Subject: Re: [harryproa] Re: Leeway Prevention
From: Rick Willoughby
Date: 8/4/2010, 11:55 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

ben

As long as you can delink the rudders you can set them to have no lift or even negative lift.  At their lowest drag they still have a bit of lift but not much.  For any area the NACA07### have less than half the minimum drag of a NACA00##.  Drag is thus very low.  The Cd is 0.004 for the section I linked to.  

An airplane does not always climb.  By setting the AoA negative they will glide downwards.  You can do the same with twin rudders on a proa.  You just need to be able to trim them relative to each other to minimise the overall drag on any point of sailing.  In light conditions there may be reason to lift a rudder.  

Most sailing boats do not have the ability to articulate the board/keel.  So the hull is forced to suffer leeway to get the lift from the symmetrical board/keel and the resultant induced drag.  The AoA is maybe 4 or 5 degrees depending on many factors. With twin rudders you can completely compensate to avoid leeway.  By going to asymmetric rudders you can get the lowest drag for the required lift to avoid leeway. The L/D of a hull is probably less than 1 and for a typical NACA00## it going to be around 12.  So the combination might be 10 depending on how it is shared.  Hence the drag  to react the sail side force is 3 times what it would be with the twin rudders.

Rick  
On 05/08/2010, at 12:21 AM, bjarthur123 wrote:

 



rick,

could you walk me through this a little more slowly? upwind, i'm totally with you. downwind, i'm still a bit confused.

a hull by itself has drag and lift coefficients. but Cd is huge and Cl is small and so there'll be a lot of sideslip. one might think this is good downwind because it adds to VMG. what you're saying though is that the resulting AOA multiplied by the hull's huge Cd more that compensates and so VMG is actually less because boat speed suffers from drag. you are better off adding a foil to the hull. the combined (hull+foil) Cd is only slightly higher but the combined Cl is now huge. so a lot less sideslip, a much smaller AOA then, and hence less overall drag. the faster speed results in higher VMG even though slip is not directly contributing. am i getting this right now?

my confusion arises because lotsa beach cat folks (H18 in particular) raise not only the daggerboards but also one rudder going downwind (H16 do this). some will tell you it lowers the drag. but i guess it really lowers Cd, and not AOAxCd. others say it makes capsize less likely-- much like sails generate a heeling force, sideslip on foils does too. the farrier trimaran people i know though are adamant about keeping the daggerboard down at all times. i figured the difference was that the spinnaker (which H16/18 don't have) needs a daggerboard to maintain helm balance. but now i'm not sure!

in any case, it seems to me that if the daggerboard is sized to negate sideslip upwind, then downwind, when speeds are typically faster, it will be oversized, lift being a function of speed. so maybe boards halfway up is the way to go downwind? and if you are overpowered and worried about capsizing sideways, or are going too fast in steep waves and are worried about pitchpoling, then boards all the way up to reduce the heeling force and increase the sideslip-induced drag?

thanks for any info. i'm embarrassed to have been sailing for 12 years and not know this stuff inside out! :(

ben arthur
weta #358, "gray matter"
ithaca, new york

> Leeway on a hull is added drag and loses VMG.


Rick Willoughby
03 9796 2415
0419 104 821

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