Raytheon ST60 Speed Calibration

[timb]

I have a Raytheon ST60 Speed instrument and have tried to calibrate it over the last few days. It seemed to be under reading, the transducer is fitted through the hull in front of the Keel on a Sadler 26. After consulting the manual I compared it to my GPS speed and calibrated it whilst going through the Crinan canal where I assumed there was little or no flow. I had to increase the reading to 1.30 ie .30 more than normal.
Does this sound strange, and can anyone suggest a way of getting the reading more accurate.

 

[gcwhite]

No it doesn't sound strange , but check the fly wheel has been fitted the correct way round. I had one some years ago where this had happened and required large adjustments to calibrate.

 

[mhouse]

My St 50 started undereading towards the end of last year by about 15%, The paddle was spotless and spun freely, I can only assume that weed etc on the hull around the hole for the paddle wheel was slowing the water flow . This season its working fine again.

 

[snowleopard]

the callibration can be completely thrown by a barnacle just in front of the impeller, 30% is perfectly feasible. to get reliable readings you need to keep the whole area in front of the paddle scrupulously clean. more realistically you will need to re-callibrate to the gps from time to time. if you have a gps feed into the seatalk bus there is a function to tell the st60 to set water speed = SOG.

 

[Budgie]

I wouldn't worry about the factor - it's only a reference number, and will vary with hull design, waterflow etc. From memory mine is set at around 1.55

 

[ShipsWoofy]

When I had a similar problem with the nasa speed I got the following advice from nasa.

With the vessel under way, loosen the top nut and twist the transducer until maximum speed is registered on the log. That ensures that the paddle wheel will at least be in the cleanest flow of water for its given position. Then tighten it down, mark with paint for future reference.

I don't know if the raymarine units can be twisted, but the above made quite a large difference for me and calibration at the head unit then was much smaller.

I have since re-located the transducer so no longer have a problem, but the method above might help a little.

 

[snowleopard]

the raymarine transducers have a notch in the housing that mates with a projection on the transducer so you can always find the dead-ahead position.