One recurring problem with plywood foils is that over time they tend to break unexpectedly.
West System says “Strip planked rudder or centerboard foils covered with fiberglass work, whereas plywood is not a good choice”. (1)
Foils below are from our OzRacer plan. The photo is of the final epoxy coat before sanding smooth and painting or varnishing.
Whereas the foils as specified in my plans are much the same as those on thousands of racing dinghies that race every week with no damage at all. They use solid timber laminated out of 8 or ten “staves” then shaped and a layer of fibreglass over.
Particularly now that Paulownia, a low density timber with good gluing and shaping properties has become more available. Western Red Cedar as a core is finally outclassed – but it is still OK as is any other moderate density timber that glues well. In Australia and Europe, Paulownia can also be cheaper than other timbers.
This is the lightest absolutely reliable way of making foils for your boat.
There is a reason for the failure of plywood foils amt it is actually similar to the reason composite foils with a foam, Nomex or other relatively non structural core fail unexpectedly as well. Many builders of high tech composite racing dinghy hulls still use good old timber on the inside of their latest carbon foils.
Why Plywood Foils fail unexpectedly
With a plywood centreboard there is a lot of side load concentrated where the centreboard exits the hull. This results in the outside veneer being creased and unable to carry load. With normal timber the load is passed to the good timber adjacent which can carry it. With plywood the adjacent timber is not in the right direction to take the load so it will miss the second veneer and move to the next one that is correctly oriented. Your 18mm thick centreboard is now effectively only 14mm thick as two veneers are now doing no work. If there is similar damage on the other side the board is effectively only 10mm thick.
Repeated reverse loading like this may snap the Ply or foam core board quickly or over time.
Detail design of centrecases and rudderboxes to reduce the risk of breakage from repeated compression.
I still think plywood foils are a bad idea for anything other than a boat built cheap and expected to last only a few years.
If you are building for keeps or need unquestionable reliablity of the rudder and other foils, the method with the track record is sawn timber for the core.
BUT … the composite manufactures found that there is a neat bit of detail design that reduces the risk of the foil folding over the corner of the case or rudderbox.
A little bit of careful sanding reduces the stresses and the likelihood of the case crimping the outside laminate or ply veneer.
I’ll explain with measurements so you get the idea. You DEFINITELY don’t want to round off the corner between interior of case and the hull bottom as it adds considerable drag to the slot/foil interaction.
Look at the underside of your case where the foil exits. Mark along the bottom of the boat a very small distance (one millimetre is fine) from the edge of the slot.
Now measure up inside the case and draw a line 10mm up inside the case.
Sand away this little triangle.
Now sand the transition between the inside edge of the triangle and the rest of the case so you can’t feel any transtion. Now the load will be distributed over a wide band rather than a sudden concentration of loads at the exit of the slot.
You really don’t need to measure – it is just so you can visualise.
I don’t guarantee it will prevent a ply board from breaking, but it must help. It is also such an easy thing to do that I always do it for any centrecase, leeboard case or rudderbox.
(1) There was a more substantial article than this mention examining the WEST System boats that are featured in their near 40 year old guide on boatbuilding which showed a number of plywood rudder and centreboard failures. It is very close to my experience as a builder and repairer as well