What to adjust?

We take a look at single spreader, fractional dinghy rigs

Although all boats are different, there are only a small number of widely used rig set-ups, particularly in the dinghy world. Almost every dinghy being raced around the world today can be put into a small list of general categories. This week, we take a look at the single spreader, fractional dinghy rigs used on boats such as the 470, Fireball or 505 and see what some of the general rules of use are.

With this type of rig there are a few different areas to set up and it is important to understand how a change in each area affects the rig as a whole. Firstly the mast step is important and usually is allowed to be moved a small amount even in the very strict classes such as the 470. However, it is not often moved from a ‘standard’ class position and this is usually very class specific.

The principle thing that has the largest effect on a single spreader rig is the mast rake. Mast rake is generally measured with no mast puller on and a ‘normal’ amount of tension on the shrouds. To measure this, attach a long tape measure to the main halyard - this is pulled to the highest point the main goes up the mast (this should be marked by a black band.) Record the distance from the mast to the top of the transom, and this measurement is the mast rake.

The rake of the mast is very simply determined by the forestay length. It is advisable to ask your sailmaker about the settings for mast rake as it will vary from loft to loft. Typically you will be using mast rake to de-power your boat as the wind increases, however a base setting is a good starting place. Some sailors use a very specific set of mast rake numbers for different conditions, others tend to go out on their base settings and rake back or more upright if they feel they need to - it is very much personal preference.

The reasons for raking a rig back can be quite complex, but essentially come down to one thing: de-powering. It is important to understand that as soon as a boat has reached its righting moment limit (crew trapezing, or hiking fully) the amount of force the sails produce cannot be increased any more without the boat heeling over and this is extremely slow. The only way to carry on increasing speed at this point is to reduce the drag being created. The longest edge of the sail is the leech which, as the rig is raked aft becomes more vertical and maximizes its span. So, with the same lift being produced over a sail with greater span, the overall drag of the rig is reduced and the boat is able to sail faster. There are a great number of other small factors that are affected when we rake the rig back, for example the clew becomes closer to the water were the wind velocity is lowered by the friction of the water and this is one of the reasons the rig should not be raked aft in lighter winds.

Rig tension is extremely important, and similar to mast rake, is something that needs to be increased as the wind builds. When the wind is lighter it is important not to over tension the rig, as this will induce too much mast bend and the over tightened forestay will encourage the boat to point too high. Conversely if the tension is too soft the rig will become floppy and the forestay and mast will sag to leeward, preventing the boat from pointing. As the wind increases, so the pressure on the rig increases meaning a greater amount of tension is required to prevent this sag from happening.



As mentioned above, the rig tension in conjunction with the spreaders induces bend in the mast. The spreaders have two main jobs; firstly they are there to create mid-mast bend but also they provide side to side stability for the mast and are acting as an effective brace against the pressure of the wind. The mast bend is created by the spreaders deflecting the shrouds out of a straight line with the mast. As rig tension is pulled on, the shrouds want to straighten up as they begin to come into line, the rake in the spreaders then forces bend into the mast in the mid-section.

As you can imagine both the angle and the length of the spreaders has a major effect on the mast bend characteristics. The more bent the mast, the flatter the mainsail is, reducing the amount of power being generated. Effectively you are trying to bend the mast in increasing amounts as the wind increases. However, a mast can become too bent and will start to slow the boat by destroying the shape of the sail as the luff is pulled to fit the mast. The classic sign of an over bent mast is back-winding of the mainsail at the luff in the lower third.



As with most things in sailing the key to achieving desired spreader settings is to make sure everything is marked and noted. Something often overlooked by sailors is ensuring the bracket that holds the spreaders to the mast is totally square. If the mast bracket is even the smallest fraction out of line as rig tension is pulled into the mast the bend will become uneven, leading to greatly reduced performance. Something that produces the same uneven bend outcome is having spreaders that are set unevenly. If one spreader is longer than the other or one is raked further back than the other it is important to even them up and ensure they stay even.



Another control which is used to adjust the amount of bend in the mast is the mast puller or ram. Specifically this controls the amount of bend in the mast in the lower quarter. It also affects the power in the rig and is critical to boat speed. At the very basic level, this is used to control how much low down bend is going into the mast, and so how much the boat will point. A very straight mast will let you point significantly higher, whereas a much more bent mast will not allow you to point as high but will make the main fuller, increasing power.

The mast puller tends to be adjusted as a matter of preference, with some choosing to keep it fully on, and keep pointing high while others prefer to let it off as the breeze builds allowing them to foot and get the boat planning. There is no right or wrong general rule here and again it comes down to what feels right for the sailor. Also the amount of mast puller needed depends on the sea-state as well as wind speed. In a slightly choppy sea the mast puller can be let off, increasing the power in the sails and helping the boat through the waves, conversely the mast puller can be left on longer in flat water as the need for power lessens and the importance of pointing increases.



The kicking strap or vang is a control that although predominantly used to control leech tension on a boat also has an effect on mast bend. Most single spreader rigs have a standard kicking strap as opposed to the more fashionable gnav so we will focus on the effects of a traditional kicker system. When the kicker is pulled on it does two things, firstly because it is attached beneath the boom it pulls down on it and creates the aforementioned leech tension. Secondly, because the kicker leads forward in the boat it also pulls the boom forward which in turn pushes bend into the mast at the goose neck. This effect can be countered to some extent by the mast puller but it is worth remembering that as we adjust the kicker a lot this will change the low down mast settings by small amounts.

Obviously choosing the correct mast section is a big part of rig control and as with many things there does tend to be some fashion involved here. Typically the motto was always to go for the stiffest section your bodyweight will allow, giving you as much power as possible, thus the heavier you are, the stiffer the mast and the more power you could get from it. In recent years, however, the increasing popularity of skiff style boats, coupled with the change to a much shorter race format has pushed a change to more of a reactive, self de-powering mast. This has made the need for a variety of settings slightly less important as one setting can be stretched to fit a wider number of conditions.

Setting up a boat is always a matter of personal preference but a few simple rules can be remembered. Firstly as soon as any boat is fully powered up, the aim of the game is to loose power while creating as little drag as possible, raking the mast back, and bending it being two examples. Conversely when we are not fully powered up the most important thing is to find as much power as possible, letting off the mast puller and rig tension a little are two ways to do this.

Understanding how your rig works and, more importantly what feels right to you is a crucial part of achieving your goals in sailing. You can learn a significant amount more about your boat if you spend a weekend tweaking controls and seeing how dramatic their effect can be than you would on many weekends on the water club racing, especially over the winter when fleets are typically smaller.