Glass or plastic fuel sight tube gauges can easily be damaged by knocks or the
heat from a fire and can therefore only be used on diesel tanks. Where they are
used they must be fitted with self-closing valves at the top and bottom (or
bottom only, if the top connection is on the upper surface of the tank), and must
be protected against damage, e.g. by use of a cage, by design, or by position.
If your fuel tank is fitted with a permanent dipstick arrangement it must be
vapour-tight to prevent fuel or vapour leaking into the boat.

Damage can be caused to the bottom of your boat's fuel tank(s) if you allow a
portable dipstick to strike it. This could eventually lead to a fuel leak. To prevent
this from happening it's recommended that you fit a cross pin arrangement,
wider than the fuel filling connection, to the dipstick. [2.8]