Backstay Windvane

For  those BCC owners that have a backstay windvane, has the boom every hit the vane when the boom comes across the boat during a gybe?  This happened to the Pardey's and the vane was damaged when they were they rounded Cape Horn.  Base on lift and moment arm calculations, for the vane to generate enough lift to turn the partially balanced trim tab, the cord width of the vane is such that it overlaps the boom.  Can anyone provide any insight into this possible problem?
 
The attached image is a preliminary "sketch" of a backstay vane - excuse the calculations.  In this sketch the boom would hit the vane if it lifted.  The leading edge of the vane is about 6" offset from the backstay.  The vane's cord is 6".  The power developed by this arrangement of the vane is higher than if the vane's leading edge was next to the backstay and the vane's cord width was increased to 12".
The backstay windvane is discussed in John S. Letcher Jr.'s book, Self-Steering for Sailing Craft, p.141, published in 1974 by International Marine Publishing Company, ISMB 0-87742-042-4.
 
"Fair Winds and Clean Fuel"
 
Rod
S/V IDUNA

— Rod Bruckdorfer <seagypsy@att.net > wrote:

For those BCC owners that have a backstay windvane,
has the boom every hit the vane when the boom comes
across the boat during a gybe? This happened to the
Pardey’s and the vane was damaged when they were they
rounded Cape Horn. Base on lift and moment arm
calculations, for the vane to generate enough lift to
turn the partially balanced trim tab, the cord width
of the vane is such that it overlaps the boom. Can
anyone provide any insight into this possible problem?

We have a Freehand Steering System windvane on the
backstay of ZYGOTE and yes, the boom has struck the
windvane at least twice when gybing. Both times were
in heavy conditions and we could have done a better
job of controlling the boom in each of those gybes. I
don’t think it has ever happened when we have done a
smooth, well-controlled gybe. So I suspect the meeting
of boom and windvane can only happen when the boom
moves fast.

In one of those gybes, the thin plywood sail of the
windvane cracked - but the windvane still functioned.
I didn’t get around to replacing the plywood sail for
some time after that, because it wasn’t an issue
preventing the windvane from working. And the boom
sustained no damage!

Cheers

Bil

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Bill:
 
Thanks for the information.  I am debating whether to use the backstay windvane or a windvane on a tower.  The backstay windvane is simple and aesthetic, whereas the windvane on a tower is significantly more powerful and probably works better in winds of 10 knots or less.  Regardless of which system I build, the windvane will have a NACA 0010 to 0015 airfoil section, i.e. an efficient lifting surface and the trim tab will be 75 to 90% balanced.
 
Please  find attach an image of POLARIS JACK leaving Tripp Creek the morning after the BCC Rendezvous.  AO built the windvane and he reports it works very well.  Note the vane's flat lifting surface.  A flat lifting surface will develop lift but not as efficiently as a NACA airfoil section.  The reason the system works is because the trim tab is almost perfectly balance.  The trim tab is a flat piece of steel.  My conclusion is, if the system has very little mechanical friction and the boat is balanced under sail, the system will work to some degree in most cases regardless of the amount of "engineering" that went into the system.
 
Can you provide some insight into the light air performance of your Freehand Self-Steering System when broad reaching and beam reaching in winds of 10 knots or less?  This would be most helpful in my decision making process.
 
"Fair Winds and Clean Fuel",
 
Rod
S/V IDUNA
----- Original Message -----
Sent: Friday, August 02, 2002 1:34 AM
Subject: Re: [bcc] Backstay Windvane

--- Rod Bruckdorfer <seagypsy@att.net > wrote:
>For  those BCC owners that have a backstay windvane,
has the boom every hit the vane when the boom comes
across the boat during a gybe?  This happened to the
Pardey's and the vane was damaged when they were they
rounded Cape Horn.  Base on lift and moment arm
calculations, for the vane to generate enough lift to
turn the partially balanced trim tab, the cord width
of the vane is such that it overlaps the boom.  Can
anyone provide any insight into this possible problem?

We have a Freehand Steering System windvane on the
backstay of ZYGOTE and yes, the boom has struck the
windvane at least twice when gybing. Both times were
in heavy conditions and we could have done a better
job of controlling the boom in each of those gybes. I
don't think it has ever happened when we have done a
smooth, well-controlled gybe. So I suspect the meeting
of boom and windvane can only happen when the boom
moves fast.

In one of those gybes, the thin plywood sail of the
windvane cracked - but the windvane still functioned.
I didn't get around to replacing the plywood sail for
some time after that, because it wasn't an issue
preventing the windvane from working. And the boom
sustained no damage!

Cheers

Bil

http://digital.yahoo.com.au - Yahoo! Digital How To
- Get the best out of your PC!



Fritha also has a freehand steering system and the boom and vane touching has not been a problem except when the vane was reefed and the line on the vane hooked on the boom.  Fortunately I perform controlled jibs so when I noticed it I sheeted the main back in and separated the line from the boom.  No damage.  From now on I'll tie the line differently so they can't touch.
 
Doug Beu
"Fritha"
 
----- Original Message -----
Sent: Thursday, August 01, 2002 6:03 PM
Subject: [bcc] Backstay Windvane

For  those BCC owners that have a backstay windvane, has the boom every hit the vane when the boom comes across the boat during a gybe?  This happened to the Pardey's and the vane was damaged when they were they rounded Cape Horn.  Base on lift and moment arm calculations, for the vane to generate enough lift to turn the partially balanced trim tab, the cord width of the vane is such that it overlaps the boom.  Can anyone provide any insight into this possible problem?
 
The attached image is a preliminary "sketch" of a backstay vane - excuse the calculations.  In this sketch the boom would hit the vane if it lifted.  The leading edge of the vane is about 6" offset from the backstay.  The vane's cord is 6".  The power developed by this arrangement of the vane is higher than if the vane's leading edge was next to the backstay and the vane's cord width was increased to 12".
The backstay windvane is discussed in John S. Letcher Jr.'s book, Self-Steering for Sailing Craft, p.141, published in 1974 by International Marine Publishing Company, ISMB 0-87742-042-4.
 
"Fair Winds and Clean Fuel"
 
Rod
S/V IDUNA



— Rod Bruckdorfer <seagypsy@att.net > wrote:
Rod: Hi!

AO built the windvane and he reports it works very
well. Note the vane’s flat lifting surface. A flat
lifting surface will develop lift but not as
efficiently as a NACA airfoil section. The reason the
system works is because the trim tab is almost
perfectly balance. The trim tab is a flat piece of
steel. My conclusion is, if the system has very
little mechanical friction and the boat is balanced
under sail, the system will work to some degree in
most cases regardless of the amount of “engineering”
that went into the system.

Our Freehand Steering System also has flat foils - a
narrow plywood foil for heavy conditions and a
sailcloth extension for lighter conditions. I don’t
know the aerodynamics, but I think you’re right and
that an airfoil section is not needed.

Can you provide some insight into the light air
performance of your Freehand Self-Steering System when
broad reaching and beam reaching in winds of 10 knots
or less? This would be most helpful in my decision
making process.

I think you need insight from someone who has used
different types of windvane on the same boat.

I think reaching and dealing with light winds,
especially if a swell is running, are about the most
difficult conditions for a windvane. In light and
variable winds, I prefer an experienced live
helmsperson, who can watch the surface of the water
and steer proactively (or at least steer to keep pace
with the wind). The windvane is always reactive. As
second-best, I use a tiller pilot on the tiller, and
ignore the variability in the wind (I’ve tried using a
tiller pilot steering to wind information from the
masthead wind instrument, but I’m not happy with the
results to date). Third-best, to me, is using a tiller
pilot on the trim tab. Fourth-best is the wind vane.

Cheers

Bil

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