Plastic or ply?

[oldbilbo]

I'm treating my little MAB to a pair of shiny new through-hulls and must replace the existing 1/2" ply packing washers. I've cut some from a Pound Shop polysomething chopping board, and have a tube of Sika 291 to hand, but wonder if there's any reason why I shouldn't use them.....?

 

[Tranona]

Very smart. Would have been better in dayglo pink though.

 

[jwilson]

My only concern is how difficult it is to get anything to stay stuck to polyethylene - it's almost waxy. I've spent years trying to seal fittings into a PE diesel tank with a variety of expensive sealants. Others have used similar materials though for through-hulls and I've never heard of a problem.

 

[sarabande]

there's some concern that polyethylene experiences polymer degradation when in contact with Chlorine and associated chemicals.


http://www.diva-portal.org/smash/re...50?parentRecord=diva2:575332&pid=diva2:590038


Now this may be superficial, but a mixture of sea water and battery acid (or vinegar, loo cleaner, etc) might trigger some chlorine, in which case you will smell it first.

The only other concern is breakdown when exposed to gamma rays at dose rates of 3, 17, and 100 Gy/h at 80°C. That may be of lesser concern.

It seems like a good material, with capability to conform to mild compound curvatures. This is borne out by searching for "polyethylene washers" which are used in industry for many applications including chemical seals.

http://www.seastrom-mfg.com/non-metallic_materials.aspx

And given the number of polyethylene canoes around, the material seems to be happy enough in a marine environment.


EDIT
some concern about 'bonding', and also resistance to heavy impact.

http://www.directplastics.co.uk/hdpe-sheet

EDIT2

That same company considers polypropylene to be tougher and more resistant to acids.

http://www.directplastics.co.uk/polypropylene-sheet

 

[Lakesailor]

I seem to remember that when we were re-lining some shower cubicles we were warned off food grade plastics (apart from the cost) as they were difficult to seal. There is a special bonding agent available in gun tubes, but not worth the cost or bother really.

 

[VicS]

there's some concern that polyethylene experiences polymer degradation when in contact with Chlorine and associated chemicals.

Now this may be superficial, but a mixture of sea water and battery acid (or vinegar, loo cleaner, etc) might trigger some chlorine, in which case you will smell it first.

Isnt hypochlorite solution suplied in polyehylene containers?

Anyway acid and seawater dont produce chlorine.. you need a strong oxidisng agent to oxidise chlorides to chlorine.

 

[sarabande]

ah, there goes another treasured myth, perhaps ?

http://fleetsubmarine.com/battery.html

and

http://www.nap.edu/openbook.php?record_id=10242&page=97

Is there something else apart from battery acid in a sub battery which makes the gas then ?

Where Old Glow in The Deeps ?

 

[David2452]

For a long time now I've used HDPE as a material of choice where I previously used ply for backing and for other small constructions without any issues at all, can't speak for the material used in cutting boards though. Nice Bosch power change arbour by the way, a real time saver.

 

[Neeves]

Wil,

I have just done the same thing, well slightly different. We installed a new depth sounder. I needed something flat on the inside of the hull to tighten the fitting against (rather than the tight curve of a cat hull). I cast filled resin into a tube, a tube sized to fit the curvature of the hull, the casting was made with the tube lying flat so the top is flat and the 'bottom' curved to the tube. I then cut a hole (for the hull fitting) with a hole saw. The only real issue was finding a tube of the right curvature, I used a rather large Tupperware type product.

Jonathan.

 

[prv]

I'd expect it to be fine. Sealing/bonding issues are a red herring, as you don't apply sealant to the inside of the nut anyway.

Pete

 

[oldbilbo]

( Note to self: 'Sara takes the task of keeping me street-legal and free from harm very seriously, for he is likely to go 'xcursing in the DB at some point.... )

I think it's worth a try, as the Sika stuff will/should provide a circular seal between the though-hull external face and the outside of the hull. I shall also chamfer the 'ole edges slightly so there's some Sika stuff compressed into the 'ole a few mmms. If you don't hear from me for some time, I may be bonded by my grubby fingers to the inside of my MAB hull, not having escaped in good time......

 

[VicS]

ah, there goes another treasured myth, perhaps ?

http://fleetsubmarine.com/battery.html

and

http://www.nap.edu/openbook.php?record_id=10242&page=97

Is there something else apart from battery acid in a sub battery which makes the gas then ?

Where Old Glow in The Deeps ?

It's batteries and seawater that don't go well together.

Simple simple electrolysis of chloride solutions will produce chlorine gas 2Cl^- → Cl₂ + 2e^-

Lead dioxide is a strong oxidising agent but AFAIK not strong enough to oxidise the chlorides in seawater although it will oxidise hot concentrated hydrochloric acid, producing chlorine PbO₂ + 4HCl → PbCl₂ + 2H₂O + Cl₂

 

[oldbilbo]

It's batteries and seawater that don't go well together. Simple simple electrolysis of chloride solutions will produce chlorine gas.

I truly promise to keep my MAB and my shiny new batteries sealed from each other. Should I ever find that seawater is overwhelming my batteries, I will give this all the attention I can spare....

 

[lw395]

In my view polythene will be fine for distributing the load from the backing nut.
If you want something to add any strength or rigidity to the edges of the hole, like if the skin is quite thin and there might be a load applied to the skin fitting, my material of choice would be GRP, either a piece cut from a sheet or laminated in place.
The sealing should take place between the fitting and the hull, although filling the threads of the fitting with sealant to exclude water may be a good idea, to reduce corrosion.

If you wanted to promote rot in plywood, a backing pad is under about the ideal circumstances, unless it is really well sealed in epoxy.

 

[Tranona]

( Note to self: 'Sara takes the task of keeping me street-legal and free from harm very seriously, for he is likely to go 'xcursing in the DB at some point.... )

I think it's worth a try, as the Sika stuff will/should provide a circular seal between the though-hull external face and the outside of the hull. I shall also chamfer the 'ole edges slightly so there's some Sika stuff compressed into the 'ole a few mmms. If you don't hear from me for some time, I may be bonded by my grubby fingers to the inside of my MAB hull, not having escaped in good time......

You are right. The purpose of the backing plate is to provide a stable flat face for the locking nut. The sealing is done between the outer face of the hull and the fitting. Logically there is no need for increasing the thickness of the hull locally as there is no load on the fitting except perhaps the weight of the valve and any force required to operate it. Backing pads like yours actually create a hard spot around their circumference.

 

[Boo2]

I'm treating my little MAB to a pair of shiny new through-hulls and must replace the existing 1/2" ply packing washers.

They go on the inside of the boat and take out the curvature of the hull, right ? Do you glass them in ? What's the outside diameter of the washer and the thru hull ?

Thanks,

Boo2

 

[oldbilbo]

They go on the inside of the boat and take out the curvature of the hull, right ?

That's right, but where these are fitted, there's very little curvature. The sealing compound which was used - and will be used new - took out what little curvature there was between the fitting's outside flange and the hull. Ditto inside.

Do you glass them in ?

No. The original setup was sealed inside and out with what appears to be old Sikaflex. It must have been in situ for yonks, remained flexible, and was not easy to tear away. The new treatment will be applied to achieve the same satisfactory result. I'm swapping old Sikaflex ( ? ) for new, and replacing ply with polywhatsit - which is marginally better able to mould to the hull profile.

What's the outside diameter of the washer and the thru hull ?

Having just measured for you, the washer O/D is 90mm and the through-hull threaded tube is 48mm O/D. The packing-nut flange is 75mm O/D and there is a 2mm gap around between the threaded tube and the inner circumference of the polyplastic washer. This gap will be filled with Sikaflex in compression, so there should be a thorough watertight seal, insh'allah, with plenty redundancy.

As advised, I'll fit the kit with the Sikaflex, do the bronze packing nut up hand-tight thus squeezing Sikaflex into gaps, leave 24 hours, then do up 'more tight'. The joint that matters is that between the external flange and the outside of the hull. All else is double and triple redundancy.

 

[DaveS]

As advised, I'll fit the kit with the Sikaflex, do the bronze packing nut up hand-tight thus squeezing Sikaflex into gaps, leave 24 hours, then do up 'more tight'.

There was a link on another thread to an opinionated Mercan who advocated cutting a small counter sink in holes to give the sealant somewhere to go then immediately doing the fitting up tight. Counter sinking a hole for a hull fitting is obviously a bit harder than doing so for a hole for a fastening.

 

[oldbilbo]

There was a link on another thread to an opinionated Mercan who advocated cutting a small counter sink in holes to give the sealant somewhere to go then immediately doing the fitting up tight. Counter sinking a hole for a hull fitting is obviously a bit harder than doing so for a hole for a fastening.

Thanks, Dave. I recall that - which focused AIR on fastener holes for deck fittings. The technique also reduced, apparently, the likelihood of gelcoat crazing.
My current 'oles are large enough to permit some edge-chamfering by small flap-wheel mounted on a power drill, so I'll try that.