Sorry I disagree Alex (unless by straight you mean more upright) in light winds you want flatter open sails which means some bend, no kicker and no cunningham. As the wind picks up you want a straighter mast and wind on the kicker to get the leech working and then as you become over-powered you need to start bending it again, loads of kicker and cunningham.  Rake is different you want the mast more upright in light winds and then rake back as the wind increases.  However, if your rake is adjustable on the water then you rake backwards for upwind and forwards downwind.  If you don't have the ability to change it on the water then you need to set up rake to get the correct compromise between upwind and down for the wind strength.  However, don't rake too much in light winds as it opens the slot between jib and main losing power.

Your class association should be able to give you some general advice.  read Lawrie Smith's tuning your dinghy book, a bit dated now but still a good source of basic general information.

That's a pretty basic summary there are bound to be specific exceptions.

I noticed in the heavy weather of the recent BNMT that all the contenders had let their kickers off somewhat in the gusty 20knot wind.  Tried it, it seemed to help.  I guess it lets the top of the sail blade off depowering the top of the sail and reducing heeling force - but surely it makes the sail fuller too.

Confusing innit.

NB: on the 3000 we raked the mast back to open the slot up to allow our new roachy jib to work better.  An open slot I'm told, is a good thing. 

Not really, because some boats can adjust mast bend and rake independently but still increase rake upwind as the breeze builds. I've never read a convincing account of why increased rake depowers the rig but it clearly does.

I've always found the genal rule 'if it looks nice then it's fast' works with sail trimming.  For example an over sheeted sail with too much kicker looks terrible and isn't quick. Generally speaking it's best to keep looking at your sails and trying to get them sitting the best you can, unless you have gone to the lengths of working out exactly what settings you need for what wind strength.Mast rake and mast bend are obviously quite different things, mast bend affecting how full the main sail is, while mast rake alters the centre of effort of the rig and has a fundamental affect on the balance and handling of the boat, again generally speaking I think more rake will improve upwind performance, and less rake improves down wind speed.

Increasing rake opens the slot (depowering) and move the Centre of effort aft. You can then lift the centreboard slightly moving the centre of lateral resistance aft.

If you get the channce go to one of Adam Bower's talks on rig tuning, he puts all this in a simple and effective way.

My experience is that mast rake makes a boat go better upwind and its not just the opening of the leech of the jib that does it.  Too much mast rake really throws away power and for this reason we always rake the mast back on the 4000 by one click on the shrouds when conditions are at all difficult, this corresponds to bringing the mast tip about 80mm closer to the transom.  Why such a small change makes such a huge difference is a mystery.

As for letting the kicker off when its windy - it doesn't work.  More wind, more kicker.  The kicker makes the rig a lot less inclined to snatch in the gusts and the power more easy to modulate.  If the boat is difficult to handle try more kicker.  If you look at the more modern roachy type of rigs you'll find you can sail with the top of the sail absolutely flat when you apply lots of kicker.  That's exactly what you want in strong winds, not a full ragging mainsail causing masses of aerodynamic drag.

Here's something else to think about.  I have found that if I power up the rig (tighter lowers, less rake and use the mainsheet for leech tension) I can get the crew out on the wire at quite low windspeeds and the boat goes faster for it, but it has very poor gust response.  By which I mean it doesn't accelerate and it is often faster overal to sacrifice a bit of power and get the acceleration.  I can only liken it to driving a car slowly in 5th gear.

It's not obvious why increasing rake would open the slot, or why that would result in depowering. If your boat allows it, try sailing with a very closed slot. More power? I suspect not.

And allowing the centreboard to be lifted, fine except that I can think of classes that, as the breeze builds, will start to rake the rig long before they'd consider raising the board.

I'm not disputing that rake works but the explanations you commonly read don't really bear examination.

Here's a funny thing. There is a keelboat class where 50% of helms are convinced that the way to handle the main upwind is to centre the traveller and ease main and kicker. The other 50% pull on maximum sheet and kicker and play the traveller. Results suggests both techniques are equally fast if done well.

Increasing mast rake opens the slot on most boats as the head of the jib moves aft relative to the rest of the sail. This opens the leech of the jib so the slot has now been opened.

No kicker in dinghys can work. Has to be really windy, and then allows power on/power off to only be a small amount of mainsheet.

Easing the sheet has the same effect with a lot less trouble. So there has to be more to it than that.

If you ease the sheet you won't point.

Garry,

A basic understanding of vectors demonstrate that the faster you go the higher you point.  so why should easing the sheet a little stop me pointing, except in the short term whilst I accelerate?

The practice of increasing mast rake in more wind is about giving the wind a different aerofoil from that used at lower wind speeds.

Rake should not be confused with bend. 

Bladed sails, caused typically by lots of cunningham and less kicker when windy are fast around the course.  This may actually be due to making a lightweight boat easier to sail as the top of the sail is controlled better rather than actually increasing straight line speed, but they are fast around the course .

Maintain some curvature in the sails in the light stuff, don't have bags in the sails but at the same time, don't have them flat. There are two schools of thought with light wind settings, but flat sails to reduce drag theory also kills all lift too and is very slow. Maintaining some curvature maintains a profile optimum for generating lift. High Performance Sailing by Bethwaite and Sail Performance by C A Marchaj, both cover the theory in great depth, Sail Performance is more specific. Best to read a book about it instead of trying to understand bits and pieces from a forum if you are really interested. We don't adjust mast rake on the 49er as we have a fixed forestay but in the light stuff we don't bend the mast extensively to move the centre of effort aft as is the effect with raking as it is a depowering method, probably why Alex was saying keep the mast straight initially. If you bear in mind what Ian was saying about wings on the 747 and Concorde, it's a good analogy of sail shape. The 747 wing shape with more curvature is designed that way to generate maximum lift to carry a high payload whereas concorde's wings are very flat with the aim to minimalise drag. The 747 shape will help on light days to maximise lift at lower speed and on the windier days when you have plenty of lift the aim is to flatten sails off to reduce drag.

My understanding of what Stefan was saying is easing the sheet opened the slot to do that he must be talking jib.  Since this thread is about depowering by having an open slot caused by raking the mast, to get the same effect either your jib is lifting at the luff or you've had to bear away, probably a considerable amount.  Yes you would accelerate,if you bore away but as soon as you sheeted in again and came up you would be overpowered and... Also a small ease of the jib greatly increases the entry angle, everything else staying the same.  Of course moving the jib lead aft is a different matter and also opens the slot by creating twist in the jib.

And less kicker more cunningham means you loose control of the top third of the sail, not get more, as a small ease on the mainsheet becomes a big ease of the top batten. 

The only reason they would be letting the kicker off is to stop the boom burying as they see-saw around in the gusts or to get under the boom and stop sticking in irons in a tack. The Contender get pretty uncomfortable to sail with little kicker in a breeze as the sheet loads increase significantly and when you sheet out in a gust you dump all the power at once (not good).

Actually the objective is completely different. Aircraft flying near the speed of sound use swept wings in order get the wing and engines working behind the shock wave generated by the nose cone. See http://en.wikipedia.org/wiki/Swept_wing - http://en.wikipedia.org/wiki/Swept_wing  for a longer explanation. This is not relevant to sailboat design just yet. 

So you think opening the jib leech by easing the sheet stops you pointing but opening it by raking the mast doesn't. Why? 

Most classes do in fact ease the jib-sheet at some point as the breeze builds. Some keelboat classes have tuning guides telling you how far the leech should be off the spreaders and these typically call for the sheet to be eased slightly in stronger winds.

Good topic guys.  Fascinating to see the different interpretations and opinions from experienced sailors. Reinforces my opinion that every setting has a drawback (or two...or three......).

Actually, reading this is very helpful if you are struggling, like me, to understand and come to terms with the basic principles involved. There are plenty of us who are a long way from reading and understanding advanced level sailing literature!

Sorry, I tend to disagree, the flat sails stall quicker as the flow de-attaches quicker where as with the curved sail, the flow will follow the shape of the sail better. 1/10th camber seems the optimum setting according to some texts for light conditions. 1/20th camber produces less lift according to research but has a relatively similar drag profile, indicating that you get less lift for comparible drag. 1/7th camber, a fuller profile than 1/10th sees more lift but also has the penalty of more drag. If you put a flat plate into a wind tunnel, it stalls quite rapidly, the curved foil maintains a lift profile quite convincingly in comparison. Increasing the flow of wind through the tunnel, ie increasing wind speed helps at higher speeds to maintain a flow over the flatter sections, so flat sails don't help at all in light winds, has the same effect as oversheeting the main in essence.

(Camber is the ratio of sail length to depth)

you have got it sorted.  substantial differences between types of rig means that most of the above rake comments could easily be misinterpreted and applied to the wrong boat.

similarly you might want to flatten your sails in light winds as your sails are very full (eg you have an unstayed rig eg Europe/OK) and the wind will not have enough energy to get around a full sail.

you need to make the original question more class or at least type of rig generic as this will be key to understanding.  Best of all discuss with your sailmaker.  I realise that if you have SMOD sails made in the Philippines or similar this is a useless piece of advice but if that is the case it probably means you just have to look at what the fastest people in the Class do / ask them as you can surely have no alternative but to do the same.

really low windspeed=near as dammit nothing?

because if thats the case im going backwards anyway because of the tide.

So, from a 505 perspective (suspect that many other classes will have a different approach) you are essentially attempting to keep the sailshape the same in most conditions.  There are 3 zones.  below 8 knots; below 12 knots; and above

8/9 knots - 12/13 knots or thereabouts is your fully powered sector.  Rig fully upright; max power, no frills.

Less than 8 knots (as the crew comes off the trapeze, and the wind gets a bit holier) you need to replace the mast bend....so more shrouds, and less strut.

Less than 4 knots - the only time when you are seeking to distort the sailshape.  Loads of shrouds - 420lbs - and destrut (upwards) to bend the mast, and make an easier shape for the wind to stick to.

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And upwards from 13 knots:

Steadily rake backwards to depower, following the rake with increasing amounts of strut (down).  If you don't pull enough strut down, you will allow the middle of the rig to "pant", which unbalances the rig, and makes the hull feel twitchy in the water.

In extreme conditions, fully nail the kicker down (we last broke a boom on Lake garda in no waves...!!) to bend the top mast, and flatten the rig....or blade out the trailing edge of the mainsail.

There are other subtleties (jib cars, flatners, cunningham, jib cunningham, centreboard etc) and obviously wave state plays a large role in deciding which gear to step into.  However, rake in this class depowers..!

Chas