[TG] [TG11] ST70 Speed Calibration

[andrewlachlan]

I am having difficulty calibrating STW using an ST70 display. The speed transducer is Airmar and its connected to an iTC-5. I also have 2 x Axiom 7's in the network. In reading the ST70 manual it initially makes reference to starting the calibration at the lowest calibration speed of 2 KTS (Start Speed Calibration item 6). I did this and needed to use the maximum calibration factor of 2.0 but even then it was reading a little low. I then moved to 4KTS and needed to use a factor of 0.9. On For the 8KTS factor I motored at my maximum speed of 7.5KTS SOG but even when using a factor of 2.0 I could not get STW reading high enough. What might I be doing wrong?

[Tom - Raymarine - Moderator]

Hello Andrew,

That sounds to me as if the flow of water around the paddle-wheel isn't smooth so that you're not seeing a linear relationship between boatspeed and indicated speed. If it were linear then I'd be suggesting that perhaps the paddlewheel weren't spinning freely enough, but that non-linear response is the kind of thing that I'd expect to see if your paddle-wheel were sited in disturbed water (behind a hull fitting, behind the leading edge of the keel for example) or if it weren't fore-and-aft or fully inserted into the skin fitting.

We sometimes come across paddle-wheels where the installer has fitted the housing skewed, so that you have a choice of either inserting the paddle-wheel insert fully so that engages into the fitting's keyway but that is then not parallel with the centreline, or inserting the paddle-wheel insert pointing straight forward but so that it's not engaged with the keyway and therefore sits a few millimeters recessed into the housing.

Any one of these 3 options will give you turbulent, eddying water around the paddle-wheel and both under-reading speed values and a non-linear relationship between measured and real speed.

If it's not this then the next most likely thing is that you've got some electrical noise signal on the transducer cable which is masking some of the speed pulses coming from the paddle-wheel. This is less likely than the paddle-wheel/water flow causes.

Regards,
Tom