May 2019 Safety of jet boats with steering systems that use locking safety wire and D-shackles
This safety update is for
- New Zealand jet boat owners and operators
- Maritime authorised persons and surveyors
- Maritime NZ maritime officers, investigators and technical advisors
The cause of a recent fatal accident involving a jet boat was identified as steering failure, caused by a D-shackle pin working loose after the wire locking on the pin became fatigued and broke. The jet boat had been used at high speed for racing and this type of use imposes significant stress on the wire connecting the shackles as shown in the image below.
The steering system for this particular jet boat had the two D-shackle pins on the tiller arm wired together. The way the locking wire was fastened between two D-shackle pins is not recommended as it can expose the locking wire to unnecessary stress.
The wire is black in the diagram.
Locking safety wire
Locking safety wire is used to prevent fasteners from loosening or falling out due to vibration and other forces. It is usually threaded through a small hole in the fastener itself and secured to a fixed object such as the shank of the shackle.
The practice of securing multiple parts together using one wire should only be used when these parts do not need to move independently.
Locking safety wire is used in both the marine and aviation industries. Best practice for safety wire tying is provided in the US Federal Aviation Administration (FAA) Advisory Circular AC 43.13-1B Acceptable Methods, Technique, and Practices – Aircraft Inspection and Repair, Chapter 7.
Like all metals, locking safety wire is prone to corrosion and fatigue and has the potential to break. Constant vibration and repeated stress may increase the risk of the wire failing.
A D-shackle is a u-shaped bolt that is secured with a pin across the opening. The pin and bolt are threaded together. The D-shackle is often used as the primary connecting link in all manner of rigging systems, including steering systems in jet boats. It allows for movement between parts without separating them while the parts it links may be disconnected when needed.
Risk to safety
The wire may break because of overloading or fatigue. This will lead to the pin of the D-shackles becoming free to rotate loose. This in turn may lead to a break in the steering linkage connection resulting in loss of steering.
What you should do
You need to physically check the steering system of your jet boat. If it uses D-shackles and locking wire check that the shackles are individually wire locked. Check the condition of the shackles. If there are nicks or tooling damage, consider replacing them.
Look at the wire that has been used in the steering system. Check that it is either aviation industry grade locking wire or mousing wire. Note that the locking wire is a single use product.
If you are unsure, talk to the supplier or manufacturer of your jet boat.
Consider adding an additional safety measure. This example shows an extra wire linking the steering cable to the tiller arm in case the turnbuckle fails.