Hi Greg,

so is this on a timber boat?

Most timber boats have bolts with nuts under the beam pads. Usualy need a spanner to hold the nut underneath while you turn bolt above. But I guess its possible that they have threaded plates in place like glass boats, in which case you can buy next size bigger in diametre of bolt and tap out the plate to same size, or put a nut on the end of existing bolt if it's long enough (or buy a longer one).