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Mast rake: explain please!
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Do Different View Drop Down
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Posts: 1312 		Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Topic: Mast rake: explain please!
    Posted: 04 Dec 14 at 8:31am
Not claiming any specialist knowledge but from purely an interested layman's point of view.
Seems to me that there are so many theories because although the concept of leaning the mast back is a simple one the effects are many and varied. 
1. Because of how the other controls are subsequently applied 
2. Widely different rig styles, FDs/505 big genoa overlap, 29er tiny jib, Mono rig.
3. The complexity of the raking setup. Mike Holt 505 & I suspect FDs move their shroud anchors back so maintaining a similar spreader deflection. This means they move the tip a lot so having a big effect on sweep and genoa overlap.
4. On a simple setup with no means to move shroud anchors or a desire to regularly adjust spreaders. A small change in rake will have a big effect of spreader deflection but less so on sweep angle or genoa overlap.
"Different strokes for different folks" 

Edited for slightly better grammar.


Edited by Do Different - 04 Dec 14 at 9:02am
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NHRC View Drop Down
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Posts: 102 		Post Options Post Options   Quote NHRC Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 14 at 4:38pm
Originally posted by sargesail

Not sure if it's relevant here but it always strikes me that sideways bend is too often ignored in the models and descriptions of how rigs work.
Certainly on soft tension rigs (Firefly, 2000) where there is a need to maintain sheet tension to hold the rig back (and the boom on the centreline to open the slot), it is my opinion that the speed that comes from extra rake is as much about the interaction of clunky sails on coarse rigs than all the detail of aerodynamics.
But why.....? 


Yes... Sideways bend is an interesting characteristic that we ignore.

At the top, a bendy rig is great with gust response.
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Posts: 84 		Post Options Post Options   Quote AlanH Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 10:15am
Re Do Different scenario 4- You rake aft, but do not move shroud anchorages and your spreaders are fixed. So what happens to effective spreader angle/ deflection/ bend of mast? Are you bending or straightening the mast as you rake?
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Posts: 1312 		Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 11:34am
AlanH.

I guess you could say effective spreader angle is increased. Certainly from observation, raking aft increases prebend:flattens the main.  

I am advised to keep plenty of mast strut on as you rake so as to maintain some power lower down in the main. 

I can rake on the water (but shroud position is fixed) and the effect of dropping the mast back is very apparent in depowering the main, in fact in what I thought were heavy conditions coming I have over done it and killed it completely. Mind you, did I pull enough strut on as I raked? Doubtless that is why the top boys "sail by numbers".
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Posts: 102 		Post Options Post Options   Quote NHRC Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 12:42pm
Adjusting rake does not effect your pre-bend.

As the mast rakes, and if the fore-aft position of the shrouds at deck level is not adjustable, you decrease spreader angle. But this shouldn't alter pre-bend either. Only your rig tension on shrouds, lowers or strut control, or changing the length and angle of your speeaders should alter your pre-bend.
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Do Different View Drop Down
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Posts: 1312 		Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 12:54pm
NHRC. As I do not know the geometry of your boat or boats in your experience I certainly will not say you are wrong.
However.
On my boat with the mast stepped about 150mm from the keel and about 250mm below the level of the gunwale/shroud anchors. Raking the mast back by shortening shrouds and lengthening forestay while maintaining the same rig tension demonstrably bends the mast more and increases prebend. 
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Rupert View Drop Down
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 1:03pm
Do Different, what boat do you sail? I'm trying to picture in my mind your setup, as I can't visualize where the prebend is coming from.

At its most basic, mast rake is simply the same as putting a telegraph pole in the ground at an angle rather than upright - it is still straight. There have to be other factors acting upon it to make it bend. These can only be the deck or lowers and spreaders, so if the lowers or mastgate are being kept in line with the new mast position, it only leaves spreaders, and I think I'll need to see that before I can work out the forces and how they change as the spreader tips move aft compared to the shroud base.
Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686
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Posts: 1312 		Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 1:50pm
Ah ha. The light is dawning Lamp everybody is correct  LOL and nobody is wrong.
Deck stepped mast with the mast base above the shroud base: raking will have little or a very small decrease in mast prebend.
Hog stepped with the mast base significantly below the shroud base: raking will increase the forward pressure exerted by the spreaders thereby increasing prebend.
See Mike Holt page 1: he has to move his shroud base aft to avoid applying increased prebend.
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Posts: 1312 		Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 2:20pm
Tried to post a sketch but been all around the houses and nothing works. For that matter it's a lottery if I can see the recently posted photos by others, sometimes they flash up for a fraction of a second and then vanish leaving large blanks.
Latest Mac OS but a slow internet connection, any ideas ?

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Posts: 58 		Post Options Post Options   Quote Mike Holt Quote  Post ReplyReply Direct Link To This Post Posted: 09 Dec 14 at 2:28pm
This is an interesting topic and as it wanders around different parts I think you need to look at each one in isolation to truly understand what is going on. 

As an example, mast ram(strut), lowers or chocks. For the longest time I thought that they increased or reduced mast bend. But really they don't. The mast bend is induced by other factors, spreaders, shroud tension, differences in height between shrouds and forestay on mast etc. In reality they control where the bend is. More ram low down straightens the bottom of the mast BUT increases bend higher up, the reverse will happen if you reduce ram, the bottom will bend more but the top section will straighten. I now think of it as a "bubble" that I can control.

So, straightening a lower mast, bends the upper mast, opening the upper leech, enabling more twist up high for an easier groove and higher speed through the water and if all else is right, a reasonably tight lower leech helping to maintain height.

Combined with rake, fast.
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