One recurring problem with plywood foils, centreboards, leeboards and rudders is that over time they tend to break unexpectedly – so are unreliable – and it is not just me who thinks so.
What WEST System Say about Plywood Centreboards and rudders
West System says “Strip planked rudder or centerboard foils covered with fiberglass work, whereas plywood is not a good choice”. (1)
https://web.archive.org/web/20170118020703/http://www.westsystem.com:80/ss/wood-epoxy-longevity
Racing Dinghies never use Plywood Foils – they break unexpectedly
Foils below are from our OzGoose plan and are typical of conventional, highly reliable sailing dinghy rudders and centreboards . 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. Accuracy gives you a measured 10 percent extra upwind performance and improves boat handling remarkably
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
Look how very neatly the out veneer has broken.
That’s a clue
Note the line drawn to show when the centreboard is out of the water … the break is perfectly parallel to that.
With any centreboard there is a lot of side load concentrated where the centreboard exits the hull.
The outside veneer of a centreboard or rudder is often creased by the edge of the case over time and is 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 (veneer) is not in the right direction to take the load.
So load 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 – as there will be – the board is effectively only 10mm thick.
Repeated reverse loading like this may snap the Ply or foam core board quickly or over time.
The board can work for a season or three and then, maybe even in light wind – it falls off with little load.
This does not happen with the strip built rudders and centreboards.
What is the correct question when a foil breaks?
My cynicism is built of long experience repairing other peoples’ boats. And WEST agrees with me.
Phonecall at work – “my centreboard has broken”
“What Happened”
“I was sailing along in not much wind and it broke off – luckily a boat was passing and they towed us 4 miles home”
“Was it Plywood”
“Yes”
99.9% of broken foils are plywood.
For a laminated timber foil to break is extremely rare.
A Trick – Detail design of centrecases and rudderboxes to reduce 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 – 1/16″) from the edge of the slot.
Now measure up inside the case and draw a line 10mm (half inch) 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.
4 Part Series – Homemade foils for better Performance and Safer Handling
(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
http://www.westsystem.com/ss/wood-epoxy-longevity/