Here are two web sites
Homepage and Information about The Pilots Association For The Bay & River Delaware. PA & DE Pilots.
www.delpilots.org
The Maritime Exchange for the Delaware River and Bay, a not-for-profit trade association, is dedicated to promoting and encouraging commerce on the Delaware River and Bay. Our members, comprising all facets of international trade and related businesses throughout Pennsylvania, New Jersey...
www.maritimedelriv.com
Depending on your location - you can hear the Philadelphia Maritime Exchange office on Channel 14 156.700 or the Lewes Pilot Tower.
What you have been hearing is the Lewes Pilot Tower (a picture of it shows up first when you open the web site of the Maritime Exchange). They have a schedule of ship arrivals and departures through the Maritime Exchange and begin to communicate with inbound ships coming up the sea lanes to the bay entrance. The pilot boat is usually dispatched to place the pilot on the inbound ship around a buoy in the area between the Delaware Bay entrance and the Atlantic Ocean.
This is one of the pictures I found with a google search. The Lewes pilot tower is located in Cape Henlopen State Park in Delaware.
The radio signal strength from the tower does change - I think they keep it on low power and then switch to high power when the ship has difficulty hearing them. So the times you are listening may have just been a high power time (if this is the first time you are hearing them).
I hear the tower routinely from my Villas NJ location and occasionally can hear the Philadelphia office as well. Here are the licenses as displayed from Radio Reference for Sussex County Delaware
The pilot tower usually identifies itself as WHW819 over the radio on channel 16 and channel 14 when talking to ships or even the USCG. Another interesting function that the tower performs is calling ships that are within the North Atlantic Right Whale Transit Zone. When the monitor a ship on AIS exceedng the 10 knot speed limit - they contact the ship and advise them to slow down and the reason why.
the area near the mouth of the bay is marked below (I circled it in blue)
The website for more details is found here
North Atlantic right whale vessel speed restrictions reduce the likelihood of lethal collisions between vessels and these endangered whales.
www.fisheries.noaa.gov
Hope this helps and gives you some good reading information.
Terry
Villas NJ