C-MAP93, Ship Constant

[pmv-2007]

hello there,
in the settings for c-map i came across the term "shipconstant", the default value is 30. from the default entries re size and draft of vessel it seem to be defaulted for a larger vessel.
so here my question: to what value does it need to be changed for a 12m steel boat and what does the term mean?
hope you can help.