Thursday, 27 June 2013

Spinnaker Trimming

On many boats, perhaps even the majority of boats it is likely that the crews’ knowledge of spinnaker trimming is not as comprehensive as their knowledge of the upwind sails. In upwind mode many people on the boat will have an opinion about sail shape and draft position, halyard tension, sheet lead position, inhauler position and so on but after the weather mark, crews will often relax as their concentration drifts or else they simply don’t know what to focus on.

Although we usually spend less time going downwind than upwind the fact remains that the distances lost or gained can be larger than when on the breeze. One of the problems with downwind sailing is that there is often no obvious or definitive groove that the boat settles into. Success in this direction may therefore depend somewhat on ‘feel’ as well as the ability of the crew.

To make big gains downwind (excluding any tactical considerations) you need to have appropriately shaped spinnakers and know how best to use them across their wind ranges.

Spinnaker Design

First of all let’s take a quick look at some design basics. In designing the spinnaker, the sailmaker has two primary objectives: firstly, to build a fast shape and secondly, to build a sail that flies in a stable manner and is easy to trim. There is often a fine line between a sail that is an inherently fast shape and something that is similar but slow because it is simply too difficult to trim. A sail that has a fast shape will not produce speed if it is continually bouncing about and needs to be over trimmed to settle it down. Speed and stability therefore go hand in hand to some degree. Stability is determined by the overall depth of the sail and the shape of the leading edge. If the sail is too flat, or if the leading edge is too straight, the lift needed to support the sail and make it project will not be generated. Maintaining a constant curl in the luff will be difficult and the resulting unstable sail will be prone to collapsing as soon as the sheet is eased and a slight curl is seen. If the sail is too full it simply won’t project enough area to be fast. Clearly the designer is trying to find a balance between a sail that is flat enough to project well but deep enough to be stable.

It used to be the case that the general rule for running spinnakers was that in order to be fast they needed to be big and deep with ‘broad shoulders’. We now know that this is not the case! When the boat is in downwind mode, i.e. you are trying to run deep without air flowing from luff to leech, you are basically trying to project the most area before the wind. This is achieved by having flatter sails that spread themselves out more readily than deeper sails that need to have the poles lifted excessively high in order to get the shoulders to project! If we just consider the IRC rule for a moment it would be possible to build two very different sails with the same rated area that performed quite differently to one another. IRC uses 4 simple measurements to define the SPA (spinnaker area). This is only a representation of the actual area of cloth that is in the sail. It is possible to build a deep sail and a flatter sail with the same rated area though the flat sail will have slightly less cloth in it (i.e. it will be smaller) but it will be faster because it presents more projected area before the wind! As mentioned above flatter sails however are more difficult to trim so the right balance has to be struck. It is fair to say that the kinds of spinnakers that you'd find on Grand Prix race boats with highly experienced trimmers would be a different shape to those that you'd find on boats used for club racing which in turn would be different to those found on cruising boats.

Trimming

There are four basic trimming tools that need careful attention in order to get the best from the spinnaker. These are pole height, pole position fore and aft (angle of attack), sheet tension and sheet angle.

Pole Height
Correct pole height and the right angle fore and
aft are critical components of spinnaker trim

Usually, the pole will already be up (or will at least be in the process of going up) when the spinnaker is hoisted. The pole should be set to a pre-determined or default height that you know is more or less correct, make sure that the pole topper is marked accordingly! Once the sail is up and filled it is time to start fine tuning. On most boats the first point of reference is usually the relative clew heights. Use this as a guide by all means, you probably won’t be too far wrong but do not use ‘even’ clew heights as gospel! Another useful indicator is the orientation of the centre vertical seam in the sail. This should be more or less perpendicular to the water. When combined with the other trimming aids which we’ll come to shortly, this is probably a better reference than relying on the height of the clews. However, the most useful indicator of whether or not the pole is at the correct height is the way that the luff of the sail breaks as the sheet is eased. What you are aiming for is a nice even break from top to bottom or a break that begins just above the mid point; if the pole is too high the top of the sail will lift and twist off and the sail will collapse from the bottom. If the pole is too low the top of the sail will be pulled towards the boat causing the break to begin higher up. In other words, if the top of the sail is breaking too early then the pole needs to go up a little. So, in order of priority, to set the pole height correctly, firstly look at how the luff is breaking, secondly check that the centre seam is vertical, and thirdly use the clew heights as a reference.

Within its range of settings, the pole height will need to be raised and lowered (i.e. adjusted) according to wind speed, sea state and wind angle. In lighter airs the pole needs to be lower to maintain some tension in the luff, keep the sail stable and hold the draft forwards towards the luff. The same applies to sloppy or choppy conditions; the sail will be more stable and easier to fly with a lower pole.

The pole height of course does have a bearing on the aerodynamic shape of the sail though this is more of an issue when reaching when you actually have air flowing over the sail, i.e. the sail is behaving like an aerofoil rather than a ‘barn door’ in downwind mode. Setting the sail with a lower pole pulls the draft forwards in a similar way to pulling on the Cunningham in a mainsail or increasing halyard tension in a headsail. Pulling the draft forwards flattens the back of the sail, encourages it to twist which helps to depower it. Similarly, raising the pole flattens the entry meaning that you can point higher but it will also let the draft creep aft which will increase drag and heeling moment. Again there is a balance to be found, this time between being able to point and managing heel.

If the inboard end of the pole is set on a track, i.e. it is movable, then make sure that the pole is set to horizontal which will help to project as much of the sail as possible from behind the mainsail.

Pole Position

As a rule of thumb, the pole should be positioned fore and aft roughly perpendicular to the apparent wind, use your burgee or wind indicator as a guide. Another useful reference point is that the main boom and the spinnaker pole should be more or less in a continuous line through the mast though in reality the spinnaker pole will likely be just forward of this point. The general idea is to square the pole back as much as possible in order to present more of the spinnaker before the wind. However, it is easy to over do this and the pole should never be squared aft to the degree where the foot of the spinnaker ends up being pulled tight against the forestay. Having the pole too far aft will also make life difficult for the sheet trimmer as the luff of the sail will be continually twitchy.

The sheet trimmer and guy trimmer need to continually communicate in order to get the most out of the sail and the sheet trimmer in particular also needs be having an almost continual dialogue with the helmsman in order for trim to be most effective. As with pole height, the position of the pole relative to its base position may depend to some degree on sea state and wind conditions. For example, in sloppy conditions when the boat is bouncing around it may well be easier for the trimmers (as well as faster) for the pole to be set just a little further forwards than usual in order to keep the sail drawing effectively.

Sheet tension


The most effective spinnaker
trimmers are constantly
adjusting the sheet
Spinnaker trim is a dynamic process; the sheet should be continuously moving in order for the overall trim to be optimal. The trimmer should ease the sheet slowly and smoothly until the luff just starts to curl and then trim it on just a touch to prevent the curl from becoming excessive. As soon as there is no curl the sheet is eased again and the whole process continues. The guy trimmer is also looking to move the pole aft where possible and needs to monitor what the sheet trimmer is doing. The sheet trimmer should be talking to the helmsman about ‘feel’ and ‘pressure’. If the sheet feels too light the trimmer needs to let the helmsman know so that he can steer a little higher (tactics and other boats permitting). Once the speed has built and the pressure in the sail feels better the trimmer can let the helmsman know that he can come down a couple of degrees. Again, the whole process is continuous.

There is no magic angle to sail, it will depend on the type of boat, wind speed, wave state and position of other boats but hard work, good communication and appropriate focus will certainly get the boat to the leeward mark more quickly than those adopting a more passive approach.

Sheet angle

On most boats the spinnaker sheets are led aft to a turning block somewhere near the back of the boat. This kind of set up may be fine some of the time but leaves no room for adjusting the downward component on the sheet tension which in turn controls how the leech of the sail twists and behaves. Therefore, assuming class rules allow it, the spinnaker sheets should be led through adjustable ‘twinning lines’ (‘twinners’ or ‘twings’) which provide the sheet trimmer with a greater degree of control over the overall sail shape. If the sheet lead is too far aft the clew of the sail might be inclined to lift up causing the leech to become too twisted with this in turning having a bearing on how the luff is shaping up. The tack of the sail might in this instance appear to low, the trimmer calls for the pole to be raised to ‘get the clews’ level and the whole sail ends up being set too high with subsequent loss of performance. Of course it is also possible to have the lead too far forwards in which case the leech of the sail will become too tight and the sail will be pulled to leeward, again compromising performance.

As with the other points above, get the luff correct first and then use the twinners to fine tune the leech. Bear in mind that the twinners will often be used as a means of moving the sheet lower away from the boom. That’s fine in principle but take care that the shape of the spinnaker isn’t being compromised at the same time.  

Other tips

Mark the pole topping lift and downhaul so that they can be pre-set into a position that is more-or-less right as you approach the windward mark and before the spinnaker is even hoisted.

As the pole is being moved fore and aft the pole downhaul will also need continual adjustment. Make sure that its default position is on hard to prevent the pole bouncing.

Similarly, put some marks on the ‘lazy guys’ at a position which you know provides the bowman with just enough slack to complete a gybe.

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