Navigational Charts: Submerged Objects & Obstructions

There’s a lot going on beneath the waves that can have a negative impact on your boating day. Your charts tell a story, and knowing what their symbols mean may save your bacon one day. Here’s a list of some of the more obscure symbols to avoid, along with their meanings.

Groins

Engineering structures perpendicular to the shoreline meant to “catch” or inhibit the alongshore movement of sand. They can be individual structures, but are usually placed in multiple structure “groin fields.”

Fishing Strakes and Fish Traps

Permanent or semi-permanent devices used for catching fish, consisting of a net or other structure that diverts the fish into an enclosure arranged so egress is more difficult than ingress. They typically (but not always) feature a row of poles that generally extend out from the shore to the head of the net.

Distributed Remains of Wreck

This is foul ground that (unless indicated) is not dangerous to surface navigation but is to be avoided by vessels anchoring or trolling. It typically indicates the remains of wreck or a cleared platform. As wreckage tends to shift position over time, always anchor well clear of these areas or risk losing your anchor.

Unexploded Ordnance

Different agencies may use different symbols. The symbol below left is a NOAA symbol; the one on the right is the NGA symbol.

Unexploded ordnance (UXO) are explosive weapons (e.g., bombs, bullets, shells, grenades, mines) that were disposed of or did not explode when they were employed and still pose a risk of detonation. Ocean disposal (dumping) of munitions was an accepted international practice until 1970, when it was prohibited by the Department of Defense. In 1972 Congress also passed the Marine Protection, Research, and Sanctuaries Act banning ocean disposal of munitions and other pollutants.

Rock Awash

The basic rock symbol looks like a plus sign (not shown here) and indicates rock that’s beneath the water surface all the time. The rock awash symbol (which resembles an asterisk) means the rock will uncover (become visible) at low tide.

Obstructions

An easy-to-overlook symbol, obstructions are where damage to keels, props, and shafts can often occur. Some charts may only display the abbreviation “Obstn,” but most will display dots around a circle or square to warn mariners that some unknown hazard lurks beneath the surface. Small circles of dots could indicate anything from old piling remnants or stumps, to submerged post and poles.

Ruins

Submerged man-made structures that can include anything from wharf foundations to buildings. Typically they’re located near shore, but that’s not always the case. Another symbol on this chart is the well-known wreck symbol to the right of the broken line structure labeled “ruins” on the chart.