seacock overkill, revisited

[jim99]

I installed a couple Blakes seacocks this week. Went in beautifully, no problems, touch wood.

A week or so ago, I asked here if I should glass over the backing plates for additional security because marine ply seems impossible to find here. Based on comments, I decided a few coats of epoxy resin was enough.

Was discussing this with a South African friend after I flew down to the marina and he said it is possible to find marine ply here, but it is crap. Also, he said the thinnest epoxy I will be able to find is the glue -- it will coat, but not sink in and saturate, a waste of time.

His suggestion, and what he did to replace four seacocks (and what I did), was to use those hard and cheap plastic cutting boards -- they don't rot, Sikaflex sticks to them like, well, glue, and they have a very slight flex that provides a better seal then wood when bolted down.

Sorry if this common knowledge, but I had never heard of it being done. It was a revelation.

PS: If you use two boards to get the right thickness (I used two 10cm boards on each seacock), they are easier to work with if you seal them together first, either with Sikaflex or glue, before cutting. My friend used two blow torches -- at the same time, giving each board a light dusting of heat before fusing them together. They became like one.

 

[sarabande]

Have you given consideration to the effect of adhesives on the plastic boards ? Some chemicals (like solvents) can attack the plasticisers used, and cause embrittlement and structural failure.

Oil and diesel in the bilges can also have similar effects.

 

[rogerthebodger]

I am currently building a steel yacht in Johannesburg and have used some plastic cutting boards not as a backing for strengh but to insulate the seacocks from the steel hull on the inside. I used neoprene rubber gaskets and sekaflex on the outside

 

[Ships_Cat]

The boards are normally HD polyethylene which is fine with both diesel and lubricating oils - is so-so with petrol and acetone but one assumes one does not have that running loose in the bilge /forums/images/graemlins/frown.gif and in any event will be more resistant to those than the Sikaflex. I have used it in bilges with no problems (as mounts for float switches, etc).

I would be very surprised if polyethylene had a problem with Sikalex and other polyurethane sealants as HD polyethylene is satisfactory with almost everything (which is probably one reason it is used for cutting boards) and Sikaflex is widely used with plastics including through hull fittings in glass reinforced nylon and other plastics (albeit that one budget marine manufacturer claims failures of its hull fittings for logs due to it?). In the unlikely event that it did affect the polyethylene (or any other plastic backing plate), the thickness of the polyethylene would protect it from other than local surface effects.

Is common to use the boards as a source of material due to its ready availability in small amounts.

John

 

[sarabande]

Yes, I'd heard reports of failures, which is why I raised a concern about using a material outside its normal (designed) operating environment.

Low risk, but high consequence ?

 

[Ships_Cat]

But he is not using the polyethylene (one assumes it is) as a thru hull fitting so no consequence at all.

I suspect that the NASA problem is more to do with NASA than the sealant - no one else seems to have the same issue.

John

 

[sarabande]

Agree totally; NASA understandably are safety fetishists.

My concern is that while using a doubled layer of 10mm decent quality HDPE board might be a satisfactory solution in one case, the thread might encourage someone to assume that all plastic boards are suitable, and by not using a rigorous approach, find and use a piece of scrap plastic which fails at some stage in the inimical environment of the bilges.

If we have highlighted the need to use good quality - albeit unorthodox or exotic - materials, then the exchange of knowledge and views has been beneficial.

 

[Ships_Cat]

NASA understandably are safety fetishists

I think you will find it is more likely to be a problem with their product.

Regarding the use of plastics and the possibility that some may be misled by the original poster, I will leave you to your chronic worry attacks.

John

 

[cliff]

Well now you have got me thinking.

Maybe I had better remove the pads I fitted below my sea cocks 6 years ago - the sika-flex might have gone for the plastic (HDPE cutting board) and made it brittle or mushy or something.

Even though I had all the cocks and/or through hull fittings out last season (the backing spacers remained stuck fast to the hull and seemed in the same condition as they were when fitted) maybe I should change them back to ply (which rotted) or maybe solid teak?

On second thoughts, I'll leave the HDPE pads in place - If they have lasted 6 years without any obvious signs of problems with the Sika or sea water or heads discharge I think they will be fine /forums/images/graemlins/smirk.gif

Or maybe I should change them - what does the panel think?

 

[philmayfield]

I think you will find that the cutting boards are polypropylene and not polyethylene. The latter is very difficult to bond with normal adhesives. The polypropylene will bond easily and should be fine for your purpose.

Phil

 

[Ships_Cat]

Most of the domestic ones I see in my home country appear or are claimed to be PE and certainly feel like it (and just bin and had a feel of a couple /forums/images/graemlins/smile.gif - very, very waxy feeling which I think is indicative of PE, compared to some number 5 plastic - PP - containers I found), but certainly PP is also used for boards. Dunno, but maybe PP is used more for commercial boards due to its higher temperature resistance?

PP and PE have very similar characteristics in respect to physical and to resistance to chemical attack as far as I know so is of little consequence for the poster's application.

No bonding problem with Sika 291 on PE (as service is mainly as a sealant) and large surface area.

John

 

[cliff]

[ QUOTE ]
I think you will find that the cutting boards are polypropylene and not polyethylene. The latter is very difficult to bond with normal adhesives. The polypropylene will bond easily and should be fine for your purpose.

Phil

[/ QUOTE ]Well, The label did say HDPE so I (perhaps wrongly) assumed the board was polyethylene of the high density variety /forums/images/graemlins/smirk.gif

The Sika-flex stuck well, "like faeces sticks to a blanket" as the saying almost goes.

Interesting that we have some (3 at the present time) comedians voting for option 4
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[sarabande]

Right, you lot of risky so and so's.

Just spoken with Sika (UK) and explained the requirements and usage environment.

Their technical guy says that Sikaflex will not bond properly to polyethylene, and the solvent and adhesive may not be compatible with polyurethane, as it may affect the plasticisers.

There's nothing like a bit of research, rather than rushed opinion, is there ?

/forums/images/graemlins/grin.gif

 

[sarabande]

Buy some wooden plugs.

 

[cliff]

Sika covering their arse!
6 years without a problem is proof enough for me.
I doubt if I could get the pads off anyway - tried one last season and the so-in-so would not move. /forums/images/graemlins/grin.gif
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[jim99]

I will keep an eye on them.

When I fit it all together, everything was so snug and the Blakes seemed really quite water resistent, I figured maybe I wouldn't bother with the Sikaflex. As time passes, if it seems vulnerable, I will yank the cutting boards and go with another sort of plastic/glass board.

 

[duncan]

I have this image of PE/sika/PE/sika/hull/sika/flange with bolts through the whole lot..............

what exactly is expected to fail here?

and catastrophically?

 

[uforea]

Why fit a pad on the inside?. I would have thought the idea was to stop the fitting being pushed into the boat therefore pad on the outside to spread the load, but then, what load. Am I missing something??

Ted.

 

[sailorman]

To take-up the hull curvature & differing thicknesses ( internally ) over the area of the fitting flange.

a simple 12m/m ply ring is usually enough, bedded in sealant

 

[Ships_Cat]

Read my posts.

I said - Even if there was a "problem" with Sika then it would not matter due to the thickness of the polyethylene (however, I am with Cliff on this, I would be most surprised if there was any significant problem as between Sika and polyethylene or polypropylene). I also said Sika is widely used with other plastics with no problems whatsoever.

I also said - With respect to adhesion it does not matter as the application is mainly a sealing one. I can add to that, as you do not seem to be aware of the fact is, in that for Sika 291 (the common variety that is normally purchased) it is not claimed by Sika to be an adhesive for ANYTHING let alone the polyethylene you asked about so you got the answer from Sika anyone knowledgable of the product would have given you. The technical data sheet for it uses words along the lines of it having mild adhesive properties (I don't have a sheet in front of me right now to get the exact wording).

So you don't seem to understand that your claims are irrelevant to the application, even should they be true (which they are not).

I think the problem you have is that you seem to be a chronic worrier finding risk in everything even though little experience with the matter, whereas I (and I suspect Cliff as well) come from backgrounds making use of these materials (and dare I say it, do not see potential disaster in everything).

John