mast....

[jason]

hi sukhdev and  everyone out there
my friend was telling mi tt he read somewhere tt in overpowered conditions  u  need to change to a stiffer mast
now tt got me really confused..shouldn't it be the case where u  need a softer mast in overpowered conditions?
and he was saying something like  depending on the degree of being overpowered..is there such a distinction?
so is it the case of a softer mast giving u  less power or does a stiffer mast do tt?
i always tot a softer  mast gives less power as it can spill the wind more as compared to a stiffer mast

[sukhdev]

Very good question, something that doesnt have a very clear answer.

First of all the right mast for that sail will give the most even distribution of luff curve (the bend) and leech tension (how floppy the sail is when downhauled).

Take this as a baseline, for example using a 460 on a 7.5 gives the most balanced feeling for top end and bottom end.

When you are overpowered a softer mast allows for less leech tension, the sail thus twists more and sailor can still get power out of it without sheeting out. If the mast is stiff, at this point you will have to sheet out.

Generally speaking a softer mast gives more control and a stiffer mast gives more power. I like to use a 490 on the Sailworks Retro 7.5 (recommended is 460). It feels fine for me at my 85kg but for Wee Goon at her 53kg its too stiff when the wind picks up, so she overpowers early.

The mast is an important tuning element. I've even tried things like putting a 430 top on a 460 bottom when sailing with wave sails in very overpowered conditions (it works).

The most important thing however is downhaul. Very very few sailors get this right.

The best quote regarding mast stiffness is from Phil McGain, the most fussy sail tuning person I ever met and the best

"I think in broad terms a softer mast will be easier overpowered and stiffer mast could be better underpowered.
Stiff mast gives more shape and tighter leech. soft mast the opposite".

There is a long thread on this subject on the gaastra forum at:

 http://forums.gaastra.com/bepartofit/ubb/Forum1/HTML/002977.html

hope this helps...
 
 [ 05. January 2003, 01:20 AM: Message edited by: sukhdev ]

[<Tom>]

I have been using a 460 mast for my Gaastra 6m and 7.5m sails. I am now looking at getting another sail, something like a 2000 Le Nitro 8.7, which needs a 490 mast. Unfortunately at the moment I don't have a 490 mast and was thinking of getting a 400 extendo.

I have been reading several posts on mast stiffness and its effects on sail performance and am concerned about reducing the sails range too much.

Would I be better to look for either a top or bottom from another mast, 490 or 520, rather than an extendo,  to extend my 460, and if so, what would you recommend. I am 80 kg and 6'2" tall.

[bwkps]

Hello, in addition to mast imcs stiffness for sail 'floppiness', I also have a question about the adjustable outhaul system and outhauling in general. I don't have one yet, but from diverse discussions I've been getting diverse opinions, and I am right now a bit confused.

Basically, the impression I get is that the adjustable outhaul is used to adjust the amount of leech tension in the sail, and also affects the draft. So when you pull the outhaul, does the leech tension increase? It seems to me that you get a 'tighter' sail- in fact I recall that the IMCOs use this flat sail configuration for upwind sailing. That would imply that the leech is less floppy and thus less likely to spill wind. Also, it appears to me that the draft is shallower. Is this true, especially for more modern sails? If so, a shallower draft would mean less power, wouldn't it? So what I'm getting is a sail with less draft but a tighter leech- what does this translate to? Conversely, when I release the outhaul, by the same argument there will be a looser leech but deeper draft. This seems bizarre - I know there must be a flaw somewhere in this reasoning, probably got something reversed somewhere, so I'd be grateful for any help here...

[sukhdev]

Tom,

I've never tried a shorter bottom and longer top (say 460 bottom, 490 top) though i've done it the other way around. If you can borrow a piece, I'd suggest going with the longer bottom, though these days on some brands ferrule fit between different sizes is a problem.

You are big guy, I'm about your height and weight. If anything I tend to use stiffer than recommended in that sail size range, like, I tested the 2003 GTX with a 520 instead of the 490 and I love the more powerful feel. A 2nd hand 490 may be your best bet..otherwise, try out a longer bottom and shorter top. My opinion is whichever hybrid you go with you will end up with less stiffness than you need and at your ht/wt that might be an issue.

[sukhdev]

Hi bwkps..my comments below

On modern sails, downhaul sets the overall leech tension. Adjustable outhauls allow the draft to vary. A lean foil is more efficent, and in my opinion actually does not penalise you for early planing as much as people think.

When you pull the outhaul you get a shallower foil.Crudely, a thick foil has high lift but high drag as well, whereas a lean foil will have a lift/drag ratio that does not degenerate as the apparent wind increases.

Its all about apparent wind, lift and drag. You go faster as you go faster (apparent wind). A huge deep foil might have lots of initial pull but will very quickly overpower. Once there is too much back hand power you can no longer sheet in and trim the sail for its best lift/drag performance (in my half baked theory at least).

A large flat sail is always faster than a small full sail.

Releasing the outhaul does not have big effect on leech tension, thats defined by downhaul. Forget "spilling wind". There are 3 reasons you need twist (ie downhaul) in a sail:
- wind gradient (wind at top of mast is different angle/speed from near the water surface).
- increase the wind range. Force generated by the sail is equal to the square of the wind velocity. This is clearly non linear. Other factors affecting the force on the sail are sheeting angle. Maintaing the sheeting angle while allowing mast bend and leech twist to work allows this non linear response to be smoothened out.
- lastly the twist allows tip stall to be avoided by reducing the angle of attack at the head (eg planes do this by using winglets at the tip).

Bottom line from all that heavy stuff..get the downhaul correct, and then use outhaul only to change foil depth. Rule of thumb, lean foil for upwind, draft for downwind.

My mantra
- lean foils are fast
- downhaul, downhaul, downhaul
- masts are tuning tools
- Adjustable outhauls are gear shifts.
- A lean drum taut foil can pump better than a full foil without lots of tension in its shape.
- small adjustments (1cm) make a big difference.

I know i've thrown a lot of theory at you, there is a marvellous work on this subject by Frank Braithwaite - book called "high performance sailing".Its about skiffs but alot of excellent sailing theory in there..if you are in singapore Kinokuniya at NgeeAnn has it in stock (whoever borrowed mine return it please!!).

[bwkps]

Thanks Suhkdev, will look up the book at kino sometime.