My question is where is the paddle wheel speed transducer mounted on other IP boats (any model), and typically how far from the centerline of the boat / how far from the side of the keel? This device is usually not installed by the factory and various locations may be used.. As shown in the photograph, the transducer on our 380, Indigo, is currently located in the starboard bilge just forward of the fuel tank, and about 16 inches from the centerline of the boat. The second transducer in the photograph, further outboard, is the depth transducer.
I am considering moving the transducer because I have never been able to get consistent speed calibration. We have a Raymarine/Airmar ST800 with Raymarine i70 instruments, which have sophisticated multi-speed calibration table capability. The transducer correctly produces voltage pulses as the wheel rotates, and the ITC-5 interface, Seatalk NG (NMEA 2000) network and instruments all seem to be working correctly, but the speed through water readings on the display are inconsistent and calibration done one day may well be wrong the next time out, or perhaps under different wind and wave conditions. A clue to the problem is that calibration requires large correction factors (up to x2) to get indicated readings anywhere near correct, which implies that the wheel is rotating too slowly. Replacing the paddle wheel with a new one makes no difference, so something else is causing the problem..
Airmar installation instructions for the ST800 say: "Full keel sailboats—Locate amidships and away from the keel at the point of minimum deadrise". My installation has the paddle wheel just aft of amidships on the fairly flat part of the hull but quite close to the side of the keel (around 6 inches), and I wonder if perhaps water flow over the transducer varies enough under different conditions of waves, leeway, angle of heel etc., to upset the calibration, though I find this hard to believe.
I welcome any thoughts on my problem. Have any other owners had problems with calibration of transducers for speed through the water? Though GPS speed over ground is fine for navigation, you do need need proper speed through water readings to get accurate true wind speed and direction.
Thanks for any advice,
Graham
