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Mast rake: explain please!

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craiggo View Drop Down
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Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Topic: Mast rake: explain please!
    Posted: 28 Nov 14 at 1:26pm
MM, you are correct regarding the forward sweep assuming a straight mast, but most dinghies run a reasonable amount of prebend. This has the effect of straightening up the 1/4 chordline from just below the spreaders to the tip.

On a boat like a 600 which runs a reasonable amount of rake with significant pre-bend, dropping the chain plates by a couple of holes probably increases the twist at the head by 5-10° which lowers the rolling moment significantly. In light airs the drag from such a swept configuration is larger than for a more upright, untwisted configuration hence why it always pays to go upright in the light.
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Medway Maniac View Drop Down
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 14 at 2:23pm
Yes, except that I'd still maintain that upright is for most boats swept forward.

I don't understand how raking a 600 mast by a couple of degrees has any significant effect on twist.  With spreaders maybe, but diamonds and indeed the mast itself doesn't even 'know' the mast has been raked.

btw, how do you get pre-bend on 600 mast without the kicker closing the leech, or by pulling on a load of cunningham?  I spent a half hour with a club-mate staring at his rig wondering how he was going to flatten off the sail and stop it having inverse twist in light airs without huge halyard loads (which was our final, less than satisfactory conclusion).  In years gone by I'd agonised similarly over my Laser 2 with its b-awful diamonds.
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craiggo View Drop Down
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Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 14 at 2:41pm
It's got nothing to do with the structure of the mast, spreaders or shrouds. Sweeping the rig back induces a change in the local flow direction. So near the top of the rig the effective flow is actually rotated as if you were on a reach. As a result using kicker and cunningham, you should setup with more twist to ensure your tell tales are streaming. You will actually develop more power at the top of the rig but the loads will largely be forwards rather than lateral, hence why it is quicker than leaving your rig upright and blading off the top.

As for pre-bend on a 600, unfortunately it's achieved via halyard tension and the luff curve of the sail. As soon as you have enough wind to sit on the racks, the shape induced by the battens is enough to prevent the leech hooking and as such kicker can be applied aggressively to avoid too much twist. Cunningham significantly bends the mast as the wind increases and as the loads increase the head twists off to match the increased upwash. In theory you could overdo the kicker but in practice you run out of range before this occurs unless you've overdone the Cunningham to early.
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Medway Maniac View Drop Down
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 14 at 2:47pm
But when you rake a rig, you are sweeping the 'wing' through what, three or four degrees.  Do you really think that will increase the effective twist (I understand you're not talking simple geometry here) at the head by 5-10 degrees as you claimed above,   I'd have thought the effect would be pretty marginal.

Edit: sorry, I've just seen that in your latest post, Craiggo, you say that you need to set up with more twist due to raking  - agreed in principle, if maybe not to the extent you suggested.   

In the previous post however, you'd said, "dropping the chain plates by a couple of holes probably increases the twist at the head by 5-10°", which I didn't get at all.  But as you meant 'enables you to increase twist without issues' then agreed.


Edited by Medway Maniac - 28 Nov 14 at 3:00pm
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Post Options Post Options   Quote dohertpk Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 14 at 7:49pm
Thanks all - an awful to take in and chew over there. I asked because I am indeed looking to depower my 600 even more. I'm also sailing a Fireball in our local Frostbites and the nuance involved in tuning that boat is more than a little intimidating. Everyone is constantly fiddling with rake and rig tension - these are entirely new inputs for me.
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Post Options Post Options   Quote Jeepers Quote  Post ReplyReply Direct Link To This Post Posted: 28 Nov 14 at 8:43pm
See the thread Nice New Laser for Christmas...

I'm sure it works on my boat...but if it's become a selling point for Laser/Kirby Torch retailers, well...for those of us who can adjust these settings, clearly we all need to take it much more seriously!
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Post Options Post Options   Quote NickM Quote  Post ReplyReply Direct Link To This Post Posted: 29 Nov 14 at 12:14pm
As Davidyacht says raking a Solo mast is effective in a blow. Solo rigging is fairly slack and without spreaders. But the Solo does have a oversized centreboard which provides too much resistance when it blows, so raising it is essential - up to 40% on a beat. So obviously the boat is a lot more balanced if the mast is raked to match. I think for this class it is as simple as that.
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craiggo View Drop Down
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Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 29 Nov 14 at 2:09pm
MM, apologies, you are right and my description perhaps missed the point that the raking of the rig in itself does not cause the top of the sail to twist, but rather allows you to set the sail up with more twist which reduces heeling moment.

As you reduce the lateral component of the lift load on the sail (with the head twisted off, more fwd load rather than lateral), you wont need to resist the same lateral forces therefore you need less centreboard/daggerboard.



Edited by craiggo - 29 Nov 14 at 2:10pm
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RS400atC View Drop Down
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 29 Nov 14 at 2:20pm
I feel it's got nothing to do with bend, because it works on boats where bend can be added separately, and in boats like the 600 where bend is not affected.
It's got nothing to do with raising the centreboard, because it works on keelboats too.
I suspect it's mostly about how the air flows up or down instead of straight back across the sail.

I just accept it TBH.
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 30 Nov 14 at 8:32am
RS400 -the thing is that people dont generally change the spreader settingswhich the only other rig control that will affect the mid and upper mast bend. Most people have their spreaders bolted up to the standard settings and they stay like that.. So the raking does it for you simply. Gives you more mast bend and balances up the boat as it changes from "point "mode in light to medium to "fast " mode as the breeze builds. Mike Holts comments are interesting and it reminds me of what we used to do a bit in 470s. The mast bend / rake effect can be too much as when the mast is in a heavy weather setting it can now be overbent and the rig needs to be stiffened up by the spreaders being moved forward. This is now a complete pain as it involves getting up the mast to do it-just possible on a 470 by standing on the spinnaker pole fitting but not possible on lots of other boats with higher spreaders( or shorter crews).
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