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Wind velocity increases logarythmically above surfaces, including the waters
surface and any obstructions (i.e. cabin tops and decks). Placing the sail
as high as possible avoids surface-induced drag and eddies. I can furnish
references for equations but they can be found in any transport phenomena
text.
Richard
P-15 Sirocco
> ----------
> From: DAVID_KAUTZ@HP-Sunnyvale-om5.om.hp.com
> Sent: Wednesday, April 14, 1999 10:59 AM
> To: ej@blarg.net
> Cc: BOBKALISZ@aol.com; wwpotter@tscnet.com
> Subject: RE: Magellan GPS and Down Haul
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - -
> West Wight Potter Website at URL
> http://www.lesbois.com/wwpotter/
> - - - - - - - - - - - - - - - - - - - - - - - - - - -
>
> In the spirit of maintaining plenty of confusion about the "right" way to
> rig
> the boat....
>
> I've read recommendations to do the opposite - The argument being that the
> wind
> speed increases with distance above the water (laminar flow). These folks
> say
> you should hoist your sail as far as it will go with the downhaul
> released,
> cleat the halyard and then set luff tension with the downhaul or
> Cunningham. The
> same claim is used to justify "Tall" high aspect rigs and fully battened
> mainsails with giant roaches near the head.
>
> Personally, I suspect on rigs the size of the Potter's 6 inches up or down
>
> doesn't make much difference. I'd go with minimizing the risk of getting
> hit on
> the head by the boom
>
>
> Dave Kautz
> P-15 #1632 Tilly Lucy
> Palo Alto, CA
>
> ______________________________ Reply Separator
> _________________________________
> Subject: RE: Magellan GPS and Down Haul
> Author: Non-HP-ej (ej@blarg.net) at hp-boise,mimegw7
> Date: 4/14/99 6:41 AM
>
>
> -
> Best performance is theoretically when the boom (and therefore the entire
> sail) is as low as possible, since the cabin and deck act to reduce the
> amount of wind spilled beneath the sail causing induced drag. I have
> experimented with this and my gut reaction is that indeed it does help. I
> suspect a few of you are going to disagree with me :) This is a cheap
> performance trick not always mentioned in the sailing books since on most
> craft, there are either fixed goosenecks or class rules dictating the
> position of the boom.
>
>
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