WHEN DO I NEED TO CHANGE SAIL SHAPE
- Boats like a 5O5 need to be sailed flat. When you are unable to do this it is generally a sign that you need to change gears.
- As wind increases and a boat becomes overpowered sails must be flattened to reduce their power.
- In light air, crew moving on and off the trapeze has a considerable impact on mast bend and generally requires a significant gear change.
- Until you become overpowered the boom should always be in the center of the boat.
- Communication between helm and crew is important to make sure you never (sailing upwind) have the boom off the centre line if helm or your crew are not fully hiking.
- As you de-power with increasing wind, the boom can and will need to be off the centre line but it should never consistently be further out than the inside edge of the side tank when going up wind.
- When you depower the mainsail you generally need to also make changes to the jib sheeting.
- Mainsail power is generally controlled by mast bend. Jib power is generally controlled by jib sheet tension.
The various Tuning Grids will help but here are some specific things you can observe that are good signals that will help you know when to change gear (adjust a control line to change sail shape).
Typically the easiest conditions most of us are comfortable in is when the crew is on the trapeze and we can keep the boom in the centre of the boat without crew and helm having to lean too hard.
Boats fully powered, boom should be on centre line, jib barber haulers in board. Crew does the work to keep the boat flat.
Imagine we are sailing in these conditions, the mast is bending perfectly (no Cunningham) and we have just the right amount of leach twist so the sails are setting up perfectly just as the sail maker designed them . The boom is centered and the boat is dead flat. As we watch the Word Champion slip further behind and put the sail-maker back on our Christmas list, we pray conditions will not change. But conditions do change. How do we know when they have changed enough (more or less wind) to make it worthwhile adjusting something?
Increasing Wind .
Your aim is to have the crew low on the trapeze, fully extended and helm hiking hard before you ease the boom, pinch or allow the boat to heel. Any of those things will impact boat speed.
The next step for this boat if wind increases is to depower. Pinching too much will slow you down without enough extra height to compensate. Heeling will disturb the flow over the foils, reduce the lift they provide and therefore increase leeway (you will go sideways more than the boat that isn’t heeling). Easing the boom from the centre will reduce the sail’s ability to keep the boat pointing as high. If you cant lean any harder and the crew has both arms above his head and is as flat as the boat, that is when you need to de-power to avoid doing any of the above. That means flattening sails. As the wind gets stronger you will just keep flattening the sails so the boat can still be sailed flat.
So the first key signal telling you that you need to change gear and reduce power (flatten sails) is when you cant hold the boom on the centre line.
As we flatten sails and de-power, it is not possible to keep the boom on the centre line. This is perfectly fine so long as you aren’t under powered. The moment the crew needs to bend legs to keep the boat flat in a lull, the boom must be pulled back to centre to maintain optimum boat speed
This boat has lost speed. The crew doesn’t feel he needs to lean but the helm hasn’t pulled the boom back to centre to power up. Communication is critical to ensure the boom is not eased when the crew has more leverage to give.
The next key signal that sails need to be flattened further is if the crew is fully leaning but the boom needs to constantly be further outboard than just inside the side tanks in order to keep the boat flat. As in the boat above.
Experience tells us that if the boom is too often any further out board than that we are probably sailing too low and too fast for optimum VMG . If we try and sail with the boom too far inboard of that point, the boat will be too pinched and not fast enough for optimum VMG. (link to description on VMG and modding from high to low, work in progress).
As wind increases and we need to depower more and more we primarily need to progressively keep flattening the sails. The tuning grids are useful but ultimately if you are overpowered you need to flatten the sails. We can rake the mast and move jib cars to the recommended settings but that alone does little to actually flatten the sails. The faster sailor is the one that is looking at their sails and making changes accordingly. Not focusing on a tuning grid. We have all experienced having the boat set up as per a tuning grid and still being slow.
One of the most challenging aspects of sailboat racing is coping with decreasing wind. If we were starting from that same perfectly set sailing described above and wind were to fall, the crew would initially be dong all the work. They must continuously be moving in and out to keep the boat flat. A key signal that you need to change gears is when the crew weight comes off the trapeze wire and sits on the tank. When the weight of the crew is no longer hanging on the trapeze the mast bend and behavior change dramatically. This has a significant impact on both mainsail and jib shape and almost always requires at least a change in sheet tension if not a series of other major changes including mast ram, rig tension and, if you have them, side stay cars. (See Tuning for light air. Work in progress)
- As with the mainsail, if you are overpowered flatten the jib. If you are underpowered make the jib deeper.
- When conditions change which result in the boom being sheeted wider, the jib generally also needs to be sheeted wider. Similarly, if the main boom is being sheeted further inboard, you should also sheet the jib further in board.
- If you change mast rake, the jib sheeting angle and therefore the jib shape is changed as well. Jib sheeting and Barber Hauler often need changing every time the rake is changed to maintain correct jib shape.
- If, when the mainsail is set up properly and the boat is being sailed properly, there is backwinding in the mainsail, the jib generally needs adjusting.
- When sailing up wind the jib leach tell-tale needs to be flowing most of the time. When it does not stream properly the jib is sheeted too hard or too far forward.
- Twist in the Jib leach needs to match the twist in the Mainsail.
- Jib sheet tension needs to be changed often (upwind) because so many things are constantly causing the jib to change shape all the time including wind strength and how hard the crew is leaning.
See the next Chapter for more on how to change settings.
 Sometimes minor backwinding in the main can be remedied by flattening the mainsail.