Rain is the single largest cause of small boat sinkings, sinking not only small boats but occasionally larger ones, too. This type of casualty is therefore one of the most frequent types of losses investigated by marine surveyors.

Smaller boats sink more frequently due to several factors: First by virtue of being smaller, second by being more open, and third because most do not have shore power operated battery charging systems.

There is a host of contributory factors, such as outboards and stern drive boats being unbalanced and stern heavy, as discussed in previous chapter.

Heavy rainfall also results in partial sinking, or hull flooding, including hull flooding of hauled vessels. Of course it is never expected that rainfall will sink a boat because almost everyone expects boats to be designed in such a way as that they will withstand the effects of what we consider to be normal weather.

That statement raises an interesting question: How much rainfall should a boat be expected to withstand?

Design Issues

Rain is usually not the only factor, but its combination with design issues, maintenance and system failures that causes a boat to sink. It is the rare case in which heavy rain, alone, is the sole cause.

This presents the investigator with the interesting challenge of having to assess the adequacy of both structural and systems design. For this reason this chapter will explore the wider variety of design issues that also cause boats to sink.

No matter where you live, unless perhaps, it is in the desert, all regions from time to time will receive extreme amounts of rainfall, amounts that are well above normal. How we define extreme will be dependent on what is defined as "normal" for a given area.

In this regard we should be cognizant of the fact that many, if not all, boat builders market their boats nationwide and even internationally. With that being the case, the builder has an obligation to design his product to withstand the normal range of weather of the region with the highest rates of rainfall where he markets his products.

The issue of proper design is often raised, as it was in a recent case involving a new $250,000 thirty-four footer that sank because the cockpit drainage system was improperly designed.

The boat suffered a bilge pump failure in addition to the fact that the aft deck was designed in such a way as it shed all rain water directly into the bilge.

After paying the loss, the insurer sued the builder over the faulty design and recovered based upon an investigation that left no doubt as to the cause of the sinking.

The faulty deck design was the primary cause because the boat should have been able to withstand a four inch rainfall even without bilge pumps. But the pumps did not work because the builder had improperly wired them.

Turning off the main battery switches cut power to the pumps when it is standard procedure to wire the pumps independent of this switch. This new boat owner made a habit of always turning the switches off when leaving the boat, not knowing that he was turning his bilge pumps off.