This month I finally got to achieve a long-term goal of exploring the cove in West Dover using sidescan sonar. In the past I've used dropped underwater cameras and ROVs to explore the underwater area but I've never been able to see more than 10 to 15 feet of the seabed at a time due to underwater murk limiting the visibility.
After a bit of research I discovered the Humminbird 398ci SI sidescan sonar unit. The 398ci SI is a high quality portable sidescan sonar unit that goes for about $450 on eBay. The unit runs off an external 12 volt battery and records the sidescan soar data to an SD memory card (which is the same type of memory card that is used in digital cameras). It can easily scan a 250 foot wide section of seabed in a single pass and records a continuous GPS record at the same time.
The Humminbird 398ci SI sidescan sonar unit is an entry level model that packs in a lot of features.
The sonar transducer and computerized display unit can run off a small 12 volt battery which means it will be really portable. When the unit is powered on, the sidescan sonar data is recorded automatically to an SD memory card.
The sidescan sonar unit arrived on a Friday and about 24 hours later the sensor was mounted on our boat and we were on the water scanning the coastal areas. The Humminbird 398ci SI product box included the required mounting hardware, a printed getting started guide, and two discs with PDF manuals and a Humminbird sidescan sonar training DVD. The training video was quite useful as it explained how to read the sidescan sonar display and interpret details in the water column like submerged trees.
The display unit has a bright LCD screen with overall good visibility outdoors. I liked the large buttons and the simplicity of the controls.
Since I wanted to explore the saltwater coves and coastal inlets, along with the freshwater lakes in the West Dover area, it was important to mount the sensor for a portable setup that could be easily taken off the boat.
Since I already have a collection of Lithium Polymer (LiPO) batteries that I use with electric model airplanes I adapted the Humminbird 398ci SI sidescan sonar unit's power cord to use a model airplane style "dean's connector" and ran the system off a single compact 11.1 volt 3 cell LiPo (3S1P) battery for the whole afternoon.
Humminbird impressed me with their compact sidescan sonar transducer design. Traditional sidescan sonar sensors are a lot larger and bulkier and would be difficult to use from small watercraft.
I mounted the transducer on a piece of aluminum box tube for portable use. The assembly can be quickly mounted or removed from a boat's transom using a c-clamp.
The Humminbird sidescan sonar display has a mode that can hide the central water column section of the plot if you want to focus on the seafloor area.
The cursor buttons on the sidescan sonar display allow you to pause the screen scrollback and mark a GPS tagged waypoint of a feature on the seafloor.
Viewing Sidescan Sonar Data
After the first sidescan soar boating trip was completed I loaded the Humminbird sonar data from the SD memory card on my desktop computer and viewed the dataset using the free HumViewer tool. The HumViewer program is very flexible as it can replay the sonar recordings using multiple view layouts, show your boat's track at any point in the sonar file, measure features in the sidescan sonar data, calculate heights based upon the shadow lengths, export Google Earth KML files, and save screen videos of your data too.
The V like shape in the water column is a mooring line coming down from the surface.
The pile of rocks on the left hand side of the sidescan sonar chart shows an infilled area that leads up to wharf.
The seafloor in West Dover is a rocky granite bottom with areas of smooth mud, and the occasional collection of glacially deposited boulders.
HumView lets you view down imaging and sidescan imaging data at the same time.