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Old 30-04-2014, 18:45   #1
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Sail Trim Article about Twist

I've seen this before from the same people at this site: Three Reasons to twist out the top of the sail | Sailing Blog by NauticEd

Interesting read.
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Old 30-04-2014, 19:19   #2
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Re: Sail Trim Article about Twist

The first reason, "Get rid of excessive healing'" is complete bunk. Adding twist to the main is a strategy for lighter air, when healing happens to be insufficient. In heavy air, when the boat heals excessively, the main should be flattened.

Sail Trim 101.
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Old 01-05-2014, 01:06   #3
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Re: Sail Trim Article about Twist

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Originally Posted by Bash View Post
The first reason, "Get rid of excessive healing'" is complete bunk.
I agree that healing can never be excessive. Heeling, however....

I disagree with your post. Your main should be flat in heavy air only if your boat is not overpowered. If you can't reduce sail area but there is still excessive heeling, you should depower the main to decrease it and adding twist helps with that.

So "complete bunk" seems a gross overstatement to me.


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Old 01-05-2014, 01:21   #4
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Re: Sail Trim Article about Twist

Twist is also important due to wind shear associated with friction as the wind nears the surface of the water. The wind can be blowing over 10degrees diffenrtly from the foot to the head of the sail.

Twist is a critical, and poorly understood by many, element in effective sail trim.
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Old 01-05-2014, 10:37   #5
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Re: Sail Trim Article about Twist

Well, i suppose that adding twist to the main on a marconi rig could in fact reduce heeling in a gust or when overpowered , but my first choice would be to drop the traveler down and then see if easing the mainsheet would keep my craft on her feet before reefing.
Sailing gaff rigged boats a much more dramatic effect is observed if pressed by easing the peak halyard so that the leach blows off to leeward and so "scandalizing" the main thereby effectively reducing sail area in the highest part of the sail and reducing weather helm and heeling. This easing of the peak halyard is one of many advantages of this "fisherman's reef" that I miss on my current craft.
And yes, healing is what comes after when one fails to tie in a timely reef.

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Old 01-05-2014, 11:03   #6
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Quote:
Originally Posted by Bash View Post
The first reason, "Get rid of excessive healing'" is complete bunk. Adding twist to the main is a strategy for lighter air, when healing happens to be insufficient. In heavy air, when the boat heals excessively, the main should be flattened.

Sail Trim 101.
Flatness refers to the camber of the sail, not the twist. You can both tighten the outhaul and twist off the leech. I don't see the two as mutually exclusive
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Old 01-05-2014, 11:52   #7
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Sail Trim Article about Twist

@mrohr

"Easing the mainsheet" = adding twist

@bash

There comes a time as the wind increses that one goes from a very flat and travelled down main to one where it is traveled back up and twisted wide open in really heavy air - also known as "vang sheeting"
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Old 01-05-2014, 12:59   #8
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Re: Sail Trim Article about Twist

First get your telltales flying if you're trying to maximize power. Wind gradient is different in different conditions. If you set your sail to your telltales then you have set your sail to the current gradient.

If you're overpowered it depends on how interested you are in efficiency and keeping boat speed at a maximum. The racers will travel down as needed to keep the boat at target speed or optimum heel angle leaving the leech tight. The untwisted sail has less drag.

A twisted sail lowers center of effort and depowers the sail. It has more drag and slows the boat more, but requires less management.

Bethwaithe's book High Performance Sailing has a nice section on this.
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Old 01-05-2014, 13:14   #9
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Re: Sail Trim Article about Twist

Thats mostly true except that "racers" are using fractional rigs and the first gear in depowering terms is the backstay - which opens the leech at the top of the main (increases twist) as well as decreases draft depth and shifts draft forward on the main and headsail by pusshing the middle of the mast forward and increasing headstay tension. Second gear is travelling down.

Ideally when racing one wants to not react with the travelor but set it to tje wind as adjusting it requires you to adjust the headsail as well. Using the backstay adjustment to react to pressure is preferred and more efficient.
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Old 01-05-2014, 14:54   #10
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Re: Sail Trim Article about Twist

Quote:
Originally Posted by foolishsailor View Post
Thats mostly true except that "racers" are using fractional rigs and the first gear in depowering terms is the backstay - which opens the leech at the top of the main (increases twist) as well as decreases draft depth and shifts draft forward on the main and headsail by pusshing the middle of the mast forward and increasing headstay tension. Second gear is travelling down.

Ideally when racing one wants to not react with the travelor but set it to tje wind as adjusting it requires you to adjust the headsail as well. Using the backstay adjustment to react to pressure is preferred and more efficient.
Yeah the guys with bendy masts and fractional rigs have that option first. Bending the mast flattens the sail so even though it's twisted it is low drag.

As far as "racers" having fractional rigs and bendy masts maybe someone should tell the half of the people that were at this race they're not racers with their cruiser/racer, IOR, and even CCA boats.

This is one of the big time series here.
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Old 01-05-2014, 15:18   #11
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Re: Sail Trim Article about Twist

Quote:
Originally Posted by mrohr View Post
SNIP

Sailing gaff rigged boats

SNIP
Do they still make gaff rigged boats?

On the other hand on a boat with a square top, like my cat with a huge traveler trimming both the traveler and main sheet allows great adjustment of the twist.

Just as an aside any of you trimmers remember hearing a square top is like the first reef. Even on my cat when a gust hits the boat will jump but the top of the sail will also twist enough to lessen a lot of the overpowering.
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Old 01-05-2014, 15:26   #12
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Sail Trim Article about Twist

IOR not bendy? They are the bendiest ever! Cant stay upright or avoid snapping without jack stays. That and those silly spreaders at 0degree rake to make the mast more bendy.

One doesnt need to be fractional to be bendy.

@tomf

Many high performance classes go for high aspect sails "square top" - they are faster amd more efficient amd really easy to adjust upwind, however in open classes there is a huge rating handicap to high aspect/high performance sails which is the only reason they are not more prevelant in the "racer/cruiser" classes and even in the cruising classes.
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Old 01-05-2014, 15:31   #13
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Re: Sail Trim Article about Twist

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Originally Posted by cwyckham View Post
Flatness refers to the camber of the sail, not the twist. You can both tighten the outhaul and twist off the leech. I don't see the two as mutually exclusive
Exactly. Well put.
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Old 01-05-2014, 15:48   #14
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Re: Sail Trim Article about Twist

I disagree though with the reasoning in the blog, under heading 1), as to why the heeling moment is reduced when the sail is twisted.

The overwhelming reason why the Centre of Effort moves downwards is NOT the geometric one suggested by the diagram (which I think is geometrically spurious: the geometric centre probably actually RISES when the sail twists off, because of the boom lifting)

It's a question of pressure distribution: when the sail is twisted off further than what would be ideal trim on an hull with infinite righting moment, that top portion is effectively undersheeted

If it's twisted sufficiently to prevent being overpowered when the wind is well over the ideal for the righting moment available, it will be luffing.

It is a nonsense to suggest that the C of E of a non-uniformly loaded sail is still at the geometric centre, as the middle diagram implies. It has to be relocated downwards in consideration of the pressure distribution, a bit like calculating a 'weighted average'.

In the instance were it's luffing, it's contributing mainly drag, whose heeling effect is minimal, so it's almost as though it's no longer part of the 'right triangle' of this oversimplistic explanation.

It is no longer a question of simple geometry, but of differential calculus, or some practical approximation.

It's an instance, I think, of a lay person knowing what the correct answer is, and devising a plausible explanation for that answer, which happens to be wrong and misleading.
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Old 01-05-2014, 15:50   #15
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Sail Trim Article about Twist

To get back to the essence of the question by the OP, to agree with Bash and Cal40, and to reference to the article he linked...

Generally your telltales, boatspeed and heel will tell you all you need in regards to sail trim.

If you are a racer you need to know your upwind polars to optimze twist as it ties into VMG since heel, drift and boat speed are all at play - but as a cruiser you are not trying to "olympic optimize" but are just trying to make miles comforably and effectively.

If you dont have a fractal rig and an effective boom vang amd backstay then twist also doesnt really apply to you either.

Lets say you have a heavy mast and are a full or 7/8 rig boat. The only real way to induce twist is to ease the vang or eas the sheet. This will reduce your heel but will also reduce your speed. It is more effective to drop your travvelor to leeward.

The idea of depowering sail trim while maintaining speed, which is the heart of the OP and the linked article, is tied to the way and oreder of events in depowering your sail. The core theory that many miss is the concept of maximum hull speed. Once your boat hits that any vector of force left over in regards to wind power generally just results in heel.

Both in racing and cruising I depower in the following order with the idea that in depowering I am still looking for maximum efficiency.

Crank on Backstay/
Travellor down
Ease main sheet
Travellor above center, fully eased boomvang (at this point cruising one would generally not be going upwind any longer)
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