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9th June 2008 11:03 PM
James Thanks Cholo and Farlo. I might take your advice about adjusting the overpowered / underpowered coefficient because it was just my own rough guess.
9th June 2008 08:05 PM
Farlo Hi James,
Congratulations, your charts are great!! Just one comment, I find your 1.05 / 0.95 ratio for fin surface and length in under / overpowered conditions a bit conservative. My practical findings suggest me something around 1.1 and 0.9 respectively. But I'm a lightweight sailor and this may play a role (1 Kg = 2.2 Lb). Best regards and good luck for your doctorate.
7th June 2008 07:43 PM
cholo James,

your formula for a slalom fin really works!

It fits what I use after years of trial and error, and also what my heavy friends do use for competition! It's amazing how your simple linear formula fitting an SX with small sails is working good also for in the big sails range (7-8m2)!
7th June 2008 05:24 PM
windfrancois Thank you Roger, I was not sure that this concept was still on the edge. I remember some experiments about a membrane on sailboats with microscopic triangular channels to retain more water layer do you know something about this research.
4th June 2008 11:45 PM
Roger Hi windfrancois,
I think the current wisdom on "surface finishes" for windsurf gear/hi speed sailing dinghys/huge racing yachts is that a polished surface is too smooth and actually cuts down your speed.
Using a "figure 8" pattern with 400 wet/dry abrasive cloth is about the best you can do.
Some folks like a straight pattern in line with the water flow.
The idea is to create a little "tooth" (microscopic maybe, but "tooth" just the same) to hold a boundary layer of water molecules on the surface.
Testing has shown that water has less "friction" on a fully wetted out boundary layer than on a polished surface with no boundary layer or "wet out".
Surface finish can have a positive effect on preventing spin out, but mostly spin out
is due to imperfections in either the foil or the planform of the fin.
Hope this helps,
2nd June 2008 09:58 AM
James Well, I'm definitely not a doctor in maths... I'm a nerd windsurfer working on my doctorate in biology.
2nd June 2008 07:46 AM
windfrancois WOW

That calculator is very impressive. You may be either a doctor in maths or a nerd windsurfer!!!!

thanks to all, things become easier to make sense now.
2nd June 2008 06:54 AM
James Francois- This spreadsheet calculates the correct size fins to be used with different size sails.


Straps forward will not help your upwind performance, but it may help you get accustomed to sailing a modern board. When you are comfortable with the forward, inboard positions on your 144 you should move the straps back and out for better speed and upwind performance. I.e. the straps should be where they are in the picture here:


As PG notes, for the hypersonic you can use a somewhat shorter fin than recommended; perhaps 6 cm shorter, but it works ok with normal length fins, too. When I had a hypersonic it came with 2 fins; a 34 cm and a 44 cm. I used the 44 cm fin with sails 7.5 - 9.4, and I used the 34 cm fin with sails 5.2 - 6.6. I don't know if you've sailed the hypersonic much yet, but I have to warn you that it's a very difficult and technical ride which is sensitive to where you put the mast base, how you rig the sail, what size fin you have, where the footstraps are, etc. It will take a lot of trial and error to figure out what settings work best for you.

Good luck!
2nd June 2008 06:25 AM
windfrancois keybord error Fins an footstraps!! I don't like Foodtraps (I am dentist).
2nd June 2008 04:23 AM
Unregistered Shouldn't be "Fins and foodtraps"?
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