Outboard Boating Engine Accident: Mechanical Failure Analysis

A severe personal injury incident occurred as a recreational fishing boat was approaching a dock at approximately 20 mph after a day on the water. The large outboard engine (75 HP) on the stern of the boat struck a fixed underwater obstruction, flipped into the boat while still running and amputated the hand of a passenger seated near the engine. The injured passenger survived because medical attention was immediately obtained but he lost his right hand just above the wrist. He later became the plaintiff in a suit in which the owner/operator of the boat, the outboard engine manufacturer and the manufacturer of the attachment devise (known as a jack plate) between the engine and the boat were named as defendants. I assisted the attorney representing the owner/operator of the boat with engineering input.

The plaintiff claimed that his "friend", the boat's owner, had not done sufficient inspection and/or general maintenance on the boat and, specifically, did not assure that there was safe and secure attachment of the engine to the boat via the jack plate. He claimed this lack of attention caused or contributed to the failure and his resulting injury. My specific charter was to offer expert opinion on whether or not the boat owner had completed sufficient inspection and maintenance on the attachment of the engine to the jack plate so as to prevent the failure that occurred. The plaintiff also claimed the design of the attachment of the engine to the jack plate by the engine manufacturer and by the related design of the jack plate by its manufacturer were each inadequate and presented an unsafe condition that caused or contributed to the failure.

My involvement started with a careful visual examination of the boat and specific failed components and review of the general design method used to connect the engine to the jack plate. The latter remained attached to the boat after the accident so the failure was between the engine and the jack plate. That attachment design included two well coated aluminum arms projecting from the engine with bore drilled holes through which four stainless steel bolts passed. Further, the assembly consisted of stainless steel nuts and large diameter washers which were tightened on the bolts to complete the attachment to the rear of the jack plate (called the transom plate). This design was established by my original visual inspection and review of specifications of the attachment method supplied by the engine manufacturer. All the components were of reasonable size for their intended function. There was no evidence of obvious mechanical or corrosion damage to attachment components other than directly at the fractured areas.

One of the engine's projecting arms was fractured at the transom plate apparently from striking the underwater object. The adjacent projecting arm was then not sufficient to keep the engine in place, it too failed and this freed the engine to flip into the boat. The fracture locations were adjacent to the holes used for the attachment bolts. Before I was retained for the case, the engine manufacturer had completed microscopic metallurgical examinations, via a scanning electron microscope, of the fracture surfaces using an independent lab. Those examinations were not repeated but they included photomicrographs of the fracture surfaces which I obtained and evaluated.

Experts for the engine manufacturer and the plaintiff differed on interpretations of the results of the lab studies. One concluded that at least one of the attachment bolts was loose and this permitted the initiation of fretting fatigue, i.e., cyclic micro-scale motion, between the bolt and its hole through the transom plate which weakened that metal so that it failed when it struck the object. The other expert concluded there was no evidence of fatigue or one or more loose bolts and the accident occurred because of small preexisting surface defects in the metal immediately adjacent to the bolt holes. These defects weakened the transom metal. The failure then occurred because of stress overload caused by the impact force. Related to that conclusion was the unknown factor of whether the vertical portion of the outboard engine struck the underwater object directly, i.e., at a 90 degree angle, or at a smaller, glancing angle. A direct hit of the object would have created at much higher force than impact at a glancing angle.

The engine manufacturer testified their standard practice was to complete full-scale testing of their engines on boats (with an attachment design as used here) in which fixed underwater objects were intentionally struck at a range of boat speeds. However, these tests were not done at a 90 degree impact angle.

From my examination of the photomicrographs of the fracture surfaces, I agreed with the expert that saw no evidence of fatigue that could have been initiated by fretting due one or more loose bolts. I did see small outer surface defects. Further, the boat owner testified that he was not given any specific instructions from the engine manufacturer (in the owner's manual) on the needed frequency of checking the bolts or for torque values necessary to maintain sufficient tightness of the bolts. However, he testified that he always visually inspected the attachment components during each use of the boat plus he shook the mounted engine by its vertical shaft just above the propeller to assure that all engine attachment connections were secure.

Independent of the exact failure mechanism that occurred, the critical issue to the boat owner's case was the specific condition of the metal immediately adjacent to the bolt holes (where small surface defects existed) and in the bored bolt holes (where the fretting fatigue would initiate). Both of these results require microscopic examination to clearly identify them. However, these areas were covered by the large diameter washers used as original equipment. The boat owner could not see in these areas or be reasonably expected to detect any problem from his routine, non-microscopic visual inspections. In addition, he was given no instructions by the engine or jack plate manufacturers to disassemble the bolts and search for potential problems. The plaintiff's case against the boat owner was dropped. I was not informed as to the liability findings for other parties in the suit.