So what's with mast rake?
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Category: Dinghy classes
Forum Name: Technique
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URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=7315
Printed Date: 27 Jun 25 at 2:59am
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Topic: So what's with mast rake?
Posted By: ellistine
Subject: So what's with mast rake?
Date Posted: 24 Oct 10 at 9:19pm
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It's one of those things that I've just accepted. The tuning guide says we should have a more upright mast in the light conditions and a more raked back mast when it's windy. I can see why an upright mast might help in the lighter conditions but what's the thinking behind raking the mast back when it's windy? What are we trying to achieve? All I know is today our mast was raked back further than it's ever been (partly from stepping the mast in a hurry and not rechecking the pins) and with the wind in the upper teens we were stonking along. Properly planing up wind and even won the race both on the water and on corrected and that's never happened!
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Replies:
Posted By: G.R.F.
Date Posted: 24 Oct 10 at 9:42pm
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Well I think its probably the same reason as in windsurfing, which is that in increasing wind, the centre of effort of the sail moves aft so raking the rig back re-addresses the balance over the Centre of Lateral resistance, it also depowers the rig a tad. With boats that can retract their centreboard, that is also a reason to rake the rig back. Also in strong wind the sail isn't always sheeted in as far, so the entire rig isn't in play, raking it back keeps the c of e over c of lr balanced, probably more important on single handers. ------------- https://www.ease-distribution.com/" rel="nofollow - https://www.ease-distribution.com/ |
Posted By: JimC
Date Posted: 24 Oct 10 at 11:17pm
| To be quite honest the more I think about it the less I understand it... There are all sorts of things going on on a conventional dinghy from jib sheeting angle to the effect of the spreaders. I have a hunch that CLR is the least of it, but could easily be wrong. |
Posted By: blueboy
Date Posted: 25 Oct 10 at 6:14am
Yep. I've never read a convincing explanation as to why raking a rig depowers it. All I know is that it works. |
Posted By: blueboy
Date Posted: 25 Oct 10 at 6:19am
The trouble with that theory is that keelboats also increase rake in stronger breeze. Also Merlins, for example, rake the rig long before they start raising the board.
That's 100% the wrong way around. If you ease the main-sheet, the luff depowers earlier than the leech. Therefore CoE moves back, so to restore balance you'd decrease rake, not increase it. |
Posted By: Fearfull
Date Posted: 25 Oct 10 at 4:44pm
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I have always thought there are two main reasons why I rake: 1. Center of effort of the sail moves forward as breeze increases, especially as the leech is doing less and less, so I rake to move this back to where it was. 2. As I rake the angle the shrouds are at makes the mast bend more. This is more noticable in rigs with raked spreaders. 3. Less tendancy to nose dive! Third is a bonus reason. |
Posted By: lynn1221
Date Posted: 26 Oct 10 at 10:34am
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All I know is today our mast was raked back further than it's ever been
(partly from stepping the mast in a hurry and not rechecking the pins)
and with the wind in the upper teens we were stonking along. Properly
planing up wind and even won the race both on the water and on corrected
and that's never happened!
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Posted By: tgruitt
Date Posted: 26 Oct 10 at 11:08am
 RAKE! ------------- Needs to sail more... |
Posted By: ifoxwell
Date Posted: 26 Oct 10 at 11:24am
I tend to agree I've always thought that the most significant factor is that raking the rig lets it all breath better, jib slot opens up, spreaders let the mast bend more etc etc. The flaw in this theory however is that raking still works for a single sail, unstayed rig... so basically what i'm saying is that I don't have a clue either! Ian ------------- RS300 |
Posted By: blueboy
Date Posted: 26 Oct 10 at 12:52pm
But the leech doesn't do (relatively) less. The luff does less as you either feather (jib) or ease the mainsheet while vang-sheeting. 2. As I rake the angle the shrouds are at makes the mast bend more. This is more noticable in rigs with raked spreaders.[/QUOTE] But plenty of boats can adjust bend separately from rake e.g. with lowers, ram or backstay. Yet they all rake to depower as well as increasing mast bend. I don't have an alternative explanation to put forward and I know rake works. I just don't buy these explanations. |
Posted By: Fearfull
Date Posted: 26 Oct 10 at 1:22pm
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Ok. Not sure I can explain the center of effort on a mainsail without a diagram BUT bear with me please. Imagine your mainsail is a rectangle like a A4 piece of paper in portrait. The center of effort would be right in the middle, ie where the force is applied assuming all 4 corners are held. And when the breeze is light/medium with the help of battens this is pretty much what your sail is like. Now when it is breezy the top corner away from the mast is going to blow out, there is no longer enough leech tension to hold it up. Back to our A4 paper, one of the top corners is no longer held, the paper is only held by three corners, the center of effort is now in the middle of these three held corners. It is significantly closer to the mast. SO center of effort of the sail has moved forward, sound reasonable? By raking we can then bring it all back in the boat. If you sail lead mines the same applies but you tend to think you see the opposite, in reality the entire leach is ussually dropping off as one big fat slab and not doing very much. With regard to shroud angle (another diagram would be good) as you increase rake you effectivley increase the distance the shrouds are from the bottom of the mast. This means for the same tension they are effectiively pulling the mast back more than when the rig is upright. If you have spreaders they will be forcing the mast to bend more, you can also carry more forestay tension. You mentioned lowers and ram, these both effect the mast low down (there is a clue in the name) raking would tend to make the mast bend between the spreader and the hounds. Backstays are good for doing everything in one hit, they will tension forestay, compress mast and force the mast to bend. Not commonly seen on dinghys though. This is how I see it anyway. FYI I now sail a 600 and rarely drop the rig back, when I do it is to mentally prepare me for (and hopefully prevent - ha) the nose dives. |
Posted By: ellistine
Date Posted: 26 Oct 10 at 1:52pm
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Thanks Fearfull. So what you are saying then is that if you have lots of twist in the sail, either intentionally or from the strength of the wind, then the CofE moves towards the mast so raking the mast back brings the CofE aft again. How much effect would having the CofE in the right place have on boat speed? I mean we were going much faster then ever before. Nothing else was much different. Trim was normal, righting moment was normal, rig tension was normal. I was making more of an effort to keep the inside jib tell tale streaming rather than lifting but I've done that before without such a massive gain in speed. ------------- |
Posted By: Fearfull
Date Posted: 26 Oct 10 at 2:13pm
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On the 4000 you also have a jib. (sure you noticed) rake will also effectively change the job fairlead position, it has the same effect as moving the fairleads back, this is all good when its breezy as now you have more twist in the jib and it is depowered a touch. The benefit to moving the Cofe back is that you are striving for a balanced boat. the foils act against the rig, if both are 'lined up' then the boat should track in a straight line through the water. If you didn't rake at this stage (well breezy) you would expect to be using the rudder more just to sail in a straight line. And using the rudder is slow. |
Posted By: blueboy
Date Posted: 26 Oct 10 at 2:44pm
Nope. I've spent a few thousand hours trimming yacht and keelboat mains and that is not what happens. As you are overpressed and ease mainsheet, you get a pronounced bubble of backwind along the luff behind the mast and the leech continues to drive. CoE therefore moves aft. Even if you ease kicker and put in twist, it's mostly the leech that's working. It's pretty obvious really that if you have a sail that's a curved foil shape, if you rotate the whole thing to reduce angle of attack, it's the luff that's going to lift first and therefore CoE moves aft. |
Posted By: Fearfull
Date Posted: 26 Oct 10 at 3:04pm
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Ok. So when you ease the main, and the front begins to luff, where exactly (exactly mind) do you think the CofE is going, do you think: a) it is going no where. b) it is moving back. c) it is moving forward. It might help if you take it to the extreme, let the sail right out, boom to the shroud. On a dinghy or high aspect ratio main then the leech will be dropping off etc. |
Posted By: getafix
Date Posted: 26 Oct 10 at 3:08pm
| it's going aft and frankly i don't get the clarification there blueboy, what you're seeing is consistent with exactly that, despite appearances |
Posted By: JimC
Date Posted: 26 Oct 10 at 3:13pm
I know that's conventional wisdom, but I have a lot of trouble believing it. The side load is shared between rudder and centreboard. If you move the mast forward and back you change the balance of the sideload between the two, but I don't believe you actually need to offset the rudder to keep the boat straight: there just isn't enough movement of the CofE between the foils. Yes it worked like that in the days of yachts with rudders on the back of the keel and to an extent on any craft that has a very large disparity in size between board and rudder, but I can't make it work for a modern dinghy.
Well perhaps so, but either way when you dump the main the jib is contributing proportionally more of the heeling moment which presumably means the net CE has a tendency to move forward... All this stuff is so complicated that I really have trouble getting my head around it. |
Posted By: blueboy
Date Posted: 26 Oct 10 at 3:21pm
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If I have a main that is not backwinding, then I ease it in a puff so the luff starts to backwind, the CoE moves aft. If I have (or expect) persistent backwinding, then I want to depower the rig and one of the things I'll do is rake. Now where we came in was the argument that a reason to rake was the assertion (which I reject) that's it is because CoE moves forward as breeze increases. |
Posted By: blueboy
Date Posted: 26 Oct 10 at 3:27pm
Yes it is complicated. Jib might be contributing more. Or less because I'm helming to feather it and twisting it more. Or less because I've changed from No1 genoa to No3 jib, or from a full jib to a flat one, or I've just flattened the jib by increasing forestay tension. Lots of variables but every single class I can think of rakes more in breeze. |
Posted By: Fearfull
Date Posted: 26 Oct 10 at 3:28pm
JimC - I don't disagree with you on the foils and CofE, and am (nearly) quoting what I have read/heard in the past. Although i think you will agree that if there is a lot of helm you are not going to be going quick. blueboy - having taken a second (should I have taken more?) to think about what you are saying I believe (and I could be wrong) that you need to think about the entire sail plan, were the jib not there the back winding would be gone, the entire main sail powered up and working then proportionaly the leech will be doing less than it is doing in lighter winds. The front of the sail is still pretty important even if it is a bit flappy and the sail wouldn't work the same it it wasn't there (this isn't the same for the leach). |
Posted By: Fearfull
Date Posted: 26 Oct 10 at 3:29pm
Just one of the reasons though. |
Posted By: blueboy
Date Posted: 26 Oct 10 at 3:36pm
| Oh yes there must be reasons! Wish I knew them is all. Rake works. |
Posted By: getafix
Date Posted: 26 Oct 10 at 4:27pm
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I think the reason that rake depowers is that it moves the CofE down as well as changing the profile of the <wing(s)> sails which means; less righting moment required = less turbulence around the foils and more speed generated by the power from the rig as power is more efficently transfered into motion = greater VMG Once, many years ago, we tried this on sailing school wayfarers, normally with their sloppy rigs and basic foils, large amounts of weather helm were present in any sort of blow, we raked the mast back and wound on the kicker and jury-rigged cunningham, result = neutral helm. the contrast was huge. Only thing I can compare it to is trying to bear away with the sails pinned in and boat heeled to leward versus bearing away with sails eased and the boat heeled to windward. To my <simple> mind the forces in play are similar, rig vs foils = turbulence & loss of speed, rig & foils working together = smoother tranference of power and greater speed hhhhhhhmmmm, sailmakers or similar pro's please feel free to tell me I'm wrong!! |
Posted By: ellistine
Date Posted: 26 Oct 10 at 4:47pm
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That's the other odd thing. I would have thought a more raked mast would create more weather helm what with the CofE moving back and everything but the tiller was still really neutral. I've been quite conscious of weather helm after Paul Brotherton said the 4K rudder is too big and can really kill the speed if not balanced. Perhaps lots of mast rake in lighter conditions would create weather helm? It's all very odd. I can see this turning into another one of those 'You Said' snippits in the Y&Y magazine. ------------- |
Posted By: getafix
Date Posted: 26 Oct 10 at 4:56pm
in that case, lets all hope we get a pro on here to give us their 'read' on it! |
Posted By: Fearfull
Date Posted: 26 Oct 10 at 5:06pm
I am still pretty convinced that when the main is fully depowered the CofE is further forward. Certainly on a high aspect ratio rig the sail above a line from the end of the boom to the top of the mast isn't doing a lot. |
Posted By: ellistine
Date Posted: 26 Oct 10 at 5:13pm
Which still fits. With a raked rig, if it's windy enough that the leach has given up and the CofE is forward then the weather helm will remain neutral. On the same raked rig, if the wind is light enough that the leach is still working then the CofE would be further back creating the potential for weather helm. All I know is I'd be useless on a Jury. I believe everybody's story!
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Posted By: ellistine
Date Posted: 26 Oct 10 at 5:19pm
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Actually, thinking back to another 'story', we had a club member (who knows a thing or two) help us setup the boats when we first got them and he told us that if we're having to sail with the boom out beyond a certain point to start raking the rig so you can maintain the boom nearer to the center again. That would put the emphasis more on de-powering than balance. ------------- |
Posted By: timeintheboat
Date Posted: 26 Oct 10 at 8:30pm
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It is complicated and surely more so when the rake is achieved with either a stiff rig (a la Merlin photo) where the mast is raked but upright or a floppy rig. In the latter case more rake also means more fall-off laterally which I would have thought aids gust response. Doesn't the modern Phantom have a floppy rake system and can offset the fall-off by tightening the lowers? ------------- Like some other things - sailing is more enjoyable when you do it with someone else |
Posted By: getafix
Date Posted: 27 Oct 10 at 12:58pm
The shroud & forestay tension and RAM/lowers won't be the main reason for the rake, the rake (to me) is the angle of the spar. After that, what you do with shroud, forestay and spreader angle will affect the deflection/bend of the spar and RAM/strut/lowers will impact on lower spar bend versus upper spar bend as well as have a bearing on sail shape (just as forestay tension) in combination with adjustable controls like halyards, cunningham and kicker. With spars having different bend characteristics, and some classes having 1, 2 or 3 sails hung off them, you'll see a divergence of rig tensions relative to each class, i.e Merlins may have more rig tension to control their relatively high aspect sails and resist the forces from the kite, while Blaze/Phantom will have 'sloppy' rigs as they don't need to worry about kites and have spar characteristics such that tensioning or loosening shrouds or forestay won't be neccessary to have the spar and sail work together the way the maker's intended. Although you need them 'on enough' to stop the mast jumping out of the foot! So what I'm thinking  is that rake could be FIRST analysed seperately from those things as a factor and then after that as a secondary factor in as much as leech profiles will be effected by say kicker tension and rake, or jib luff profiles effected by both rake and jib lead/car position. |
Posted By: redback
Date Posted: 27 Dec 10 at 5:16pm
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I sail a 4000 and the rake makes a huge difference to the power of the rig. I've experimented with easing lowers, sliding the fair leads back and altering the strop length. They can all decrease power but nothing works quite as well as mast rake. One hole downwards on the shrouds makes the boat so much easier to handle in a really strong winds but you very thoroughly lose a lot of power and its a real disaster if the wind is marginal. Unless its survival I generally go for half a hole and maybe move the fairleads back a click. Now I'm a great theorist but I can't explain this. Sure it opens the jib leach but so do the fair leads. It eases the lowers but I do this anyway in strong winds. As for rig height it must reduce it by about 1cm which I'd say is insignificant. I can't explain it but it works a treat and I recall it worked nicely on the 800 that I crewed. If anybody can find a good reason why this is so I'd love to hear it but I've heard nothing convincing yet, however it does work beautifully.
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Posted By: Garry
Date Posted: 27 Dec 10 at 5:23pm
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IMHO, If you change nothing else raking the mast back 1 full hole increases the bend considerably = flatter sail. ------------- Garry Lark 2252, Contender 298 www.cuckoos.eclipse.co.uk |
Posted By: Skiffybob
Date Posted: 27 Dec 10 at 7:03pm
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As a starting-point, you need to think of your jib and main as a single foil with a slot in it. The jib essentially provides a lot of the powr for the main, and they need to work in harmony to get the best from them. By raking the mast, you increase the slot and push the slot effect further down the main, hence reducing the venturi effect on the back of the main an making the power eer so slightly lower down. This has an enormous effect on the power that the main creates. It allow you to keep your critical angle of attack on the jib, and also lets you keep some leech tension on the main (for stability), whereas moving the job cars back opens the slot bu also ruins your pointing. Easing the barber-haulers (if you have them) has a similar effect, although not as great it can be done on the water.
It also have a really bigeffect on the weight distribution, and takes a lot of weight and inertia away from the bow, meaning that you can stand/sit further forwards and drive the boat hard without it mining.
Sail a short, over-powered boat for a while, and you'll learn all about it... ------------- 12ft Skiff - Gordon Keeble and the Furry Fly-by AC - GBR271 - Whoosh B49 - Island Alchemy |
Posted By: redback
Date Posted: 24 Jan 11 at 6:34pm
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Skiffybob - you may be right but I understand the rake angle has a large effect on Contenders and they don't have a foresail. I'm afraid I'm not convinced about the weight distribution argument since we are only talking about moving the mast head a few inches for and aft. However as I have said before it works.
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Posted By: NickA
Date Posted: 25 Jan 11 at 8:33pm
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Don't contenders just go faster with more rake in pretty much all conditions? I think they rake as much as they can whilst still being able to get under the boom! Contenderers please confirm. Plus in stronger winds you might raise the centre board .. which pushes it's centre of effort back - so the mast rakes back to place the sail COE back over the board's. On the two boats I sail: The Javelin has a tall very bendy mast, a huge jib and a swivelling centre board. Rake makes a massive difference. In about F4 a 24'10" rake can become un-manageable, whilst a 24'8" rake is fast as we can keep the boat flat with the boom centred (and hence leech well in). I think the depowering comes from a wider slot and a shorter mast - but also we might pop the centre board up a tad and unless we rake the mast back to match the boat ceases to point properly. The V3000 has a short stiff mast, a small / medium jib with no / little overlap and a near vertical dagger board that generally stays full down regardless. Rake makes a bit of difference to power and quite a lot to pointing ability, but it isn't life and death. I usually single hand it with lots of rake ... but mostly 'cause it looks kinda cool  . ------------- Javelin 558 Contender 2574 |
Posted By: Garry
Date Posted: 25 Jan 11 at 8:48pm
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Contenders = as much rake as you can handle and still get under the boom. ------------- Garry Lark 2252, Contender 298 www.cuckoos.eclipse.co.uk |
Posted By: tim grasse
Date Posted: 25 Jan 11 at 11:25pm
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From what I gather in higher winds the hull starts to plane and the lateral resistance of the bow is decreased so the centre of lateral resistance is moved aft. To compensate for this you rake back the rig to keep the centre of effort above the centre of resistance so the bat is balanced and sails better. |
Posted By: 17mika
Date Posted: 29 Jun 11 at 4:43pm
Sorry to highline an old discussion. I pretty much agree with this point in the 4k case upwind, toghether with the opening of the slot.
Thing I have never properly understood in the 4k is why a raked mast in low/medium breeze is ridicously bad upwind in terms of height (and not only speed). All times I went out with a raked mast and wind didn't go up as expected, even when I adjusted the lowers for the wind, upwind i felt everyone else like would go 5 degrees higher than me.
Looking also at this discussion, it may be a combination of too much wheather helm to counterbalance with the tiller + jib leech opened up too much. Or something else?
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Posted By: x1testpilot
Date Posted: 30 Jun 11 at 3:24pm
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In I14's a few years ago the rake became the main control (continuous systems led to the gunwale ere common) to adjust for wind strength. Since then things have changed a bit, but the raking theory really worked. Rake was adjusted so much that a "flattener" had to be applied to the main leech to lift the boom so you could get under it. If all else is kept the same raking will open up the jib leech (twist off at the top as the sheeting angle is effectively changed. The shrouds angles + the lowers (or mast ram) would do less to keep the mast straight, so it would bend and flatten the (luff of the) main. These also open the slot. A little extra Cunningham and or uppers would twist off the top a bit more. Centre of effort as mentioned above. I have heard argument that some lift is generated, but I'm not convinced. In the X1 we rely on the automatic flexing of the topmast / square-top and add Cunningham to increase this in wind. It is even possible to play the Cunningham instead of the mainsheet. The effect is very marked - pulling rapidly ion the Cunningham can alarm the helm! We do ajust rake on the day but so far not by much at all. ------------- I like to take pictures of sailing, but I'd rather be sailing! http://www.LCSC.org.uk" rel="nofollow - LCSC.org.uk http://xdinghies.com/" rel="nofollow - Xdinghies.com/ X0 & X1 |