Backing plates

[whipper_snapper]

Plywood does not last long in boat bilges; ....

My bilges are made of plywood, as are my backing plates! My backing plates are plywood, coated in epoxy and bedded on thickened epoxy.

At the risk of stating the obvious, may I add that in addition to the good advice above, don't forget that the shape of the backing plate is important. It's all about spreading load and avoiding points where loads can concentrate. So use rounded blacking plates never square, if you stack multiple layers, taper the stack from widest at the hull to narrowest at the top of the pile. For the same reason bedding is important to take-up irregularities on the faces.

I also quite strongly disagree with the initially attractive idea of a manifold to tap off all thru hulls from a single hole in the hull. A good thru-hull is perfectly sound and the additional plumbing, complex pipe runs and competition for water flow frequently makes a manifold less safe and less convenient in my experience.

 

[oldsaltoz]

Having replaced a few through hull fittings in my time, I came to the conclusion that to get a rounded hull section flat so the valve base seats nicely you only need a flat area a little larger than the valve.

I just measure the valve base width, add about 15mm, cut a small bit on timber the required length, mix up some resin and micro fibres and use the stick as a guide, it fills the rounded section and leaves a good flat area for the valve, I let part cure and add 3 layers of 250 glass patches starting with the small one first.

Result, nothing will rot, hull is stronger, never needs replacing, takes less time and effort than other methods like shaping blocks or cutting and sealing ply.

Good luck.

 

[john_morris_uk]

The mastic keeps the water out not the backing pad

I think that Yoda meant 'keeps the water out of the plywood'. I didn't mean that the epoxy coating would keep the water out of the boat and as I happen to know Yoda I am fairly confident that I knew what he was getting at. You still need to bed the epoxy coated circular (I forgot to mention circular in my post) backing pad on some sealant. Paint over the whole lot.

Out of interest this is how our seacocks are mounted and our boat was built to 100A1 which I assume included the seacock fitting...

 

[rivonia]

Coat the Marine Ply with epoxy resin, bed into silka flex. When cured paint with bilge paint.

Peter

 

[Deleted member 36384]

.... Out of interest this is how our seacocks are mounted and our boat was built to 100A1 which I assume included the seacock fitting...

I understand that hull and structural bulkhead penetration reinforcements have to be integral with the hull and bulkhead. On a GRP hull bonding with an appropriate glue is the only method of achieving integrity and not just sealant. I believe that this will be written into the 100A1 code, not because I have read the code, but because Lloyds would only approve new bulkhead penetrations that were integral on a project that I have been involved with.

The reason for the integral bonding is two fold: replace the loss of strength as a result of the hole, distribute any additional loading caused by the fitting around the hole.

A backing pad that is sealed in place with just a sealant is structurally weak and provides an additional leak path compared to bonded pads.

The method that oldsaltze and others describe by bonding the backing plate to the hull with a thickened (if there is a curve) epoxy adhesive will provide a superior interface between the seacock and hull.

I dare say that there are plenty of boats with sealant only backing pads that are quite safe and sound.

Marine ply can be purchased with 25 year guarantees from Robins timber.

 

[john_morris_uk]

I understand that hull and structural bulkhead penetration reinforcements have to be integral with the hull and bulkhead. On a GRP hull bonding with an appropriate glue is the only method of achieving integrity and not just sealant. I believe that this will be written into the 100A1 code, not because I have read the code, but because Lloyds would only approve new bulkhead penetrations that were integral on a project that I have been involved with.

The reason for the integral bonding is two fold: replace the loss of strength as a result of the hole, distribute any additional loading caused by the fitting around the hole.

A backing pad that is sealed in place with just a sealant is structurally weak and provides an additional leak path compared to bonded pads.

The method that oldsaltze and others describe by bonding the backing plate to the hull with a thickened (if there is a curve) epoxy adhesive will provide a superior interface between the seacock and hull.

I dare say that there are plenty of boats with sealant only backing pads that are quite safe and sound.

Marine ply can be purchased with 25 year guarantees from Robins timber.

I have no in depth knowledge of the Lloyds regulations - and actually for all I know my backing pads were bonded to the hull when the skin fittings were fitted as I have never removed a backing pad when I have been replacing them. I also know that I have always used Sika Flex when I have bonded underwater fittings - usually on the grounds that I don't worry too much about ever taking them off - and you can break the bond with a bit of effort!

 

[spuddog]

Thank you one and all,John.

 

[Bajansailor]

A pal of mine recently renewed all of the through hull fittings and seacocks on her boat, and she did a pretty fine job of it - here is a typical example :

This is an American Groco seacock - real belt and braces, where the seacock base is through bolted rather than just being screwed on to the through hull fittings like most ball valves.