Backing Plates and doublers- Aluminum or Fiberglass?

[George DuBose]

My 1970 Pearson 26 came with 1/4" backing plate of fiberglass and they were all cracked. I replaced them with 1/2" HDPE. I replaced all the backing plates on my Pearson 36-1 with HDPE.

Also used it under the foredeck to back the windlass and the two large cleats. I was towed up the Rhine river by a freight ship and all my deck fittings held during the long 23 hour tow.

Don't know about creep.

[Chotu]

Originally Posted by Ken Pole

plywood epoxied all 'round.

Did that 5 years ago based on internet lore.

Doesn’t work AT ALL. Everything that was encapsulated with epoxy falls apart in 5 years or so. Wood is awful.

And... see signature

[zstine]

Originally Posted by George DuBose

My 1970 Pearson 26 came with 1/4" backing plate of fiberglass and they were all cracked. I replaced them with 1/2" HDPE. I replaced all the backing plates on my Pearson 36-1 with HDPE.

Also used it under the foredeck to back the windlass and the two large cleats. I was towed up the Rhine river by a freight ship and all my deck fittings held during the long 23 hour tow.

Don't know about creep.

I suggest you tighten those nuts with a torque wrench and log it. Go back every 6mo. and re-torque. If you have to physically turn the nut to obtain the same torque as last time, that's no bueno. Loss of preload makes the fastener susceptible to cyclic failure. Maybe not an issue on your windlass, but definitely a concern on mooring cleats... you don't want those breaking off.

[mitiempo]

Originally Posted by sagablu

I have used FRP 1/4 and 3/8th sheets as a backing plate, a bit less expensive than G-10 I believe. Hard as a rock. I used a sawzall with a metal blade to cut it. I was able to cut round holes with a regular hole saw for the thru-hulls. https://www.mcmaster.com/fiberglass/...rs-and-strips/

That is what I use. G10 is overkill in my opinion for backing plates.

I have always thought that a backing plate - of whatever material - by itself will only prevent the bolts from pulling through the deck or whatever they are attached to. By using fiberglass and epoxying it to the deck underside it actually strengthens the deck. I always make backing plates large enough to spread the load. Attaching them with thickened epoxy (or vinylester in Chotu's case) also prevents any point loading if the surface is not smooth - it rarely is perfect.

[Island Time O25]

+1 for G10. Basically no more worries ever. Backed up mooring cleats and above deck windlass about 7-8 years ago. Still as solid as the day it was installed. No corrosion, no cracks, no compression issues, etc. IMO not an overkill but a long term insurance policy for a few more dollars.

[Chotu]

Originally Posted by mitiempo

That is what I use. G10 is overkill in my opinion for backing plates.

I have always thought that a backing plate - of whatever material - by itself will only prevent the bolts from pulling through the deck or whatever they are attached to. By using fiberglass and epoxying it to the deck underside it actually strengthens the deck. I always make backing plates large enough to spread the load. Attaching them with thickened epoxy (or vinylester in Chotu's case) also prevents any point loading if the surface is not smooth - it rarely is perfect.

Exactly how I see it.

Some of these plates will be used as doublers up on the cantilever davits as well.

[lexowgrant]

Ok. Say you're using the off the shelf fiberglass backing plates under a compound curved part of the deck. How do you make the backing plate conform to the curves of the deck/ With a metal backing plate you could maybe pre-bend it into shape or use the bolts to pull it into shape.

[guyrj33]

Originally Posted by lexowgrant

Ok. Say you're using the off the shelf fiberglass backing plates under a compound curved part of the deck. How do you make the backing plate conform to the curves of the deck/ With a metal backing plate you could maybe pre-bend it into shape or use the bolts to pull it into shape.

Same answer as the previous two posts, bed the backing plate in thickened resin of your choice.

[Capt. Don]

Extren is a polyester fiberglass sheet and rated at 30 thousand PSI. It comes in white with a UV inhibitor so will not need painting. It can be drilled and tapped.
Available inexpensively on eBay in any thickness cut to size.

[Chotu]

Originally Posted by Capt. Don

Extren is a polyester fiberglass sheet and rated at 30 thousand PSI. It comes in white with a UV inhibitor so will not need painting. It can be drilled and tapped.
Available inexpensively on eBay in any thickness cut to size.

Extren was used, is already installed and painted black outdoors to go with the theme.

Inside, the hidden plates aren’t painted at all.

I just got a 24x24 sheet and cut my own plates out of that because it was way too hard to specify exact cuts and wait for the lead time.

I tried using a jigsaw at first. The blades don’t last (didn’t have Jedi’s blades on hand).

So I brought out the sawzall and it sliced through like butter. Or maybe like a thanksgiving turkey and an electric carving knife. Ha ha. Perfect cuts. Straight, accurate. Second time I’ve been really surprised what you can do with a sawzall.

Note: my davits are also the same material.

[Chotu]

Originally Posted by lexowgrant

Ok. Say you're using the off the shelf fiberglass backing plates under a compound curved part of the deck. How do you make the backing plate conform to the curves of the deck/ With a metal backing plate you could maybe pre-bend it into shape or use the bolts to pull it into shape.

You glass it in there if necessary, using a thick resin behind it if you must.

I haven’t had that come up yet. It’s all been flat surface for this stuff.

Compound curves were about finish work inside the boat.

[mitiempo]

Originally Posted by lexowgrant

Ok. Say you're using the off the shelf fiberglass backing plates under a compound curved part of the deck. How do you make the backing plate conform to the curves of the deck/ With a metal backing plate you could maybe pre-bend it into shape or use the bolts to pull it into shape.

As posted above attach the backer to the deck with thickened epoxy. Backing plates should be larger than the item on deck they are for to spread the load. They also should be epoxied to the deck which will make the area stronger. If not all they accomplish really is preventing the bolts from pulling through, not adding strength where needed.

[s/v Jedi]

To add to the above: also avoid stress risers. Use rounded corners instead of sharp 90 degree corners. Also, for high load applications like deck cleats, consider tapering the backing plates around the perimeter.

These simple techniques prevent cracks, even if just in paint or gelcoat, but potentially in the laminate as well.