How do you use your boat’s fishfinder to help locate a good spot for anchoring and avoid dropping the anchor over rocks, kelp and eelgrass? When anchoring for the first time in a bay or cove, it’s sometimes hit-and-miss to find the right spot. Charts are very general and not granular when it comes to bottom characteristics. Discover another valuable use for your fishfinder:

Overview. When anchoring in an unfamiliar location, it is helpful to know more about the bottom than is available from navigation charts and a depth sounder. Charts show general seabed characteristics (e.g., S = sand; M = mud; R = rocky; K = kelp; Grs = grass) and known rock locations. But they don’t show the exact edges of grass or kelp beds and don’t identify subtidal rocks and hard spots located within an otherwise soft seabed. A fishfinder (sometimes called an echosounder) helps provide this information.

Fishfinder vs Depth Sounder. A depth sounder typically gives only a digital reading of the depth under the boat. Some navigation programs track recent history of depths that helps understand the topography. A fishfinder not only gives the recent depth history, but, more importantly, also gives information about the strength of the returning acoustic signal. Every object has a unique “acoustic signature” and every fishfinder has a unique way of presenting that signature (usually in the form of a color scheme). Generally speaking, setting the frequency higher, say from 50 KHz to 200 KHz, is usually better.

Seabed Hardness. The harder the seabed the more acoustic energy it reflects back to the transducer. If the seabed is hard enough, the returning energy bounces off the boat, back to the seabed, and back to the boat creating a second echo. Second echoes are weaker and twice the depth of primary echoes. Fig. 1 shows soft seabed in Roche Harbor that does not create a second echo. Fig. 2 shows a harder seabed in Montague Harbor (note the second echo at twice the depth). The harder the bottom, more echoes will show up between the first and second echoes.

Screen image showing soft bottom

Screen showing a hard bottom with a second echo

Rocks. The seabed is not always uniform. Fig. 3 from Gowlland Harbour shows a hard or rocky section of an otherwise soft seabed. Note the second echoes under the hard spots. This information will help you avoid putting the anchor down on a hard spot.

Screen showing hard spots on a soft bottom, with a second echo

Eelgrass. As noted in the main Waggoner text, cruisers should avoid anchoring in eelgrass. Fig. 4 shows the eelgrass at the west end of Roche Harbor. Note that the eelgrass gives a weaker echo than the seabed and that the reported digital depth is 12.8 ft (the top of the eelgrass) when the true depth to the seabed is 15 ft. Reporting depth to the top of thick vegetation is a characteristic of most depth sounders and fishfinders. Here the fishfinder helps to see the true depth.

Screen showing eelgrass

Kelp. Fig. 5 shows understory kelp on a hard substrate (note the weak second echo) at Point Partridge. You don’t want to anchor here. The bottom shown in Fig. 5 is the thick deep brown/rust colored line. The kelp is the same color as the seabed but has a yellow/green edge to it seen easiest on the right half of the photo/image. The short tails sticking down from the seabed indicate the hardest part of the seabed and are most likely rocks that the kelp is attached to. The blue/yellow targets in the water column above the seabed are most likely the bulbs of the young bull kelp. The second echo is the mostly yellow fine line at the bottom of the screen.

Screen showing kelp on hard bottom, kelp bulb images above the bottom and second echoes

Article by Jim Norris
Waggoner Field Correspondent