Well I've been experimenting again. I've been struggling downwind with 'standard' rudder blade pretending it's not there. Tried deeper ones and still the same. 

Just want to check that tubercles, or bumpy leading edge, on a rudder blade do not fall foul of rule 17(a) ...winged rudders, double rudders and similar contrivances....

What is included in 'contrivances'? Does it simply mean mean blades should not have more than two 'surfaces' 

I'm pretty sure what I made is OK, it works very well, but no prizes where the inspiration came from.

I know it has been cold, but it looks like an icicle, or is the date wrong?

The blade profile shape is unrestricted. Just don't fit any additions - wings, tabs, flaps, etc. 50 deg North / Composite Craft were doing these on a professional basis. The rudder I saw was a bit smaller in area than a "standard" rudder for the particular boat, but it was fantastically thin - a significantly reduced blade thickness. No problems with stalling I am told.

Thanks Andy

What is the theory behind this?

I have a nice Milanes rudder on eBay. It came off a National 12 but is 1 metre deep so would work for a Merlin. Search Milanes Rudder.

The theory is based upon the fact that a humpback whale has remarkably small fins for its size and can turn in an equally remarkably small space. People have been playing with the bumpy shape on windsurfer foils for a while.

Smaller / thinner foils = reduced drag
Bumpy foil = smaller foil without stalling tendencies!

Does rather rely on keeping the boat flat though .... :-)

The bumpy foil as you call it, also induces a series of pressure waves rather than one big one, so reducing energy/drag to move forward. The concept is great and well proven on bulbous bows on ships. My only concern when I saw the one Toby (N50) had at the 2012 Dinghy Show, was how did you get rid of debris once collected in the concave parts. Great idea in clean flowing water. With sything blades on the leading edge game on if brave enough.

Remove seaweed etc. with carbon golf club shaft that has snapped at head end,(available free from most Golf Pro Shops) just put a U shaped debris remover on end.

These also make good tiller extensions, the come with grip as standard!

Like to see you doing it in a force 4 - 5 upwards downwind!!!

Have had seawead problem on a straight blade. As they say, will not know till it happens. May be a pop-out razor sharp strip down leading edge activated from tiller extension? :)

Cutting edge technology???

that would be a force fore!!!