A NEW module introduced by Olex of Norway to its navigation, plotting and mapping system allows water currents to be detected and displayed.

The SB module analyses five types of NMEA data input - position, speed and course over ground, true heading and speed through water - to calculate vessel drift over ground.

This is then shown as an arrow pointing away from the vessel to indicate the divergence from the steered course.

According to Ole Jorgen Orsnes of Olex, this drift is assumed to be representative of the water current. He tells FNI that, with the system only limited by instrument resolution, accuracy levels are very good.

He explains: "In addition to the real time water current, the values are also mapped in a celllike structure much like the seabed depth, albeit at a somewhat coarser resolution.

"Mapping with the SB module takes the main tidal phase into account.

That is, for a given position, currents are mapped in several time slices synched to the moon's angle from the earth, the so-called M2 tidal constituent."

"The effect of this is that, as the vessel sails around and collects current readings, a detailed map of water flow is built up; at any time, the assumed currents are shown as small arrows related to the actual point of the tidal phase. There is a slider available to test what the flow will be like at a certain time into the future."

While the next versions of Olex will allow sharing of current data, the present version 6.4 contains a demo and a data sample collected from the company's vessel a few months ago.

Says Ole Jorgen Orsnes: "This is a surprisingly simple instrument for current mapping. It is possible because Olex calibrates the sensors very efficiently, recognising various short-term errors and using GPS positions for prediction validation.

"On our own vessel, we use a Furuno SC-60 GPS compass for position, heading, course and speed over ground, and a simple paddlewheel water speed sensor from Airmar for speed through water.

"These two instruments are sufficient. Of course, a higher grade of water speed sensor can also be used, but careful sensor placement and high-grade true heading may be more important. Maybe a midprice electromagnetic solid state sensor would be a good option."

He tells FNI that, regardless of what water speed sensor is used, it has to be calibrated for nonlinear speed response due to water flow patterns.

"The Olex has a special mode where it asks the skipper to steer opposing straight lines at various speeds, automatically working out a graph of apparent water speed and correction factors.

"The same has to be done for the heading sensor. It's likely that this procedure will need to be carried out only during the initial sea trials, but can be repeated by the skipper if he feels it may be necessary."

Ole Jorgen Orsnes says comments that, at last year's Nor-Fishing exhibition, Olex spoke to skippers who use highefficiency doppler sonars capable of giving water currents at various depths.

Ole Jorgen Orsnes adds that a natural development would be to interface to such instruments, allowing currents close to the sea bottom to be mapped.

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Mapping currents made easy