Reference to Code of Safe Working Practices for Merchant Seafarers (COSWP, UK): 24.7.3 Earth return clamp must be connected as closet to welding piece and not hull.
The return clamp should always be placed as close as possible to the welding area, or on the work piece itself if this is not an integral part of the hull.
The return connector must be properly dimensioned to provide good contact for the welding current load to be used. There are four types of return clamps available: the 200A, 400A, 600A and 800A.