Why Do Boats Sink?  

Well, the simplistic answer is a failure of the vessel’s structure to maintain watertight integrity and keep the water on the outside of the hull where it belongs! Although there can be contributing factors such as weather and sea conditions, collisions, groundings, operator and navigational errors and fires involved; in all cases the watertight integrity of the vessel was compromised in some way. The real question is then: what types of failures contribute to the loss of watertight integrity and which are the most common and how can they be avoided?  

In a recent study by a well-known marine insurance company of 150 sinking claims, several interesting statistics were brought to light. Probably the most interesting is the fact that for every boat that sinks while underway, four boats sink at the dock in their slips. There are two basic reasons for this. First, most recreational vessels spend considerably more time at the dock unattended then they do underway, and secondly when underway someone is onboard. This affords the possibility that the leak could be discovered and stopped before the vessel is lost.   

Boats That Sank At The Dock

 In looking at vessels that sank at the dock, the overwhelming majority sank do to failures of through-hull fittings below the waterline. Of these the causes were given as leaks at stuffing boxes (31%), failure of outdive bellows or shaft log hose  (28%), failed hoses or hose clamps (21%), failure of sea strainers (10%), and failure of drain plugs (10%). For this reason the surveyor’s standard recommendations to remove, disassemble, inspect and re-bed seacocks and through-hull fittings as part of any preventative maintenance program should not be taken lightly. The use of proper seacocks and through-hull fittings as defined by the American Boat and Yacht Council (ABYC) Standard H-27 is also critical. This standard specifies that all through-hull fittings meet the requirements of the American National Standards Institute and Underwriters Laboratories Marine Through-Hull Fittings  (ANSI/UL 1121). Be wary of cheep plastic through-hulls that do not meet this specification. There are many of them out in the market place that many boat owners have mistakenly used as through-hull fittings with disastrous results. The only type of plastic fitting that meets ANSI/UL 1121 is constructed of RC Marine’s Marelon ® and marketed by various marine distributors such as Forespar. If in doubt, replace it!

The second most predominant cause is surprising- rain and snow! What is even more startling is that the claims involved boats with self-bailing cockpits! Scuppers clogged with leaves or debris, where the water backed up until it found a path to the bilge, caused five of the sinkings in the study. Two of the vessels sank because of cracked or broken scuppers that leaked into the bilge.

In the above table the third major cause of dockside sinkings is the failure of through-hull fittings above the waterline. Why? Fittings that are above the waterline aren’t always above the waterline. Consider this- during a rainstorm water finds it’s way below deck, and at only 280 gallons per long ton, the vessel rides a few inches lower on her lines as the bilge pump tries to catch up. A fitting that was slightly above the waterline before the added weight- the transom cutout for an outboard, scupper drains, or raw water outlet for the air conditioner- is now submerged below the water. Any failure of these through-hulls now allows water to enter the boat from below the waterline as well as from above and the bilge pump can be easily overwhelmed. Other possible scenarios would be the weight of snow and ice causing the vessel to ride lower on her lines, or a sailboat that is heeling due to strong winds blowing thought the rigging as she sits at the dock. For this reason the ABYC recommends that through-hulls be equipped with seacocks below the maximum heeled waterline. This is considered to be 7° for powerboats and for sailboats the, the level of the sheer amidships.

In the category “improper mooring”, ten vessels sank as a result of being caught under docks as the tide came in or impacted the dock or pilings with sufficient force to hole the vessel. The use of fenders cannot overcome a poor mooring arrangement.

The “other” category listed four vessels that had sunk because of the boat’s dockside fresh water system. This was due to either a hose in the system that had burst, a broken fitting at the vessel’s water heater.  

As we look at dockside sinkings a couple of things are also evident. Automatic bilge pumps can hide problems from the boat owner as the bilge pump periodically pumps the bilges. The owner examines the bilge and finds that the bilges are dry or contain very little water and therefore assumes everything is normal. The owner is unaware that the pump has been frequently cycling on and off dewatering the bilge because a shaft log is leaking, or water is entering the bilge from a cracked fitting or hose. This continues until the pump clogs with debris, the batteries go dead, or a fitting fails completely. How can this be avoided? In addition to routinely inspecting bilges, through-hulls, hoses, and hose clamps, the addition of a bilge pump counter can keep track of how many times the pump actually runs. If you notice that your pump has run, say, 50 times since the last time you checked it a week ago, this is a clear indication that something is wrong! Also the addition of a bilge level alarm with an audible signal can let others at the marina know that there is a problem on board. Some can even place a phone call just as your home alarm system does alerting you or a security service of a problem.

Now let’s examine the studies findings on vessels that sank while underway. The single most critical reason boats are flooded on open water has to do with transom height. Thirteen of the 15 boats in this category were outboard-powered and the engine cutouts were often only inches above the water. The motor wells in some cases were too low, too shallow, and too slow to drain. The lack of freeboard aft allowed seas to board, “pooping” the boat as water came over the stern. Slow draining wells caused the boat to sit lower on her lines due to the weight of the water, further compounding the problem of low transom height.  Another contributing factor is typically weight distribution. The addition of trolling motors, large coolers, and bait wells, among other things, decrease buoyancy aft and trim the vessel down by the stern, in turn decreasing freeboard aft.

Of the two remaining boats, one was a sailboat that took a knock down and sank when water entered an unsecured cockpit hatch, and one was an inboard powerboat with very low freeboard. In all cases inherent design flaws and/or operator error had compromised the watertight integrity of the hull.  

Boats That Sank While Underway

   Although through-hull fittings and hoses did not rank as the predominant cause of underway sinkings, as it had in dockside losses, combined with failure of outdrive boots (in essence a through-hull) and leaks in raw-water cooling and exhaust systems (a through-hull sub-system) they accounted for 34% of all sinkings. Through-hull fittings and the systems they serve should be looked at as a whole. Any part of the system can be subject to failure allowing uncontrollable amounts of water to enter the vessel in a surprisingly short amount of time. The table below will give you an idea of just how fast this can occur! 

 Flooding Rates of Various Size Holes At Differing Depths 

 Figures Are In Gallons Per Minute (GPM)

If you were to consider a hole as small as 2” in diameter, the size of most fathometer transducers, 3’ below the waterline the flow rate into the boat would be136.1 gallons per minute or 7,896 gallons per hour! A boat equipped with three 2,000 gallon per hour bilge pumps would sink! Now if this vessel is equipped with a bilge alarm, it would be able to warn those on board (or ashore) of a problem and give them precious time to find and plug the a hole that may have otherwise gone unnoticed until it was too late.

The missing drain plug and grounding categories can most assuredly be attributed to operator error. Learn and practice basic navigational skills and techniques. Keep current editions of the proper charts for your area of operation on board, review and keep onboard the most recent USCG Local Notice to Mariners, install a GPS and fathometer. Keep constantly aware of your vessel’s position relative to navigational dangers such as shoals, rocks, reefs, etc.  Oh yea, attach that drain plug to your boat keys so you don’t forget to install it!

The striking of a submerged object, even at low speeds can have disastrous consequences. Transducers can be ripped from the hull, shaft logs, propeller struts and rudders damaged, not to mention the hull being holed. In the event of striking a submerged object, immediately have someone inspect the bilges for the accumulation of water and contact the U.S. Coast Guard and other vessels in the area and inform them of the situation. Don’t wait until you find that your boat is taking on water, by then it could very well be too late! Check all shaft logs, shaft struts, through-hull fittings, transducers, and speed logs for damage. Be prepared to effect emergency repairs by plugging leaks with rags, towels, cushions, or damage control plugs. The key is obviously to slow or stop the flow of water by any means available, even if it results in having to use other parts of the boat to due so. Continue to monitor bilge levels until you are completely confident that your boat is not taking on water.

As far as structural failures are concerned, a proper marine survey by a fully accredited marine surveyor is instrumental in finding areas of potential structural failure, as well as evaluating the condition and installation of through-hull fittings, their associated sub-systems, and dewatering devices. The marine surveyor has the knowledge, expertise, and tools at his disposal to analyze these systems and components. Talk to your surveyor and be sure you are completely aware of his findings, and understand his recommendations for any corrective action that may be needed. In the middle of the night, with water lapping over the deck boards is not the time to wish that you had done this!  

As this study has pointed out your first line of defense is the seacock. When leaving your vessel unattended (or even when underway for that matter) close all seacocks that are not absolutely required to be left open. Also shut off dockside water supplies, and ensure that all scuppers and cockpit drains are clear. Hose clamps, hoses, fittings can and do fail. Inspect them often, and ensure the proper operation of all seacocks. Bilge pumps can be easily overwhelmed by even small holes or leaks and should be considered only as your second line of defense. The installation of bilge pump counters and bilge alarms can go a long way in further protecting your vessel, by identifying a problems and alerting others of the situation, both underway and at the dock.  Be sure that the dock master and others in your marina are aware that you have a bilge alarm installed on your boat, and are prepared to take action in case of emergencies. It is also a good idea to leave a key to your vessel with marina personnel to provide access to your boat if problems do arise.

The next time you are onboard your boat take a look around and familiarize yourself with the location of all through-hull fittings, their sub-systems, and their purpose. Again, and I can’t over emphasize this point- check their condition. If in doubt contact your surveyor. Make mental notes of what you have at your disposal to plug a hole in the event of an emergency. Have a contingency plan and before you get underway make sure everyone onboard is aware of it, and what actions are to be taken in the event your vessel starts taking on water. And remember, even the best marine insurance policy does not protect lives at sea!             

Captain Scott Thompson, AMS

Ocean Marine Services

© Copyright June, 2000 by Ocean Marine Services. No part of this document or web page is to be reproduced or distributed by any means, including electronically, without the express written permission of Capt. Scott Thompson and Ocean Marine Services.