In the new millennium, Sea Tow has set Safety on the Water as our theme. In keeping with this theme, I would like to provide some insight into two interesting areas in Boston Harbor. You may want to explore them or just avoid them. In any case you should be aware of them. As a Sea Tow Captain I see some of the most infamous spots in the harbor "up close and personal" on a regular basis. My job is to assist boaters experiencing problems and often that means going into areas to assist them that I would otherwise avoid. Two of these spots, in particular, come right to mind when your talking about Boston Harbor.
They are: Lower Middle and Can #5A at Castle Island. LOWER MIDDLE Lower Middle is a rock outcropping that is located in the middle of Boston Harbor between Deer Island Light and Castle Island. (see the excerpt from Chart #13270) It is located on the chart adjacent to green cans #1, #3 and #5. Of most concern is the area 500 yards long by 150 yards wide adjacent to #3 where the water is, at the most, less than 6 feet deep at low tide. The underwater rocks gets shallowest near can #3 about 50 yards in from the channel, where some rocks normally expose at low tide. Even though the area is extremely well marked with several aids to navigation, it is none the less, a troubling area because of its general location and lack of obvious rocks showing. As you proceed eastbound out of the Boston Inner Harbor toward sea, with the Airport on your port side and the South Boston waterfront off to starboard, the expanse of the Harbor apparently seems to open up in front of you as you approach Main Ship Channel lighted buoy R-10. This expanse of what appears to be wide open water is what makes Lower Middle so dangerous. The area may look inviting but it isn't.
As you look left toward the north you will see the shallows of Governors Island Flats with shallow water depths at low tide. In the center straight ahead looking east is Lower Middle itself with its rocks. Then off to the south you will see the Main Ship Channel with its safe water. As you're coming out from the Inner Harbor, therefore, almost 2/3 of the area you see opening up in front of you may present navigational problems at low tide. The large tides themselves can cause a false sense of security about the Lower Middle area. At least once or twice each season I get a radio call from a boater that states "I've hit a submerged log and bent a propeller in the middle of Boston Harbor". When asked their location they tell me they're right next to green Can #3. That location is usually a dead giveaway that they have probably hit the bottom on Lower Middle rather than a log.
When shown their location on a chart many of them cant believe it. "I come through here all the time and have never hit anything before." The reason I explain is because of the average 9.5 foot tidal range in Boston Harbor. With this large fluctuation in water depth twice a day the average recreational boat can get over the rocks at high tide and maybe even half tide but not usually at low tide. The false sense of security came to the customer because he had made it through the area several times on previous occasions without incident just never at low tide. The surest and safest way to get around the Lower Middle area, for the casual boater, is to avoid it completely and make your transit out the Main Ship Channel to President Roads. On the other hand, there is a well marked channel that runs between Lower Middle and Governors Island Flats that can be used. This channel is usually referred to as the "Small Boat Channel" or the Lower Middle Channel."
If your comfortable with your navigation skills its a great way to go and avoid a lot of boating traffic. CAN #5A, CASTLE ISLAND The rock strewn spit that comes out from the seawall at Castle Island to the Main Ship Channel, marked by Green Can #5A, is a very nasty place! Can #5A is part of a series of aids to navigation that mark the limits of the Boston Main Ship Channel, the deep draft channel for large commercial ships entering Boston's Inner Harbor (see the chart above). Fifty feet to the north of Can #5A you can literally navigate the channel with an Aircraft Carrier with a draft of 40 feet, but just fifty feet to the south of Can #5A there is zero water. Personally, unless I have to go in there to assist someone I never ever cut inside Can #5A even at high tide. BOSTON HARBOR Between Deer Island Light and Castle Island from Chart # 13270. Note the rocks that expose at low tide on "Lower Middle." Although this is an extremely well marked area, it is often hit by unaware boaters because it sits right in the middle of this heavily traveled area. Also note the area in the lower left showing the spit between Green Can #7 and Castle Island with its rapid depth changes.
Click to enlarge chart
Again the problem is perception. As you are transiting outbound from the inner harbor going to say Dorchester Bay it looks like an expanse of wide open water in front of you especially if you have large commercial traffic in the area. It may look open but its not. There is a radical change in water depth just a few feet outside the channel. As your coming out of the Inner Harbor your in the Main Ship Channel with a controlling depth of 40 feet and then if you leave the channel and try to cut inside Can #5A within a few boat lengths you have run out of water. Just look at the water depths on the chart to see how quickly it changes. Remember, transiting inside Can #5A will ruin your whole day. I hope, through this article, I have a raised your awareness about these two interesting spots in Boston Harbor. The waters around Greater Boston are full of little areas like these but they're all very well marked. As long as you use your navigational chart in conjunction with the aids to navigation you should always be able to transit the area safely and enjoy boating.