Gasoline is very dangerous. This explosion left two people with very serious injuries and destroyed the boat.
Some 10 days before the accident, during a short cruise, it was noticed that the engines were not working properly. Upon reaching the dock, the owner examined the port engine and noticed water in the fuel. The day before the accident, he tried unsuccessfully to start the engines. Hence, he decided to bring a small portable electric compressor and 10 - one-pint containers of dry gas. The boat was alongside the dock, connected to the shore power. He opened the rear hatch on the cockpit, opened the sliding doors, opened the gate on the railing, opened the engine hatches and opened all windows that could be opened. He disassembled the carburetors on both engines, drained water from the fuel filters and reassembled the engine fuel lines, using paper towels to avoid any gasoline drips from spilling, and discarded the used towels in the trash dumpster at the marina. Then he added 10 pints of "dry gas" to the fuel tank through the filter line at the aft port side of the vessel. He put the compressor on the dock and, in order to mix the dry gas with the gasoline in the tank, he removed the fuel tank level gauge, inserted the air compressor hose deep into the gasoline tank and ran the compressor for 15 to 20 seconds. At that time, he smelled fumes. After putting the compressor and tools away on a picnic table, him and his wife went shopping for 2 hours.
After returning to the boat, they never noticed any smell of gasoline and no gasoline smell was noticed while cooking or eating dinner, which they prepared in the galley in the lower cabin. They smoked during dinner. The weather in the marina was calm. About 6:45, he went to the lazarette, which was still open, as well as all the other hatches were still open and his wife asked if it would be okay to turn the hot water heater on to wash the dishes. After receiving approval from her husband, she reached for the switch of the water heater and an explosion occurred immediately. The owner was thrown from the boat into the water and his wife was seriously injured and was trapped in the boat remains by flying debris.
The boat had been all open for some 5 hours after the application of compressed air and the vapors were probably substantially dissipated in the area of the hatches above the engines and in the area of the hatch of the lazarette, but the vapors were trapped under the floor of the lower cabin and in the compartment where the water heater was located.
The information contained in the documents reviewed indicates: The lower decking and floors were blown upwards by overpressure in the bilge area and the upper deck was blown away from the center of the boat. There was melted wiring insulation in the vicinity of the electric water heater forward of the non-tight bulkhead located under the cabin floor, indicating that flames were present in the vicinity of the water heater.
The wife testified that her clothes were not burned, therefore no flames developed in the lower cabin or in the 3 steps connecting the lower and the upper cabin, except for some burning of the edge of the carpet under the refrigerator, and no flames were developed anywhere in the rest of the cabin, because the owner's personal effects were not scorched or burned in anyway.
Gasoline as a liquid does not burn. It is the vapors emanating from the liquid gasoline that can form an explosive mixture when mixed with oxygen in the proper proportion. These fumes are several times heavier than air and will "sink" or displace air to occupy the lowest position respect to the surrounding air. In the case under consideration, as the fumes were "pushed out" of the gasoline tank by the effect of the compressed air used to agitate the gasoline, a very small amount may have been propelled up by the air stream to acquire an elevation of several inches to be smelled.
If these fumes went up more than 8" to 10" above the surface of the liquid gasoline, these fumes may have reached the cockpit deck (6" above the top of the tank) and should have drained overboard thru the open gate and thru the scuppers. The fumes that were unable to reach the cockpit deck will fall to the bilges and will advance, like molten lava of a volcano, toward lowest elevation on the bilges. Upon reaching the transversal bulkhead forward of the engines, some of the fumes will drain thru the limber holes, like a liquid, towards the forward end of the bilges where the bilge pump was located, while more fumes would have continued to accumulate against the bulkhead, leaking forward thru the numerous penetrations through the bulkhead framing non-tight seams of plywood and holes cut into the bulkhead connecting the space where the water heater was located.
One of the most popular books regarding boating, CHAPMAN, which has been reprinted more than 56 times since 1912 indicates a concentration of gasoline in air as low as 1 ¼% - a half a tea cup, a few ounces of gasoline, can create enough explosive vapor mixture to destroy a large boat completely. The actual explosive range of vapor mixture is from 1.4 to 7.6% in volume.
As the gasoline fumes "fall" to the lower part of the bilges, the vapor will form 3 layers of vapor diffused with air. The lowest layer would be too rich for flammability, the next layer above will be flammable and the upper layer will be too lean. Of course, a sharp and obvious division between layers is not possible unless that a condition of calm winds and repose exists for a period of time. Turbulence created by people walking among the vapors and a natural draft created by differential temperatures (sun and shade) and the availability of two open hatch areas, will result in a variation of vapor concentration, but the situation in the bilges and the compartment containing the water heater, is that no walking or other means of creating turbulence are possible because of space restrictions, and these "pockets" will preserve the existence of the 3 layers.
Hector Pazos, is a Naval Architect, Marine Engineer and a Registered Mechanical Engineer and has been engaged in Accident Investigation/Reconstruction for more than 40 years. He has been retained as an Expert Witness in over 1,200 Maritime cases, related to both commercial vessels and pleasure crafts, for both defense and plaintiff.
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