Real life AIS

At the beginning of a recent run between Annapolis, Md., and Southwest Harbor, Maine, I was pleased to find that Consulting Time II, the Morris 48 I was aboard, was now equipped with an Automatic Identification System (AIS) unit. This was especially comforting given the fact that we would be sailing through fairly congested waters and many of the vessels we encountered would be equipped with Class B transceivers. Prior to AIS, a radar equipped with a mini-automatic radar plotting aid and a vigilant eye were the best we had. But now with the availability of AIS, the margin of safety has broadened. AIS supersedes some of the limitations of radar &mdash AIS can identify traffic masked by land features like a protruding headland &mdash and is a welcome addition to the nav station.

“Because my six-year-old Raymarine chartplotters will not support AIS,” Consulting Time II‘s owner, Doug Diehl, said, “I installed a black box AIS with a dedicated VHF antennae on the navcom tower. The AIS black box feeds a signal into the CAPN navigation program on my PC. I had to upgrade the CAPN (Computerized American Practical Navigator) program to version 8.3 in order to support AIS and then, of course, had to update my raster charts to support that version of CAPN.”

“In order to get the AIS and GPS data into the PC,” Diehl continued, “I had to install a combiner by SeaLevel. This is a piece of hardware with software for the PC and is a bit complicated. It also takes the SSB modem signal into the single USB port. It would have been possible to install a splitter on my existing VHF antennae, but there would have been some signal loss for the boat’s radio. Most navigation programs for PCs will now support AIS. Our AIS is receive-only but Class B transceivers are just now becoming available for recreational vessels.

“There are two disadvantages with my system,” Diehl said. “First, it requires running the PC all the time, which is a significant power draw (alternatively, shutting down the PC entails a significant start up time, possibly when you need the info the most). Second, it is located at the nav station and not at the helm. I have looked into adding Class B AIS, but that would involve having to replace both chartplotters and the radar at a total cost of about $17,000, including labor.”

On our recent passage from Maryland to Maine, from north of Cape Cod to Southwest Harbor we were in 0.1-nm visibility fog. When approaching the outbound Boston traffic separation lanes, our AIS detected a 960-foot freighter making 15 knots.

“I first noticed it on AIS at 9 nautical miles,” Diehl said, “before we picked the ship up on radar. We had a constant bearing and I did not want to pass in front of the ship. I called the vessel by name and it answered on the first call. This is one of the big benefits of AIS. Calling another vessel by name virtually ensures that they will answer you.”

“Once we were in radio contact, I informed her that we had her on AIS and radar and asked her to maintain her course and speed while we passed astern,” Diehl continues. “We had to alter course and slow down. AIS continually showed us the closest point of approach (CPA), and time to closest point of approach (TCPA). It also showed a dotted line presentation of her course and our position at CPA. We passed 1,200 yards behind the freighter and never saw her. I felt more comfortable than the freighter, since my information was probably more accurate than his radar-only information. He would have benefited from a Class B transceiver.”

By Ocean Navigator