Stability factor in mini jet boat crash
Lookout! Issue 35, June 2016
Mini jet boats, with a hull length of 3.6 metres or less, have a narrower beam than regular jet boats and are therefore less stable. When the driver is the sole occupant the lack of balance can produce a considerable lean, which is exacerbated when the boat is in turbulent water.
In this case the driver was a heavily-built man who, at 125 kilograms, weighed about 50 percent more than the average male, which further affected stability. He was accompanied only by his dog when he set out on a Southland river, having driven the mini jet boat only about three times previously. The boat struck a rock at high speed. His body was found upstream of the jet boat – which was located in the middle of the river upside down on a shallow rock bar.
Damage to the hull indicated that the first impact was on the bottom plate, which caused the jet boat to go ‘nose down’. The port side then appeared to have crashed into a large rock, with the speed and momentum causing the boat to flip end-over-end, ejecting the driver into the water.
Sun-strike was ruled as likely to have affected the driver’s ability to assess the river, due to the winter sun being low in the sky in the direction he was headed.
Another possibility canvassed was that the throttle jammed due to debris, including a small section of carpet, found in a compartment under the seats that also housed the throttle mechanism. The driver had reportedly expressed reservations about the foot throttle accelerator being very touchy during the test drive, and there was some evidence of carpet being jammed in the throttle mechanism.
A keen outdoorsman, the driver was experienced in operating small jet boats when he decided to test drive and purchase the mini jet boat for multi-purpose activities, including carrying equipment and game. It was 3.3 metres long and 1.24 metres wide at the stern transom.
While there is no evidence that a substantial load was on board at the time of the crash, the Coroner heard that load-carrying would further affect stability, due to the design length and waterline beam of the craft. In addition, the location of the primary load-carrying capacity – a tray on top of the engine compartment with the bottom about level with the gunwale – added to the boat’s unsuitability for some of the purposes the driver intended.
- As with any new vessel, operators must be cautious when familiarising themselves with the particular handling and stability characteristics of the boat they are in. Different boats handle differently.
- Operators of mini jet boats need to be aware of potential handling difficulties and limitations.
- Mini jet boats are not simply scaled-down conventional boats. The use of a jetski motor and propulsion unit means the weight of the jet units is lower than a regular jet boat, which alters the centre of gravity and handling.
- Placing extra weight on the top of the engine cover, such as spare fuel or an animal carcass, further affects the centre of gravity; as does a lone driver’s weight on the port side of these short, narrow craft.
- There is some consensus that to mitigate the risk of increased roll-over, and end-over-end tip-overs, mini jet boats need to be made wider and with a lower centre of gravity.
- It is vital to thoroughly check any vessel before taking it out on the water. In this case debris was found in a compartment that housed the throttle mechanism.
- The Coroner heard that investigators found it surprising to see that this compartment had not been sealed off, to prevent ingress of items such as a beer bottle top, cable ties, insulation tape and the small section of carpet. However, it is the responsibility of the skipper to check the vessel for all hazards that may pose a risk to safety.
- Winter sun riding low in the sky can be one of factors that affects visibility. Drivers need to adjust their speed accordingly.
- Jet Boating NZ has commissioned a report to look into the complete design, construction and recommended guidelines for use of mini jet boats. This is expected to be made public mid-year.