Peter,

I was nearing completion of converting my K26 'Antares' with an Arne style cambered sail when I learned that a sister, 'Chopsticks' had just been fitted with a Scanes built cambered sail and was found to have severe weather helm. With this in mind, I fitted end plates projecting about 6 inches to both the skeg and the rudder and was happy to find that there was no more weather helm on any point of sail than there had been under her bermudan rig, except when over-canvassed. Dropping a single panel of her 370 sq ft sail cured this.

I suggest that before going to the trouble of making a new rudder, you try end plates, a simple job that took me less than a single day. Mine were made out of scrap ply, encapsulated in multiple layers of epoxy and glass. 

Asmat

I found some K26 photos, and these are showing a rudder that's way too far forward. Clearly, the modern form of spade rudder that has a vertical rudder stock at the aft end of the waterline is the best fit to this kind of reverse raked counter. But this would require some seriously expensive engineering, and I doubt whether the cost could be justified. Also, I would hesitate to put such a spade on a shoal draught boat - even moderate pitching while taking the ground would carry the risk of damage.

There seems to be little sensible alternative to a transom hung rudder, parallel sided, with a draught 20cm less than the keels, and a balance of 10% - 15%. Weaverbird's rudder hangings are designed such that the rudder can be lifted by 15cm in shoal water, and I would do the same for the K26.

Vane steering with this kind of rudder, to a standard of performance that I would be happy with, would require a pendulum gear, preferably a modified form of Weaverbird's . No problem, I could design that.

At the risk of hijacking this thread..... I'm about to replace the rudder on Tara (K26)  I intend to move the rudder to the transom, and completely discard the barn door that currently exists.  

Perhaps there might be some suggestion as to what I replace it with..

all thoughts welcome... 

Peter

Of course, Scott  -  something that simple can't be right.

I once suggested to a JRA member that he should fit an endplate to the shallow keel-hung rudder of his boat. He aired the idea for the designer. The designer was not impressed: He replied that if he thought the rudder needed an endplate, he would have fitted it in the first place. End of that story.

A digression: A couple of days ago, the rudder fell off a friend's Athena 34. He (Allan) had just started on a voyage from Stavanger to Denmark. They were towed into Sirevåg (58:30.223N 5:47.008E), a bit down the coast.  Safely tied up there, Allan managed to tie a rope to the head of the rudder stock and lower it out of the boat and recover it with a boat hook. The rudder stock appeared to have broken in a weld (surprise, surprise) about 20cm inside the rudder blade. No trace of the rudder blade itself. On the photo below, taken this evening, one can see the temporary rudder he had made and bolted to the broken rudder stock. The rudder was re-inserted from below, by me pulling the line and Allan moving the rudder back and forth until it entered the hole. A motoring test in the harbour showed mediocre, but sufficient control. Before he makes a new, permanent rudder, I will have him modify this temporary rudder with an endplate. I will make a note on it here, afterwards.

Arne

Anti-cavitation plate - same word used for the plate often mounted over the top of a propeller especially on outboards - its a misnomer. Should be anti-ventilation plate. (Cavitation is a different phenomenon from ventilation, as I am sure you already know.) Just being picky, I suppose.