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I have a ST 670 speed transducer that has just started to register speeds of approx 60% of true GPS speed (in a zero tide environment).
I tested it with a multimeter and it shows that there are two magnets working.
I assumed that the internal electronics weren't good so purchased a replacement
The replacement does the same thing, registering 3 knots at 5 knots true.

Any suggestions?

Thanks Conrad
Hi Conrad,

Thank you for your enquiry.

The speed transducer can be tested as per this link as you can see there should be four magnets active in the paddlewheel. If the paddlewheel is defective, then a service kit can be purchased from our approved dealers.

if the problem persists, then further tests on the transducer can be carried out described here .

If the replacement transducer still under reads, check there is no fouling under the boat which is affecting the water flow across the transducer, also have you checked the calibration? click here .
Thank you Derek,

There is no fouling, I cleaned the bottom beforehand, and I have checked the calibration, it it set at 1. To get the correct speed it needs to be changed to 1.6 which makes it unstable.

I definitely get 2 pulses per rotation, not 4 with both the the old and new transducer

My concern is that the replacement ST800 P120 transducer is doing exactly the same thing. What is the chance of the new one being faulty?

HI Derek

Thank you for your reply.

There is no fouling under the boat and no obstruction near the paddlewheel.
We tested the paddlewheel and it contains 2 magnets, as does the ST800 P120 replacement that we purchased.
We took the new transducer for a test run and it read 3 knts when the GPS registered 5 knts, I can correct this with a calibration of 1.6, but the speed becomes very unstable.
As both the new and the old transducers read the same, could it be the display unit? If so, what unit should I use to replace it (ST50 or ST70)?


The display can be replaced with an i50 Display, this allows the transducer to be connected directly to the back of it using 1/8" spade connectors.

I have just checked myself and the current Raymarine 4 lobe paddlewheels have a magnet in each lobe - so 4 magnets . .
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