Notes on Sail Trim from North's On Line Lecture

Some Notes I made from a YouTube featuring North's N,American management team racing a j35 back in the 80s or 90s sometime.

1) Traveller and Forward Drive Vector; Slot Closes Last - Stronger and building Winds

In stronger winds we tend to think of the traveller as a depowering device, and an out-right brake pad when we have too much power on. If we dump it all the way down and let the majority of the main "bubble" of flogg, then yes, we have closed the barn door on the slot so-to-speak.


HOWEVER- Food for thought: Just as twisting off the top of the mainsail creates a greater forward vector in that part of the sail, so does dropping the traveller off the centre line as the wind builds.

REMEMBER: "design wind" for many racing boats with the no.1 genoas/jibs and a full main is often less than 12knts, very often 9knts TWS!!! Thereafter you are depowering the boat, and there up to you are powering up to the point where leech tension is maximal, pointing is optimised and the slot is perfectly trimmed. So in a good racer of 25 to 40 feet the Apparent wind AWS is 15-20 knots when you are needing to depower. 

Because forces on the sail increase and decrease by the cube of the windstrenght, the relative adjustments either side of "design wind trim" are a little more exagerrated than you may imagine. A couple of knots either side is enough to mean you need far more than just adjusting the mainsail leech tension on the sheet alone to keep the boat going optimally for the wind strenght.

In effect we twist the top of the sail off to create more forward drive (and less heel)  in appropriate conditions. The shorter chord legnth up there will fly in less wind or at a greater angle of attack in more wind, thus pointing it's deeper section forward and creating a vector with more thrust fore-over.

The same is true to an extent with the main traveller dropped down - as the wind builds and we start to heel too much, then we can ease the traveller down (and use twist on the top, see later for testing twist set up, leech tension) . The sail will fly at a greater angle of attack to the wind for the same bow course, and the boat can be trimmed to point as high if desired, but use more speed.  The main then creates along more of its height, a greater forward vector and the boat speeds up.  As the wind lulls, the main will no longer fly at this angle, and should be tracked back up towards the centre line.

Eventually though, the mainsail will start to close the slot and the boat will slow, or at least not gain any speed, and become a little unstable as the wind and waves flick the power on and off.

To judge if the trim is ok, you will see that back winding of the front third of the main just begins, or is intermittant, at which point you can trim up a little or see if you can tolerate a bit more mainsheet.

This point at which backwinding happens will vary between different rigs and sail plans, but also to mitigate it and build more power and less heeling we can alter the jib/genoa set up- running the cars further back to open the leech, and even sheeting out a little, or sheeting in to flatten the bottom of the sail if it is a long, deep genoa creating a superpowerful slot.

Usually in a breeze which is building, the sea state will get choppier and so we need more forward drive as described above, and we need to steer through the waves a little more. Thus the jib halyard can be taken on a little more to draw the draught forward to make for knuckle in the sail. The sheet must be controlled to allow this to happen firstly, and then to ensure the sail is both deep and a little twisted. On most sails I have trimmed, the genoa top will close a little with more halyard, so the sheet-car will mowst likely need adjusting even further back (if you have laready opened the slot a bit)

At this point when the boat is balanced and driving, while you can also steer through the worst waves and react to the oscillations in the relative wind, you can on a fractional rig use more backstay as a control to depower in the gusts - try this and also fire on more kicker such that you can use backstay and the fine adjust of the mainsheet to open the top of the sail more and reduce heeling, while also increasing twist. Finally you can feather the boat more by steering up a little- thus you avoid that "dump and stall" of dropping the traveller and closing the slot. At this point the traveller is often very highly loaded and you need to take one person down off the deck or rail to fetch it up again

At some point though, as the wind builds more, you need to consider changing headsails on a frac' rig, or perhaps reefing the main on either type of rig if your foresail will tolerate the wind strength.   One disadvantage of easing leech tension on many sporty rigs is that it eases the forestay and thus powers up the jib and makes the rig a little more unstable. However a compromise can be reached here to your own boat and preferences for depowering in the highest gusts for the day prior to stripping down sail area.

2) Traveller Part II _ Light Airs Revisited

We all know how to trim a main in light airs, Right ?? Traveller up, sheet out, let the top tell tale fly between 75% and 100% of the time depending on the boat and sailmaker's recommendation.

Well let us say that often we actually get the twist a little wrong and lower down the sail is either too tight or too slack. North's offer a tip in the video on assessing this which any Idiot Savant from their first day at sailing school could pass on!!!

As an interesting aside, preamble to cut a short story long: When I turned up at Tighnabruaich sailing School in the mid ninetees as a yacht crew eager to learn how to helm in dinghies, I was shocked that the school were so badly prepared as to not have tell-tales on the jibs! Quelle horreur, methought, IRC genoa trimmer as I had been. Further to my amazement they had taken a HACK SAW to all the main sheet cleat flanges and the mainsheets were all free to run !! ( a trend which of course became the norm in many light, high performance dinghies) The reason for this was because they taught the absolute basic, to trim in the sail and then let it out again to see if the luff bubble developed. Thus when sailing  to a course we could always find out if we were over trimmed, or of course just bear away if the bubble appeared of its own devices in a shift. I have used this countless times on some pretty serious amateur racing boats, especially in light airs or when the sails are sucking their own wind on, making it look like we are still close hauled when in fact we are in a freer wind!

In light airs ( or any wind for that matter) you can actually judge if the leech tension and thus twist in the sail is set up right by dropping the traveller until the luff backs in ("bubble") Ease the traveller down and see if the luff breaks more or less evenly along 90% of its lenght. Check for:

A) If it breaks high first, then you have a little too much twist, and you can trim on the main sheet once the traveller is up again. You can then use the top tell tale and note if it breaks say 50% of the time which may be the set up for your boat in that specific condition.

B) If it breaks in the lower half first then you have one of two effects or a combination, with a third possibility of rig tune.
i) Firstly you may not have enough twist in the top of the sail. Try sheeting on a little, and then ease the traveller back up and note as above. Check boat speed on the log, or feel on the helm which should be more pressured.
ii) Secondly your slot may be too closed, or a combination of too little main twist as in i) and too closed a slot- Trim the Genoa cars back and if you can move them outboard or barber haul outwards, do so. Then check the main again as above.
iii) You may have too much fullness in the lowerhalf of the main sail: the depth coul also be a little too far forward. There are a number of different faults to find here- are the sails old and baggy? Is the cunningham or kicker inadvertedly on? Can we take in more outhaul than we think (often the case!!) Is the mast pre-bend wrong? Is there too much rake in the mast (ie too long a forestay)? On boats with carbon rigs or very spindly alloy rigs, is there too much side bend (main luff sag) due to too slack a set up with the inner lower shrouds?

This is a lovely technique I will look at using, and of course it is quite similar to checking for a regular spinnaker height when reaching, by seeing if the kite breaks more to the foot or the heead when you ease out.

3) Sails Too Full!

Following on from the point above about checking the set up of main twist, often in fact mainsails on performance boats are set too full to perform optimally. Very soon they start to back around the slot area, or pointing versus other boats in a fleet is poorer.

As mentioned above, OUTHAUL is often too slack on racing yachts - it can be upto 2 inches off the boom at max power for all but the worst chop. In light airs, flat water, the foot is probably slowing the wind too much by being deep, and the lower batten may be hooking the leech especially if it is quite low down. Take a sighting up the mast from cheek on the boom, and look at the sail from the transom or a following boat. Mainsails are nearly always deeper and more twisted than you would believe when you see them from new angles!!

Also the middle of the sail may well be too full actually, as revealed by letting it down the traveller in the technique in two above, or if the mid section backs as the boat accelerates or meets a gust.  

As mentioned on the Melges this is often due to the lower (inner D1) Shroud being too slack and the boat no longer needing power by luff sag. On most boats though it is because the sail needs to be flattened (and may be cut too deep in the first place or blown out if it is Dacron) Under way this is acheived by backstay but at the expense of opening the top of the sail a little. When you then compensate with mainsheet to close the leech again, it is very likely that the draught of the main has moved backwards and there is more rake and heavier weather helm due to the mast being not only bent but its top point moved aft. Cunningham may well need to be used earlier than you believe to correct this. of course this can further open the head of the main.....so it goes on

The draught on the jib /genoa will also most likely go aft, and need halyard or jib cunningham on, and a corresponding trim

All this comes out very much in the North's YouTube vid on the j35: the team are new to the boat, and try lots of different settings, with lots of nattering until they are happy, and then of course the wind changes strength again!!!  Such is sailing!

4) Vang Sheeting Off Wind is the Norm!

In most fractional boats at least, when you turn off onto a reach or a run off wind, then the main sheet loses most of its function in actually trimming the leech tension and the kicker aka VANG takes over.

The traveller can in most boats be set at two thirds of the way down the traveller , allowing for some extra dump, while shortening the sheet and weight of rope suspended. Some boats may suit the traveller all the way down for both a more rapid response sheeting in at least, while other boats may get a better angle and less things in the way with the traveller up in the centre line.

Some boats can be set up for this upwind too, especially dinghies without a traveller of course, but it may be useful to you in some conditions to have a lot of leech tension on and adjust just a little of this upwind by dropping the main sheet rather than the traveller. Upwind the main sheet will have some percentage of effect on leech tension with the vang on most keel boats. This may be say of use in a Melges 24 in puffy conditions on flat water, when you want to drive the boat forward when the gusts come- I am going to try this when next out in the Melges which has quite a powerful kicker.


5) Depowering a Traditional Spinnaker and Stopping "Death Roll"

As the wind builds we probably want to bear away anyway more to DDW, but keeping the wind nicely over the quarter - Here we can over guy the pole such that

i) more of the wind comes round the windward side and sklies off the sail.
ii) The sail is flatter in the foot and less powerful in its drawing double curled edged effect
iii) The wind sklies off the sail on that side quicker too as a result of ii)
iv) We can bear away in a big gust even more just on the helm while the sail keeps stable

Death roll can also be stopped by this method in combination with lowering the foot height such that the centre of gravity is lower - this is done firstly by using the policeman/barber hauler which will bring down the leeward clew, likely to be flying high and unstable compared to the guyed clew. Thereafter or if lacking a leeward hauler, we can lower the pole, N:B:on both the mast and the outward end!!

The helmsman can also steer up , less than 5', to get out of the worst wave top effects on the hull.

Spinnaker aside, the mast man should check that the main top is not so loose or twisted off that it can go forward of the mast, inducing roll, nor that it is too tight inducing heel which can also make roll. It should be flying at an angle just aftwards of the luff track.

Remember to fly the spinnaker at 90' to the wind, which will be the AWA, using the windex and the luff as your guides, and further back in more wind is better as above to stabilise things. The main boom may not be a very good approximation as for the continued angle of the pole (ie straight line along both) because you may need it inboard to avoid too much twist and hence rolling developing.

Depowering When Reaching- Although not fancied by our friend Hans Fogh in the video, the barber hauler is also a very good way of powering on and off on the tighter reach with the kite flying.

In the lulls you can often trim the top third of the kite in towards the main, capturing more power and also making more of a slot effect as it draws wind over the main which is on a reach too of course. It should be eased first as the gust builds before sheeting out is required.

Also you can on most boats I have sailed on, very successfully render your spinnaker assymetric to a large extent by lowering the pole as far as it will go on the outward end and mast. This will flatten the entry of the spinnaker and open the leech - you can then trim with the barber hauler as above.

 Some boats with a very high point on the mast for the spinnaker, which will have the effect that dropping the pole on the outward end too low actually powers up the spinnaker beacuse the "j" length shortens - so although the entry may seem a little flatter, the draught of the spinnaker is deeper. You can over come this by flying the spinnaker a little furtther back than the course may suggest,  with the pole, and letting the luff curl more often, even flying it with a constant bubble on the luff in stronger winds.