Co-incidentally there is another kind of PD boat, mentioned in a couple of small North-East of Scotland songs.
One has
"Wha saw the PD boats? Wha saw them comin hame?"
In this case it is the abbreviation on the Peterhead fishing boats, that in together with their number painted on the bows to identifie them.
Plus, in the Northern Islands 'peedie' is a loving diminutive, indicating small size -'Peddie Betty's the one for me'.