Monday, February 9, 2015

Your Centerboard and Rudder Need You!

Our rudder and centerboard are below the water, out of sight, and out of mind. They also aren't wear items like sails and lines so that default reminder doesn't exist. Like your sails, your foils create lift and generate drag, countering the force of your sails to help your boat move forward. Most people don't realize that the effect of your foils underwater on your performance is just as significant as your sails.

I'd heard all kinds of ideas on the best and fastest ways to keep your foils. A popular theory is to wet sand them so the water adheres to the surface and creates a sheer layer. Last year I decided to educate myself on foils and found some great takeaways. It turns out the wet sanding with moderate grit sandpaper is just to justify being more lazy in the preparation of foils. Your foils should be as polished as possible, with no imperfections, especially on the leading and trailing edges. 

Use these tips and give your blades a little love before MWE or your first regatta and have the confidence your foils are performing at their best.

Common Misconceptions:
  • A wet sanded blade with 400-800 grit provides the best performance (least drag)
  • Thistle foils cannot achieve laminar flow, which is very good for drag reduction (they can, on the pressure side)
  • Small nicks in your foils don't have much effect on performance (imperfections on leading and trailing edges massively effect drag)
  • Your foils are probably polished enough already (you should have a reflection when looking at the surface straight on)

Performance Tips:
  • Fix all nicks in your blades, especially in the first 4 inches and trailing 2 inches
  • Sand and polish your blades as finely as possible. Start with 800 (or 400 if your blade is really rough) and progress up to 1200 grit. Then use a fine compound followed by a fine polish. A good guide is that you should see a reflection straight on, not just from an obtuse angle
  • Try to sand down any abnormal convex bumps in the form (concave areas are not nearly as detrimental)

  • A foil can never be too smooth - the smoother the foil the less the drag and higher the lift - and the faster you go!
  • A foil polished to a mirror finish has 1/3 the drag of a foil wet sanded perfectly to 1200 in the majority of sailing situations (imagine what the comparison is to 400 or 800 grit)
  • Only a highly polished foil is capable of laminar flow. Less polished foils will have turbulent flow on both sides in all conditions
  • My real world experience is that you gain a little bit in speed upwind, but the biggest change is in height, which we all want more of.

Your blades should be as smooth as you can get them and as polished as you can get them. Imperfections in the leading and trailing edges cause the biggest detriment to performance. At +/- 3 degrees angle of attack, highly polished blades have 1/3 the drag of blades sanded perfectly to 1200 grit (imagine the improvement over 400 or 800!). 

Use the last few weeks before your first event to take a look at you blades and make some small repairs. It can give you that extra little bit to live in that lane when you couldn't have otherwise.

I tried to keep this brief and somewhat less technical, but if you would like more of the technical background behind these conclusions please let me know and I will be happy to share more info with you and discuss it. 

"Higher, Faster" is always a great thing to hear on a boat, so put in some time on your foils and see the difference for yourself.

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