Terry B 71 Posted February 13, 2012 Share Posted February 13, 2012 Hi team, I want to make various sizes/shapes of backing plates for a number of deck fittings that have poor backers on them at the moment. Rather than ply, I thought I'd use aluminium - I happen to have a perfect piece to cut up into the various shapes I need. And its thick enough that it wont bend. My question is "Does this pose an issue - mixing alum backer with s/steel bolts?" I'd use s/steel but it's a hell of a lot harder to cut/drill than alum. And I don't have any lying around in the garage saying "use me, use me!" I believe there's a finish/paint or something you can coat alu in to stop the "clash" between the 2 metals? Or it's not really an issue worth worrying about? As always, all advise appreciated. Link to post Share on other sites
Battleship 100 Posted February 13, 2012 Share Posted February 13, 2012 You would probably be better off with ply and plenty of epoxy, generally alloy will act a bit like an aspirin if salt water is present especially where it meets stainless, most of the aluminium you see used on boats are special alloys. Link to post Share on other sites
Guest Posted February 13, 2012 Share Posted February 13, 2012 Carbon? Link to post Share on other sites
chic014 0 Posted February 13, 2012 Share Posted February 13, 2012 Or glass? Link to post Share on other sites
ab1974 1 Posted February 13, 2012 Share Posted February 13, 2012 At minimum you'd want to use some lanocote to keep the metals apart. We cut up the plastic / nylon kitchen chopping boards. No need to paint or seal. Was cheap and quick. Easy to work with (used a jig saw / bandsaw). Link to post Share on other sites
tuffyluffy 76 Posted February 13, 2012 Share Posted February 13, 2012 TB, Ive got an alloy yacht which ive recently refitted all the deck fittings on. I made all the new backing plates out of marine grade alloy simply because a had some spare stuff lying around - its probably preferable to standard alloy but i wouldnt have hesitated using standard stuff if it was all i had. There are a few products out there that isolate the stainless and alloy, but i used Lanoline and it seems to work just fine. Most of the scaremongering about alloy is based on igorance or a poor understanding/installation of the product. Link to post Share on other sites
Grinna 2 Posted February 13, 2012 Share Posted February 13, 2012 Is the application of tefgel enough to isolate the stainless from the aluminium? Link to post Share on other sites
Terry B 71 Posted February 14, 2012 Author Share Posted February 14, 2012 None of the backers are holding huge loads. So glass and carbon not reqd. They just have small washers currently which is ridiculous - they need proper backers. Why do people go to all the trouble to install hardware and not: a - seal the hardware/deck properly b- Use appropriate size washers or backers? Really pi$%es me off. I was really just trying to find a simpler way than ply, cut out, coat in epoxy etc. I know, I know, lazy bugger I've always made them with ply in the past. The thought of a simple cut of Alum, drill holes and "Bob's your aunty" appealed to me! No waiting on epoxy to harden etc. Soooooo - compromise. I'm good at those. I'm married I'll do the two easiest to get at (and therefore to check/replace) with the alum and coat it in lanoline (thanks for that advice - I found Prolane NZ website which had good stuff on it's usage etc). The others (there's only 4) I'll stick to ply. Thanks for all the advice. I'll report back on the comparison and results in, say, 10 years Link to post Share on other sites
Steve Pope 243 Posted February 14, 2012 Share Posted February 14, 2012 alloy should work well, lanoline / lanocote /teffgel and as well perhaps a plastic washer under the bolt head / washer etc should cover any possible corrosion issues. Link to post Share on other sites
wheels 543 Posted February 14, 2012 Share Posted February 14, 2012 Steve is right. Use the Alloy as a backing plate and place some lanacote on the thread of the screws. It gets trapped in there when the screw is turned home and stops the corrosion. Don't use treated ply, it has too much copper and arsenic in it and will cause corrosion. If you ever did use SST backing plates, you need to place a spacer of plastic or sealant inbetween to stop corrosion. Link to post Share on other sites
Terry B 71 Posted February 14, 2012 Author Share Posted February 14, 2012 Thanks Wheels (and Steve). Wheels, when you say "treated ply" - do you include marine ply? And a related question - if the surface is a little uneven - as the underneath of fibreglass topsides always/often are, do yo put some form of compound between the hull and the backer so there's no pressure points? I've used epoxy before and made a "mask" that fits perfectly on high load jobs, but these jobs aren't very high loads. So can something else suffice? Link to post Share on other sites
wheels 543 Posted February 14, 2012 Share Posted February 14, 2012 Marine ply is not treated. Ummm, I may have misunderstood though. Is the backing plate going against GRP?? If that is the case, it doesn't matter what you use. Sorry I thought it was Aluminium you were going against. So it is an Aluminium backing plate against GRP. To fill up the spaces from the ruffness, yes you can use some Sealant and if is is very ruff, then you may want to fill it with some epoxy filler then push the plate up against it and let it harden, then drill and screw. The treated ply comment was in regards to placeing treated ply up against Aluminium. The ally will corrode away if water should get inbetween in that case. Link to post Share on other sites
Terry B 71 Posted February 14, 2012 Author Share Posted February 14, 2012 Thanks Wheels, yep the hull is grp. I'll make an epoxy pad I think - the area is pretty rough and a large backer straight on to grp with no "levelling" done will just create pressure points. Something to do in the rain this weekend Link to post Share on other sites
Battleship 100 Posted February 14, 2012 Share Posted February 14, 2012 I know the others don't agree, but to me seems like a lot of effort going in only to use a corrosive material, if it were me I wouldn't use aluminium on my boats as the material cost is minimal compared to the time needed to do it, for a grp boat I would use wood or composite even the white plastic board suggested would be superior IMHO. Link to post Share on other sites
ab1974 1 Posted February 14, 2012 Share Posted February 14, 2012 I'm with you Willow. Even buying the plastic chopping boards at the big red shed would only cost $30 for a few of them. I made backing plates for the all stanchions and push / pull pits with three $10 boards and still has some left over. No need to go and buy and tub of lanocote. No risk of corrorion. No need to paint or seal. Easy to work with. Link to post Share on other sites
Terry B 71 Posted February 14, 2012 Author Share Posted February 14, 2012 Okay AB - but is the plastic chopping board stuff rigid enough? My understanding of backing plates is that they should be as rigid as possible so you don't get joint movement - which can break the water sealant (whichever one your using) over time, and put tensions on the wrong places. Or am I just being over fussy? Ain't yet seen a really rigid chopping board..........?? Not that I buy them that often Link to post Share on other sites
TimW 1 Posted February 14, 2012 Share Posted February 14, 2012 I know the others don't agree, but to me seems like a lot of effort going in only to use a corrosive material, if it were me I wouldn't use aluminium on my boats as the material cost is minimal compared to the time needed to do it, for a grp boat I would use wood or composite even the white plastic board suggested would be superior IMHO. This post has really cheered me up. It really got me down reading all that bad advice about Alloy. terrible stuff actually. Hard Anodising the stuff makes it better. And using Lanocote or Duralac or Tefgel makes it better and reduces corrosion. But bottom line it is still a battery and all the time it is still corroding. Might last 5 years or it might last 30 years but it is corroding all the while. It doesn't belong near Salt water and if you have a Composite boat (especially if it has Carbon) try very very hard not to use Aluminium at all. your alternatives of Either a Composite or Acetal backing pad will be way better and last a lot longer. Link to post Share on other sites
ab1974 1 Posted February 14, 2012 Share Posted February 14, 2012 I'm sure there was something stiffer or better. However I don't work with composites (i'm an accountant) so I wanted something simple and easy. I probably could have gone to one of the plastics places mentioned on other threads and got something more suitable in offcuts. However once I'd cut the backing plates to size (matched the foot of the stanchion - so say 80mm x 80mm and it was 15mm thick), it felt pretty stiff and suitable enough as a low load backing plate. Heck you can buy angled backing plates from Burnsco etc for most cam cleats that look like basic plastic and I'm sure the loads on those equal or exceed the loads on my stanchions except when they are stopping the occassional bowman going over thes side. All in all I did it as I thought the plastic was denser that a ply backing plate, and I didn't have to mess around sealing / painting them which would have meant another weekend before the job was done. Link to post Share on other sites
Terry B 71 Posted February 14, 2012 Author Share Posted February 14, 2012 Fair enough AB, I'll give it a go. I sure like the easy use aspect. And if I don't like it then it's easy enough to change and no cost. thanks. Link to post Share on other sites
ab1974 1 Posted February 14, 2012 Share Posted February 14, 2012 I still used appropriate size penny washers for the job so the nuts couldn't pull up into the plastic. Link to post Share on other sites
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