Mast stiffness and sail shape question
What are the consequences of using a stiffer vs softer mast in a sail? Equivalently using longer extension to substitute for longer mast. Many sailmakers give option of say using 460 mast or 490 mast with longer extension in a given sail size.
Does the stiffer mast make the draft shallower or deeper when rigged correctly? What is the tradeoff as far as improved low end or top end when going with stiffer/softer masts?
I've heard of heavyweights using stiffer masts. Is that to create a deeper draft to get the heavier sailor going, or is it to reduce the rigs deflection as the heavyweights greater mass moves in gusts or through chop.
I guess the first part of my question refers to how a sail responds to mast stiffness in a static sense - how does the sail look when rigged. Would you apply less downhaul when using the stiffer mast?
And the second part refers to the dynamic aspect - does the sail feel soft or stiff when pumping or when sailing through chop.
I'm most interested in the effect on race and salom sails. It's interesting how North tells you to downhaul until there's a crease at a marked point on the sail. Maui sails tells you to downhaul to a specified measurement relative to the extension. So, if you substitute a stiffer mast into a North sail, would you still downhaul to the same point on the sail?
Thanks for any insights!
There are probably several factors at play. Using a shorter/softer mast with long extension may result in a bit more top flex. Air release at the head will be easier without affecting the lower body too much. Adversely a longer/stiffer mast will give a steadier profile but also a more direct, harder feel. Not necessarily more low end power.
The more downhaul I give the sail, the flatter it gets and the leech gets looser. That's a relationship I understand clearly. Less downhaul, makes the sail fuller and the leech tighter which gives it more power.
I still don't understand the effect of stiffer mast regarding the looseness of the leech and the depth of the draft.
II think that harder or stiffer masts do not directly relate to the static aspect - how does the sail look when rigged. For instance, the Loft sails can be rigged with either the stiffer SDM mast or the softer RDM mast. When rigged, those sails look quite the same. It's when they are used that you can feel a rather different behaviour. The loft sails recommend - especially for the larger sails - SDM masts for heavier riders and RDM masts for lighter riders.
The bend curve of the mast is more directly related to the static aspect, although it will also have it's influence on the dynamic aspect of the rig.
The bend curve can change seriously when using a longer extension.
In general, like already said by Farlo, longer extensions tend to give the mast more flex top.
While downhauling sails on Flex top masts, they release generally more the head (more loose leech) while changing less to the lower body (deep belly). That's the theory. The sail design cut needs to accompany this of course.
Wether a sail will feel softer or harder is again a combination of a lot of factors. In general, the more a sail can release the excessive air flows (more twist/flex), the softer it gets.
No too long time ago, there was an amazing thread on the Dutch windsurf forum about all this stuff, also referring to the recent speed contest in Luderitz.
For those of you who understand Dutch, I can highly recommend this to read.
Thanks, BelSkorpio - through the magic of google translate I can try to understand that EPIC thread...
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