What does mast rake do?

Originally posted by giraffe 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? [/QUOTE 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

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. 

Edited by Garry

There is some confusion about flat and full sails in light winds.  Generally the fuller the sail the more power it generates - hence we start flattening it when we get overpowered.  But at really low wind speeds (a drifter) the full sail that you would use in light winds stalls too easily and in such light winds this is difficult to detect.  Hence the rule is:- full sails for light winds but in extremely light winds a flatter sail is better.  Personally I think the bar is better.

Originally posted by Garry Originally posted by Stefan Lloyd 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.

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.

Edited by Stefan Lloyd

Originally posted by redback But at really low wind speeds (a drifter) the full sail that you would use in light winds stalls too easily and in such light winds this is difficult to detect.  Hence the rule is:- full sails for light winds but in extremely light winds a flatter sail is better.  Personally I think the bar is better.

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)

Originally posted by radial179102 Depends entirely on the boat. For example, the Europes boom is on the deck all the time up wind, so raking the mast back opens the leech and spills wind, raking it forwards makes the leech very tight and helps pointing.

 

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.