Threaded inserts for below-the-waterline fasteners

A

Adeline

New member
As I prepare to install a new transom-mount transducer I am considering the most secure way to install fasteners below the water line. Customarily, a transducer-bracket is simply screwed into the transom with 5200. I've always worried about water migrating up the threads into the coring. Another method is to bed the screws into holes drilled into West System Epoxy. However, these COULD crack. I am considering a third way. Bedding threaded-inserts into epoxy and securing an acrylic panel (3"x5") with machine screws. The transducer-bracket could then be screwed to this panel. If it needed repositioning that could be done w/o new holes in the hull. And when I upgrade my sonar at some point I could easily replace the panel and remount a new transducer. I am also considering inserts to secure my new 25 gallon fuel tanks to the cockpit sole. Here is a picture of the inserts http://www.c-brats.com/albums/Adeline/Molded_In_Insert.sized.jpg I would appreciate comments before I proceed.
 
I think the idea of installing the transom mount sensor on a plate and then bolting the plate to the transom is great. In fact that's what I did. A 1 ft. piece gets you above the waterline.

I thought that plastic would break. I wanted to use starboard, but they claim that's not a structural material. Acrylic seems to be even weaker. The plate has to take all the water pressure on the sensor, and the sensor wasn't cheap. The lever arm is the 1 ft. height between the sensor and the bottom bolt through the transom. so I used metal.

The only problem was to select the right material for the mounting plate. I got a piece of aluminum channel, and used that. Put a rubber gasket between the channel and the transom, and used two bolts above the waterline. However, that bare aluminum corroded rapidly ( in just a couple of days) so now I'm going to use a piece of stainless channel I found.

the idea is great, and that's the way to go

Boris
 
Boris,
I am just starting the same project on my boat. I recently bought a new GPS/DF and the last step is to install the transducer. I planned to mount the transducer on a piece of 3/4" starboard or similar and mount the board to the transom with plenty of 5200. I had always thought of threaded inserts as being most useful when hardware needed to be removed and replaced often. I hope the piece of starboard will become pretty permanent and future additions can be added or removed with out any additional holes in the hull. I have no experience with them but it seems logical to me that the solid epoxy bond between the insert and the hull would be just as susceptible to cracking as the bond around a screw? The 5200 layer should remain flexible forever. Another thought: my boat is always stored on a trailer. If it were moored year round in the water I might be a little more concerned with the mounting method, but screws well-bedded in 5200 seems to be a common and well-respected method. I would be very interested in other's thoughts.
 
Pete and Boris,

I would go with SS screws bedded in 5200. And if you want a mounting board, install it with screws and 5200. I would clean the screws in acetone first to ensure a good bond. 5200 is considered a permanent glue, not just a sealer. Wood trim installed to fiberglass with 5200 cannot be removed without damage to the wood or fiberglass.

The stainless mount to replace the aluminium may cause corrosion problems on the outboard.
 
Pete - some of my experiences mounting transducers or removing them from C-dorys. A carefully drilled hole and the correct size SS screw plus an underwater sealant works just fine. I have used 5200 with good success. Believe me, 5200 properly cured will seal the holes and surround the transducer enough to keep water migrating into the hull. In terms of removal, 5200 presents a few challenges but they are not insurmountable with proper techniques. I just removed a transducer stuck on a hull seven years ago with 5200. There is enough elasticity to carve away the bonding material with a very thin blade (such as double edge razor), without scratching the gelcoat or the transducer. With a little bit of effort, the transducer can be removed without damage. Of course residue remains and that is when it gets to be a bit of work. Use the single edge razor blade held in a holder at a very slight angle and slowly (carefully) shave away the sealant. Eventually, you will get to a very thin flim. Don't try to shave that with a razor blade, or you will scratch the gelcoat. Use a combination of acetone and gum remover on a coarse cloth and rub away. The sealant will wipe away leaving a perfect gelcoat surface. I have never damaged the gelcoat using this technique. Of course it takes elbow grease. If putting on a plastic mounting board gives
you peace of mind, by all means use it, but in my judgment, it is really unnecessary for properly sealing holes. John
 
I have stern mounted transducers and sensors on each side. I bought a pair of mounting boards at a local marina. These are similar to starboard but denser material and came gray colored about 5/8, 3/4 inch thick. Apparently very popular with the bass boat crowd so Cabelas and Bass Pro should have them. I painted them dark blue with Rust Oleum plastic paint that worked great. I attached each mounting board with just two long thin ss screws and a good bit of 5200. Don't think any water can get through. I then have a handful of short screws attaching the hardware on each side.
 
Sort of a PS
I think it was Ken on CAVU that did something similar by embedding nuts or inserts in the deck to reattach fuel tank mounting cleats. There was some discussion there about this. Also there has been discussion, and confirmed by the factory, that any water in the balsa core cant migrate very far.

Regards,
 
And as a reminder, don't forget that WM sells a little spray can of debonding agent that works well on 5200. I just used it to remove a couple pieces of SS I had glued on a couple years ago. Just spray it on, let it set 5-10 mins. and presto. It may still take a little elbow grease, but the stuff works!
Al
 
I for one would just cut the board to match the curve of the hull and use 5200 to glue it to the transom with 1/4inch up from flush. then just screw the transducer to the mounting board. that way if you need to adjust it you can screw and unscrew from the mounting board as many times as you wish. I ddont think that screwing intothe hull is nessasery. 5200 realy works well.just give it three days or so to set up . make sure that you get all the bubbles out and placea bead ofit around the edgeof the board to. cutting board material works really good because its very pores. you can always find some atthe thrift stores.
 
Mark,
There was a lot of discussion last year about repairing the fuel tank hold down boards. I ended up embedding bolts upside down in the floor and used these to attach the covering boards.
Tom,
I keep hearing of people using 5200 to glue starboard or cutting boards to fiberglass. My experiement trying this did not work well. Several sources I have read indicate that 5200 will not bond this material. Another problem with using the cutting board material is it is only 1/2" thick which would not give much screw length to hold the transducer bracket. I would feel much more comfortable with 3/4" thick material. I would also want the board mechanically fastened to the transom. The force of the water or striking an object at cruising speed might peel the board off the transom otherwise.
 
I myself plan on mounting a new transducer in the near future. As far as I know, if one is going to use starboard for anything, according to the West Marine catalog, it is nearly impossible to bond it to anything with any kind of marine sealant/adhesive. If anybody out there has been able to successfully bond starboard to fiberglass with any kind of adhesive, I would like to hear about it. At this point I do plan to screw the 1/2" thick starboard into the transom in combination with 5200 to seal both the new screw holes as well as some old screw holes that will be present when I remove the transducer that was screwed directly into the hull by the former owner of my boat. This is the method for mounting a transom mount transducer that was recommended in the April 2004 issue of Trailer Boats. I like the idea of not having to drill still more holes in my transom, the next time I have to change the transducer.

Bill Showalter
 
Bill,
That is exactly what I was planning to do when I mount mine. As I said above I am concerned that 1/2' long screws won't give much holding power and any longer and they will penetrate the transom. I going with 3/4" thick plastic. Just this afternoon I found a source locally that will sell me a small piece without buying the huge piece at WM.
 
A plastic mounting plate is available from Bass Pro as shown here plastic mount board Overpriced of course but thick enoough to do the job.
I scored the back with a sharp knife, maybe even drilled a few shallow holes in it to give the sealant a better purchase.

I have bonded Starboard to fiberglass with 5200 with good success. At least the stuff hasn't fallen off yet. I score a lot of lines. Someone suggested a dovetail bit and routing grooves, that would be great.
 
Bill, I haven't used 5200 alone to attach starboard type stuff to the hull below the waterline. I did attach two of those starboard type pieces below the waterline at the stern. I scored them up, I used a lot of 5200, and I used two long thin screws that went maybe an inch into the hull on each piece.

I have used 5200 above the waterline in the cockpit, lazarettes, under the sink and etc. In those instances I put up a piece of starboard so I would have something to screw into and the starboard is affixed to the hull or bulkhead with only 5200. My battery charger is installed that way and is fairly heavy. I attached a piece of starboard to the bulkhead, that piece had some small pieces of starboard on it to act as spacers and then the charger attached, etc.

Regards,
 
Thanks to everyone for their thoughts. I talked To West System about this and they believed that inserts were unneeded. However they advised that screwing into cured epoxy is not recommended. Brittle. They advised that I coat machine screws with Pam Cooking Spray and insert them into uncured epoxy. After curing, remove the screws and the thread grooves will remain. I did some mockups and found that drilling/tapping into cured epoxy offered better results with no cracking. So I went this way. First I removed my old transducer bracket and the 5200 that bedded it for 16 + years. Thankfull there was no water infusion. I filled the old holes with thickened epoxy + milled fibers(added strength). I made a cardboard template of my new transducer mounting panel and had Tap Plastics in Portland create one out of 1/2" acrylic. This stuff is really dense and I'm sure it will hold my bracket. If I'm wrong I'll redo it with 3/4"( 1/4" piece onto 1/2" piece with Testors Modeling Glue). I drilled four 3/8" diameter(.375") X 1" deep holes into the transom and filled them with the epoxy mixture. I then drilled and tapped 3/4" deep for the 8-32(.013 diameter) machine screws. I mounted the plate with a dab of 5200 at each of the holes. Finally, I mounted my new transducer bracket to the plate using 10-32 machine screws. I know some folks may think this is overkill. Especially since I'd not had a problem using 5200 alone. However, I can't see any downside and it wasn't much trouble doing it this way. It puts my mind a little more at ease. As to the threaded inserts, I still think they're a great idea and if I doubted the integrity of the epoxy mixture I would've used them here as they'd help distribute the loading. I may still use them when I install my fuel-tanks. Anyway, I posted some pictures in Adeline's photo album in case anyone wants a look. http://www.c-brats.com/modules.php?...ame=gallery&file=index&include=view_album.php
 
Like Pete, I've had good success with drilling and tapping cured epoxy. Over a very long time, it might get a little brittle, but I bet it will outlast the 'ducer! For me, the easiest way is to wait a day or so after initial layup, so the epoxy is still a tiny bit green, when it cuts easily. Another couple days and the stuff is bull strong.

Filling with microfibers is an inspired way of handling the strength issues.

I really believe in filled epoxy.
 
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