sinkings at the dock are due to thru-hulls above the waterline failing in some way. The reason, he says, is that many thru-hulls are only slightly above the waterline, and with added weight due to gear or stopped up scuppers, occasionally end up being below the waterline. Check them as noted above.

    b) Make sure that hoses that start below the waterline (such as those for a bilge pump) and exit above the waterline, have a loop that extends significantly above the waterline to prevent siphoning if the exit thru-hull should become below the waterline.

    c) As an added precaution, you can add an anti-siphon device at the top of the loop, which should be substantially above the water line.


    This anti-siphon “valve” demonstrates the concept. The device prevents a vacuum or negative pressure from being created.

  5. Outboard motor transom cutouts.

    a) Typically outboard motor transom cutouts are only a few inches above the waterline at rest. This is an extremely vulnerable place on your boat which can cause sinking for any number of causes. If your boat does not have an outboard well which matches the freeboard of your boat, then you must be particularly vigilant.


    Note that while this boat has a cut down transom for the outboard motor, it also has an outboard well with the same freeboard as the rest of the boat. This is a good design, and some interior volume is given up for safety.

  6. Dock Entanglement.

    a) If you are in a tidal location, make sure that there is no way your boat can float under a dock or an obstruction and become trapped.

    b) On a mooring, particularly in areas with a long fetch, make sure that high seas will not come over the bow of your boat while moored. Ski and wakeboard boats with low freeboard at the bow are particularly vulnerable to this scenario.


    This boat could have sunk at the dock for any number of reasons. By using our check list above you can greatly reduce your chances of sinking at the dock.


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