Raking Rigs
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=11183
Printed Date: 15 Jul 25 at 4:13am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: Raking Rigs
Posted By: boatshed
Subject: Raking Rigs
Date Posted: 02 Nov 13 at 8:23am
This from the R Speculation topic
How does raking a rig back a little bit de power it ? ------------- Steve |
Replies:
Posted By: Mister Nick
Date Posted: 02 Nov 13 at 8:36am
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http://forums.sailinganarchy.com/index.php?showtopic=103845" rel="nofollow - http://forums.sailinganarchy.com/index.php?showtopic=103845
There's some pretty good information on there. The general consensus seems to be that it reduces weather helm, lowers the centre of effort and helps to flatten the sail. Most people seem pretty uncertain about the actual physics of it though.
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Posted By: Dark Harris
Date Posted: 02 Nov 13 at 9:48am
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Ok here's my twopence worth...Lowering the CofE on whole rig is not due to the mast being lower its due to the power in the sail being lower, the shrouds are slacker the top of the leach leans to leeward with boat still upright power is lost high in the rig, hey presto even a short a... like me can keep it upright. To match the rig going back then raising centreboard/daggerboard a little will offset any weather helm you may induce due to CofE moving back but often the loss of power up the rig will be enough. ------------- When they say you are too old you probably are, but do it anyway. |
Posted By: Rupert
Date Posted: 02 Nov 13 at 10:17am
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But raking the rig back works even when you keep the rig tension on, so it can't just be losing power because of slack shrouds. It works for boats with jibs, and those without, and really I've never seen a convincing argument for it that hasn't been shot down by others, with their own slightly unconvincing argument. Best I've come up with is "its complicated, I don't understand it, but it works". ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Dark Harris
Date Posted: 02 Nov 13 at 11:05am
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Good point well made, but the boats that leave rig tension on have a jib, different rules, as you now open slot. Single handers without jib slacken the rig. Don't know of a single hander that is able to rake rig and keep tension on. You are right that it is subjective but it definately works or I would be in the bar more often. ------------- When they say you are too old you probably are, but do it anyway. |
Posted By: Rupert
Date Posted: 02 Nov 13 at 11:14am
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Does the Contender keep the rig tension on? They are the classic raking rig singlehander. I've always assumed it did, because of the trapeze, but have never studied the boat.
------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Dark Harris
Date Posted: 02 Nov 13 at 11:26am
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Don't think they do although, obviously, the trapeze is going to give some support. ------------- When they say you are too old you probably are, but do it anyway. |
Posted By: andymck
Date Posted: 02 Nov 13 at 11:43am
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Raking the rig has multiple effects. From increasing mast bend in the majority of boats to opening the slot, allowing the top of the main to blade off etc. many of these can be achieved in isolation, but as you do you drag the centre of effort forward. In the majority of boats we sail this leads to a very lurching response to gusts which is inefficient, and requires more rudder corrections.
Raking the rig allows the boat to come back into balance. It will also increase other benefits such as opening the slot more. Next time you go out in a breeze, if you have the ability to rake, de power as much as you can by pulling the usual controls. Then sail up wind and adjust the rake till you feel comfortable. Then adjust the centre board, and see if you need to adjust again, on single handers with stayed rigs it has a big effect. On two sailed boats also pay a lot of attention to the slot. If you get into that main flogging situation, and others arn't or you want to be first to regain control, ease the jib, and flow will be restored. This is a good indicator to either go for more rake or move jib cars back. This fine tuning is effectively what the good guys are doing when they are doing that practice beat, as well as keeping an eye on winds shifts. Last thing to remember is if the wind strength changes, re adjust everything. You will then end up sailing the boat rather than fighting it. Rig tension depends on the boat and rig. You can induce a lot of mast bend further up by keeping it on, but some rigs need less, as it allows more lateral bend at the top. Andy (wish I was fit enough to do that practice beat. Am one of those who drops off after first lap against the top guys looking a bit too red) ------------- Andy Mck |
Posted By: rb_stretch
Date Posted: 02 Nov 13 at 1:02pm
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I've often wondered about what the precise effects are and I'm sure that they vary boat by boat. The phantom is an interesting case in that raking the rig is the single biggest thing you can do to de power, however that could be as much the other effects it creates rather than the rake on its own, all other things being equal. As mentioned in the other thread the most immediate thing rake does is to loosen the tension in the lowers therefore allowing the mast to bend a lot more and flatten the sail. Next, due to the fixed shrouds the whole rig falls to leeward a bit therefore allowing more air to be spilled. Then the daggerboard can be raked back a bit to balance the CoE that has moved back and the reduces power in the sail. I'm sure there are more effects, but I can't be certain beyond these. The point is that the cumulative knock-on effects of adjusting one control turns out to be quite dramatic overall effect.
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Posted By: Oatsandbeans
Date Posted: 02 Nov 13 at 2:59pm
| I think that the main effect of raking the rig is the mast bend effect. For every 1/2 hole that we rake the mast back ( equivalent to ~2" of mast tip movement) we get ~7mm more of mast bend. So if you drop the rig back 4" at the tip, by dropping the shrouds 1 full hole you can get 15mm more prebend. If you want to know how this effects the mainsail shape this will take 1.5% camber out of the sail which has a significant flattening and depowering effect when its breezy and you need the sail as flat as you can. |
Posted By: Rupert
Date Posted: 02 Nov 13 at 4:16pm
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Why do you get more mast bend when you rake? You've obviously measured the effect, so I'm not suggesting you are wrong, just trying to figure out the relationship.
------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Do Different
Date Posted: 02 Nov 13 at 4:24pm
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As you lean the mast back the change in geometry means spreaders push more hence more mast bend, assuming you keep the same rig tension. Edit. Oh yes and if you don't reset the lowers, chocks or strut they will ease as well giving more low down bend.
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Posted By: Do Different
Date Posted: 02 Nov 13 at 4:29pm
| BTW. A friend of mine sails a 505 with light to medium crews and he says it's a must have. If the wind gets up and it's all getting a bit much he just dumps the rig back, eases the board and hey presto, well mannered and flying again. |
Posted By: JimC
Date Posted: 02 Nov 13 at 5:04pm
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The stuff about centre of effort is obviously a nonsense: the sums demonstrate its minimal. Think, by jamming on the downhaul, blading out and twisting off the top of the rig (assuming a reasonable rig) you drop the effective centre of effort by feet, so why on earth would moving it at best three inches down make a happorth of difference? Similarly the actual effect on the sail chord isn't going to be that great either.
On the other hand the posts on SA from SHC seem to actually provide some sort of rational explanation - the first I've seen, because I must have missed that thread. I don't doubt that for many types of rig there are secondary effects due to the way other bits of the rig interact though. |
Posted By: iGRF
Date Posted: 02 Nov 13 at 5:14pm
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The other point you have to consider with a raked back rig, the foil section of the sail is no longer acting as efficiently as it's designed parameter, there is also an element of vertical lift, obviously we get this more on racing sailboards as we rake our rigs much more, to reduce the 'railing' (you'd call it heeling)effect. the obvious benefit to us is we can hold onto bigger rigs and can balance the heel exactly by multiple options in the rake.
Honestly a good dose of sailing a long race board makes it easier to comprehend if only by anecdotal 'feel' evidence as to what happens. I wish it could be organised on a dinghy as easily as it occurs on a board with one push to a foot pedal. A Merlin dude walked me through their system at the Dinghy show, it does what I need, but jeez how complicated the ropes and had I forgotten how it worked within yards of leaving the stand? ------------- https://www.corekite.co.uk/snow-accessories-11-c.asp" rel="nofollow - Snow Equipment Deals https://www.corekite.co.uk" rel="nofollow - New Core Kite website |
Posted By: Mike Holt
Date Posted: 02 Nov 13 at 5:33pm
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I am sure every boat is different, but on a 505 when you rake, it is firstly a considerable move of the rig. The boom at the outboard end moves more than 3 feet between fully upright and fully raked, the mast moves aft 4 to 5 inches at deck level. We also maintain mast bend as we rake, but importantly move where the bend is. We "ram down" hard, this moves the point of max bend up the mast helping to flatten and twist the upper 1/3 of the main. You are also opening up the slot between main and jib. The result is dramatic. In 18-20 knots of wind you would not be able to sheet the main inside the quarter of the boat and it would be back winding dramatically with the rig upright. The boat would be triply and slow. Drop the rig back, board up slightly(not much though with the very high aspect foils we now use) cunningham on hard and the main will be back in the boat, no back winding and the boat will now go fast forward and easy to control.
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Posted By: Do Different
Date Posted: 02 Nov 13 at 5:47pm
| Thanks Mike. Stuff I can relate to and use. |
Posted By: Oatsandbeans
Date Posted: 02 Nov 13 at 5:48pm
| I think that Mike ( and GRF!) have a good point regarding the effect of rake changes on the balance and feel of the boat. As the rig is raked back and the board is raised you then feel the boat "lighten up" and you lose the feeling of being overpowered and it tripping over itself. You can now sail off marginally and go for a fast mode getting speed without sacrificing too much height. The 505 sailors have this well sorted as they have a very well defined set of numbers to work to ( have a look at the the articles on the web on this-they really know how to get the most out of the rig through the wind ranges). GRF will also intuitively know about this from his long board sailing. Here the effect of knocking the rig back on the track 2" or raising the board by a similar amount can really be felt and the board will change completely. So more rake will all help to make a dinghy faster and easy to sail in a breeze, from both the balance and reduced heeling force from the increased mast bend. |
Posted By: ChrisI
Date Posted: 02 Nov 13 at 6:57pm
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I'm keen to understand this one - still don't properly.
Some questions for you Mike: - 3 feet is huge. Can you get under the boom when it is raked? Or do you have a flattener? - would a carbon mast on a 505 depower better with this technique, or is the design of the current alu one such that it is almost as good as a carbon one would be? - would there be any difference between a pin-head and a square-top main in terms of the extent to which they depower from mast-raking? |
Posted By: Do Different
Date Posted: 02 Nov 13 at 8:50pm
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I found this article a while back. http://northsails.com/RADUploads/How-Sails-Work.pdf Reading it is easy. Understanding it, hhhhhmmmmmmm. Remembering to put it all into practice, aagggghhh. But then if was easy............................  |
Posted By: mole
Date Posted: 02 Nov 13 at 10:31pm
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It reduces the chord length so reducing the power,drag and heel. Bit like reducing the richness of the carburetor, if anyone can remember what they were! There is no such thing as dynamic lift. Remember, dinghy rigs are all about reducing drag. The power generated is roughly all the same. Hence why new sails seem so efficient, they have less drag (stretch) We are running at numbers far below normal aerodynamic speeds compared to aircraft including gliders. One bad tack can eliminate all the gains from a slight gain in rig set up. Go and practice :)
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Posted By: andymck
Date Posted: 03 Nov 13 at 12:39am
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Let's think about the things that move the centre of effort forward in a sail as we get overpowered.
1. Cunningham. The Cunningham is designed to flatten and pull the centre of effort forward. Increased by blading off if the trailing edge higher up. 2. Mast bend. This again drags the chord and therefore centre of effort forward. 3. Easing the sails. Again this brings the centre of effort forward. I am sure the canoe and several of the other boats Jim sails have the ability to adjust the rake to reset the centre of effort. Like I said, it is complicated. But a fast boat up wind is balanced. You have to re balance the boat with each major adjustment. This can also be done in various different ways. I have already alluded to one way in a two sailed boat which is to depower the jib by increasing twist or easing it. I Have also suggested its a trial and error issue for each variation of boat and rig, but actually we are achieving similar results by doing different things. To answer Rupert's question about rake and mast bend. This is a dynamic between rig tension and spreader angle, the amount of forward pressure by the vang or main sheet arrangement, the luff tension, and the low down mast support. As the mast gate is often fixed, or the lowers are often non adjustable, as the mast is raked the vang puts more relative forward motion on the lower mast. Merlin's for example have their lowers involved in the one string taking system to reduce this. Some classes such as the phantom can adjust lowers to get same effect. This makes a huge difference to the mast tip position. On a N12 mast you can get the mast tip to move a good 6 inches by playing with the lower mast supports, hence tuning numbers for initial rake set up are normally given with lowers and mast ram in an unloaded position. By manipulating these you can change where the bend occurs in the mast. For instance more lower tension and a lot of Cunningham encourages higher up bend and tip response. Release both and you get a very unresponsive flat sail with a tight leech. This can be good for pointing. Bottom line is you have to experiment. But that is half the fun. ------------- Andy Mck |
Posted By: Mike Holt
Date Posted: 03 Nov 13 at 1:15am
We have a flattener that moves the boom back up about 10" when the boat is all the way "upright" the boom is easy to get under, all the way back and the flattener it is starting to be a challenge! In extreme conditions I will ease the kicker to tack. I think carbon would improve the gust response and weight aloft but would probably take a year or two to get optimized on the sail cut, same for changing to a square top. As for the "one bad tack" a well set up boat will allow you to make a lot of bad tacks in my experience and the winners have good boat speed and don't tack too badly!
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Posted By: Do Different
Date Posted: 03 Nov 13 at 8:40am
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Posted By: RS400atC
Date Posted: 03 Nov 13 at 9:54am
Does it? Not sure that's true in a Merlin. |
Posted By: Dark Harris
Date Posted: 03 Nov 13 at 11:02am
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Sorry Oats but when on the water we are not half way up the beat removing clevis pins and adjusting the shrouds! The single handers are just easing the rig back by releasing forestay tension. Rig tension goes to nothing, then pull the kicker on till your ears pop. Generally, unless it's blowing dogs off chains this will be just up the beat or a tight fetch and then, downwind you would ease the kicker and slap some rig tension back on to bring it upright again. So mast bend doesn't come into it. ------------- When they say you are too old you probably are, but do it anyway. |
Posted By: Oatsandbeans
Date Posted: 03 Nov 13 at 5:31pm
| D H -yes I don't adjust rake on the water, its not impossible but I like to concentrate on other things once I have left the beach. If you have a decent set of double row shroud adjusters and a good weather forecast you should be able to sort it out pretty close before you launch. Lots of sailors don't adjust the rake because its a hassle and they cant be bothered, and opt for an average rake position, and use the secondary controls (vang and cunn) to adjust the power.I have 4 shroud positions and most of the time I get it quite close to be optimal. I think it makes a big difference to how the boat goes (speed, pointing, power and feel) when the rake is right for the wind strength, and its no more hassle than picking the right gear to wear. |
Posted By: craiggo
Date Posted: 03 Nov 13 at 5:43pm
I'm with you 400atC. It amazes me how little people know, but how much they are willing to spout as if they knew it. Raking the rig increases chord, fact. As to the effect this has, well it really depends on how the mast is supported and how you trim the sail out. Most classes will rake the rig on the shore by dropping the shrouds down a hole or two, and then applying normal rig tension. Lowers will need to be adjusted but differently on various classes due to geometry. Merlins have shrouds, forestay, lowers and kicker all connected, but most of us have to adjust all this stuff independently. It can be done on the water, but most will need to adjust for the day! |
Posted By: Slippery Jim
Date Posted: 03 Nov 13 at 9:08pm
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Keep it simple, why not have a automatic rig AKA 49er, 29er 59er... once set up, sail and smile... You spend more time looking out of the boat than pulling on bits of string. ------------- Pass the skiff, man! |
Posted By: boatshed
Date Posted: 04 Nov 13 at 11:45am
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Umm, seems like lots of twaddle being spouted. What is clear is that moving the mast back a bit doesn't materially increase the chord. Neither does it decrease weather helm or give vertical lift because the rig needs to be canted to windward fore this, windsurfer style. It seems the only boats that practice rig raking are medium to high performance boats with swiveling centre boards. As these boats begin to plane upwind, they need much less area of their boards in the water and the only way to reduce board area is to rake it back. To re balance the boat, the mast is raked back plus the cunningham is applied to the main sail. Now the drive is much lower down and the C of E further back. I'm not sure but when the cunningham is applied hard and the the top of the main is twisted off , does the C of E move backwards in the remaining, driving part of the rig ? So, 5o5s, Contenders, Fireballs etc all benefit. But low performance boats probably don't benefit as they never go quick enough upwind to start tripping over their board. There is a secondary benefit of raking a mast as it opens loosens the jib leach making it twist but that can be achieved other ways without raking. Tack height, jib cars etc Boats with dagger boards are much simpler. They just lift the board to reduce area and the Cof E just moves up and not backwards. Therefore, the mast doesn't need raking, just the cunningham applying to to twist the main to lower it's C o E. The jib is twisted off by lowering the tack. One of the great advantages of dagger board boats is the ability to change gear so easily. As the breeze builds, pull the dagger board up, pull the cunningham hard on and crack the jib off. Job done. ------------- Steve |
Posted By: jeffers
Date Posted: 04 Nov 13 at 11:58am
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Adam Bowers explained one effect of raking the rig in conjunction with using the cunningham at a Fireball training day. When you rake and pull the cunningham on the effect of slackening the leech (or opening the 4th corner as some chicken followers might say) offers a path of least resistance to the volume of air in the sail so it can exhaust out of the 4th corner. This combined with adjusting the slot and raising the board helps reduce the total power in the rig and balance the boat. I would guess allowing the 'excessive' volume of air on the high pressure side of the sail means the pressure differential between the 2 sides is less and hence less power. Someone with more knowledge of aerodynamics may disagree though.....
------------- Paul ---------------------- D-Zero GBR 74 |
Posted By: RS400atC
Date Posted: 05 Nov 13 at 9:53am
Well, I agree with the first statement. Plenty of slow old yachts and keelboats rake the rig to good effect. I am not sure why it works! |