What does one prolonged blast plus two short blasts every two minutes mean?

One long two shorts is a sound signal made in restricted visibility by the following classes of vessels:

• Not Under Command
• Restricted ability to manoeuvre
• Constrained by draught
• Sailing vessel
• Vessel engaged in fishing
• Vessel engaged in towing or pushing

A vessel engaged in fishing would make this signal at anchor and underway.

All other classes of vessel listed would make the signal only when underway.