Tufnol for backing plates?

M

mattp

Active Member
Joined
10 Dec 2020
Messages
54
Visit site
I need to remove my stem head due to lifting. There is a 3/4” ply backing pad below that the stem head is bolted to. This has rotten over the nearly 50years of service. I’m thinking of removing this and replacing with G10 or alternatively 20mm Tufnol sheet. This would be bedded in place with thickened epoxy to the underside of the fibreglass deck and then be overlaid with a heavy weight biaxial cloth and epoxy.

My question is would Tufnol be ok in this location? It would need to be able to take compression from the bolts.
 
mattp said:
I need to remove my stem head due to lifting. There is a 3/4” ply backing pad below that the stem head is bolted to. This has rotten over the nearly 50years of service. I’m thinking of removing this and replacing with G10 or alternatively 20mm Tufnol sheet. This would be bedded in place with thickened epoxy to the underside of the fibreglass deck and then be overlaid with a heavy weight biaxial cloth and epoxy.

My question is would Tufnol be ok in this location? It would need to be able to take compression from the bolts.
Click to expand...
In my experience, Tufnol is a great material for a backing plate when bedded onto thickened epoxy as long as the whole lot does not create any hard spots around where it meets the deck. I see no benefit in glassing over on the inside. You do not give any indication of the size of boat or rig loading which will dictate the area to reinforce, the thickness of the Tufnol or the size and thickness of stainless washers/plates under the nuts but as the existing assembly has lasted 50 years, other than preventing a repeat of the rot, that will give you a starter for those dimensions.

Www.solocoastalsailing.co.uk
 
Last edited:
If it's replacing plywood, then I'd be happy, in fact likely to use it. I haven't done a stem-head fitting but have backed two sets of genoa winches with a tufnol-type board, one for new/resized winches and the other due to rotted ply as you're suffering; I simply bedded them in sealant and fitted s/s penny washers beneath, but would've used those with plywood backing plates too.
 
Tufnol and other forms of SRPB are basically expensive, not ever so strong and difficult to make things stick to.
It's not light.
If you happen to have the right size lump of it, it's OK for lots of things.

I think for something as critical as the stemhead, it might be a good idea to think about the structure as a whole, how do the forestay loads etc get spread into the hull and deck? What loads are actually on this backing pad? How did the original work? was the glass below the ply critical? How is that bonded into the hull or deck? How big is the backing piece?

I would be wary of taking advice given on the basis of the scant information the OP has given, not knowing how the structure of his boat works.
 
Made lots of things out of Tufnol, one of the early composites, directplasticsonline do most sizes. Be a bit careful about the dust when drilling, not good for your health.

As a backing plate... lots of other things, but it would be good. Over the top compared with ply, but solid.
 
The one thing that ply has going for it is it can be shaped to conform to the shape of the foredeck, stem fitting pad , or whatever. I replaced my rotten ply foredeck core with plywood for that reason. Bonds well for reinforcing the deck to hull joint and spreading the stresses. If that shaping and bonding feature isn't critical the Tufnol is very, well, tough

I made sure to drill the holes for fittings well oversize, wet out the ply and fill with thickened epoxy before redrilling for the bolts, for both compression and water resistance
 
Wing Mark said:
Tufnol and other forms of SRPB are basically expensive, not ever so strong and difficult to make things stick to.
It's not light.
If you happen to have the right size lump of it, it's OK for lots of things.

I think for something as critical as the stemhead, it might be a good idea to think about the structure as a whole, how do the forestay loads etc get spread into the hull and deck? What loads are actually on this backing pad? How did the original work? was the glass below the ply critical? How is that bonded into the hull or deck? How big is the backing piece?

I would be wary of taking advice given on the basis of the scant information the OP has given, not knowing how the structure of his boat works.
Click to expand...

Eh?

If a ply wood backing pad lasted for that length of time I would guess with G10 or Bear brand Tufnol either would easily replace the ply as a backing pad.

Ink
 
Plum said:
In my experience, Tufnol is a great material for a backing plate when bedded onto thickened epoxy as long as the whole lot does not create any hard spots around where it meets the deck. I see no benefit in glassing over on the inside. You do not give any indication of the size of boat or rig loading which will dictate the area to reinforce, the thickness of the Tufnol or the size and thickness of stainless washers/plates under the nuts but as the existing assembly has lasted 50 years, other than preventing a repeat of the rot, that will give you a starter for those dimensions.

Www.solocoastalsailing.co.uk
Click to expand...
Perfect, thank you.
its for a Nicholson 32. I intend to fill the entire underside of the stemhead area; so roughly a 500 x 300 triangle; with 20mm Tufnol to create a nice flat surface for a Stainless backing plate rather than just washers to bear on. I’m also having a strap welded to the stemhead which will go over the bow fabricated to through bolt from the bow as well. There is a known weakness on the Nicholson 32 mk10 where the stem pulls up off the deck, which mine is doing, rather a poor design really. It may be overkill but peace of mind is priceless when you need it.
thanks for the reassurance. If you think I’ve missed anything or can make a further suggestion I’m all ears.
I was thinking whale brand Tufnol, I am rather confused as which type would be best.
 
mattp said:
Perfect, thank you.
its for a Nicholson 32. I intend to fill the entire underside of the stemhead area; so roughly a 500 x 300 triangle; with 20mm Tufnol to create a nice flat surface for a Stainless backing plate rather than just washers to bear on. I’m also having a strap welded to the stemhead which will go over the bow fabricated to through bolt from the bow as well. There is a known weakness on the Nicholson 32 mk10 where the stem pulls up off the deck, which mine is doing, rather a poor design really. It may be overkill but peace of mind is priceless when you need it.
thanks for the reassurance. If you think I’ve missed anything or can make a further suggestion I’m all ears.
I was thinking whale brand Tufnol, I am rather confused as which type would be best.
Click to expand...
Sounds like the 'if rotten ply was OK tufnol will be fine' mob are doing ill-informed guesswork.

I suspect the skin of grp below the ply is structural, it's a GRP sandwich with ply as the core.
Adhesion of the core to the skins is important.
If the current design is know to be weak, I'd look at it and consider an internal knee or web of some sort to take the forestay loads etc down to the hull.
To stiffen the deck, I'd consider laying up a piece of solid GRP, it will probably be cheaper than the tufnol, much stronger and only a little heavier.

But there's no point making the deck really strong and not attaching it to the hull equally strongly, if the forestay loads go to the deck.

A backing pad under the deck is a poor approach for the stem fitting of a dinghy, let alone a yacht.

But we're all guessing to some extent as we haven't seen the structure.

Whale Tufnol is, AIUI, phenolic resin and cotton.
There is 'tufnol' with glass in it. It's a whole family of materials.

There is some really dodgy advice on this forum on some subjects.
 
Wing Mark said:
Sounds like the 'if rotten ply was OK tufnol will be fine' mob are doing ill-informed guesswork.

I suspect the skin of grp below the ply is structural, it's a GRP sandwich with ply as the core.
Adhesion of the core to the skins is important.
If the current design is know to be weak, I'd look at it and consider an internal knee or web of some sort to take the forestay loads etc down to the hull.
To stiffen the deck, I'd consider laying up a piece of solid GRP, it will probably be cheaper than the tufnol, much stronger and only a little heavier.

But there's no point making the deck really strong and not attaching it to the hull equally strongly, if the forestay loads go to the deck.

A backing pad under the deck is a poor approach for the stem fitting of a dinghy, let alone a yacht.

But we're all guessing to some extent as we haven't seen the structure.

Whale Tufnol is, AIUI, phenolic resin and cotton.
There is 'tufnol' with glass in it. It's a whole family of materials.

There is some really dodgy advice on this forum on some subjects.
Click to expand...
I think you are inventing arguments to satisfy your requirement to denigrate other advice. The OP clearly says the ply is a backing pad. There is no suggestion whatsoever that the ply is structural, in which case it would be bonded and encapsulated in the construction.
 
Use ply and it will be good for another 50 years, with Tufnol it's good for infinity and beyond.

I would use ply, it's easy to shape and you will probably make a better job of the thing than the original builders anyway.

.
 
Having used various plastic in my workshop may i suggest that you go online to
Plastics direct
They give details of the various products & you can select accordingly
Then I suggest you look at RG 1000 as it is much cheaper & available in 250 * 250 for about £17 plus P & P
It would certainly meet your requirements. I use it quite often for machined parts & bearing/spacer material
I tend to order by phone as my account with them always seems to crash their website
 
RG1000 is polyethylene,
Notable for being hard to bond and not very stiff as plastics go.
Also known for creep under continued load.
"There is a known weakness on the Nicholson 32 mk10 where the stem pulls up off the deck, which mine is doing, rather a poor design really. "
The original design may be poor, but the forum can help the OP make it worse.
 
You must log in or register to reply here.

Share this page

Top