The build-up of static electricity, from fuel flowing through the filling pipe, has
been known to cause a significant spark at the filling point, which in turn has
ignited petrol vapour in the area. To reduce the risk of this happening petrol
tanks must be electrically bonded to their deck filling connections by low
resistance metallic conductors.

If the boat has a non-conducting deck or hull, such as wood or glass-fibre
reinforced plastic (GRP), fuel tanks must also be electrically bonded to an earth
point, e.g. sea cock or stern gear, which is in direct electrical contact with the
surrounding water, creating an earth and discharging any static electricity.
If the filling pipe is already of a conducting material, such as copper, there is no
need to provide an additional conductor. It's recommended that you follow these
precautions if your boat has diesel fuel systems. [2.10]