This isn't a very complicated project, but it is much better than drilling holes in the transom of your boat, only to find you made a mistake and have to drill more. I've done that.
For this project, you will need a piece of polyethylene at least 3 x 6" in size and at least 1/2" thick (5/8" or 3/4" is better). I bought mine at a marine hardware store. You will also need a couple of stainless steel screws, #8 x 1" or so (depending on the thickness of your plastic. Lastly, you will need some "5200" (an adhesive sealant, made by 3M TM), or some other sealant made for marine duty.
I'll tell you right up front that you can buy one of these mounts, if you want. In fact, I have seen the commercial version offered by one of the major outfitters online for just about the same thing as I paid for the scrap I bought. The difference was, I had already used part of the piece I bought for what I purchased it for; I had enough left over to make this project, so in reality, it didn't cost me anything.
The steps in making this mount are as follows:
- Figure out where you want to put the transducer. This should be determined by careful study of the instructions that came with your depth finder.
- Knowing where you want the transducer, figure out where to put the mounting block. Mark the outline with masking tape, leaving about 1/8" between the tape and the block.
- After making sure you won't drill into anything important, and that you won't void your hull warranty by drilling holes in it, drill pilot holes through the block and into the boat's transom.
- Enlarge the holes through the block so your screws fit very loosely. Counter sink for the heads.
- Enlarge the holes in the boat so your screws will get a bite but not so small that you will strip the heads when you install them. Stainless screws strip very easily.
- Apply 5200 to the holes, in a circle around the holes, and in a rough rectangle on the underside of the block, near the edge, but not so near it will squeeze out when you tighten the screws. You want to ensure water doesn't leak into the boat.
- Tighten the screws.
- 5200 around the block neatly. "Neatly" and "5200" are not words found too often in the same sentence, so be careful. If you did like I said and used masking tape,you can now peel it off and there will be a straight line around the sealant.
- Let the 5200 dry.
- Now, mount your transducer to the block, using stainless screws that aren't long enough to penetrate the hull.
Once you get your transducer aligned properly, you will be ready for serious navigation and fish-finding. While aligning it, you will quickly find that using the block makes it much easier to do - if you flub up, just drill another hole.
Additional comments: You can see from the photo that I made my block rectangular, and it follows the bottom of the hull. That makes the top of the transducer, which is level, look crooked. I guess I could have made it level along the top and the bottom follow the hull; it might have looked a tad better, but I didn't have enough plastic to do that. Also, it would have looked better if the plastic was tan or olive drab or something, where the boat is camo. I could have painted it, perhaps, but the plastic is pretty slick and I didn't think it would last without peeling off. Thirdly, as you see, I used sticky cable tie pads to hold the transducer cable. I didn't want to drill any more holes in the hull, and they are holding up well.
NOTE: After more than two years, I am still very satisfied with the mount.