# Another thread on leeway and upwind performance

Discussion in 'Hydrodynamics and Aerodynamics' started by erdben, Jan 22, 2017.

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### erdbenJunior Member

Hi all, I tried to dig up one of the several interesting old threads on this topic, but the system didn't let me reply to those old threads.

As a winter project, I got the rudder and centerboard off my boat to do some repairs/improvements. Since it's too cold in the garage now, instead of getting the epoxy out, I started reading about foil shapes and optimal planform area, then one thing led to another, and I ended up making a VPP for my boat so I can play with different foil shapes and centerboard sizes.

Let's say the question is what's the optimum centerboard size for a boat to achieve best upwind performance.

I understand that changing leeway angle by changing the size of the centerboard just rotates the hull relative to direction of movement, which will influence resistance of the appendages and the hull, and generally you'd try to find the centerboard size that gives you the best lift/drag ratio.

However, what I haven't seen to be discussed in detail is how the leeway affects the performance of the sails. In the VPP, I use the ORC sail coefficients to calculate aerodynamic forces, and those tables are based on apparent wind angle.

Should the AWA input for these aerodynamic calculations be corrected by the leeway angle?

Are the CL and CD numbers given for varying AWA in these tables independent of hull dimensions and sheeting angles (most importantly minimum sheeting angles)?

Here is an example: the max CL for the jib is at 27 degrees of AWA from the ORC tables. For angles less than 27 degrees, CL drops very quickly. Let's say my boat sails at 42 true wind angle, 12 kts true wind speed, at ~ 6.5 kts of boat speed with an AWA of 27.6 degrees. Leeway is ~5 degrees.

Would you calculate the main and jib CL / CD values from the 27.6 deg AWA or because the hull is rotated into the wind by 5 degrees, from an AWA reduced by the leeway (22.6 deg)? It makes a pretty big difference!

If the AWA for the sail forces need to be corrected by leeway, should it be done for all sail angles or only for angles where hull/rig geometry limits trimming of the sails? Should it be done only for the genoa, which is limited by the shrouds or the main, too? The main could theoretically be oversheeted with the traveler, but its interaction with the genoa will change, since the genoa can't be sheeted in more.

Considering the effect of leeway this way, would greatly influence optimum size for a centerboard, since leeway will not only affect hull/centerboard resistance, but also sail forces.

I'd be very interested to hear your opinions.

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### brian eilandSenior Member

Perhaps you will get more replies if you do post your question(s) on a subject thread thar already has many such discussions, and often by some knowledgeable people.

So let me know which subject threads you found interesting, and I will see if I can reopen them for you.

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### erdbenJunior Member

Hi Brian,

there are several interesting discussions on this topic, the closest one is maybe this:

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/thoughts-resistance-leeway-51130.html

Thanks a lot!

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### brian eilandSenior Member

Done, ....you are welcome

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