Bye, Bye, Birdie #5

The Need for Speed

- Robert Reeder

In our last installment we discussed the basic techniques of ocean and coastal dead reckoning, which is determining our location based on our course and speed since the time of an earlier known position. In this installment we will discuss how to obtain our boat speed in the absence of GPS.

SPEED OVER GROUND VERSUS SPEED THROUGH THE WATER

GPS speed, meaning Speed Over Ground (SOG), is a measure of our total speed over the surface of the earth, regardless of how that speed was accomplished, whether from our own propulsion, wind, currents, or any other source. Speed Through the Water (STW) is only the speed of your boat relative to the moving medium of the water.

For example, imagine that you are floating in a barrel in three knots of tidal current. Your SOG would read 3.0 knots, but your STW, however derived, would (always) be zero, no matter how fast the currents are running. For dead reckoning (and many other navigation applications) we must use STW rather than SOG, so even with an operational GPS, we need to be able to derive our STW.

Here are several means of determining Speed Through the Water.

DST TRANSDUCERS

For a cruising trawler, the simplest solution is to have an electronic transducer that tells us our STW. There are stand-alone speed transducers, but for a few hundred dollars you can get an all-in-one depth, speed, and seawater temperature transducer. There really isn’t much more to say about this one; it’s plug-and-play, completely independent of GPS, and a superb investment for any cruising boat.

HULL SPEED