SJ23 Tech Tip E05, (Updated 2021-09-30) Bob Schimmel


Install a Depth Sounder.
INDEX - Display, Transducer, Dead Rise, Water Bath, Oil Bath, Epoxy, Panache Installations.

There are two things to consider when replacing a depth sounder; installation of the bulkhead display and the location of the transducer.

DISPLAY - Consider yourself lucky if you can buy a replacement depth sounder with a display that fits in the same size hole as your original.  There is some obscure law of technology evolvement that states a device must get smaller with time.  Problem is that as a device gets smaller the aging population has difficulty reading the small font.  To maintain functionality in a small unit a manufacturer will add "multi-function" switches that unfortunately sacrifice simplicity.  This combination guarantees frustration for a user during an emergency.  Best to take the time to read the manual and add extra labelling to satisfy your needs because this problem ain't going away.  Take heart.  New instruments are more water tight, consume less power and have greater functionality.  You just have to use some clever thinking to install the smaller display in the larger hole.  There are a couple of choices;

  1. Restore the larger hole with a fibreglass plug, gel coat then drill a correct size hole to install the new display.  This is a lot of work but if you match the gel coat colour it looks very good.  It is difficult to colour match aged gel coat.
  2. Fabricate a reducer plug of an acceptable material to plug the large hole and house the smaller display.  However, the material has to look good otherwise it looks, well tacky! 

I choose the latter option because my colder climate in the shoulder seasons does not give me the luxury to spend oodles of time to cure epoxy and repair gel coat outside.  It would be easy if I had a heated shop to put the boat into but working in the bush is just too much.  So I opted to make an acrylic plug because I always wanted a wee bit of a window to the cockpit anyway. 

I chose 1/2" smoked acrylic because it matches my companionway drop boards.  A word of caution about night sailing with a clear acrylic "window."  Remember to switch off the cabin lights or run red lights otherwise the light shining in your eyes may affect your night vision!  I've never had a problem with smoked acrylic though.  As an alternative you could use wood or UHMW.

  1. Select a piece of 1/2" thick smoked acrylic that is large enough to cover the original 4 5/8" hole plus an additional 1" diameter for a 1/2" mounting flange.  You could use 5/8" thickness because the SJ23 bulkhead is 1/4" to 3/8" thick where a display is usually installed.  Good idea to have a thick flange.  Panache's new display requires a 2" hole.
  2. Cover both sides of the acrylic with green Frog Tape to protect the surfaces from scratching.
  3. Use a compass to draw a circle on the masking tape.  Drawing on it is the second reason for the masking tape!  Draw the circle big enough to overlap the original hole by at least 1/2" to allow for a mounting flange to adhere the plug to the bulkhead. 
  4. Very carefully cut along the circumference of the plug.  I used my jig saw with a very sharp blade and cut slowly.  If you use a hole saw you still have to drill slowly because the rotating blade has a significant tangential speed and could take off on you when it grabs.  It could also be cut with a band saw with deep gullets in the blade.  See Tech Tip B08, for cutting techniques
  5. Using the same center as the previous step, drill out the center hole for the required size of your new display.  I used a hole saw for this cut.
  6. If all components fit during the dry fit you can safely proceed to finishing.
  7. CAUTION - Use a face shield to operate the router table in steps 8 & 9.  If the acrylic explodes on the bit you can't back away from it fast enough to save yourself.  The acrylic will cut deep like glass and will hurt again when the doctor digs it out.  Don't be greedy when feeding the acrylic into the blade.  Feed slowly. I can't emphasize this enough.
  8. Use a router table to remove a 1/2" wide swath of acrylic from the back circumference of the plug.  Remove about 1/8" at a time till you have a 1/2" wide flange to overlap the fibreglass bulkhead for a secure seal.  Ensure that the center portion (full thickness part) fits just inside the bulkhead hole.  If you have an oblong hole just round it out with a drum sander.
  9. Flip the plug over and bevel the outside circumference on the router table.  This corner should be bevelled to remove the sharpness and to improve the appearance.  This is when you pick up that wow factor!
  10. Once you are satisfied the plug will fit, sand the sharp edges and the perimeter with 200 grit sandpaper and then remove the protective masking tape for assembly and installation.

At this point I installed the display through the acrylic plug, sealing it with butyl rubber so it would be ready to install on the boat during warmer weather.  The plug was then sealed  to the bulkhead with butyl rubber under the flange.  The gauge was oriented vertical and pushed it into place till the butyl rubber oozed out.  It held very quickly at 200C outside and never slipped.   If you feel it slip, support it with tape across the face.  I really like working with butyl rubber because it adheres better than marine sealant and isn't as sticky or messy to work with.  Full cure comes in about 24 hours but I'm not really sure if it ever hardens.  

So that's it.  Now all you have to do is install the wires and fasten the transducer to the hull. 

Below you can see the sounder installed from inside the cabin.  The wiring is finished so it gives you a good idea what it looks like.  I will likely cover the back of the display with a paint spray can lid or similar cap if for no other reason than to muffle the power on and alarm beeps a bit.  Wow is it loud in the cabin.  I don't mind the alarm being loud but the power on can easily be 10dB quieter.  The terminal strip to connect the display power is just visible below the radio.  Each terminal will be labelled to facilitate repair.  I hate having to reverse engineer one of my installations especially when labelling is so easy while it is fresh in my mind.  Not getting any younger!

WINTER STORAGE - "The HawkEye D10DX display is rated for storage at -400C.  The liquid crystals in the display will return to normal operation once the display is above freezing, with no abnormal effects.  The transducer in our lab is regularly subjected to -400C and tested for operation afterwards."  NorCross Marine.


TRANSDUCER - Ever wonder where to install a transducer on an SJ23 and if it will work after all your efforts?   Should you drill a hole through the hull to get the best performance? 

To begin with, you don't have to drill a hole through the hull to achieve good performance.  If you are like me, I dislike having another hole through the hull just to measure the depth of the water.  Another hole is simply another step closer to sinking a boat!  I would rather heave a lead line than drill a hole in a perfectly good watertight hull.  Consider what happens when the transducer packs it in, or worse yet, springs a leak, through itself?  Well I guess you can always ask the scuba divers how deep the water is!  So put your drill away, there is an easier method.  What follows is the personal experience of some SJ23 sailors.  But first some theory to held understand. 

THEORY - A depth sounder uses a transducer to convert an electrical signal to an acoustic (ultrasonic) signal to transmit pulses through the water to the bottom.  The pulses bounce back to the surface to be received by the transducer which converts them back to an electronic signal.  The difference in time between a transmitted pulse and a received pulse is equivalent to the depth of the water.  "There's math involved in knowing the propagation delay through the water and the electronics to measure it, but I'll spare you the details."  In order to receive the very weak reflected pulses the transducer must be mechanically coupled to the water through a uniform high density material like fibreglass.  The pulses are readily passed by a dense material like fibreglass and absorbed by a low density material like wood or a foam cored hull.  A high density material equates to low signal loss and low density material like wood or foam equates to high signal loss.

Since a pulse can be transmitted or received through the fibreglass hull, you don't "need" another hole through the hull.  The location of choice for a transducer in a sailboat is forward of the keel, under the forward berth.  In order to pass the pulses through the hull efficiently (least signal loss) you must bond the transducer to the hull with no air bubbles (uniform density) between the two, otherwise you will receive a very strong echo from the bubble resulting in no depth reading being displayed.   This technique is quite easy to install on a solid fibreglass or metal hull.  There is an alternate technique to install a transducer on a wood or foam cored hull having multiple layers.  Replace a small section of the core material, under the transducer, with solid epoxy to create a dense functional installation.  Fortunately the SJ23 has a solid hull and this is not required so I don't have to describe it! 

TRANSDUCER INSTALLATION - The key to a successful installation is to install the transducer out of the way where it can be serviced.  The transducer and the wiring MUST be protected so an object can't slide against it to shear it off the hull.  Now aren't you glad you didn't install it through the hull!   The second point is to aim it vertical so the signal passes through the hull to measure the depth accurately when floating level.  Compensating for an angle will be a lifelong frustration.  The preferred location is under the forward berth just in front of the keel.  This spot is a well protected, relatively easy to access and is the furthest forward for piloting your way through shallow water.  It is the best you can do without having forward looking sonar.  It is better to read the water depth ahead of the keel, than behind it!  

Here is a simple test you can perform to confirm a suitable location or to test the sensitivity of a transducer shooting through a hull. 

  • TEST a LOCATION - Place the transducer in a water filled bag and press it to the inside of the hull.  You should be able to get a depth reading with little fuss.  Although, you might have to sprinkle some water on the inside of the hull to get a reading.  If you don't get a reading or it seems intermittent, try moving over a bit.  You may be dealing with an air bubble in the fibreglass.
  • TEST the SIGNAL LOSS - This comparison technique is a good way to determine the signal loss through the hull.  Lower the transducer over the side, into the water, and record the depth.  If you have adjustable sensitivity turn it down so it just shows a reading.  Place the transducer/bag on your desired spot inside the hull.  If you still get the same reading without changing the sensitivity, there is basically no signal loss through the hull.  This is the best performance you can ask for.  If you have to increase the sensitivity slightly to show the same reading, this is also OK, especially if the sounder can still measure to its full depth rating.  If your depth sounder does not have adjustable sensitivity the next best thing is to determine how deep you can measure.  There is basically no signal loss through the hull if you get a reading at the rated depth of the sounder. 

DEAD RISE CONSIDERATION (SLOPED HULL) - The most suitable location for a transducer on an SJ23 is just forward of the keel under the V berth.  The dead rise here is ~20 making for a simple installation.  Most manufacturers claim a wedge is not  required unless the dead rise is >200.  That being the case, make a dam of Silly Putty to pour a batch of epoxy into, creating a wedge so the transducer can shoot straight down.  This way the transducer can show an accurate reading.  While still pliable push the transducer, covered with a thin plastic food wrap, into the top to create a depression that perfectly matches the business end of the transducer.  You might have to use some tape to hold the transducer in position while the epoxy cures.  Once the epoxy has cured, you can test the depression with a thin film of water in it.  You should get a reading.  To make the installation permanent spread a thin layer of epoxy on the surface of the transducer and insert it in the depression. 

TRANSDUCER IN WATER BATH - Lower the transducer into a thin plastic grocery store "baggy" filled with water or antifreeze and seal the top of the bag around the cable with a tie wrap.  Place the transducer against the hull with the depth sounder on and sensitivity at max (assuming it is adjustable).  The display should show a depth reading.  If it does, turn the sensitivity down to determine how strong the signal is.  If you don't get a reading, sprinkle water on the hull or move the bag/transducer to another spot till you get a reading.  You may have to experiment with this.  The water in the bag is very effective at coupling the signal between the rigid hull and the transducer.  This technique works so well that I know of several people who use it on a permanent basis.  Empty the bag for winter freeze up or use antifreeze!  

TRANSDUCER IN OIL BATH - A variation of the water in a bag technique is to put oil in a bag or oil directly in the fitting as my buddy did for on his Cal 25 for a X25B Lowrance depth sounder.  He used it for at least 20 years with no problems.  Description below: 

"I shoot through the hull by installing a 1.25" diameter transducer inside a 1.5" diameter grey PVC plumbing cleanout fitting.  The grey fitting is equipped with a screw cap and affixed to the inside of the hull with Sikaflex.  Allow it to cure for 48 hours then pour in the mineral oil.  Lower the transducer inside the cleanout and pour in enough mineral oil to immerse the head. 
(Use mineral oil because it doesn't create air bubbles easily where automotive oils can foam.  An air bubble can stop an acoustic signal dead.  Alternatively you can use vegetable oil.  By the way, baby oil, mineral oil, liquid paraffin or lamp oil is basically all the same oil with slight variations in viscosity or clarity.  Most compasses use the same stuff). 
To facilitate the signal wire that has to go through the cap, cut a slot in the side of the cap, just wide enough for the wire to slide through.  Installation of the transducer is then as simple as: push the wire in the slot, lower the transducer in the oil and screw the cap on.  The slot also acts as a vent to release air pressure as you screw the cap down and to allow for temperature expansion or contraction of the oil.  You could press some sealant over the slot, sealing all but a tiny hole at the top for venting.  With the slot sealed like this you stand less chance of spilling when the boat heels excessively.
NOTE:  If you have to install the transducer on a sloped bilge, then simply cut the end of the tube to match the slope so the tube is vertical.  Sikkens Sikaflex and mineral oil are compatible with each other so the fitting will not separate from the hull.  Butyl rubber is NOT compatible.  The oil cannot freeze over winter so it is OK to leave the transducer in the oil over winter." 

TRANSDUCER IN EPOXY - If you intend to epoxy or glue the transducer directly to the hull, then switch the depth sounder on and monitor the display to confirm a functioning installation as you push and twist the transducer into the pliable goop to squeeze out any air bubbles.  If you have an older depth sounder with manual sensitivity control, reduce the sensitivity it just detects a signal.  This way you can be guaranteed of an installation that will work to full depth after the epoxy has cured.  I suspect it will be a very thin layer so the transducer is very close to the fibreglass.  If the unit is sensitive enough and the signal loss through the hull is low enough it should detect the bottom similar to operating the transducer hanging in free water.  To remove an epoxied transducer, set a wood dowel against the bottom edge of the transducer and give it a sharp rap with a hammer.  The transducer should just pop off.


PANACHE INSTALLATIONS - For my first installation I used a 4" ABS cleanout fitting filled with oil to house and protect the transducer.  It was installed forward of the keel for maximum "ahead look" while gunk holing.  The bottom of the fitting was shaped ever so slightly to follow the curvature of the hull.  The transducer cable lays safely on the hull with no flexing due to boat motion.  The "fluid bath" technique is a good system to install if the temperature is too cool to epoxy the transducer directly to the fibreglass.  (My situation).  Although you could use cold cure epoxy provided the temperature is above freezing.  (Didn't have any).  For this reason I used butyl rubber since I may also have to move the fitting out of the way to remove the water tank.  Cutting through sealant is a lot easier than cutting through epoxy. 

After about 6 years the oil leaked from under the ABS fitting.  (That was a mess to clean).  Problem is, I had sealed the fitting to the hull with butyl rubber which is incompatible with mineral oil.  The best oil resistant sealant to use is silicon or a polysulfide like Sikaflex or 3M5200 (Fig 1).  But one oil leak was enough for me to try water in a thin grocery store "plastic baggy", so I installed this in the Spring of 2021.  I let the Sikaflex cure for a few days before placing the baggy filled with water in it.  (Cont'd below).

Fig 1 - ABS fitting as it was for liquid filled versions 1 & 2.


Fig 2 - Transducer epoxied to hull for version 3.


Fig 3 - ABS fitting glued to hull with 3M5200.


Fig 4 - Cap screwed on ABS fitting to protect transducer.


(2021) - While the water filled baggy showed a fairly steady reading, the fear of forgetting to remove the water at year end had me consider epoxy again.  So in the Fall 2021 I took advantage of the still warm summer water to epoxy the transducer to the hull.  NorCross has a really good description of the procedure.  The J-B Weld epoxy worked very well, being just creamy enough to stay put.  I later glued the ABS fitting to the hull to protect the transducer (Fig 3).  The cap has a slot to pass the cable through and twist it on the base. 

The epoxied depth sounder showed a constant reading while sailing all afternoon.  This is far superior performance to the inconsistent readings with either of my liquid filled versions.  At right is proof that shooting through the hull actually works.  I've since found the deep holes and several mounds on the lake bottom that I had only previously heard of.  This is a whole new way of navigating for me. 



  • Tom Dew - "I epoxied my transducer to the inside of the hull just to the right of center under the V-berth and it works fine.  The trick is to make sure there are no air bubbles in the epoxy.  You can't use this method on a cored hull since the wood has air in it."  
  • Jack Carr - "I installed my transducer several years ago under the V-berth and it works very well.  It was suggested to me to use Marine Goop, approx 1/4" or thicker, in one solid mass WITHOUT ANY AIR BUBBLES TRAPPED INSIDE.  I then pressed the transducer into the goop and the deed was done.  It has not budged since despite pounding through waves, etc.  To ensure nothing would hit the transducer, I covered it with a thick round plastic dome.  Very important for me since I added 15 gallons of water ballast in the same area."
  • Ray Merritt - "The previous owner used a sealant to attach a small plastic bucket (without a bottom) under the V-berth.  He then sealed the transducer inside a zip-lock bag filled with water and the wire coming out the top of the pail.  The bag, filled with water, rests against the inside of the hull.  The bucket keeps the transducer from tipping and if the bag breaks the water will stay in the bucket.  The water in the bag couples the acoustic signal to/from the lake bottom.  It has worked fine and when checked against other boats it shows comparable readingsWhen I showed my "system" to another local sailor he said that he attached a plastic jar to his hull.  The jar is smaller than my bucket, also with no bottom, and the wire ran through a hole in the lid.  However, instead of filling a plastic baggy with water he filled his jar with mineral oil.  He said that his unit also gave accurate depth reading.  I didn't change to mineral oil because my depth sounder works well and cleaning water seems easier than cleaning oil in case of a spill.   However, it freezes around here and it might be prudent to switch to mineral oil."

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