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Old 3rd April 2009, 07:16 AM   #1
johnk
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Question Short mast w/ long extension vs long mast?

Hi Roger,

I have always been curious about the performance differences of using a shorter mast with long extension verses a longer mast with short extension. My 2000 Retro 9.5 for example is rigged on a 490 with a decent amount of extension. I inherited this sail with a 520 mast that subsequently broke as you may recall. I've not really been able to discern any significant difference between the two masts other then the 490 feeling lighter.

I've recently acquired a new old stock Gaastra Swift 10.0 which has a 526 cm luff and calls for a mast with 34 IMCS. What performance would I be giving up to use a 490 cm mast with 30 IMCS instead of a 520 with 34 IMCS?

Thanks in advance,
John
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Old 3rd April 2009, 08:16 AM   #2
Roger
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Hi John,
I do not have any personal experience with the 10.0 Gaastra Swift, but my educated
guess would be that you would get significanlty more top twist (the sail is much looser and the looseness will extend in much closer to the mast) and a flatter mid section if you use the softer 490 mast in place of a stiffer IMCS 32-34 520 mast.
I've done a bit of testing using stiffer/softer tops on shorter masts to "tighten up" the top section of several sails to give more shape and power and that has been my experience.
So, if you put more extension in the bottom, it does nothing to stiffen the top section of the mast.
So, more twist/ less power up in the top panels of your sail.
Yes, you can use less downhaul, and do some "creative rigging" and you might like the results, but the Swift has 2 cams, and you need to get the downhaul pretty near the design optimum if you want the cams to fit/rotate correctly.
Give it a try if you like, but also remember that if you get too much extension in the bottom of the sail (sounds like at least 36 cm for this sail on a 490 cm mast) you begin to run the risk that you will "point load" the mast at the end of the extension and the mast could break at this point. I've seen over extended masts break in this manner a significant number of times.
Also, if you can borrow a good 100% 520 mast, be sure to give that mast a try in the
10.0 Swift.
I think you will find it has more power, and the overall balance will be better.
Hope this helps,
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Old 3rd April 2009, 08:47 AM   #3
johnk
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Thanks Roger for the prompt response! Would the same thing apply for my 2000 Retro 9.5... that a 520 would provide more shape and power?

Thanks again,
John
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Old 3rd April 2009, 10:23 AM   #4
Roger
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Hi John,
It would apply if your 2000 Retro was as "twisty" as the newer Retro models.
If the recommended mast is written on your 9.5 m2 2000 Retro, I'd follow that
guideline.
I have not been able to find the rigging spec's for the 2000 Retro, so I'm unsure if it took a 490 or a 520 as the "best mast".
You 2000 Retro had a much tighter leech than later versions, so if it's working well on the 490 cm mast, with good handling and an appropriate amount of twist, might be better to continue sailing it that way.
I think we decided earlier that the recommended mast was a 490 cm, didn't we?
The amount of twist, and the stiffer IMCS 32-34 520 masts have affected the design of
the later Retro sails.
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