National 12 Tuning Guide

Transcription

National 12 Tuning Guide
Introduction .................................................................................................................................. 2
How the rig works ......................................................................................................................... 2
Sail Basics................................................................................................................................ 2
Jib Slot ..................................................................................................................................... 3
The influence of the spreaders. .................................................................................................. 4
Fore and aft bend ...................................................................................................................... 4
Sideways bend .......................................................................................................................... 4
Jib sheeting position .................................................................................................................. 4
Deck level control ...................................................................................................................... 5
Outhaul..................................................................................................................................... 5
Cunningham ............................................................................................................................. 5
Kicker ....................................................................................................................................... 5
Mainsheet ................................................................................................................................. 5
Set up method.............................................................................................................................. 6
Coarse set up............................................................................................................................ 6
Fine Tuning - jib ........................................................................................................................ 8
Fine tuning – mainsail ................................................................................................................ 9
Dynamic tuning ....................................................................................................................... 10
Choosing your settings ................................................................................................................ 12
Collect some figures ................................................................................................................ 12
Standard settings used by various people ................................................................................. 13
Other considerations ................................................................................................................... 14
Making a dangly pole ............................................................................................................... 14
Centreboard ............................................................................................................................ 14
Rudder ................................................................................................................................... 15
Mast – stiffness comparisons and how to measure .................................................................... 15
Mast – stiffness comparisons and how to measure .................................................................... 16
SAILS ..................................................................................................................................... 17
Black bands ............................................................................................................................... 20
Thanks to................................................................................................................................... 21
1st revision
2nd revision
National 12 Owners Association
1991
2004
? www.national12.org
Various
Kevan Bloor , Graham Camm
& Zoë Ballantyne
? [email protected]
1
Introduction
Do you feel frustrated in your boat? Does sailing fail to bring the satisfaction that it used to? Has the
sensual pleasure of yachting paled to a former shadow of its glorious past? Is there something
missing? Boat speed? Boat speed is what you get from tuning or, conversely, tuning is how you make
your existing boat go faster. So many articles and books have been published on tuning that anyone
ought to be able to tune a boat.
The effect of boat speed on sailors is intoxicating. Racing becomes a doddle. Tactics fall into place
naturally and the crew's ego travels in a large cloud above the burgee. Every situation is faced with
confidence and a dominating ability to escape and continue to the head of the fleet. On the days when
the boat is the wrong shape or the rig will not behave, when the boat speed has evaporated the
intoxication becomes more like a hangover. Tactical situations invariably lead to an even worse
position and the crew's spirits are somewhere about the level of the water in the bilge. Racing let
alone winning, is a struggle.
Tuning is about getting many parts of the boat working in harmony. It is the combination of mast
stiffness, rig tension, spreaders, sail shape, centre board position & section and rudder (to name but a
few) that need to be set up to match each other. With so many variables (and we haven’t even got to
the fickleness of the wind yet) it should come as no surprise that tuning is an iterative process. Whilst
the coarse rig set up can be achieved fairly quickly, it can take a season to get it perfected as you
need to sail in a fleet in all conditions from zephyr light breezes to honking gales.
The ability to tune and change the boat is one of the best features of the National 12. So the best plan
is to read this guide, get a basic set up then go sailing. Make notes on the conditions and your
performance, analyse the performance then make some changes. Then keep on iterating until it feels
right.
There are many books on how sails work and how to get the most out of a boat in general. This guide
is written to describe some of the subtleties of “getting the most” out of a National 12.
How the rig works
Here are a few basic points on how the rig works and how it is controlled.
Sail Basics
The rig on any boat is the engine which
drives the boat through the water. An
understanding of how a sail works is
necessary in order to get the best out of your
tuning and sail setting. The sails work by
generating aerodynamic forces much like
the wing of an aeroplane. The lift that
keeps the plane in the sky drives the boat
forward. The air flow is deflected round the
sails and in simplistic terms the greater the
deflection (the fuller the sail camber) the
greater the amount of drive generated.
FIG 1
FIG 1 is a diagrammatic representation of the
drive generated by the wind passing round the sail.
If we transpose the above graphic on to a
boat and note the direction in which we wish
to travel, we see an interesting fact. When
the sail is set on the centre line with no
twist, the after part of the mainsail is not
helping the boat’s progress forward and is in
fact creating drag.
? [email protected] FIG 2
2
We need, therefore, to induce or regulate an
amount of twist so that we minimize the
drag from the sail whilst generating as much
drive (deflection of airflow) as possible.
The optimum is to set the sail with enough
twist so that the leech is as close to the
centreline as possible (thereby obtaining the
maximum deflection in the airflow) without
the leech of the sail hooking round more
than parallel to the centreline of the boat.
The twist in the sail will increase as we look
up the sail, close to the centreline at the foot
and progressively more open. Below is a
diagram showing the section of the sail near
the foot and at mid height showing the wind
direction and the desired leech position,
FIG 3
Note how much flatter the sail section
needs to be near the foot and the fullness
further forward. Higher in the sail the shape
is fuller with the camber further aft.
Jib Slot
FIG 4
The jib slot is very important in accelerating
the air round the leeward side of the
mainsail and is paramount in delivering
good boat speed. If the slot is too open,
the air is not squeezed enough to obtain
maximum energy. If the slot is closed too
much, the air flow is constricted and the
mainsail will backwind. In light /medium
winds use the lower luff area of the mainsail
as an indicator that the slot is set
correctly, a very small amount of panting
of the sail should be observed.
FIG 5
?? Major back winding
would indicate that the
slot is too closed.
?? Use the jib sheeting
angle and sheet tension
to achieve the optimum.
Possibly inboard/
outboard adjustments
may be necessary.
?? Use the sheeting angle
marks on the jib as below
to calibrate your fairleads.
FIG 6
FIG 7
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The influence of the spreaders.
The length and angle of the spreaders and, to a small extent, the height of the spreaders will all affect
the mast. The spreaders should be long enough to keep the mast straight sideways from deck level to
the hounds. If the mast bends to windward, pointing ability be affected though the boat may be easier
to hold upright. If the mast bends to leeward, this will close the slot, affect boat speed and make the
boat more difficult to hold upright.
The length of the spreaders control the lateral stiffness and the fore and aft angle of the
spreaders control the fore and aft bend. The mast is stiffened sideways by deflecting the natural
line of the shroud outward: the further the deflection, the greater the stiffening effect.
The angle of the spreaders affects the mast stiffness. When the spreaders are angled forward
and deflect the shroud forward of its natural line the mast is made stiffer fore and aft, when they are
angled back the mast is made more flexible.
Using different spreader lengths and angles the object is to obtain the optimum bend characteristic for
your crew weight and sail fullness etc. Mast rams / screws, etc, are used to restrict or control overall
bend, particularly in the lower region.
Fore and aft bend
Fore and aft bend controls the amount of camber in the mainsail and the tightness of the leech (and
therefore the power). The actual amount and position of bend required for optimum performance will
vary from boat to boat and different sails and crew weight will also require different settings. Since the
amount of power in the mainsail is controlled by fore and aft bend then it follows that if you are either
under or over-powered in windier conditions then you may be able to remedy this with different
settings. If the boat is difficult to hold upright (especially when compared with other boats with
similar rigs and crew weights) then possibly the mast needs to be softer (move spreaders aft). If you
feel under-powered, i.e. that you should be sitting out harder and earlier, or the boat lacks
acceleration in puffy conditions, then the mast may need to be stiffer (move spreaders forward). When
assessing mast bend it is important that it is done on the water in a sailing situation, in the dinghy park
spreader settings don't have the same amount of effect.
Sideways bend
This can only be checked on the water from outside the boat (obviously by another person). When
sailing upwind the mast should stand straight between deck and hounds. If the mast bows to
windward, this de-powers the rig by opening the jib slot and freeing the mainsail leech, which makes
the boat easier to hold up but also leads to lack of pointing ability. Remedy - longer spreaders.
Should the mast bend to leeward, the jib slot is closed and the mainsail leech is tightened. The light
weather performance will be adversely affected and, in heavy weather, the boat very difficult to hold
up. Remedy - shorter spreaders.
Having found a particular setting for the spreaders on your boat, record the information as
(a) A measurement from the mast to the spreader tip (spreader length) and
(b) The distance from the aft side of the mast to a straight line between each shroud where they
pass through the spreader (spreader depth)
In this way it is easier to make accurate changes without having to find a long ladder or suitable roof
since the adjustments can easily be checked with the boat on its side. Similarly mast rake should be
checked by a measurement from the mast head to the top of the transom (on the centre line) or other
suitable datum at the aft end of the boat. Any changes can then be easily done without the problems
of trying to level the boat and accurately measure from the mast to a halyard blowing around in a
force eight gale!
Jib sheeting position
The jib sheeting position affects pointing ability, the shape of the jib and also the size of the slot and
backwind in the mainsail. Jib sheeting should be fairly steep, the line of the jib sheet if extended
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4
forwards and upwards would normally bisect the line of the luff at a point 40-50% up the luff from the
tack (It can be useful to mark this jib sheeting angle on to the sail near the clew).
Deck level control
The mast should be restricted in its travel in the mast slot using either chocks, mast ram or strut etc.
Having set the boat up with the required mast rake, pre-bend and rig tension the mast position at deck
level should be marked and calibrated and the deck level control should hold the mast at the set up
position. In strong winds when the rig needs to be de-powered the mast should be allowed to travel
forward by around 15 – 20 mm at deck level by removing a chock or changing the ram / strut setting.
For boats with struts which can induce pre-bend in very light winds it is usually helpful to increase the
nominal pre-bend to flatten off the mainsail as much as possible without having to increase rig tension
(Excessive rig tension can distort jib shape in very light winds).
Lowers should only be used to control the mast offwind. The use of lowers upwind can have a
detrimental effect on boat speed especially in a large chop. They will keep the mast too stiff and not
let the mast absorb the shock as the boat hits a wave, thus stopping the boat.
Outhaul
The outhaul is an important control and therefore should have an efficient slider and purchase
system. The normal minimum purchase would be 4:1. The outhaul should be pulled tight in all
conditions when going upwind and should be eased considerably (maybe up to 6/7 inches) when
sailing downwind, except for running when the outhaul should be pulled tight again to project
maximum sail area.
Cunningham
The Cunningham is used to modify the sail camber and de-power the rig in strong winds. Very little
Cunningham tension is used in light/medium conditions as maximum power is required. Once the
boat is overpowered the Cunningham can be used to flatten the mainsail and open the leach.
Remember that in most conditions you should see slight wrinkles running perpendicular to the mast at
the luff. If you tension the Cunningham and remove these creases you will almost certainly have
compromised your sail shape and power.
Kicker
The kicking strap is an important means of controlling leach twist and mast bend and thereby
adjusting the power in the rig. It is important that the type of purchase used enables the boom to lift in
very light airs and also be capable of bending the mast as required in very strong winds. Many
systems don’t give enough range of adjustment. In light winds with the kicker off you should be able to
lift the boom around 6/10 inches at the outer end before the kicker pulls tight. In strong winds you
should be able to bend the mast with the kicker so that the sail will just start to distort with creases
running from the clew towards the spreader bracket (though in practice you may never actually apply
this amount of kicker). So that the kicker is easily adjustable a purchase of around 16:1 should be
used.
Mainsheet
This is also used to control twist in the mainsail as well as controlling the angle of the boom.
Remember that tensioning the kicker will bend the mast whereas pulling the mainsheet harder will
close the leech without bending the mast to the same extent. Therefore when you need to power up
the mainsail in light/medium conditions use the mainsheet in preference to the kicker.
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Set up method
There are three stages to the set up process; first start with a coarse set up to get the rig roughly in
the right position, then move on to fine tuning and finally optimise the dynamic performance so the rig
responds as you want in gusts and various wind conditions.
Coarse set up
Once you have decided what figures to go for, this is how to set the rig up to those settings.
(a) Fix the mast heel position,
(b) Rig the boat and measure the forestay tension. Next increase the forestay tension to the
figure decided. This tension is necessary because the jib will normally be cut with a 15mm luff
hollow and under average conditions the forestay will sag 15mm if thus tensioned when the
jib is sheeted in.
(c) Adjust the spreader aft deflection (this is best done from a garage roof or balcony) and clamp
the spreaders in position with a couple of mole grips.
(d) Measure the static mast bend (NB. at this stage the mast must be unrestricted in the fore and
aft plan at deck level). The easiest way to measure the static mast bend is to fix the end of
the main haIyard to the aft of the mast at the gooseneck, apply enough tension to straighten
the halyard and then measure the maximum deflection between the halyard and the mast.
(e) Check the mast rake by setting the boat up level to its water line (a spirit level on top of the
plate case is normally good enough here) and using the main halyard as a plumb line
measure the rake from the aft of the mast at the gooseneck to the main halyard. If the rake is
not enough decrease the length of the shrouds, increase the length of the forestay and repeat
the whole exercise again.
NOTE: It is imperative that the forestay tension is measured with the mast chocked at deck
level and that the spreader aft deflection, static mast bend and mast rake are measured with
the chocks removed. It is more important to achieve the correct static bend than the correct
spreader aft deflection but normally the two should coincide. Once the correct measurements
have been achieved fix the spreaders and check everything again.
(f) Finally with the level chock / mast ram removed mark the neutral mast position at deck level
and then arrange fittings so that you can lock the mast in this position for medium wind
strengths. For light and strong winds arrange fittings so the mast can be held up to 10mm to
15mm forward of this neutral position to provide pre-bend as necessary.
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Spreader
Deflection
Measured
from end of
spreader to
line
strength
from
Mast-bend,
measured from
mast head to
gooseneck
with no kicker
Rake, plumb line
from mast head to
gooseneck
Distance from mast
foot to transom
Rig measurements: in profile
Rig measurements: from rear
Note: Pre-bend in the mast should be measured without
the sail hoisted (as the halyard tension and sail weight
will affect the bend)
A = Spreader length, measured from
centre of wire at tip of spreader to the
middle of the mast track.
B = Spreader depth, measured from back
of mast track to a line stretched between
the tips of the spreaders
Spread measurements: plan view
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Fine Tuning - jib
Jib fully powered up good for strong winds on flat water
Jib sheeting angle too close to the centre line. When the jib is pulled in tight, it closes off the slot between
the jib and mainsail preventing the air travelling through. The jib will backwind the mainsail and the luff of
the mainsail will lift
Jib sheeting angle too far aft. This flattens the bottom of the jib too much, reducing power, and allows the
top of the jib to twist off too much, spilling wind.
LEFT: Jib sheeting angle for light winds. Jib sheet slightly eased
to create a small amount of twist.
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Fine tuning – mainsail
Perfect pre-bend to create ideal main sail shape for
medium winds. Mast bend matches the luff round of
the main. Max draft 1/3 along sail (indicated by
arrow.)
Main sail powered up for the reach, outhaul let off to
power up the foot, leeward shroud released to
straighten the mast to power up the middle and top of
the sail..
Fully depowered, cunningham tight, shroud tension
tight, kicker hard on. Sail is flattened off but retains
enough power. Max draft about half way back.
Mast too straight, maximum draft too deep and too
far forwards, mainsail will back wind, pointing may be
affected.
LEFT: Mast too bent, sail too flat, position of
maximum sail depth has moved too far back towards
the transom.
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Dynamic tuning
It is always pleasant to have pictures of your boat in action but photographs are also a useful tuning
aid. If you have some influence over the photographer get them to take pictures of the boat - directly
broadside from windward and leeward (fore and aft mast bend, jib luff sag, mast rake, boat trim);
directly behind (sideways mast bend, mainsail leech tightness, boat trim); directly in front (jib luff sag,
sideways mast bend); and from leeward quarter (jib slot).
Boat tuning is often easier and more enjoyable if you can involve another boat and crew. On the water
tuning is often more valuable if you have a pace maker to assess your changes against. Swapping
boats and the discussion of the problems involved all adds to the enjoyment.
The two things that can be set up on the water without the aid of another boat or photographer are
balance and gust response.
(a) Balance – when sailing flat there should be a small amount of weather helm (i.e you should
have to pull the tiller towards you slightly) With the boat heeled slightly to windward (3-5
degrees) the helm should be neutral. The position of the mast foot, amount of rake and board
position should be adjusted to achieve the neutral balance. If there is lee helm then the mast
foot can be moved backwards, the mast raked further back or at a last resort the board
moved further forwards.
(b) Gust response – the rig should de-power from the top of the sail down, so when it is windy
and a gust hits the top sail should de-power first. This requires the right amount of twist in the
sail and bend in the mast so the sail is flat.
Using tell tales to adjust sheeting angles
There is little that can be done to adjust the overall fullness of the jib or the angle of attack of the luff
as these are cut into the sail by the sail maker, though minor sheeting adjustments will make sure that
fullness and the angle of attack are balanced all the way up the sail. This balance can be checked
using luff telltales which should be fitted to all jibs. Many articles have been written on how to sail with
luff telltales but as far as balance up and down the sail is concerned just make sure that all the
telltales on the windward side all do the same thing at the same time and the telltales on the leeward
side likewise. This should happen anyway if the leech is set correctly. Getting all the telltales to
stream parallel is then just a matter of pointing the boat in the right direction.
Remember patience is the watchword and only adjust one item at a time
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Photo 1 is a rear view sailing to windward,
because of the mast rake the slot between the
main and jib is open at the top. The wind is light
so the clew outhaul is pulled tight to flatten the
bottom of the mainsail. (in the medium winds the
clew would be eased 25mm and then pulled tight
as the wind strengthened). The mast is chocked
12mm forward of its neutral position giving a nice
even mast bend. The overall result being
maximum drive with no back winding of the main
by the jib.
Photo 1 – side view, light winds
Photo 2 is taken from the front in medium airs,
note that the kicker has been pulled on to achieve
a parallel top thus giving maximum power. The
boom is eased to give drive and the boat is being
sailed dead flat with the transom just immersed to
reduce drag.
Photo 2 – rear showing top batten,
medium winds
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Photo 3 is taken from the front in strong
winds. The jib leech is still tight although it is
starting to open up a little at the top. The
mainsail is flat at the top and developing
twist, this reduces the drag in the sail and
also the power.
With heavy weights it should not be
necessary to chock the mast forward of the
neutral position in strong winds so for
medium and strong winds chock the mast
neutral and pre-bend by 15mm in light airs.
Photo 3, upwind in strong winds
Choosing your settings
So far the guide has talked about how the rig works and how to set up the rig in a repeatable. Now
comes the time for you to decide your settings. You can always copy someone else’s but there are
probably some subtleties – your combined crew weight, your sail cut, the bend properties of your
mast and the power that your hull design can take.
The table on the next page gives some typical figures used by top National 12 sailors.
Collect some figures
Go sailing in the conditions where you are just starting to de-power. Pull on the shrouds so the leeward shroud is just starting to pant (i.e just starting to take up the tension). When you come ashore
measure the rig tension. This is a critical rig tension as up to this point you are searching for power
and after this point you are trying to lose power. So it is a good tension to use as your benchmark.
Perform the tool box test on your mast to understand how bendy it is
Measure the luff round in your mainsail.
Adjust the settings to make your mainsail look right across the wind conditions
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Standard settings used by various people
Setting
P&B
Design 8
Dave Peacock’s
Baggy
Alverbanks
Graham & Zoë
Numinous
Crew weight (Stone)
P&B
Foolish
18 stone
Stevie Sallis
Foolish
19 stone
Aluminium
Proctor Kappa
Aluminium
Proctor C
Carbon
Angell
Carbon
SuperSpar
Aluminium
SuperSpar M7
Mast to transom (mm)
2555
2500
2540
2500
2500
Centreboard bolt transom (mm)
2120
Mast type
Rake Light – Windy (mm)
200 – 400
2100
380
300
200-400
2200
Spreader Length (A) – (mm)
345
320 - 360
345
333
Spreaders Depth (B) – (mm)
157
130
110
85
Shroud deflection outwards (mm)
40
30
30
Shroud deflection forwards (mm)
25 (according to
pre-bend)
10
25
3040
2960
2900
2940
15 – 20
30 (according to
sail luff round)
20
250lB on forestay
200-250lbs on
forestay
150-400 lbs on
shrouds
enough to keep the
shrouds taught
when sailing.
2200
2050
380
320
Spreader height (mm measured
from shear line)
2950
Mast pre-bend – (mm) 1
Rig Tension; Light – Windy (Lbs)2
350-400 lbs on
shrouds
Jib sheet (mm from transom) 3
1800 (to back of
track)
1900
Jib sheet (mm from centre line)
400
375
360
40
1
Pre-bend should be set to suit the sails, in particular the luff round.
Rig tension is best measured on the jib luff (rather than the shrouds) this is because the shroud tension tends to vary more with different shroud bases (width of boat and distance back from the mast)
3
Note: The jib sheet position is further forwards on double bottom boats as the floor is higher.
2
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Other considerations
Making a dangly pole
The idea (like most good ones) is blindingly simple. The pole slides up
and down the mast on a piece of string that is tied from the spreaders to
the jibstick eye or gooseneck. It is launched with a line that runs from the
clew of the jib through the length of the pole and then down to a cleat on
the deck. Retrieval is simply done by a piece of elastic dead ended at
the top of the pole round the spreaders and down the mast to the goose
neck area. Velcro on the bottom end of the pole is sufficient to keep it
snug to the mast up the beats.
The dangly pole certainly increases offwind boatspeed. The crew needs
to adopt a new technique as they are using both the launch string and
the jib sheet to control the twist and shape of the jib. (see the technique
guide)
Centreboard
The key properties of the centreboard are lift (i.e. prevent leeway) whilst minimising drag. To achieve this the
area of the board, profile (i.e shape when looking from the side) and the section can all be selected. There are
also options for gybing boards, laminar flow boards or standard NACA sections.
The theory of laminar flow foils
Laminar flow foils attempt to prevent the flow going turbulent therefore reducing drag. To achieve this laminar
flow boards generally have a finer entry and the maximum width further back (say at 40% of the chord rather
than 33% as per the standard NACA sections). Laminar flow sections can operate at a very small angle of
attack (<4 degrees) so a National 12 is just in this range with leeway less than 4 degrees. Starting from
stationary can be a problem as the flow stalls more easily.
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The theory of gybing boards
The gybing board works by the sideways pressure of the water pushing the front edge of the board to windward,
twisting in the centre-board case. The theory is that the water increases its angle of attack on the foil and
therefore more lift is produced. Typical leeway is 2 to 4 degrees so a gybing board may reduce this. The gybing
board is most effective in light to medium air and flattish water. In windy conditions you do not want the
centreboard to gybe. Generally the centreboards are designed so that by bringing them up very slightly, part of
the aerofoil blade goes inside the centreboard trunk, and jams at full width, stopping the centreboard from
gybing.
Aspect ratio
Generally drag on foils is caused by the turbulence at the tip, so for a given area then the longer the foil, the
smaller the impact of the end turbulence will be. Higher aspect ratio is therefore better - but you are limited by
the class rules.
Parameter
Rudder section NACA
Approx area
Length (below waterline)
Chord
Width
Profile
Standard board
0010
Laminar flow
Max width 40% of chord
Gybing board
0008
1070mm
400mm
25 – 32mm
Elliptical
1070mm
400mm
20mm
Elliptical
1070mm
350mm
50mm
Elliptical
Rudder
The key properties of the rudder are to produce a little bit of lift but mainly to turn the water without stalling whilst
also minimising drag in a straight line. As with the centreboard the key parameters are area, profile, section
(generally NACA). There are options for buoyant rudders which provide static lift or winged rudders which
provide lift (or downwards force) at speed.
Buoyant rudders are hollow and can provide lift whilst static in the region of ~10KG. This provides lift at the
transom and can be a benefit in light winds preventing the stern dragging. The only downside is the bigger cross
section and increased overall area which will increase drag slightly.
Parameter
Rudder section naca
Approx area
Length (below waterline)
Chord
Width
Profile
Standard rudder
0010
Slim line rudder
0010
Buoyant rudder
0014
800mm
200mm
25 – 32mm
Elliptical
750mm
175mm
20mm
Elliptical
800mm
250mm
50mm
Elliptical
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Mast – stiffness comparisons and how to measure
The key properties are weight, stiffness and springiness (how quickly the mast drops off in a gust and how
quickly it returns). The mast can be well controlled with the spreaders so while the mast section has a certain
stiffness the overall stiffness is very much influenced by the whole rig set up.
Materials
Carbon is lighter and springier, which gives better gust response than aluminium. Aluminium is stiffer, cheaper
and some may argue more durable. Production carbon masts are available from SuperSpars and Angel. There
are three common aluminium masts available at present. Proctor "C" and SuperSpar Ml sections of medium
stiffness and the Proctor Kappa, slightly stiffer for more power downwind, but harder work upwind
?? The shrouds and rig tension along with the mast stiffness all combine to make the perfect rig set up.
Some people sail with very stiff masts and do well, others sail with more floppy masts and do well
whether sailing with heavy or light crew weights.
?? The mast must be a tight fit sideways at deck level. We recommend a mast ram or strut at deck level is
used to control fore and aft bend upwind.
?? Limited swing spreaders are a must for maximum speed downwind. The outer tips should move around
40 mm forward when sailing downwind, which in turn will keep the mast straighter.
Table of mast comparisons
Mast bend comparisons
Load = One sailors tool box (18Kg)
Carbon Original Proctor
Carbon Superspar
Carbon Angel
Carbon (Mike Cooke’s N3489)
Aluminium Proctor Kappa
Aluminium Superspar M1
Aluminium Superspar M7
Fore-aft
(mm)
55
62
64
68
55
46
47
Side
(mm)
87
Weight
(Kg)
5.5
5.5
6.0
82
69
64
7.5
?
?
Note: There is no easy way to measure springiness hence no figures in the table.
The tool box test
The above measurements were taken using the “tool box test” as follows:
1) Support the mast at its tip and also at the gooseneck
2) Find a tool box that weighs 18KG
3) Hang the tool box at the spreaders
4) Pull the halyard tight from the tip to the gooseneck
5) Measure the maximum deflection of the mast and measure the position of this relative to the gooseneck.
6) Take lots of other measurements along the mast and plot this relative to the luff round of the main sail.
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16
SAILS
Sail Construction
All sails are designed as three dimensional shapes and aren’t just flat areas of sailcloth that just ‘blow into
shape’. Most of the panels in the sail are designed to give a specified shape and fullness to the finished product.
When you lay a mainsail on the ground it will lay fairly flat with a noticeable convex curve at the luff. You may
choose to measure the amount of curve (as shown in the photo below). This gives you an idea of how full the
sail is (bigger curve = more sail camber) but do
remember that this curve is influenced by the bolt
rope (in a used sail the bolt rope will gather and
cause wrinkling along the luff tape and change the
apparent curve). This curve should be called the
total fullness curve, as it represents the point at
which the mast bend would pull the sail flat and
destroy the designed shape.
In reality this curve is the combination of the sail’s
built in fullness and the actual luff round cut into
the sail. In a sail that shows around 150mm of total
fullness curve, the luff round would in fact will be
far less than this (dependent on the cut it could
50% less) approx 80mm.
The luff round is the key to determining how much
prebend your sail will need.
Measuring the fullness of your main sail
Comparing the luff round relative to the mast
bend
Typical main sail luff round compared to fore-aft mast bent
Mast bend and main sail luff curve along the length
180
160
Main sail luff round (mm)
140
Angel Fore-aft Bend (mm)
Bend (mm)
120
100
80
60
40
20
0
0
1000
2000
3000
4000
5000
6000
(mm)
Above: Purple curve shows mainsail luff round, green curve shows fore aft bend for an Angel mast, both against length
A jib, when laid on the ground would show an S bend convex in the lower half (approx) and concave higher up
towards the head. In reality the sail has no convex curve at all cut into the luff, in fact it will have a luff hollow
which would normally be around 15-20mm. The apparent lower convexity is the built in fullness distorting what
you are observing.
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FIG 8 Right Illustrates:
the panel shaping,
fullness curve
with sail laid flat
and the true luff round
with sail flaked to
remove the influence
of the built in shape.
Materials:
Material
Dacron
Woven Polyester fibre with resin coating to
increase cloth stability (resistance to stretch
especially on the diagonal / bias).
Examples:
Laminates
Laminate sailcloth is usually made up of 2
layers of Polyester film, approx 1mm thick, with a
scrim of woven fibres sandwiched between. The
weights commonly used Polyester laminate: all
the scrim fibres are Polyester.
Contender Polykote;
Bainbridge NYT (now discontinued);
Dimension HTTP +.
Pentex Laminate: the scrim fibres are a mix of
Polyester and Pentex (Pentex is a modified
polyester with very low stretch)
Kevlar laminate: The scrim fibres are a mix of
Polyester Kevlar and Spectra
All the laminates rely substantially on the film for
the basic stability of the cloth. The scrim adds
tear strength to the cloth and supports the film as
the loads on the cloth increase. Kevlar gives the
highest resistance to stretch, though the fibre
does have issues of durability.
Weights
Usually 3.9 or 4.46 oz or similar
Twelve sails range from 2.8 to 3.4 oz.
Shape &
life
Sail shape changes slightly initially, and later
in life more pronounced changes take place
and bring about a decline in performance as
the finish of the cloth degrades. The power
and pointing ability of the boat is reduced as
the sail flexes and stretches in stronger
winds.
All laminate sails, apart from being lighter in
weight, are more consistent in their shape
retention during their racing life, but suffer from
the film fatigue and delamination, due to constant
flexing and fluttering. Consequently their physical
life can be much shorter than Dacron.
Dacron sails may lose their shape in a
relatively short time, but they have a pretty
long physical life.
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Ratios and Dimensions
??
??
??
??
Most current Ratios are between 2.2 sq mtr jib - 6.2 sqmtr main or 2.4 sq mtr - 6.0 sq mtrs.
Mainsail luff lengths range from 5400 to 5486mm.
Jib luff lengths are pretty standardised at 3800mm.
Relationship with boom Under the current Class rules the bottom mast band is permitted to be
above the boom(check with the current rules). This gives some margin for optimising the rig. A
reduction in the luff dimension = a potential increase in the foot length and consequential wider cross
width allowances for the mainsail. Having the band above the boom does also enable the loose foot to
flip from one side of the boom to the other when gybing etc.
Calibration
Mainsail
Fit the usual leech telltales below each batten pocket (others can be fitted where you wish, but are not
necessary and beware of getting into a ‘telltale watching’ syndrome) You can mark the boom and the mast with
number strips for different outhaul and cunningham settings.
The top batten should be set at one tension ( reasonably tight) for most conditions. If the top batten becomes
difficult to gybe in very light winds, ease the tension to allow 3mm of slackness in the pocket and this problem
should ease. You can mark the correct tension for the batten with a felt pen on the batten corresponding with
the leech.
Jib
Telltales should be fitted about 120/180mm behind th luff (3 sets required) A single telltale can be fitted about a
third down the leech where it is visible through the mainsail window (if fitted).
Mark the three sheeting angle lines on the clew ( as shown in fig.6). The fore and aft sheeting position in the
boat should be capable of achieving any of these angles (particularly the 40% and 50% angles). Any
adjustments should be made with reference to these lines.
You should start with the sheeting at about 45% of the luff. Then check that all the luff telltales react in unison. If
the top telltale lifts before the lower ones move the jib sheeting forward (steeper angle towards the 50 % mark)
and vice versa.
Using the sheeting angle marks on the jib rather than the jib track position etc is important because, if you
choose the change the mast position/rake, the jib clew will rise or fall and throw your jib track reference out.
Also if you are comparing track positions with another boat it is irrelevant if the jib size and geometry is different.
If you are experimenting with mast rake you should re-adjust your jib sheeting. Otherwise your change in
performance may be due to the sheeting angle change and not the difference in rake.
Check the slot between the jib leech and the mainsail. It should be parallel from top to bottom and the
twist/curve in the jib leech should match the mainsail curve. When the setup seems ok on the land, mark the jib
sheet as an indicator for the correct sheet tension. The settings should be checked on the water and adjusted
as necessary and additional marks used if required for angle/ tension changes.
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19
Black bands
As you know the 12 has a maximum total measured sail area of 8.4 m2. This may be divided however you
choose between the main and the jib. This means that you can have any size of mainsail or jib you like provided
that their combined areas total 8.4 m2 or less. The way that we measure the area of the sails is also very
simple. As you will remember from way back when you were at school (or not so way back) the area of a
triangle is half of the length of the base, multiplied by the height. Now, for even the most mathematically
challenged, using a calculator to multiply two numbers together and dividing the result by two is not a roblem.
When we measure the rig all we do is pretend that the main and jib are both simple triangles, find the size of
each of them, add them together and voilá.
So where do we get the numbers that we need to multiply together to find the size of these two triangles I hear
you all asking; obviously this must be the complex bit, but no…
For the jib
· The length of material measured along the luff of the jib.
· The shortest distance from the clew to the luff (usually a point about two feet up from the tack.
For the main – we don’t actually measure the sail, we measure the mast and boom bands and insist that the sail
must stay within these bands. The bands allow everyone to see at a glance that a boat is being sailed legally.
· The distance from the bottom of the top band on the mast to the top of the bottom band on the mast.
· The distance from the inboard edge of the boom band to the mast.
(You should look at the rules or ask your friendly local measurer to see exactly how to take these
measurements.)
This gives you two areas to add together to find out how big your rig measures.
A=BxH/2
As you can see once you’ve got a suit of sails the jib
area is fairly fixed (but it’s amazing what you can do
to a jib’s area by taking a sharp knife to the head and
tack). Because we measure the bands, not the sail,
moving the bands on the spars can easily alter the
measured mainsail area. When people talk about
making the sails ‘fit the boat’ they are moving these
bands on the spars.
H
H
B
B
You can’t quite put the bands wherever you want to
get the correct mainsail size to match your jib, there
are two things to remember.
•
The sail must not be capable of being pulled
beyond any band. This could be because the
mainsail is not big enough to go that far, or because
there is some form of stopper on the spar or in the
control line pulling it.
•
The cross widths of the actual sail used must
not be bigger than those allowed by the length of the
foot (i.e. where you put the boom band).
The cross widths are used to control what we call ‘unmeasured area’ and give the mainsail the profile/aesthetics
that it has. As you will remember we measure both sails assuming that they are perfect straight-sided triangles.
Obviously they are not, as the foot of the main and jib curve down, some of the jibs have ‘fat heads’ and the
leech of the mainsail sticks out in a large roach supported by the top batten. All of these bits combine to make
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20
up the ‘unmeasured area’. The ‘measured area’ and the ‘unmeasured area’ added together give us the ‘actual
sail area’ of approximately 11 m2, which we go sailing with.
To provide some control over the amount of unmeasured area in the mainsail we have something called
‘allowable cross widths’. These govern how big the roach can be and are measured across the mainsail ¼, ½,
and ¾’s of the way up the leech. Again look at the rules or ask a measurer to see exactly how to take these
measurements. The maximum allowable cross widths on your boat depends on your boom band measurement,
the longer the measurement the bigger the allowable cross widths. The actual allowable cross widths for a
given boom measurement can be calculated from the formulae in the rules or taken from a table that all the
measurers have. So now you can see why it is important that you have marked the bands on your spars,
actually it’s the bands that govern how big your
mainsail is seen to be not the sail. When we
measure the mainsail we’re only making sure
that it can be used with the bands that are
marked on your boat.
You can check what the measurements on your
boat should be by looking at your certificate. The
max luff length, max foot length (both between
the bands on the spars) and consequent
allowable max cross widths (on the sail) and the
resultant maximum jib area are all shown. If you
don’t have a certificate a new copy for your boat
can be obtained by contacting Kevan Bloor on
the telephone number in the yearbook.
If you want to alter the numbers on your current
certificate (foot, luff, max jib, or allowable cross
widths) you need to have it signed off by a
maintenance measurer. Remember, it’s up to
you to ensure that your boat is legal and meets
the certificate at all times…
Thanks to
Tom Stewart
Dave Peacock
Kevan Bloor
Graham Camm
Zoë Ballantyne
Alan Bax
John Sears
Patrick Elcombe
Jon Brown
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21

National 12 Tuning Guide

Transcription

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