New rudder for Corribee Mk2 from scratch

[Stephen Pimley]

The rudder on Corribee Mithril had already been replaced due to the well-known Corribee design flaw of the corroding rudder head attachment. Aka disappearing rudder syndrome.
Unfortunately the replacement rudder had the fluid dynamics of a brick. So I set out to design my own.

I selected a NACA 0008 profile and drew up a basic plan.

Rudder section corribee mithril.pdf (687.18 KB)

This is the rudder after having been pieced together, planed, sanded and epoxied. More to follow when time allows...

 final sanding and after some coats of Epoxy ...>   

To be continued...

[Declan McKinney]

Interesting, and impressive work Stephen! How does the plan compare compared to the original Newbridge one?

[Stephen Pimley]

Hello Declan,


Great to speak with you earlier. To your question, I'm not quite sure but I took the dimensions from the 'brick' which I suspect was made up in sympathy at least to the shape of another Corribee rudder (just not the aerodynamics!). What I will do is map out the angles and the lengths - would be interesting to see if others can confirm its the same. I sailed a friend's fin-keeled Corribee 'Florence' throughout 2018 and her rudder seemed very similar in shape to what Mithril had.

As for the aerodynamic profile I selected NACA 0008 after looking at the max thickness I would like together with consideration of the mean chord of the rudder. 0008 seemed a good fit.

I do intend to put everything up about the conception, design and building of the replacement rudder but have had to replace the motherboard of my laptop, so am awaiting reloading the original photofiles. Apologies for the delay but once I have these back on I will put the whole project up here (need to get it onto my own website too).

Stephen

[Stephen Pimley]

This was the rudder that was on Mithril when I bought her. She was made up as a replacement for the original and did not have fine aerodyamics. At anything approaching hull speed, vibration could be felt on the tiller. I'm sure it didn't do her final speed through the water any favours either.  During the refurb project I also noticed the glassfibre exterior of the leading edge was cracked, so decided to bite the bullet and strip it down to see what was beneath.  Very quickly into that I decided a replacement rudder would be needed, and that it could be a good learning exercise.  I had been researching the design of rudders and found this at ybw forums helpful.  And this.

Note the position of the rudder well forward and close to the leading edge. And the brick aerodynamics..

       

Next I noted the approx dimensions and angles:
          

Finally I stripped all of the glassfibre off to reveal the underlying design which was marine ply.
The stainless steel shaft and welded tongs were in excellent condition but the glassfibre had become saturated and was rotting (black marks around the stock shown below).

       

[Stephen Pimley]

Next steps: Design, select materials, select build method.

Research focused on sailing forums such as ybw.com and the american ones. Nathan Whitworth who made some great youtube videos sailing his Mk1 around Britain had dealt with rudder problems. His old website is still accessible via the wayback machine. Search functions on each offer plenty of reading and links.

I decided on the stock position at 18% Mean Aerodynamic Chord, and selected a symmetrical aerofoil pattern.
The rudder would be built up of 6 sheets of marine ply, epoxied together around the original shaft and tongs, routered to suit.
This aerofoil plotter website was excellent in allowing me to accurately set out my wood cuts.


The boards were made up from 9mm Marine Plywood sheets from Gilmour and Aitken.
Epoxy used was West System 105 with 205 hardener.

[Stephen Pimley]

The following photos show the boards being cut and then assembled.
As per the design plan / diagram above there were 6 boards.

Boards 3 and 4 would have the central shaft area completely cut out, and each would need careful routering work to accommodate the tongs.
Boards 2 and 5 would have to be routered to accommodate the edges of the shaft.
Boards 1 and 6 were the outers, the smallest in area.

Cutting out:

          

Laying out

           

Marking up the areas to be routered for the tongs:

              

      

Record hand router - excellent piece of kit



Sections 3 and 4 prepared and ready for the shaft

       



Result of joining sections 3 and 4 (mid sections) to shaft...

        

[Stephen Pimley]

Shaping up boards 2 and 5...more routering for the sides of the shaft where they protruded from the middle section.

        
all boards glued together


Out with the electric planer:

[Stephen Pimley]

Further shaping out in the sun - moving on to a 80 grit sanding disc...

          

       



            



Addition of Kevlar strips on leading edge and bottom - for strength. Mithril is a bilge keeler so I wanted strength on the leading edge for hitting objects that might get between the keels.

   

Wrapping the entire blade in glassfibre woven cloth prior to further epoxy coats.

          


         

[planktos]

Lovely job! It looks good too.

I hope you dry the boat out lots do that people can see that.

[planktos]

Also I just realised that I’ve spent a long time looking at the lovely photos on your blog!

[Stephen Pimley]

Thanks Planktos,

Little Mithril has been stuck under a barn for some time now - although she looked lovely from the outside she was suffering badly from leaks and that rudder, so I knew a fair bit of work was required. Looking forward to her getting back out on the water early next year. In the meantime I bought a friend's fin-keeled Corribee, Florence, which I sailed last year before selling her to concentrate back on Mithril.

[juergen]

Have you taken a skeg into consideration? Would this improve the sailing course stability?

[Stephen Pimley]

Hello Juergen,

I did consider it but I have enough to do for the moment. Perhaps next year it would be worth having a go. It's an interesting question about whether it would improve stability, I suppose it would.

[Stephen Pimley]

The rudder was finished by layering up with more resin (West System 105 & 205) together with 406 Colloidal Silica.





Then grinding / sanding back down






Into the bench, wipe with acetone prior to underwater primer:





After several coats of underwater primer, the coats of antifouling were applied:






Finished rudder out in the yard to dry before fitting

[Declan McKinney]

Hi Stephen. How did the rudder fare on Mithril's recent peregrinations? Satisfyingly unbricklike?