FAQ – Crimes against boatbuilding – never put a plywood deck join on top of a deckbeam

This is a sortof long, but I think quite well written rave about Plywood decks – what to do and what not to do.

Just jump down to the headings about joining decks if you want to cut to the chase.

In brief don’t join plywood or any other sheet on the boat the way you join things on a house. Houses are weak and stiff. Boats are strong and flexible.

It also sticks in a bit of history for your amusement – about L Francis Herreshoff – one of the great designers of the classic era.

This article was inspired by the restoration of a Knud Reimers designed Tumlaren that was being restored.

Are Plywood decks appropriate for historical craft – My Take.

Orange boat before redecking.

The "orange boat" before restorationa and redecking

Click for Orange boat after redecking. Redesign by Storer. Work by DuckFlat Wooden Boats Adelaide.

Orange boat after redecking. storerboatplans.com

The End of Laid Decks . . .

(Sorry guys – I have got enthusiastic about things again – but I do get to the chase later)

There is a really good set of arguments for going with a plywood deck, but before I cut to the chase I want to give you a little bit of history.

One of the very greatest of the classic yacht designers was L Francis Herreshoff. He was the son of Nat Herreshoff but had very much his own design direction.

He designed dozens of boats that are classics – that will hold you spellbound with the sheer gorgeousness of every curve and line. And they sailed like rockets too.

On the racing side – he virtually gave up designing racing boats after he destroyed the Universal Rule, “J” Class, “R” class and other letter classes by driving a truck through the rules. The J Class that used to race for the America’s Cup. Anyway all the hot designers worked in the R class (about 45ft long).

When L Francis came along with his R design “Live Yankee” – rotating wing masts, Super lightweight hull with over 80% of the weight in the ballast keel, sails with two surfaces (ie one surface attached to the right back corner of the mast, the other attached to the left back corner) forming a thick, efficient airfoil the boat’s performance was so high that it outdated the existing fleet instantly.

Francis Herreshoff's Live Yankee R class drove a truck through the rules. And how to join plywood properly. storerboatplans.com

Oh, by the way – wing mast, double luff sails, super lightweight hull and something like 80% of the boat weight in the ballast (more normally 50% or less) – this was in 1929.

So they changed the rules and LFH dropped out of the racing scene soon after.

Plywood and L Francis Herreshoff

OK – LFH was around before plywood. His designs are among the most sophisticated traditionally built boats ever conceived.

He HATED plywood with a vengeance – he called it “stinkwood” because os the smell when you cut it with a powered saw. The only thing that he hated more was fibreglass – he was one of the first if not the first to call it “solidified snot”

But by the end of his life he was happily specifying plywood for decks and bulkheads.

Herreshoff concedes. Plywood deck advantages. Structural, cheaper, easier

In the end Francis Herreshoff had to concede that plywood for decks has a number of great advantages.

1/ Plywood is so structurally superior.

Having the deck of one piece allows the significant rigging loads to be carried in it as a single beam. There are huge compression loads carried in the deck as the two shrouds on either side of the boat try to squeeze the boat together. The same thing happens between the forestay and the mast – the forestay is trying to pull the bow of the boat up and back – leading to huge compression loads as the deck resists.

The problem with laid decks in a structural sense is that to handle the lateral compression loads there is a huge “squeezing” effect on the caulking. As the boat manouvres around relative to the wind the squeezing effect varies enormously with the caulking between the planks going through cycles of being squeezed and released. This is very tough on the caulking (fibre jammed between the deck planks and then sealed by putting tar (or a more modern polyurethane sealer) over the top between the planks.

The twisting of the boat (as the mast pulls it one way and the keel pulls it the other) also causes significant tensile and shearing loads between the planks.

But the squeezing means that over a few years the caulking becomes compressed, the deck starts moving more and the tar or other sealer gets pulled and pushed in all sorts of weird ways and finally starts splitting.

The deck leaks.

The old way of dealing with it was to canvas cover a traditional deck. So you would caulk and pay (tar seal) it then paint on a heavy coat of white lead paint and lay canvas down over the top and paint it. This increases the life as the canvas soaked with paint is quite waterproof – but the cycles of compression will result one day in the canvas tearing. Or it just becomes worn out from people walking on it.

When fibreglass arrived on the scene it became popular to use fibreglass and polyester resin instead of the canvas. It is a slightly better solution because the glass can sustain tensile loads (if it is thick enough – which causes problems with expense and weight) and the old polyester resin doesn’t stick to wood very well – see …

Epoxy or polyester resin for wooden boats.

With a properly installed ply deck – it is all one piece – so all the loads – tensile, shear and compression can be carried easily without the problem of having to transfer the loads across many joins that are prone to movement.

2/ Plywood is cheaper

 – it is starting to become hard to find timber suitable for decks at reasonable prices – you should use quarter sawn fine grain stock (all the woodies are salivating right now). The quarter sawn stock means that the timber will swell and shrink evenly as the moisture content changes.

Ply is much cheaper and more available. Also because of its structural efficiency the thickness of the deck can be reduced – reducing cost and weight.

3/ Plywood is easier

Because it covers such large areas it cuts the amount of labour enormously. Using modern epoxies you can eliminate all screws and nails too – and just use some temporarily to hold things in place while the glue sets up. This cuts cost and also reduces the number of points where water can intrude into the timber structure.

Eliminating fastening also makes deck repairs a dream. Just set a router to the thickness of the deck ply and cut out any damaged sections without the slightest bit of damage to underlying structure. FANTASTIC.

Eliminating fastenings

When not to use plywood on a Classic Boat

It depends on your view of the boat and the boat’s history. If the boat is historically significant or you really like the idea of keeping the boat in a traditional form – then do it! It will mean more maintenance and might mean a bit more inconvenience – but if you LIKE traditional boatbuilding process – that is the way to go.

Another area is that the ply deck is so structurally efficient it will reduce the loads that the rest of the boat has to deal with.

Joins in deck plywood. Totally different from house building methods.

What not to do with ply – OK – this is a lesson for the housebuilders amongst us…


ie some people join the ply or deckplanks and butt them together by having the end of each sitting halfway over a deckbeam or frame. This is not adequate for boats.

If your deckbeams are 50 mm wide and are spaced 400mm apart what happens is if someone puts their foot between the beams then the edge of the beam that the plank rests on becomes a fulcrum and as the deck is pushed down by the foot and the fulcrum takes the load the edge of the plank is levered up from the surface of the deckbeam. The mechanical advantage in this case is 400/25 – or 16:1 so you will easily break the glue join or pull out any fastenings.

You need to use properly designed scarf joins (minimum of 6 to one glued with epoxy) or butt straps (20 times the plywood thickness (so 9mm ply needs a butt strap 180mm wide made of the same plywood thickness glued underneath) or for planking a properly designed and installed butt block (you will have to do some research – McIntosh and Rabl’s books are very good – also there have been a number of articles in WoodenBoat magazine over the years).

How to choose between Scarf and Butt Strap joins.

Plywood selection – if the boat will last – choose plywood and other materials that will last too.

Using good quality Marine Ply or planking timber. The Tum is a fantastic boat – great, fast sailor – excellent heavy weather boat. She is going to be around for years. Because of her narrowness there is not a great deal of material involved in the decks – so get the best quality marine ply that you can. She is going to be around for another 30, 40, 50 years and cheap ply and other low quality material will come back to haunt you – and faster than most people think too.

The deck should be sheathed in fibreglass fabric (NOT chopped strand matt) using the same good quality epoxy (Bote Cote, WEST, System Three and some others in the same price range) that is used for gluing the ply in place.

Traditional looking ply decks

You can make a ply deck look like a traditional laid deck by gluing planking over the top. For a boat like the Tum it is important to not go too heavy – the boat will perform much better and be dryer when sailing in a seaway. See a copy of “The Gougeon Brothers on Boat Construction” – (big book just reprinted) or get a copy of a cool little pamphlet put out by WEST “Wooden Boat Repair and Restoration” – it is now downloadable for free.

Teak overlay decks

Choosing the correct fibreglass

Depending on the deck frame spacing 9mm ply with 10oz (300gsm) woven glass cloth over the top would be adequate for the TUM. It ends up looking like canvas decks

If going for a timber veneer laid over the ply it might be good to try and locate some 8mm ply. The veneer should not be too much more than 4 or 5 mm after sanding.

In fact have a look over my FAQs on using epoxy with boatbuilding


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