UK, Cornwall. Earlier this year, a RIB accident that took place in the Camel Estuary, Cornwall resulted in two fatalities and serious injuries. The accident occurred when all six occupants of the eight-metre rigid inflatable boat called Milly were ejected from the boat into the water as it was making a turn. At the time of the accident, it was found that the engine's kill cord mechanism was not active and was not been attached to the driver. Consequently, the boat continued to circle with no one at the helm, at full power, striking some of those in the water. The Marine Accident Investigation Branch (MAIB) has released the full report on accident, which raises some serious design considerations for RIB manufacturers.

"Just prior to returning to the boat’s mooring, the adults changed over at the helm but the kill cord was not attached to the new driver," the report states. "A short time later, the adult who was no longer controlling the boat reached across in front of the driver and operated the helm and engine controls to execute a tighter, high-powered turn, and the accident followed almost immediately. Post-accident trials of Milly showed it to have some undesirable handling characteristics in certain circumstances, which could be mitigated by design changes."

Milly was manufactured in December 2011 by APV Marine Limited, trading as Cobra Ribs. As part of the investigation, MAIB undertook water trials to better understand the accident scenario and how the boat acted during high-speed turns. Reporting on those turns, a naval architect, commissioned to make a detailed examination of the vessel, noted that: “When executing the turns, the craft initially would take up a high-heel angle. It would proceed to turn, but if the speed was slightly higher than a particular threshold and the turn tighter than a certain degree, the heel angle would increase during the turn, and the aft end would lose grip and slide – thus initiating a ‘partial spin’ or ‘hook’ since the bow did not slide by the same amount. This rapidly took the craft to a position, which was appreciably diverted from its original course. The craft would execute a sideways slide and grip suddenly when it landed. Thus the hull’s sideways motion was suddenly stopped.”

The naval architect observed the performance of the boat when turning at the end of the straight line runs, and noted that; “the boat did take up a high angle of inward heel – towards the more extreme end of the scale for planing deep vee craft – and also, that this could happen in two stages – a certain angle of heel at the start of turning and then an increase to a more extreme angle part way through the turn. This is not unknown and some other craft have exhibited this tendency, but it is preferable to engineer it out.”

In his report, he concluded that; “the craft did not seem to show any bad handling characteristics, although banking, tail sliding, side-skip when the craft reaches its limit… was noted. It would be nice to develop out the above characteristic from the craft or reduce it. It is undesirable and it should be possible to reduce the effect or the suddenness of it. Reducing the degree of heel angle would help.”

In determining the reasons for the high angle of heel, the naval architect considered why the dynamic righting moment of the hull did not overcome the heeling moment in a turn. He suggested that, “either the round tube or the unusual outer panel of the hull (just in board of the chine) are picking up water by ‘coander effect’ (the force exerted by sticking to and being drawn round a curved surface), which is actually sucking the hull over beyond its natural banking angle. If this is the case then it should be possible that hull modifications/additions could be fitted which would separate the flow and break the suction.”

“It is evident that the boat was put into a high-powered tight turn, which was an exceptionally unusual manoeuvre for a recreational boat,” concluded the report. “However, the boat’s tendency to adopt a high angle of inward heel when turning was described by the naval architect as ‘undesirable’.” As one of the conclusions of the report, MAIB has recommended that the manufacturer reviews the design of the hull of its Cobra RIB range and makes modifications to reduce the steep angle of heel that the boat adopts in tight turns.

The full report on the incident can be read here.