Installing on Wafer Thin Liner

[Terry]

Well I finally bought the Raymarine ST4000 MK2 plus AutoPilot before it became extinct. I have it mostly installed but have run into a challenge. I am mounting the rudder sensor arm upside down on the underside of the liner just in front of the rudder control arm and beside the engine well area way back at the transom inside the boat. Hope that makes sense. Anyway the problem is that I have the arm mounted on a block of wood to bring it down a bit lower and I will secure the block of wood with attached rudder sensor arm to the underside of the liner. Well it appears the liner is much too wafer thin to put screws through as I fear they will only be torn out over time, the fiberglass is not thick enough for screws to get a bite on. I wondered if screws combined with 3M 5200 gooped all over the wood mounting block might help secure it in place but that is a gamble. My other thought is to use a hole saw and cut a 2 inch hole to get my fingers on the other side then I could insert 4 bolts from the inaccessible side, glue them in place so that they won't turn, let dry, then place the block of wood with holes drilled to match the bolts up to the glued in bolts and use nuts to secure it to the wood block to the surface tightly. Hope you get the idea, hard to explain but I need a way to secure this thing to the underside of the deck where it is inaccessible from the opposite side, I think there is flotation styrofoam shoved up in that area at the transom corners. There in no access to the other side to put a washer and bolt on unless I make a hole to put my fingers through then put bolts and large washers on the other side for holding power. Anyone got a better idea before I start drilling?

[DLT]

Terry,

Sounds like a pretty good solution.

An alternative solution might be:

1 - go to the hardware/metal store and buy a little bit of thin sheet metal, preferably aluminum or stainless.
2 - drill your 2" (or whatever) access hole.
3 - cut the metal such that it will fit through your hole, but then surround the hole, on the backside. Probably just need two-four strips, depending on how you want to arrange it.
4 - epoxy the metal in place.
5 - after it cures, mount the sensor over the hole, using screws through the metal.

[kmclemore]

What about using Rivnuts or (better still) Rawl nuts? Rawl nuts would also provide additional vibration insulation.

See earlier forum link here.

[DLT]

My concern about the Rivnuts or Rawlnuts would be that you're still concentrating the loads.

I don't know if this would be a problem, but it seems to me that spreading out any loads, when mounting something on "wafer thin" fiberglass, would be an issue in addition to getting the fastener to stay put...

Another issue with those particular fasteners would be that they work better where the material is robust, like sheet metal. I'd be afraid of just crushing the fiberglass when trying to set them, such that I'd end up with a big hole where I wanted a little one...

[Pouw Geuzebroek]

Yeah I know your problem. Whenever I have to attach something to the thin liner I use standard building plugs, like these

Works great and keeps the stuff in place. However, I only have experience with attaching relative light items. I wander how heavy your block is and that might be a diiferent story. But I suggest you just have a go and try it out, if it is fixed, pull the block a little to see if it holds and if not unscrew the block and you are only left with some small holes.

[Frank C]

Terry

I have that same rudder xducer in my hand, pending install above decks. Its internal return spring will impose some forces on your liner. I'm unsure what the M is like back there, but I'd be thinking of sandwiching a piece of plywood between deck and liner for stability. The rudder reference will be lots less useful if the liner is flexing. Unfortunately, it might mean cutting a slot in the liner to permit sliding-in a plywood backer ... lots of work in a very uncomfortable location.

[AJ]

Terry,

I've just posted what I did on the Mods page. The liner back there doesn't appear to flex too much and the transducer works fine. Make sure you position it so it can swing through it's arc without hitting the hull.

I use self tappers to hang numerous things from the liner - sail bags, radios epirbs etc etc. The secret is not to overtighten. Screw them up firm but no more or they will cut out - this will happen even in thicker fibreglass. I've never had one come loose.

You can also use proper taps to tap a female thread to accept bolts. I don't do this in critical situations where you need through bolts and backing washers or plates but they will take high loads where access is a problem.

Cheers,

AJ

[Catigale]

>>>a small dab of 4200 on the threads will also give the screw some more holding power into the liner, and stop it from backing out (since you arent tightening them that much)

Get some of the screw head underside so it contacts the liner - wipe off the excess before it dries