Re: B&G Sonic Speed sensors replacement

Gary Silver

Hi Michele: 

I will echo what Jason Rutledge posted and perhaps amplify the procedure.  I provided about a dozen photos to Bill and perhaps he will post them if he hasn't done so already..  Jason and I did this job together.  It is a two person job.  

The sensors and cables run in smooth plastic chases (pipes) from their position below the waterline to a position above the waterline so no sealing is necessary.  Our sensors (SM Hull # 335) were not sealed in any way, however, they are a loose interference fit in their recesses/counter-bores.  I have been told by Bill that some were sealed with 3M 5200 or other type sealant.  The plastic wire chase for the forward sensor comes up just in front of the forward seat of the saloon table.  The wire exits the chase at this point, makes a "U" turn and re-enters a chase that goes to starboard, enters a fiberglass chase that end in the cabinet where the sonic speed computer is located (next to, and forward of the starboard side fore-aft saloon seat).    The rear sensor, that is on the leading edge of the stub keel has a chase that appears in the aft, under-table, athwart-ship locker beneath the saloon table.  It is encased in a semi-clear hose that takes it further above waterline and enters the same fiberglass chase that takes the cable to the computer.    It is easiest to remove this hose at the fixed chase in the under-table locker so you are only working with 2-3 feet (1 meter) of cable.  

Start by identifying the routing of both cables.   Next, remove (gently pull on) the 1/2 in (1.5cm) long rubber covering of the transducers at the hull forward and aft transducer positions.  You will find underlying those rubber transducer covers a ceramic like plastic face of the actual sonic transducer.  The  rubber acts  as a protectant "sonic coupling".   Use a drill to gently and patiently drill a small hole in the face of the transducer to a depth of about 1/2 inch.  Into that hole thread an appropriately sized sheet metal screw (coarse thread) such  that it has good grip on the coupling but doesn't fracture it or expand it creating excess drag.  Next, while a person inside the boat pushes on the transducer cable (close to the chase so as to exert pressure on the cable without kinking it), gently and patiently pull on the previously inserted screw in the transducer to extract it from its counterbore (recess).   In our case these slid out with minimal traction.  There was no evidence of marine growth or other "cementing" adhesives.  The blunt end of the chase tube is positioned such that it provides a shoulder for the counter-bore that exactly positions the transducer so that it CANNOT be seated too deeply into the recess.  Exact distance between the transducers is important and has been expertly calculated by Amel.  

You use the old cable as a messenger line to pull the new wire into place.  In our case I cut off the old transducer, stripped a small section of cable cover and I soldered the wires of the old and new together.  It takes minimal traction to pull the new wire into place.   I lubricated the new transducer and counter-bore with silicone grease and it slid into place easily.   I WOULD NOT SEAL THIS TRANSDUCER WITH ANY KIND OF SEALANT AS IT ISN'T NECESSARY AND WILL ONLY MAKE THIS TASK MORE DIFFICULT IF IT NEEDS TO BE REPEATED IN THE FUTURE.  AGAIN, the chases ends above waterline and there is no need to seal the transducers.  Thread the respective cables thru their respective hose and/or chase to the computer making sure you somehow label them as fore and aft (as this will be required for proper connection).  Use the old cables as messenger lines  Cut the cables long enough to have a proper service loop (copy what Amel did).  Strip the insulation and extract the wires from the shielding.  Attach at the computer in the same manner as previously.  See the information in the files section for setting the dial in the computer for illumination of the calibrating red LED.  Calibration is also done on the B & G Multi-function display at the nav station (instructions in the installation manual for that unit as provided by Amel).    If you have any questions don't hesitate to email me at garysilver at mac dot com.  

This was a relatively easy job that I had dreaded (unnecessarily it turns out) for years.  You need someone to push/feed and someone to feed/pull at the same time, hence the two person job.   There is a bunch of information in the photos section and the files section on the Sonic Speed.  I did trim the rubber coupler fair with the face of their hull and keel faces (at a more marked angle for the keel coupler) by gently slicing with a razor blade.  Try to make a smooth cut.   Also read this:

All the best, 
Gary S. Silver, M.D.,  Farmington, UTAH,  USA
former owner of s/v Liahona (from new to age 20 years)    Amel SM 2000 # 335   now owned by Jason Rutledge

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