Thursday, July 19th, 2018

Personal Watercraft Explosion Hazards

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Lake Wylie Pilot 07/13/2012–

“According to York County Sheriff’s Office, a 31-year-old male from Waxhaw, N.C., was riding a Sea-Doo personal watercraft near Buster Boyd Bridge when it stalled. When he tried to re-crank, the vehicle exploded. Witnesses say he was blown to bridge height.”

WESTMINSTER, S.C. 05/28/2012–

“A watercraft explosion is responsible for injuring a man in Oconee County. They have been called out to the Chickasaw Boat Landing on Chickasaw Drive in Westminster where Fire Chief Charlie King says at around 11 a.m., what looks to be a jet ski blew up. The rider was thrown off into the water.”

Ferguson Falls Journal 07/31/2012—

“One person was transported to Perham Health Monday after a reported explosion of a jet ski on West McDonald Lake, northwest of Dent. The explosion was reported around 7:45 p.m. near Meyer Beach Drive. It apparently happened while the jet ski was being started.”

Headlines like these appear all too often during the summer months and in temperate coastal areas year round. What causes personal watercraft to explode and why are they different from a regular boat?

When the weather warms up, many people will be headed to lakes, rivers and beaches to enjoy boating and riding personal watercraft (PWC). While both PWC and boats with inboard engines are designed to float and carry passengers, there is an important design difference which can make a PWC more likely to explode when starting it. Since the operator of a PWC sits directly on top of the engine, an explosion involving a PWC is more likely to result in significant injuries to the operator and any passengers.

The United States Coast Guard (USCG) defines personal watercraft as small (less than 13’ in length), watercraft capable of carrying 1-4 people, and which are propelled by a jet of water. PWC can have either a 2-cycle or a 4-cycle inboard internal combustion engine. In the 2-cycle variants, the gasoline and oil are held in separate containers and premixed prior to combustion.

The USCG requires all boats which have inboard engines to have bilge blowers installed. A bilge blower is a fan which forces air into the enclosed space/s housing the engine in order to vent any accumulated flammable vapors. The USCG requires that prior to starting a vessel, the operator must turn on the bilge blower for a minimum of 4 minutes, then open the engine compartment and smell for gasoline fumes. They also require a warning label be placed by the ignition switch which warns the operator of the risk of explosion and fire should they fail to vent the engine compartment in case there are flammable vapors present.

In contrast, personal watercraft are not required to have bilge blowers per the U.S. Coast Guard. The only standard addressing the issue is SAE Surface Vehicle Standard J2034-08: Personal Watercraft Ventilation Systems, which requires only passive ventilation. According to the standard, passive ventilation can be achieved by providing two openings each having internal cross section exceeding 3 square inches. The two openings must be separated either at the fore and aft sides of the engine compartment, on opposite sides of the PWC, or fitted with cowls which are pointed in opposite directions.

In a passively vented system as described above, any accumulated flammable vapors in the engine compartment can only escape when the PWC is in motion, or when wind is blowing towards one of the openings. When the vehicle is stationary, there can be no positive air movement and therefore, no removal of accumulated gas vapors. This is the exact situation which occurs every time the PWC is started. If any vapors have accumulated, the danger exists for their ignition by starting the watercraft.

In such a scenario, how would the flammable vapors get in the engine compartment and what could be the ignition source?

The source of flammable vapors is the gasoline used to fuel the PWC. Liquid gasoline produces flammable vapors at temperatures above -45° F. Fugative gasoline can get into the engine compartment through spills during tank filling or through cracks or holes in the tank, fuel lines, or gaskets due to abrasion, age, poor design, manufacturing methods, or poor servicing.

For there to be ignition of the gasoline vapor-air mixture, the following four criteria must be met:

  1. An ignition source having adequate energy must be present;
  2. The amount of fuel-vapor must be present at a concentration within its flammability range (for gasoline the percentage in air is between 1.4% -7.6% in air;
  3. The ignition source must contact the fuel while it is in its flammable range; and
  4. The contact between the ignition source and the fuel must be of sufficient duration for a transfer of energy to occur.

As little as 1TBS of gasoline fully evaporated within the engine compartment, can be enough to create a concentration of vapor in air which is within its flammability range, thus capable of causing an explosion. (This calculation is based on estimates of the average free air volume in a personal watercraft engine compartment.)

The primary ignition source for gasoline vapors is the PWC’s electrical system. The spark plug wires typically carry 30-80 kV of electricity between the coil and the plugs. When the plug covers and wire insulation get covered in oil and dust and are in a wet environment, their outer surfaces can become unintended pathways for electricity, allowing some of the electricity to “leak” out onto their outer surfaces. Engine vibration can cause wiring to loosen at connections or chafe against another component causing the insulation to wear resulting in a short and subsequent arcing. The sparks produced by both shorting and current leakage are sufficient to ignite the fuel-air mixture.

A careful examination and analysis of a personal watercraft which has been involved in an explosion should reveal: 1) the source of the gasoline, and 2) the ignition source and sequence.

Our research has determined that none of the currently manufactured personal watercraft come equipped with bilge blowers, nor do any of the manufacturers offer bilge blowers for aftermarket installation.

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