The End of the Plywood Boat?

Bypk

The point where foam and glass construction is cheaper than plywood may be upon us soon.

There was a recent post on the John Welsford Small Craft Design Facebook group by someone who had built a Pathfinder using carbon composite construction rather than plywood . I thought this pretty extraordinary, imagine the cost!! In my opinion, the use of carbon foam sandwich construction is quite inappropriate for a cruising dinghy. The high strength of carbon is totally unnecessary and carbon is very stiff and brittle, not the best for handling bumps and impacts on your cruising boat.

However, this got me thinking. A foam and glass cruising dinghy would make a lot of sense.

Having owned three foam and glass racing dinghies in my time (building two of them) I know them to be very strong and durable. They do not mind water being left in them and they are quite impact resistant, you might dent the skin and underlying foam but its integrity will be retained. I know of several local foam and glass Cherubs and I14s that are decades old and are still in excellent condition. My old skiff Moth built locally by John Ilett in the early nineties reappeared at the Walpole Regatta in 2022 still in very good condition. I was delighted to see it again!

So what about the cost? First, let’s consider the price of plywood. I am currently building a plywood Welsford Sweet Pea. When I ordered the plywood for it at the end of 2023 Tony O’Connor said “Brace yourself! Prices have gone up enormously” and indeed, the prices were bracing!

Joubert Plywood prices, including GST, in December 2023, sheets are 2.5m x 1.2m = 3m2
6mm $169 per sheet. This corresponds to $56 /m2
9mm $195 per sheet. This corresponds to $65 /m2
12mm $239 per sheet. This corresponds to $80 /m2

Note that shipping for 30 sheets of ply was $400 , so roughly $13 per sheet or about $4 /m2 should be added to the costs above.

Now consider foam: Gurit 80kg density PVC foam panels 2180 x 1020 = 2.223 m2 from Composite Warehouse in O’Connor. Prices as of July 2024 and include GST

  • 6mm $120 per sheet. This corresponds to $54 /m2
  • 8mm $125 per sheet. This corresponds to $56 /m2
  • 10mm $135 per sheet. This corresponds to $61 /m2
  • 12mm $145 per sheet. This corresponds to $65 /m2

So the foam core is a bit cheaper than plywood, more so when you add in shipping costs for ply but we have to add fibreglass and resin costs. However, not that much glass is needed. Prior to carbon fibre becoming common for dinghy construction the layup on an 18′ Skiff would be 2 layers of 6oz glass cloth on each side of 10mm foam. My 14′ Skiff used 2 layers of 6oz glass on the outside and 1 layer of 6oz on the inside. For a cruising dinghy we could perhaps increase these to use the equivalent of 3 layers of 6oz on the outside (~600 gsm) and 2 on the inside (~400 gsm). See the layup schedules on pages 19 to 21 in the Klegecell Boats and Sandwich Construction booklet. Here are some fibreglass costs at Composite Warehouse:

  • 6oz (180 gsm) glass cloth 1m wide $12 per metre
  • 10oz (330 gsm) glass cloth 1.3m wide $17.50 per metre, corresponds to $13.50 m2
  • 600 gsm double bias glass cloth 1.27m wide $18.50 per metre, corresponds to $14.50 m2
  • 1708 multi-axial fabric 600 Double Bias with a 225 CSM backing $19.50 per metre

Remember a plywood boat will typically be sheathed in 6oz cloth on the outside, maybe more on the bottom, and every other surface coated with 3 layers of epoxy. It will also require considerably more solid timber in its internal structure. A foam and glass boat will certainly use more glass but perhaps not that much more epoxy. So, overall, it will be more expensive, but I would argue not hugely so. Future price movements may well make the two construction methods comparable. Also, the eventual resale value of a foam and glass boat is likely to be higher.

Foam is available in a good range of thicknesses, 6, 8, 10 and 12mm. Having only 6mm and 9mm plywood can be problematic. I am certainly finding in the construction of my Sweet Pea that there are plenty of situations where using 6mm ply would perhaps not be strong or stiff enough, but using 9mm, being 50% thicker and heavier, would be overkill. What I find myself doing is laying up glass on 6mm ply to bring its strength and stiffness up. Unfortunately we do not have the equivalent of 5/16″ plywood that used to be available.

So, maybe it is the time for cruising dinghies to start thinking about moving from plywood to foam and glass construction. Certainly racing dinghies have not been built in plywood for over thirty years now. For us there is no need to use any of the advanced composite construction techniques. Basic foam and glass construction on a male mould is all we need. It is simple with no special tools required and, with care, the process is not too messy. If you are capable of sheathing a plywood boat in fibreglass then you are also capable of sheathing foam panels affixed to a male mould. It is the same skill. Converting existing plywood cruising dinghy designs to foam and glass construction should be reasonably straightforward (as long as they are not lapstrake!). Possibly designers of cruising dinghies will eventually start designing specifically for foam and glass construction. The main downside is that sadly, compared to plywood, foam is rather a lifeless material to work with. Also, you will not be able to take it to the Tasmanian Wooden Boat Festival!

Resources:

Looking around on the web I found there were a surprisingly small number of sites devoted to basic one-off foam and glass dinghy construction. I guess because it has been significantly more expensive than plywood construction not so many people have been using it.

Response from Jim Black

The End of the Plywood Boat?  I don’t think so!

An interesting and thought-provoking blog, but I beg to disagree and would offer the following arguments, particularly based on my experience with building the Ness Yawl Wee Birlinn and my previous experience with various FRP construction methods:

  • A glued clinker (lapstrake) construction has so much inherent strength within it’s laps that very few additional timber structural members are actually required.  And a good exterior grade plywood is really quite acceptable for most of our small boats, rather than needing the top grades of marine ply.
  • The amount of epoxy glue can be limited by only using it in the actual mating surfaces – there is actually no need to add all the “recommended” beads of epoxy above and below each lap.
  • There is no need to sheath in glass: all you need to do is an application of International Everdure (or the less expensive Norglass equivalent) inside and out, paint with single-pack oil-based paints/varnishes, and annually touch up as required – Wee Birlinn is now entering her fourteenth sailing season with her structure fully intact.
  • When actually damaged, which can happen to the best of us, the extent of the damage to a timber/plywood boat can be much more readily established than that of a foam-cored FRP boat where the water penetration along the kerfs in the foam, or the separation of the foam from the skins, can be very difficult to accurately assess and then fully rectify.
  • And then there is the real satisfaction of working with wood using traditional hand tools – FRP work just cannot hold a candle to this!

Enough said?  Happy to debate!

JIM BLACK
Wee Birlinn