Robert A. Adriance, in his book Seaworthy, states the following:
These 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.
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.
b) Check all hoses from thru-hull fittings below the waterline. With age, rubber hoses become brittle and will eventually split. Hoses for engine cooling are particularly vulnerable.
c) Check stuffing boxes for shafts and rudders. There should be no more than a few drips per minute from shafts, even less on rudder stocks.
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 they are often 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 especially vulnerable. Rubber degrades due to UV, changes in temperature, and time.
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.
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, have a positive metal close and 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 boatyard to replace them with ball valves.
b) Although you may have ball valve seacocks, you must still 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.
a) Rain and snow sink 32% of boats at the dock. You must first be aware if you have a self-draining cockpit. If you do not, obviously rainwater 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. Even if you have an automatic bilge pump, it will eventually 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 (7.62 m) 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.
c) Examine the size of your cockpit scuppers and drains and ask yourself how easy it would be for them to get clogged. If it seems 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.
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.
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 especially vigilant.
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 especially vulnerable to this scenario.
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