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Increasing mast rake in stronger winds

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redback View Drop Down
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 16 Sep 04 at 1:57pm

Getting back to Andi's rig tensions.  I believe the kicker is terminated very close the the bottom of the mast on a RS200, there will therefore be little extra tension applied to the forestay.  Just look at the Laser - because the kicker is actually terminated on the mast it does nothing to pull the mast back - it simple bends it.  My Enterprise however has the kicker terminated at the bottom of the bulkhead which gives considerable leaverage over the forestay of its deck stepped mast.

Like Andi my suspicion is that the increased kicker bends the mast and thus shortens it, which is why the shroud tension is reduced.  Mainsheet however should increase the forestay tension, but since it is applied through a flexible lever (the mast) it doesn't increase it by very much.

On larger boats they use runners to tension the forestay.  This whole discussion is very enlightening, since I have just twigged that my kicker on my Laser 4000 does virtually nothing to tension my jib luff.  What my kicker can do is break my lower shrouds but that's another story.



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Post Options Post Options   Quote Andi Quote  Post ReplyReply Direct Link To This Post Posted: 16 Sep 04 at 9:52pm

You are correct the kicker in an RS200 is terminated right at the mast foot so there is no direct effect on the jib luff wire tension when you pull the kicker on.  I would have thought mainsheet tension would have affected jib luff wire tension but apparently not.  When you think about it though you have a 2:1 mainsheet system with say 35 pounds sheet tension.  Multiply by 2 = 70pounds.  This however is applied to the the end of the boom and then to the upper mast via the mainsail leach which gives a mechanical disadvantage ( mast is longer than boom ) of say 2:1 giving 35 pounds again.  The mast pulls the forestay at an angle which gives again a mechanical advantage of !!guess!! 2:1 which should give an increase in jib luff wire tension of 80 pounds.  Phew!  BUT, the mainsheet causes a lot of mast bend on a 200 which I have measured as reducing shroud tension by 80 pounds ( each side ) 160 pounds reduction in shroud force applied to the mast at a disadvantageous shroud angle ( empirically arrived at mechanical ratio of 2.5 : 1 ) gives a reduction in jib luff wire tension of 64 pounds.  This just about totally cancels out the increase injib luff wire tension caused by the aft pull of the mainsheet via the boom via the leach.

I haven't had time to do any proper maths, I need angles measuring and I need to try to remember all my old physics stuff but I think my basic point is valid that the jib luff wire tension is left pretty much unaffected by mainsheet tension because of the two opposing effects.  A stiffer mast would give very different results as the mast would not bend so much when mainsheet tension was applied so it would bend-shorten and loosen the shrouds..  A taller mast or rather higher aspect ratio would also reduce the effect of mainsheet tension on mast bend ( you are pulling back the mast aft from a more disadvantageous angle.)

   - Andi        PS  That lot has done my head in and it has left a major piece of maths work to calculate the effect of mast rake on the inter related angles and how they increase or decrease the opposing forces.  It also might be simply beyond me!

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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 9:10am

Of course the really good sailors get by without the maths and just seems naturally to do the right thing.  Less able guys like myself have to work it all out in the hope that it improves our performance.  What I have noticed though is that it is very easy to concentrate too much on one aspect of sailing to the detriment of overal performance.

Years ago when I moved from the sea to a lake I really got into windshifts and for the 1st season I used to tack myself to a standstill.  However the following season when I had got things back in proportion I did have the boat speed of a sea sailor and the cunning of a lake sailor - enough for club level trophies anyway.

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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 10:24am
Originally posted by Andi

Interesting that the Merlins are convinced about raking as a means to depower. ( I wonder if you mean de-power or mean re-balance the helm? )  .

- Andi

I mean de-power.

 

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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 10:29am
Originally posted by redback

On larger boats they use runners to tension the forestay. 

True up until a few years ago. Modern designs have almost all dispensed with runners because user-error with them results in a broken mast, which spoils your day.

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Post Options Post Options   Quote Phil eltringham Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 2:01pm

I guess (and tell me if this is completely wrong) that the basic idea behing all the 'depowering' techniques is to get the mast to match the luff curve of the sail thuss making the front part of the sail flatter and depowered.  Raking the mast induces more bend (If the base of the mast is fixed - does this apply to the Meerlins??) and so the luff curve of the sail produces less shape or belly in the sail and so less 'power' is produced (also less drag). 

As far as moving the centre of effort of the sail around is concerned I guess in strong winds you are looking to move it lower predominately and also frowards to balance the probable heel on the boat (though it should be flat) and the fact that the rig is now further aft to get the sail flatter.  Pulling the kicker on tentions up the leech (especially the lower half to two thirds) curves the bottom of the mast and flattens the sail further.  The cunningham, especially on sails with a full top batten allows the top third of the sail to 'blade off' as jeffers explained, which will help to move the centre of effort down reducing the sail's heeling moment and thus the amount you have to hike

Having said all of this, there is no substitue for good boat handling, a couple of dodgy tacks or a swim in the breezy stuff will cost you at least as much as not quite having the boat set up 100%. 

FLAT IS FAST!
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 3:09pm
Agreed.  I would like to point out that all this raking the mast backwards and forwards does not affect weather helm very much at all.  A far more significant cause of weather helm is boat heel.  If the rig is over the leeward side of the boat its bound to luff the boat.  Flat is fast - especially when its windy.
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Post Options Post Options   Quote FreshScum Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 4:19pm

What is the thinking behind boats that drop the rig back as they sail into gusts, 470s 505s?

Is it just about acceleration and opening the slot combined with moving the cars back or something else?

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Post Options Post Options   Quote Andi Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 4:35pm

Aft mast rake doesn't do much of what every on is claiming for it.  Lots of people have talked about sail setting techniques and sailing the boat flat, but no-one has put up a significant arguement supporting aft rake in strong winds.

Raking our mast from fully forward to fully aft ( within the range described by all the tuning guides)  moves the top of the mast 300mm.  Centre of mainsail effort therefore moves aft about one third of this.  Centre of effort from jib moves aft hardly at all  This might affect boat balance - weatherhelm slightly. 

With (guess) 20 foot mast this 300mm mast tip movement corresponds to an increase in rake of 2.8 degrees.  This ain't going to cause wind to flow differently enough to depower the rig.  Neither is it going to lower the height of the centre of effort much ( about 8mm I guess!)

With a deck stepped mast the spreaders induce an extra 10mm prebend with the extra rake.  ( negligible when compared with the kicker which when on tight causes about 300mm extra bend )   With a floor stepped mast the extra prebend might get to 20% of spreader deflection ie about 30mm?  Not much then.

The only strong point in favour of mast rake is that the jib clew comes lower giving a lower sheeting angle - more foot tension and more twist but moving the fairlead back does this on most boats.

I'm not convinced about mast rake!!!!

   - Andi

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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 04 at 6:27pm

On my boat (L4000) I have lower shrouds and therefore rake is a quick way of adjusting them.  When I rake the mast back it opens the jib leech and eases the lowers.  It barely affects mast bend aloft.  We have discussed the benefits of opening the jib leech but easing the lowers is doubly beneficial.  1. eased lowers allow more mast bend and this flattens the mainsail and makes the slot wider.  2. eased lowers makes the rig more flexible, it has a more springy feel and this help absorb the gusts which are more violent in windy weather.  Indeed I find the boat goes better even in gusty light winds with eased lowers (which I ease by moving the pins).  It doesn't point as well though and it is less powerfull, but it accelerates much better.

Talking of flexible rigs.  You can get the same mainsail leech tension in two ways. 1. kicker, 2. mainsheet.  Using kicker gives a more accelerative rig more able to respond automatically to gust, less powerfull ultimately but makes life a lot easier for the crew since they don't have to move around so fast.  With the kicker slack and main used for leech tension the helm has to play the main a lot since the boat is less forgiving and the crew is in and out like a yo-yo, however the boat will feel more powerfull in the gusts.  Combine this with mast rake and changing the tension of the lowers and you have a fair few variables to get wrong.

On my boat it is possible to have an upright mast, tight lowers and leech tension by mainsheet and in any sort of gust the boat produces massive heeling force (even in a F2), the crew goes out on the wire, the centerboard stalls and the whole lot starts going sideway.  Those with a more forgiving rig simply sail away from you. 

I remember crewing a RS800 on several occasions when we were already moving with speed and the helm pulled the main in too far.  The boat slowed so quickly it felt like you'd sailed over a mooring warp, it then went into irons and we went swimming.

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