The electronic fish-finder nearly fills the inside of Mark Rice's boat — not because the electronics are so large, but because the boat is so small.

The vessel is a remote-controlled model of a tugboat, just a bit longer than a yardstick. The fish-finder mounted inside the boat allows Rice to measure the sludge depth of lagoons without the danger of launching a regular-sized boat on them.

Measuring sludge is a legally mandated imperative in North Carolina where Rice works as a North Carolina State University (NCSU) agricultural engineer. In some states, the tracking of sludge depth is mandated only at problem sites.

“The remote-controlled boat is faster and safer than getting out on the lagoon in a boat,” Rice says.

In about 10 minutes of navigating the boat across a lagoon, Rice takes some 30,000 measurements, backed by global positioning system (GPS) locations. The data is stored on a memory card similar to those used in digital cameras, then plugged into a computer and uploaded to calculate an average depth.

When done by hand, Rice says NCSU recommends a minimum of eight measurements to determine average depth, which certainly would take longer than 10 minutes. Per measurement, those eight hand readings technically would be more accurate, but the fish-finder allows many more points to be measured.

“So you have a much more accurate sludge survey with the remote-controlled boat and the fish-finder,” he says.

Safer Sampling

Rice first raised the question of using a fish-finder for sludge-depth measurements at a meeting where swine company representatives were present. After he began his work documenting the concept's accuracy potential, one swine company began surveying lagoons with a fish-finder mounted on a johnboat.

But Rice felt a remote-controlled boat would better address safety concerns.

“My original concept was to put together common, relatively cheap components that producers could duplicate,” he adds.

A fish-finder with GPS capability costs about $900. The boat can range from as little as $300 to the $2,000 Rice spent to have the model boat completely prepared and the fish-finder mounted.

The university is in the process of protecting Rice's “sludge boat” design from patents or trademarks so it can remain in the public domain. To contact Rice, call 919-515-6794 or email him at

Sludge measurements

A guide to proper lagoon sludge measurements is available from North Carolina State University at: