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So what's with mast rake?

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Fearfull View Drop Down
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Post Options Post Options   Quote Fearfull Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 1:22pm
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.
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Post Options Post Options   Quote ellistine Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 1:52pm
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.

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Post Options Post Options   Quote Fearfull Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 2:13pm
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.
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Post Options Post Options   Quote blueboy Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 2:44pm
Originally posted by Fearfull

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.


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.
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Post Options Post Options   Quote Fearfull Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 3:04pm
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.
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Post Options Post Options   Quote getafix Quote  Post ReplyReply Direct Link To This Post 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
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 3:13pm
Originally posted by Fearfull

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.

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.

Originally posted by blueboy

it's the luff that's going to lift first and therefore CoE moves aft.

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.

Edited by JimC - 26 Oct 10 at 3:16pm
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Post Options Post Options   Quote blueboy Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 3:21pm
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.
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Post Options Post Options   Quote blueboy Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 3:27pm
 
Originally posted by blueboy

it's the luff that's going to lift first and therefore CoE moves aft.

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.


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.


Edited by blueboy - 26 Oct 10 at 3:28pm
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Post Options Post Options   Quote Fearfull Quote  Post ReplyReply Direct Link To This Post Posted: 26 Oct 10 at 3:28pm
Originally posted by JimC

Originally posted by Fearfull

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.

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.

Originally posted by blueboy

it's the luff that's going to lift first and therefore CoE moves aft.

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.


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).
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