Tugs
 

Please excuse my ignorance, but what does 'SD' stand for on a tug?
Roger.

I wondered (as one I saw was a services auxiliary) if it could be Serco-Denholm? I think Denholm may have sold out their share but a left over harking back to the start up.

Yes, true, Serco Denholm was the joint venture which took over operation of RMAS vessels.
Serco have since bought out the Denholm share.
Serco having no previous experience in shipping needed a partner with the required skills.
It would probably be too much trouble to rename them all again as Serco... so SD remains.

Can also define Stern Drive as with the Dutch Kotug company which also have RT prefix for their rotor tugs