Boom Spar Curvature

Discuss measuring an IOM and being a measurer

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Andy Stevenson
GBR NCA Officer
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Boom Spar Curvature

Post by Andy Stevenson » 04 May 2006, 12:51

IOM CR C.7.5 wrote:Boom spar curvature measured between points on the top of the spar 10mm from each end.................... Maximum 3mm.
Boom spar curvature is in bold text, so we must look at ERS 2001 – 2004 for the definition...
ERS 2001 - 2004 F.12.2 wrote:The shortest distance between the top edge of the spar and a straight line between the outer point and the top of the fore edge of the spar at a specified distance from the outer point, when the spar is resting on one side.
Several things strike me as not quite right here.

1] I can find no reference to outer point in the IOM CR. Given that, one can’t expect to find an outer point on an IOM boom spar. That would seem to make the ERS definition difficult to measure.

2] ERS F.12.2 states “top of the fore edge of the sparâ€
Andy Stevenson
"A little pain never hurt anyone!" Sam, aged 11

ralph kelley
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Location: USA 41

Post by ralph kelley » 05 May 2006, 20:35

Without tension of any rigging

But why use such a small spar? The larger diameter arrow shafts are very light too, and are likely to be easier to outfit. The additional stiffness of the larger diameter tube should make it easier to perform sail adjustments since the spar is essentially rigid

Ralph

Andy Stevenson
GBR NCA Officer
Posts: 772
Joined: 15 Sep 2005, 13:08
Location: UK

Post by Andy Stevenson » 05 May 2006, 21:45

Thanks Ralph.
But why use such a small spar?
The price was right (free), and it struck me as a reasonable experiment.
...larger diameter tube should make it easier to perform sail adjustments
Actually, the rig is reasonably easy to tune. I’ve only sailed it once, in very light wind, but my initial impressions are good.
Andy Stevenson
"A little pain never hurt anyone!" Sam, aged 11

Andy Stevenson
GBR NCA Officer
Posts: 772
Joined: 15 Sep 2005, 13:08
Location: UK

Post by Andy Stevenson » 12 May 2006, 18:24

For anyone interested, I sailed with this on Sunday, in top of A rig conditions (a little past on occasion) with some success. And it sure is light!

Image

Cheers
Andy Stevenson
"A little pain never hurt anyone!" Sam, aged 11

Steve Landeau
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Joined: 26 Nov 2003, 08:25
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Post by Steve Landeau » 13 May 2006, 16:54

I have always found it difficult to "predict" how much the lighter jib booms will bend while under way, making it difficult to get correct jib twist. With a boom that flexes, you're either too flat in the light stuff, or are carrying too much twist in the puff. I find the same truth with my mast. I use the Easton 12.4 mm mast, and our local group have dubbed it "the tree trunk". Much simpler to set up. No pre-bend to worry about. It works for me!
I know many top skippers continue to use the light boom with great success, but I have found it much simpler to use a boom with zero flex.
This is quite typical with the IOM though... there are many ways to make it work. Some ideas will work for one skipper, but not for another. In the end, most boats end up very close to equal when sailed and tuned properly.
Steve Landeau
AMYA 10859
IOM USA 112
Finn USA 112
Cal 25 #548

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