Raymarine ST60 depth offsets

[jwilson]

By measuring the depth below water of my transducer when boat ashore and consulting the Raymarine manual my depth offset to get actual depth of water SHOULD be +0.3 metres. By leadline however (measuring either side of the boat as near to transducer as possible) I need to set it at -0.4 to get an accurate reading. This was happening both with a now-partially-faulty ST60 and a definitely working as-new ST60+ which has replaced it.

The only thing I can think of it that when moored there is softish mud underneath (thick enough to sometimes trap me in the berth at LWS though) with a harder layer some distance below that is reflecting pulses better than the mud.

Not really keen to look for a hard shoreline to go aground on and check!

 

[jdc]

Do you not think it could be a confusion about the sign of the offset? Supposing you measured the transducer to be 0.3m below the waterline, and when reading with the lead-line you have to set offset it to -0.4m to get agreement, is it not possible that you should have set the offset to be -0.3? this would give an 'error' of only about 0.1m, which is more plausible.

 

[jwilson]

Do you not think it could be a confusion about the sign of the offset? Supposing you measured the transducer to be 0.3m below the waterline, and when reading with the lead-line you have to set offset it to -0.4m to get agreement, is it not possible that you should have set the offset to be -0.3? this would give an 'error' of only about 0.1m, which is more plausible.

No: both logically and according to the manual it should definitely be a + offset to read to water level. However checked again this afternoon ago before and after moving to a different berth and it read correctly with a -0.4 offset in the first berth, but after moving had to set it to +0.3 as it should be to get it correct. I can only assume the mud was not reflecting but something harder underneath was.

 

[RichardS]

No: both logically and according to the manual it should definitely be a + offset to read to water level. However checked again this afternoon ago before and after moving to a different berth and it read correctly with a -0.4 offset in the first berth, but after moving had to set it to +0.3 as it should be to get it correct. I can only assume the mud was not reflecting but something harder underneath was.

My ST60+ is set with a -ve offset to give me the depth under the keel so a +ve setting must be correct to give water depth. I've never noticed any error in soft mud although, to be honest, I don't check it against a lead line and, over soft mud, I obviously wouldn't be worried either way.

Richard

 

[johnalison]

Whatever the theory, the offset that gives the right reading is the correct one. Simpler still is to do what I do and sail with no correction and just add 0.3 or so when it suits me (I'm not concerned about the difference between 32.4 and 32.7). Having sailed with a whirly Seafarer for yonks I just don't trust offsets.

 

[ghostlymoron]

As far as I can remember, you can either set the offset at zero which will tell you how much water is below the level of your transducer, set the offset to minus the depth from transducer to keel bottom to tell you how much water below the keel or set offset to plus the depth of the transducer below the water surface which will be the depth of water. I always set it to tell me how much below the keel (+ 300mm to be on the safe side) so my offset would be negative (-).
The instrument only knows how far above the seabed the transducer is. I think maybe your confusing the meaning of offset. Try drawing a diagram.

 

[Baggywrinkle]

I leave the offset at zero, I know my boat draws 1,8m ... so I never let the depth go below 2m which gives me a good margin for error .... but I don't sail in tidal waters where 10cm can make the difference between getting home and waiting for the next tide.

 

[jwilson]

The instrument only knows how far above the seabed the transducer is. I think maybe your confusing the meaning of offset. Try drawing a diagram.

I promise you I understand the offset + or -. As I said in Post #3, a second check in a different berth after moving the boat and it suddenly became correct. Must have been an instrument error, probably mud too soft to register.

In the days of Seafarer spinning-neon echo sounders (I was sailing when they were state of the art) you could get false low readings such as 66 feet reading 6 feet - switch to fathoms range and you saw it was 11 fathoms because the spinner had gone round past 360 degrees. After lashing out and buying a (possibly £29.99) new Seafarer 2 I then went aground twice shortly afterwards because instead of being very cautious I was cutting corners more to cheat tides.

 

[ghostlymoron]

Sorry, didn't mean to attempt to teach grannie to suck eggs.

I promise you I understand the offset + or -. As I said in Post #3, a second check in a different berth after moving the boat and it suddenly became correct. Must have been an instrument error, probably mud too soft to register.

In the days of Seafarer spinning-neon echo sounders (I was sailing when they were state of the art) you could get false low readings such as 66 feet reading 6 feet - switch to fathoms range and you saw it was 11 fathoms because the spinner had gone round past 360 degrees. After lashing out and buying a (possibly £29.99) new Seafarer 2 I then went aground twice shortly afterwards because instead of being very cautious I was cutting corners more to cheat tides.