Improving my Simrad Sidescan – Part 3 (Re-mounting the Transducer)

Okay, so if you’ve been following along, I had identified a new location for my transducer – under the step and near the drain plug.  I REALLY didn’t want to drill more holes in the hull, especially since I wasn’t sure it was going to work in that location.  I did turn to Google to see if there were any posts about moving a side-scan transducer there, with marginal success.  Based on detaching the transducer from its current location and pulling some slack in my cable, I was able to eyeball the new location and see that everything seemed to be okay – the main concern being clearance of my lower unit when trimmed fully down and turned both ways. In that location it was clear that I would have an unobstructed view for a full 180 deg.  

Again, some Googling brought me to this site, that describes a way to mount Starboard (a.k.a HDPE) on a fiberglass hull.  HDPE does not like to stick to anything, usually.  I pretty much followed the instructions on the side to a “T” and had very good success with it.  Luckily I had some 3/4″ HDPE lying around already from other project(s).  If you need to get some HDPE, you can get it here.  Just in case the website link disappears in the future, I’ll list the steps below.  


  • 3/4″ white HDPE (a.k.a $tarboard)
  • 1/2″ drill bit or end mill
  • West System G-Flex 655 Epoxy
  • Blue painters tape
  • 80 grit sandpaper (or thereabouts)
  • Acetone or denatured alcohol
  • Propane torch

Cut the 3/4 HDPE to size for the places you want to make a mounting plate.  I chose to make 2 plates, one for the new location, and one to make a plate along the transom where transducers are usually mounted (and also to cover the old holes).  I figured while I’m at it, I might as well make a second plate “just in case” and covering the old holes was a second motivation.  

Once you have test-fit the HDPE block(s), drill 1/2″ +/- holes approx half-way through the thickness of the back part of the plate.  No special rule but put them randomly throughout the piece.  The link has some pics of what it should look like.  Unfortunately I didn’t take any pics of mine.  I then took a hand drill and 1/8″ drill bit and made short holes in the back of these holes angling to the side.  This is for the epoxy to grab onto.   The website says to make a lip underneath the holes.  Its really the same thing.

Next, use your sandpaper and rough up the back surface pretty good.  I also held the blocks on my transom and put painters tape around the perimeter of the desired location.  Then removed the block and sanded the gel coat so it was roughed up.  

Use your acetone and clean the gel coat and back of your block.  Get rid of any hanging pieces of plastic.

Using a propane torch, flame treat the back surface of the block.  Apparently this is the key to having the HDPE stick.  Pass the flame over the surface in overlapping passes at a rate of 2″ to 3″ per second.  It should not show any visible difference (ie. do not melt the surface – you can melt it if you go too slow).  The heat creates a chemical property change that aids the bonding.

Mix up the G-Flex (super easy to do, its roughly 50:50 mixture) and using a Popsicle stick or similar, fill the holes in the back of the block completely.  I also buttered the entire back of the block with a thin coat.  

For the next step, get your painters tape, paper towels and acetone ready.  Place the block in the desired location.  It will stick a little but will want to slide.  Using the painters tape, tape it thoroughly to keep it in place.  Don’t worry if its not perfect.

Blocks set in place and taped up while the epoxy sets

Wait about an hour and come back and remove the tape.  Make slight adjustments in the location.  Wet a paper towel with acetone and clean up excess epoxy now.  Re-tape to hold the block in place.  Give it a good 24 hrs to set.  Should be a very solid hold now.

With the blocks being set I went ahead and placed the transducer with my motor trimmed as far down as it would go in the driveway making sure it cleared with the wheel turned all the way to the left.  This was a goof – as described below (I do a lot of dumass things)! 

HDPE blocks placed and transducer set in new location. Wire secured with temporary double-sticky anchors

Took it out to trial it, and images were great, both left and right.  I did a check when trimming the motor ALL the way down and turning the wheel and quickly noticed that it would hit the transducer IF trimmed all the way down.  Something that I really never do (lowest I go is 40%) but Murphy’s Law says “if it can go wrong, it will go wrong”.  When I got back home, I lowered my trailer jack all the way down so that I could trim the motor to the locks and turn the wheel.  Luckily I was able to shift the transducer about 3/4″ right and avoid an interference.  (AH, the benefit of having a block – no new holes in the transom!)

After shifting the transducer to the right. Note the fish line used to align it to the boat hull


Clearance is very tight when trimmed all the way down and the wheel turned all the way left. But – – no possibility of interference in this location

After all this, the only thing I did was replace the temporary cable ties with smaller profile ties secured with a tiny bit of G-Flex.  Again – no holes in transom. 

I’ll just need to be careful of having the transom get beached (I do this anyway) but it is a little more vulnerable in its current location even though its above the bottom of the hull.  For me, having the advantage of a full view of my side-scan and getting everything out of it is worth the additional risk of it in its new location.  My last post on this subject will show some of the images I’m able to get now.  Stay tuned!

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