Sunday, April 05, 2009

Fairing the daggerboard and rudder blades

I've started fairing the daggerboard and rudder.  I had thought that the blades were actually quite fair but I'm finding that there are a number of areas that need building up.  I want to keep the final quality of these parts quite high.


I'm also quite sure that the trailing edge thickness will be greater than 2mm when I am finished fairing.  So... I'm told that to prevent humming when the boat is planing, the sides should be made asymmetric.   The following picture is from a web page that Menno kindly brought to my attention.  


Ian Farrier also describes this approach in his sailing hints document.  

5 comments:

Anonymous said...

Hi Grant , the desirable trailing edge thickness is actually a function of the chord of the foil - can't remember the exact number off the top of my head ( If you are interested the Marchaj book "The Aerohydrodynamics of Sailing" is informative ) So I think the rudder t.e. needs to be down below the 2mm mark but the centreboard can be thicker because it is wider . All the best . Regards , Jim Buckland .

GK said...

Thanks Jim, that makes sense. I'll try looking that up.
Grant

TexLex said...

Grant:
Did you find an answer to this? I am building an i550 (planing hull 5.5m sportboat) and I would love to know the answer so I know how to prepare my foils.

GK said...

I just had a look at the i550 design and pictures. What a wonderful design idea. This boat would have been on my short list when I was looking, if I had known about it.

I'm still fairing the foils. The whole point of it all is to keep the flow of water laminar around the daggerboard and rudder. So, I am concentrating on first getting them fair and smooth. The perfect foil would have the sharp point but is not practical. If the flat on the trailing edge is too thick (>2mm) I can shape as in the posted picture. From my research this seems to be the best engineering compromise to delaying the onset of turbulence at that point.

GK said...

I just found this common sense comment on hummmmmm.
http://www.j24class.org/usa/tips/humm.htm
Grant