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 Post subject: Sail/Mast measurement
PostPosted: Wed Oct 13, 2010 2:43 pm 
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Joined: Tue Oct 02, 2007 10:42 am
Posts: 83
To the chief measurers,

assuming I would have a sail that runs in an internal sailtrack of a rotating wing mast and the track is so far up front in the mast, that maybe 100mm of the sail span is inside the mast. Would this sailarea inside the mast be wasted, because the mast area is measured as well?

Best regards,
Arne

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 Post subject: Re: Sail/Mast measurement
PostPosted: Wed Oct 20, 2010 11:18 am 
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To be sure we are on the same wavelength, a drawing would help.
My view would be to measure the projected sail area only. I would measure soft sail from the trailing edge of the mast, just as we do with all sails now ( eg exclude the bolt rope.) I would deduct 75mm x mast length as set out in the rules from the final calculation together with the 0.6 square metres
The sole object in the measurement is fairness - measure the projected area, less allowance for mast of 75mm.
Any other views welcome please.


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 Post subject: Re: Sail/Mast measurement
PostPosted: Wed Oct 20, 2010 8:22 pm 
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This is what I was thinking about (just a rough sketch to show my problem):
The track is deep inside the mast or has a fairing at the back to allow more bending for a wingmast.
Image

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 Post subject: Re: Sail/Mast measurement
PostPosted: Thu Oct 21, 2010 4:53 pm 
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I would measure the soft sail from the exit point of the mast. To do this, with the sail rigged and hoisted inside the wing mast, I would run a pencil line down the sail cloth as close to the trailing edge of the mast as possible. Then measure area as set out before.
Any additional comments welcome.


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 Post subject: Re: Sail/Mast measurement
PostPosted: Thu Oct 21, 2010 5:08 pm 
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Joined: Fri Dec 22, 2006 10:32 pm
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Acting as devils advocate - wouldn't the mast section behind the sail rear of a line at 90 degress to the sail bolt rope be classified as a fairing as it is not actually part of the structural needs of the mast and should therefore be measurable?

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 Post subject: Re: Sail/Mast measurement
PostPosted: Fri Oct 22, 2010 11:31 am 
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Er......... not sure what you mean here Steve , can you either explain again or ring me.

Colin


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 Post subject: Re: Sail/Mast measurement
PostPosted: Sat Oct 23, 2010 10:32 am 
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No forget it. I see that the rules are fairly clear, the area of the mast which extends beyond the 75mm depth is measured and must be included in the calculation for allowed sail area but if iot overlaps the sail I guess it makes no difference.

Interesting though Arne if you look in Bethwaites recent book he now believes that thin round sections are practically better as the wing shape mast is only efficient as long as the air flow around the mast is correctly aligned to the apparent wind for which you need to rotate the mast.

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 Post subject: Re: Sail/Mast measurement
PostPosted: Sat Oct 23, 2010 1:24 pm 
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SteveC wrote:
[Bethwaite] now believes that thin round sections are practically better...

Umm, that's a bit of an over simplification Steve.
I just wrote (while waiting for some big photos to scan) 1100 words of reply to this which was still about half complete so perhaps I'd better find another medium for my pontifications on the subject.

Suffice to say I've got some experience playing with wing masts, I own one of Bethwaite's original wooden ones, and whilst I think that the Canoe platform would be a good one for an overrotating wing mast the sheer amount of development work required puts me off.

Arne, I would applaud and do my little to help anyone who wanted to take on a development project on this scale. I can tell you more about things that haven't worked than things that have though.

I reckon with finger in the air that you ought to reckon on building at least three complete spars, plus a lot of gluing on and sanding off of unidirectionals before you get one that really works.

I would be very wary of a pear shaped section, I think you probably want something with a finer leading edge.

I think you're on exactly the right track by seeking to separate the aerodynamic section from the mechanical section, but I would be inclined to put the fairing on the front and the structure and track at the back.


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