Sheathed ply cold molding

Capt. Clueless

Capt. Clueless

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Having been involved in the building of Leisure 17's, I fully get and understand 2 part GRP construction. (Hull/Superstructure).
My "one off" Guy Thompson superstructure is of sheathed cold molded ply though. (I don't think it is any other type of wood).
I'm currently googling this process, but know that there will be many here possibly experienced and maybe had the ownership of this type of constructed boat.
I'm kind of wondering on how the superstructure is joined to the GRP hull, I'm presuming it will be with GRP, but have read of many leaking in this area.
I have some small repairs to do, especially where an interior cross member has rotted through, and therefore this small particular part of the superstructure (entrance to the cabin) is a little spongy to say the least. (The rest is rock solid). I am trying to find the best guide as to how to carry out this type of repair. Can anybody offer any advice or point me to the best tuition info please? Also, what thickness roughly would the sheathing be, considering the boat was built in the brick outhouse days of the early 70's.? If I have also misconstrued my understanding of this sheathed Cold molding method, can someone put me right as well please?
 
Probably the best resource on cold moulded construction is the Gougeon Brothers guide to wooden boat construction which is available online for free from the West System epoxy website
 
pmagowan said:
Probably the best resource on cold moulded construction is the Gougeon Brothers guide to wooden boat construction which is available online for free from the West System epoxy website
Click to expand...

I'll have a look at that, many thanks. Any info is always helpful to increase my knowledge and therefore my skills.
 
Likely the hull has wooden shear clamps screwed or bolted to the hull and the wooden super structure glued/screwed to them.
As for your beam, probable after cutting it out, a new laminated one can be assembled in place.
Second reading the Gougeon Bros.

A photo would help.
DW

If it is glass and polyester resin, be prepared to find other 'soft' spots...
 
Last edited:
DownWest said:
Likely the hull has wooden shear clamps screwed or bolted to the hull and the wooden super structure glued/screwed to them.
As for your beam, probable after cutting it out, a new laminated one can be assembled in place.
Second reading the Gougeon Bros.

A photo would help.
DW

If it is glass and polyester resin, be prepared to find other 'soft' spots...
Click to expand...

I don't have many close up pics currently. The first were taken in her "better days" and then the rest are the recent horror pics I took. The pic of the cockpit shows the cabin entrance which is spongy due to a rotted cross member. I expect I could cut it out, sheath the replacement ply and laminate in.
 
Yes, all the Guy Thompsons I've seen were from the T24/27/31 group of desihns, but for reference they have an inwhale which supports all the traditional woodwork for the decking. The deck and superstructure are all ply, screwed to the timber frame beneath and then sheathed wit cloth/resin. However the age of the build makes it more than likely that the resin is polyester. This loses its grip on the ply substrate eventually, leaving the sheathing brittle and subject to breakage.

Although I've done many patches on one of these decks, in truth if the substrate is sound it would be best to strip it altogether and re-sheath in epoxy. This will last at least twice as long, so long as you paint over with deck paint which will protect against UV degradation.

As to the soft beam, one of the beauties of these designs is that the ply will maintain the shape (unstressed} whilst you cut out rot or damage and laminate in a new section. I always found laminating best as be choosing the thickness of each laminate, you can avoid having to do a scarf joint. During my mate's ownership of T24 Hasty, we repaired the deckshelf in the cockpit lockers and the bulkhead frames that make up the mast support amongst many other repairs.

Rob.
 
It is not cold moulded, but just ply sheathed with GRP. The downside of this is that the GRP does not stay stuck or cracks, or is badly sealed at edges. Water gets in and rots the ply and then the supporting beams. No different really from straight ply construction except that you have to remove the GRP before you can do repairs.

If you are lucky you may find the delamination is only local to the rot underneath, but more likely as you strip off the GRP you will find it comes off in sheets. Then you will likely find that the rot has spread through the ply. You won't know until you get it off.

Once you have repaired the underlying wood structure, resheath with glass cloth and epoxy, using peel ply to get a reasonably fair surface then use fairing filler to get it ready for painting. All this is well covered in the West publications from Wessex resins.

Do not underestimate the amount of work involved. The ideal would be to remove all the GRP, make good underneath and resheath, but this is a massive labour intensive job and you may decide the effort and time is not worth it. I have a ply boat and find the make do and mend using epoxy is OK as you can deal with small jobs one at a time. However I did decide to sheath the deck and coachroof top - but in glued on teak. In retrospect not a route I would recommend.
 
Tranona, while the cockpit is probably sheet ply with grp covering, the coachroof's double curvature is where the 'laminated ply' came from. It looks a little like the Atalanta's hot moulded one.
 
DownWest said:
Tranona, while the cockpit is probably sheet ply with grp covering, the coachroof's double curvature is where the 'laminated ply' came from. It looks a little like the Atalanta's hot moulded one.
Click to expand...

Still not cold moulded. Using two sheets of 6mm instead of one 12mm is common with the amount of crown in that coach roof. If it was cold moulded it would be done with strips at 90 degrees to each other, or even same axis but joins offset, either ply or like the Fairey solid veneers. Cold (or hot) moulded can be built with very little framing as many of the Fairey boats were. Very different proposition to repair than conventional ply on frame.
 
Looking at the OP's side elevation pix it seems like there is a definite 3D form , which is simple in grp but surely impossible in any timber construction other than hot or cold moulding unless carved out of blocks like a half model. As Fairey were making Atalantas by hot moulding agaba veneers with resorcinol about that time, and the coach roof is quite similar in shape my money would be on that being the construction method. Also my 1968 T31 has a moulded coach roof made that way. At this point I don't know how many veneers are in the construction since I'm not at that point in the job yet.
The original sheathing on my boat was Cascover btw, which is quite clear because the weave is about 25 to 30 threads per inch and quite loosely woven. Some parts have been patched with glass cloth or mat since and this is noticeably different. Dunno that any of this answers the original question though
 
The hull of my boat is Cascover sheathed up to the rubbing strake. Still sound, since 1963 with one area repaired about 10 years later. The original was then coated in Cascote and painted. I had the paint blasted about 5 years ago, but the Cascote did not like it and ended up a less than smooth finish. Wessex Resins recommended fairing with an epoxy fairing mixture. I used International Watertite. I have also done other repairs where the cloth has been breeched using glass cloth and epoxy with 100% success.
 
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