Boat Trim and Balance

Written by Godfrey Clark for the “Green Book”.

General

As the Moth is a small 11 foot boat, you will usually weigh more than the boat and your position will make a radical difference to hull trim. In a boat of this size, you cannot sit like a sack of potatoes, but must move around a lot to trim the hull for changing speed and wind.

Even upper body movement of the head and torso will tilt the Moth in light airs, as will letting the boom run all the way out. The Moth is a tricky boat to trim correctly with its bluff bow which slams the smallest wavelet and makes a lot of noise.

While the (original 751,752 and 754, Why, Wein and When . ed) Claridge built Moths can be sailed consistently flatter at all times, one thing you will notice is that when heeled, the frontal area of the hull can be reduced by half, with a corresponding reduction in wetted area. This situation is reached when the weather chine is flying just clear of the water and can be used to good effect when beating.

You seldom get something for nothing and the well known down-side of a heeled boat is losing leeway (i.e. the boat slips sideways in the water). This particularly applies in the first boat length following a tack, when the centreboard stalls and the boat just slides sideways until the foils start to bite.

Upwind, the Moth can be allowed to heel just a little, unlike Merlins/National 12s which are sailed plumb upright or even with a degree or two of windward heel. Just balance the boat in the usual way and work up the speed, hiking off the widest point of the hull in a breeze. In light airs I move forward to the bulkhead to remove drag from the transom. Transom drag is not much of a problem when the boat is moving fast enough to create its own wave pattern.

Downwind the Moth should be sailed level both laterally and fore and aft for optimum trim and speed. This means moving fore and aft a good deal in a gusty breeze. The basic technique of moving aft at the beginning of a gust and returning forward in the lulls is essential. You may even want to move aft almost before the gust hits you, so that when the gust drives the nose down, the hull only ends up in a normal attitude and planing fast. As the gust passes, you will be able to slide forward again and trim the hull level. It is vital not to remain sitting on the transom when the lull comes, because the boat will not only sink by the stern with horrendous drag, but with the Moth’s narrow transom, she will lose a lot of stability as well. A flat boat does not just mean sitting on the transom if it is windy.

In light airs, you should be sailing the runs with perhaps 0-5 degrees of windward heel to enable you to steer without using the rudder. This will make it possible to sail a straight line from “A to B” without inherent weather helm trying to luff the boat off course all the time. You see so many races where the fleet sails a curved track between marks (and that’s before all the boats start luffing one another!) each one trying to bear away all the while by using the rudder.

In severe conditions you should try to do the same, but you may find the neutral helm disconcerting. Don’t forget that a steady pressure from the rudder may make you feel secure but it is slowing the boat and taking you above your intended course which may cause other problems as you approach the leeward mark (i.e. you are too high and need to bear down or even gybe twice to get round the mark  a nightmare!).

Tacking

Tacking the Moth can be quite a challenge to someone used to something docile. You need to use three basic styles according to the conditions:

1. In light airs we use the traditional roll tack to great (and sometimes illegally great) effect. You are crouched in the boat, heeled nominally to leeward and decide to tack. Using your body weight taken on your feet at all times, allow the boat to heel much further than normal. Let the helm go down and as she begins to luff, you lean out, pulling the boat over on top of you during the arc of the tack. When she is pointing on the new course , and when the gunwale you are sitting on is in the water, swivel smartly round on your feet (those feet come in handy at times like this), and by transferring all your weight across to the new (very) high side pull the sail into the wind again. Don’t overdo this action. The boat should come up as far as its normal sailing trim angle. The cycle consists of a preparation stroke and two power strokes, and a nice tack can be executed quite gracefully under full control. However, the centreboard is asked to work very hard and will stall. The Moth will sideslip badly but this is a small price to pay for two bursts of acceleration from very low speed.

2. The price of leeway becomes too great when sailing at normal speed. If a lot of acceleration is not to be had preservation of momentum and the course through the water are more useful, so the second style of tacking is used. Sailing to windward at top speed, the Moth is tacked in a more yacht-like fashion, luffing very gently to begin with, keeping the hull level at all times to give the centreboard a chance to do it’s job, and keeping the underwater hull shape efficient. Momentum is maintained and the boat is steered around the corner leaving a clean trail of bubbles in a 90 degree tack.

3. In windier weather, strangers to the Moth get caught out by trying the above style. The Moth is so light; add the huge drag of a flapping sail and it will stop dead in two yards! Especially if you try to tack and are unlucky enough to hit a wave head on. Heavy weather tacking requires that the tack is executed as quickly as possible, and that lots and lots of forward drive is re-applied to get her going again. Luffing gently into the tack as before, the main is eased and the Moth is then flung hard through the eye of the wind using quite a lot of rudder. You have to spin her round before she stops dead in irons. Once through the eye of the wind, you turn a little more (quite a lot more if it’s blowing old boots). The Moth then finds herself a little below close hauled but safely on the new tack. All that remains to be done is firstly to get your weight over the new side and the boat roughly level before pulling in the mainsheet. Once you are sailing again and up to speed, you can bring her up to close hauled.

Gybing

A thorny topic if ever there was one! Much has been written in the sailing books on gybing Olympic 470s, 505s and Finns but the Moth has its own particular problems and is one of the hardest dinghies to gybe in any sort of breeze. The boat is more prone to nose-dive than anything else I know and I suspect that transom mainsheets only serve to apply a lifting force to the transom which is pretty counter productive. The centre main gives so much more control that it totally transforms the boat. Gybing styles change drastically according to the conditions.

In light airs the roll gybe is essential, using all the techniques of the roll tack. Too much centreboard may cause the boat to luff on the exit of the gybe and not enough will leave the boat very skitterish and lacking in grip in the water. Weight on the feet is still important, as is not over-doing the roll. While the roll itself needs to be quite pronounced, it is essential not to swerve off course which would involve covering extra distance and necessitate the use of the rudder (steering brake) to straighten things up again. If only gybing from run to run, try to economise on all movements and actions; just enough that you can tug the sail back against the new wind pressure in order to get the top batten to flip and the sail to fill on the new gybe.

Gybing from run to run is very hard to do with a transom main; so much so that people will gladly run by the lee rather than put in one, or even several gybes, so as to tack advantageously down a long run. The ability simply to set the sail on the other side is so important on some river courses, which meander such that you either gybe several times or face running by the lee. You may also like to gybe to avoid close-hauled boats coming the other way.

With centre main, you should not need to change course hardly at all. Stand up and face forward with the tiller in the hand you will steer with after the gybe. Grasp either two stands of mainsheet or all of them at once. This means you will have changed hands before doing the gybe. Take up the slack and with one tug on some, or all of the mainsheet purchase, fling the whole lot across the boat and if the batten does not “ping” you are perfectly poised to give the main another yank (you still have a fist full of mainsheet) and the job is done. As the wind increases the batten will flip just under the pressure of wind. As the wind increases more you will use another technique here, using that final tug not to flip the batten but to regulate the bite and resultant heel when the whole thing slams across.

If it looks as though the boat may suffer a knockdown, you just ease the wind pressure in the sail (using that fistful of mainsheet that you are grasping), or, if you are heavy or there is a lull in the wind, or you realise you are sitting down on the new side a bit too early, you can always hold the sail in a bit to steady the boat. Some people fall in to windward actually after the gybe has taken place; that haul on the main could save the day.

The key points to remember are;

  • Weight on the feet for balance and mobility
  • A fistful of mainsheet for throttle control in the closing stages
  • Keep the hull level at all times

There are two other techniques worth covering here for windy weather gybing when the course is a reach to reach:

  1. One is what some people call the Power Gybe. Beloved of “fat boys” who can do it best, you simply steer round the mark until pointing roughly in the new direction, initiate the gybe and as the whole thing slams across on the new side, you bear away onto a broad reach that even a strong wind cannot knock the boat over without a partial pitchpoling. You may get a wave over the foredeck but that’s as bad as it gets.
  2.  If you are light, there is a time when the above approach may appear suicidal, but more importantly when the second leg is a beam or close reach this is not the way to go. Then we resort to what may be called the survival gybe. Approaching the mark downwind; level the boat, change hands before and all the good things we usually do; wide approach/close exit; we steer the boat round until her new heading is that of a fetch (a close-hauled course where the next mark can be reached without tacking) on the new tack. As the boat alters course, the boom will fling wildly across with very little prompting from the helm (no need for the fistful of mainsheet here), but will meet with no resistance and will not really fill with wind on the new gybe. You end up with the sail flapping in a hove to position, the helm on the new side ready to find the toe straps, shift weight aft and bear away onto a screaming plane. It is essential that the gybe is initiated with hull dead level or even heeled slightly into the wind with the boom close to the water. That gives the wind even more work to do when the gybe actually happens.

Controls and Foils

Normal winds (approximately 6-16 knots):

Control

Beating

Reaching

Running

Kicker

Variable to keep sail top batten
tell-tail flying 80% of the time

Full sail/straight leech

Top batten at 900 to
centre-line

Outhaul

Medium/slack

Off

On

Mainsheet/boom

Boom between centre-line and
transom corner. Control leech tension with mainsheet tension

Boom pressed on shroud

Cunningham

Off until over powered

Off

Off

Rudder

Fully down

Fully down

Fully down

Centreboard

300 from vertical

600 from vertical

800 from vertical

Boat trim

Flat/50 leeward heel

Flat

Level fore and aft. 2-50
windward heel

Light airs (approximately 0-6 knots):

Control

Beating

Reaching

Running

Kicker

Remove slack only

Remove slack only

Remove slack only

Outhaul

Medium

Off

On

Mainsheet/boom

Boom between centre-line and
transom corner. Control leech tension

Totally free

Cunningham

Slack. Enough tension to make
sail readable

Off

Off

Rudder

Slope to increase turning moment
in very light airs

Slope to increase turning moment
in very light airs

Slope to increase turning moment
in very light airs

Centreboard

300 from vertical

600 from vertical

800 from vertical

Boat trim

2-50 leeward heel

2-50 leeward heel

Level fore and aft. 2-50
windward heel

Strong winds (greater than 16 knots):

Control

Beating

Reaching

Running

Kicker

Tight

Tighten leech and keep sail as
full as you can sail with

Tight. Top batten at 900
to centre-line. Tighten more if on death roll

Outhaul

Tight

Off until wiping out

Tightish

Mainsheet/boom

Play mainsheet to control heel.
Boom as close as possible to centre-line

Flat boat/maximum power

‘Pump’ in response to death roll

Cunningham

Tight

Off until wiping out

Off if you have a chance

Rudder

Make sure it stays fully down

Make sure it stays fully down

Make sure it stays fully down

Centreboard

Eased 450 from
vertical

600 from vertical

800 from vertical. If
boat unstable, keep it down

Boat trim

Avoid frontal wave contact

0-50 leeward heel

Flat. Level fore and aft

British Moth Boat Trim

British Moth Boat Trim