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How does mast bend flatten a mainsail?

kirky1

New member
Joined
11 Jul 2021
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9
Hi there

It's counterintuitive to me that bending your mast flattens your mainsail. I would have thought it would make it fuller instead of flatter.

Does anyone understand why mast bend flattens your main? If you could explain in lamens terms that would be great.

Regards, Kirk
 

Wing Mark

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29 Sep 2021
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179
The luff of the sail is made convex. (luff round) That extra material creates fullness in the sail. bending the mast takes up the extra material.
Some dinghies, you also pull the tack or cunningham down at the same time, which straightens the sail in a line from head to tack.
The mast is generally supported to mostly bend in the fore and aft plane, the spreaders limit side to side bend. The mast must bend forwards at half height/back at the top
 

Daydream believer

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Hi there

It's counterintuitive to me that bending your mast flattens your mainsail. I would have thought it would make it fuller instead of flatter.

Does anyone understand why mast bend flattens your main? If you could explain in lamens terms that would be great.

Regards, Kirk
If one has a straight mast the distance from leech to luff is, say, length "X" & the curve in the sail suits that length regardless of where the fullness is.
If one now bends the mast the length "X" becomes "X" plus a bit. It stands to reason that the sail now has to adjust to that extra length thus the amount of curve reduces.
To demonstrate further. Take a piece of string & hold each end between finger & thumb of each hand. Let it sag a little between the hands.
As one moves one's hands apart the sag reduces. This is the same effect on the sail.
 

Robih

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Boat - West Scotland, Home - Tamar, Devon
If one has a straight mast the distance from leech to luff is, say, length "X" & the curve in the sail suits that length regardless of where the fullness is.
If one now bends the mast the length "X" becomes "X" plus a bit. It stands to reason that the sail now has to adjust to that extra length thus the amount of curve reduces.
To demonstrate further. Take a piece of string & hold each end between finger & thumb of each hand. Let it sag a little between the hands.
As one moves one's hands apart the sag reduces. This is the same effect on the sail.
I’m not sure I get that. Surely the mast is the same length? The mast doesn’t stretch in any sense, it simply goes around a corner a little. Same length but different route? The aluminium extrusion doesn’t magically become longer?
 

Daydream believer

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I’m not sure I get that. Surely the mast is the same length? The mast doesn’t stretch in any sense, it simply goes around a corner a little. Same length but different route? The aluminium extrusion doesn’t magically become longer?
Perhaps I should have said that this is the middle section of the height of sail, which is where the mast is bending
However, In my post I did say, from leech to luff. Not head to foot. Take a point horizontally half way up the mast & then try it. That is nomally where the majority of the bend occurs. Depending on the rig setup.
 

stranded

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I’m not sure I get that. Surely the mast is the same length? The mast doesn’t stretch in any sense, it simply goes around a corner a little. Same length but different route? The aluminium extrusion doesn’t magically become longer?
The mast doesn’t have to get longer - the centre of the mast moves forward, away from the leach, taking up some of the curve in the sail.
 

James_Calvert

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Here's another way of looking at it.

Before the advent of high technology, sails were made from ordinary sail cloth, which is flat. To get them the right shape when set, they were cut with the luff in a curve. If you could bend the mast to match the curve cut into the luff, they would be flat again.
 

Birdseye

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Hi there

It's counterintuitive to me that bending your mast flattens your mainsail. I would have thought it would make it fuller instead of flatter.

Does anyone understand why mast bend flattens your main? If you could explain in lamens terms that would be great.

Regards, Kirk
Its not clear what you are asking - do you mean bending the mast when it is set up already to match the curve of the sail? The sailmaker builds a curve into the luff of the mainsail, a curve which is supposed to match the bend in your mast set up to the standard for your boat. You can alter that mast curve under sail, much more on a fractional rig than on a masthead rig. But if you bend the mast further and touch nothing else then you dont flatten the sail - you let the top "blow off" to depower the sail.
 

RJJ

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2,937
I’m not sure I get that. Surely the mast is the same length? The mast doesn’t stretch in any sense, it simply goes around a corner a little. Same length but different route? The aluminium extrusion doesn’t magically become longer?
Length of mast and luff is the same (as long as the halyard is fixed).

Think about the horizontal distance from leech to luff, halfway up.

All three corners are fixed relative to each other. So the foot and the leech don't move. The only bit that changes is the luff, at any given point on the bend, moves further from the leech, stretching out the camber of the sail and flattening it.
 

flaming

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Length of mast and luff is the same (as long as the halyard is fixed).

Think about the horizontal distance from leech to luff, halfway up.

All three corners are fixed relative to each other. So the foot and the leech don't move. The only bit that changes is the luff, at any given point on the bend, moves further from the leech, stretching out the camber of the sail and flattening it.
Well, kinda...

If you pull on the backstay, then the first thing you are doing is reducing the distance between the top of the mast and the back of the boat, by moving the top of the mast aft and down. It is of course this movement that causes the mast to bend. This reduces the leech tension, as it reduces the distance between the head and the clew. So with no other adjustments, the 2 things that pulling on the backstay (on a fractional rig) does is to reduce the camber and reduce the leech tension, thus adding twist.

If you then pull on the sheet to get the leech tension the same as before, with the same profile, you will find that the boom is a few cm lower than it was before. Conversely, if you let off the backstay you often find you then need to ease the sheet to prevent the leech from being too straight.
 

Ianthediver

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4 Dec 2020
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38
Hi there

It's counterintuitive to me that bending your mast flattens your mainsail. I would have thought it would make it fuller instead of flatter.

Does anyone understand why mast bend flattens your main? If you could explain in lamens terms that would be great.

Regards, Kirk
I have sailed dinghys for many years, the luff is pulled down by the kicker pulling against the mast the rig is under tension. The mast is acting as a spring and the sail is forced flat. That is my interpretation.
 

Daydream believer

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I have sailed dinghys for many years, the luff is pulled down by the kicker pulling against the mast the rig is under tension. The mast is acting as a spring and the sail is forced flat. That is my interpretation.
I beg to differ
The kicker does not alter the luff line length. between gooseneck & head. Both are fixed points( assuming that the halyard does not stretch) The kicker pulls the end of the boom down which applies tension to the leech.
This tends to pull on the top of the mast causing it to bend within its length. This alters distance between leech & mast thus flattening the curve of the sail. Which is what I described way back in post #4 & enhanced in #6
 

RJJ

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I beg to differ
The kicker does not alter the luff line length. between gooseneck & head. Both are fixed points( assuming that the halyard does not stretch) The kicker pulls the end of the boom down which applies tension to the leech.
This tends to pull on the top of the mast causing it to bend within its length. This alters distance between leech & mast thus flattening the curve of the sail. Which is what I described way back in post #4 & enhanced in #6
Also the kicker pulls the mast forward at gooseneck level, unless resisted by lower shrouds or a strut.

Risks confusing the issue imho. Just talk about mast bend. Mast bend flattens the sail for the geometric reasons discussed.

Those reasons are a function of pre-bend from the shrouds and spreaders; or of backstay tension; or of the kicker; or of the downhaul on a windsurf-style sail. All those devices have other effects, but all of them can bend the mast which flattens the sail.

Which is the right way to think about it. The thought process shouldn't begin with "pull on the backstay". It should go "what sailing mode am I in; what sail shape do I want (how much depth/twist/draft position and at what angle); how do I use sail controls to achieve that shape?
 

RJJ

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14 Aug 2009
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2,937
I beg to differ
The kicker does not alter the luff line length. between gooseneck & head. Both are fixed points( assuming that the halyard does not stretch) The kicker pulls the end of the boom down which applies tension to the leech.
This tends to pull on the top of the mast causing it to bend within its length. This alters distance between leech & mast thus flattening the curve of the sail. Which is what I described way back in post #4 & enhanced in #6
Also the kicker pulls the mast forward at gooseneck level, unless resisted by lower shrouds or a strut.

Risks confusing the issue imho. Just talk about mast bend. Mast bend flattens the sail for the geometric reasons discussed.

Those reasons are a function of pre-bend from the shrouds and spreaders; or of backstay tension; or of the kicker; or of the downhaul on a windsurf-style sail. All those devices have other effects, but all of them can bend the mast which flattens the sail.

Which is the right way to think about it. The thought process shouldn't begin with "pull on the backstay". It should go "what sailing mode am I in; what sail shape do I want (how much depth/twist/draft position and at what angle); how do I use sail controls to achieve that shape?
 

kirky1

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11 Jul 2021
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Hi guys a big thanks to everyone who posted on this, I've got my head round it now.

Many thanks, Kirk
 

steveeasy

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12 Aug 2014
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Its not clear what you are asking - do you mean bending the mast when it is set up already to match the curve of the sail? The sailmaker builds a curve into the luff of the mainsail, a curve which is supposed to match the bend in your mast set up to the standard for your boat. You can alter that mast curve under sail, much more on a fractional rig than on a masthead rig. But if you bend the mast further and touch nothing else then you dont flatten the sail - you let the top "blow off" to depower the sail.

Always a place to start. Basics.

Steveeasy
 
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