Rudder foil shapes

I am looking to build some new rudder blades for my Gemini 3000 cat, the existing ones are a wood plank 74" x 11.25" x 1.25" with not much of a foil shape, they are a daggerboard type which slide up and down in ss cages, the blades are cocked forward at the bottom so they are balanced when fully down. I am planning to layup some composite blades in a sheetmetal mold the way Malcolm Tennant used for many of his foils, ive built quite a few this way over the years and it makes a nice blade. What i am wondering is what radius to use for the leading edge, i think Malcolm used a radius of 4mm on the Great Barrier Express rudders which are similar dimensions, however it seems a bit sharp to me for slow boat like the Gemini, i am tied into the thickness and chord but could change the leading edge radius and the location of the maximum thickness. Any thoughts?

Steve.

 

Search NACA 0012 offsets

 

When I was researching this excellent forum for details for my own rudder,I came across the figure of '60 degress of a circle'.It took me a while to figure out that this means-draw a circle,put in the vertical (12 o clock/6 o clock) line.From the centre,draw lines 30 degrees up through the diameter on either side of the vertical line so that the lines cut the diameter.Where the lines cut(approximatly 11 oclockish,1 oclockish),draw a horizontal line to join the two points.Thats the leading edge.Scale up/down the circle so that the flat side is correct for the rest of the board.I managed to buy some wooden moulding from our local DIY shop that pretty much matched what I was after.
I don't know if this is the optimum,but it's what I used.
Hope this helps,but everyone,please correct me if I'm wrong,or has better ideas-I enjoy learning.
Cheers

 

You want an ellipse, not a circle.

 

This will produce section offsets just enter the cord length

 

There's a bit about rudders on the marine design tips page of Competition Composite Inc. (ex Phils-Foils)

http://www.fastcomposites.ca/site/marine/design-tips-fabrication-overview/design-tips/

 

Thanks Doug, looking at that i guess the 4mm that Malcolm shows for the leading edge radius would be about right, my chord is 11.25 (285.75) so 12% would be 34.29, my existing blades are about 1.25" (31.75). These blades will be laid up in a sheet metal mold so when i have it folded on press brake they need to fold it around a radius, after laying up the blade a beam is positioned at the point of maximum thickness and then it is inserted nose down into a fixture which sets the thickness and the trailing edge is clamped together and it is left to cure,at this point the nosecone will be pretty much an ellipse. Thanks for the help.

Steve.

 

I have a formula for you. r=1.1019c(t(squared)). R is the radius of the leading edge. C is the cord. T is the max thickness (expressed as a percentage of the cord).

This comes from the NACA 0012.