Offshore Optimising
October/November 1996
The balance of a sailboat is the output of a complex array of dynamic forces acting on the sails, hull, keel , and rudder. On every point of sail and in every weight of breeze the dynamics will change and the well sailed boat will make adjustments to bring the balance of the boat back to a level considered acceptable by the helm. In fact the helm is the critical interface in the whole equation and that is why a good steerer is more than just someone guiding the boat through the waves. The best steerers are constantly updating the trimmers and the rest of the crew as to what they need. What they need is to keep the balance right. I will give you the basics of how balance works in this article and also tips on how you can effect changes to your own boat
The forces acting on a yacht on any particular point of sail are enormously complex. There is the side force of leeway acting on the appendages and to a lesser extent the hull. The magnitude of these forces is constantly changing as the yacht heels and yaws even on the same point of sail. The centre of effort on the sails will vary with trim and design shape as well as the sail selection for the wind strength. If a strong gust hits the yacht the physics of the "steady as she goes" situation get thrown out as the yacht and her crew attempt to resolve all the forces back to equilibrium. The common scenario is the big ease on the main and the helm well up in an attempt to allow the boat to accelerate up to the next level. The opposite scenario is when the breeze goes light. Suddenly the helm lacks feel and the trimmers madly set about putting "grunt" in the sails. This may all occur intuitively with experienced sailors but how does all this get dealt with in the design equation?
The fact of the matter is the complexity of the balance equation has defied successful mathematical modelling even at the America's Cup level. For all the complex equations and the large computers required to solve them this most critical element of design has successfully resisted technology. Even AC boats have balance problems and those that don't are the fast ones all other things being equal. Think about the effect a few degrees of helm angle can have on performance . On the plus side it is lift but the down side of lift is always drag. The perfect balance of these two forces results in the best speed.
The large design balance issues revolve around the total sail area, the distribution of that area between main and foretriangle, the actual location of the mast, the canoe body shape, and the size and location of the keel and rudder. In lieu of complex solutions designers rely on the concept of balance of areas divided into two main groups - the sail plan and the underwater profile. These are both calculated as flat geometric areas which you can cut out of paper and balance on a pin if you like to keep it simple. The horizontal centres of these areas are drawn down or up to the waterline and the distance between them is noted as a percentage of the waterline length. Since the centre of the sail plan is always forward of the centre of the underwater shape the term used to describe this distance is lead, i.e., the lead of the sail plan. This percentage value varies depending on the type of yacht with a modern fractional rigged yacht at around 8% and a masthead yacht as high as 22%. See the diagram to clarify this in your mind.
If the designer puts everything where he thinks it should go and the lead is not an acceptable value then things must be moved. If the value is to high then moving the rig aft can help. Alternatively the keel and rudder can be moved though normally the keel, due to its location near the centre of the boat and its large mass is both less effective as well as upsetting the flotation trim. The rudder is quite powerful being at the back of the boat. Moving it forward in the boat will increase helm with aft producing the opposite effect. The diagram shows an interesting situation where an old style keel and rudder are being replace with a more advanced configuration. Note that the mast and sail plan are fixed as this is an existing boat. The plan was to invert the areas of the keel and rudder somewhat so that more of the work would go onto the adjustable foil (the rudder) and less on the static foil (the keel). Also the overall area of the two foils is less as they are more effective in their shape. You can see that, despite the radical transformation, the balance of the two set ups is identical.
This standard approach to balance is simple but does require a base of experience to back it up and give it meaning. Yachts with identical underwater geometry may need to be set up very differently as the forces generated by different foil shapes or faired vs. unfaired shapes may create a completely different dynamic array that will load or unload the helm in an unacceptable manner. Therefore, the simplification only works if all the important variables are constrained. For example the geometric centre of a keel is quite a different thing from the centre of pressure on the keel which will migrate about the place depending on the speed and the point of sail. However , it is this centre of pressure where the real load is acting on at any point in time and every type of foil will have a different centre of pressure. Whereas sails can be trimmed (i.e., the centre of pressure adjusted) a keel cannot be trimmed as its shape is locked in. However, every type of foil will produce a different result and a designer's balance equation based on years of experience and countless boats can be rendered useless if the foil cross sections that all that experience is based on are changed.
You, however, already own a boat. What tools do you have available to you to adjust helm balance ? First, without touching a thing, you have your sails. Old sails typically see the drive move aft and this can have a gradual but negative effect of increasing helm. Trimming has a big part to play here as well as overly full sails will load up the helm as the wind increases.
The big tool most people turn to is mast rake. Rake can be anywhere from 0 to 5 degrees typically and the basic effect is that increased rake adds weather helm and decreased rake reduces it . Beyond a point you may need to re-cut your sails as the boom may end up dangerously low if you go back too far. It is always worthwhile to try a few different rakes at the beginning of the season or when you get new sails to make sure you have bracketed the options. Never assume it is right since a small change could really bring your boat alive.
Finally, the keel and rudder can have a role to play here. I have seen a rough antifoul job on a rudder turn a well balanced boat into a nightmare of excessive weather helm. The solution was a good wet sand and everything returned to normal. Another well known yacht had tried virtually all the standard tricks to bring the helm to a manageable level to no avail. At this stage it was a major move of the mast or a new keel or anything- very expensive. However, a quick check of the static balance against our approach suggested the boat was fine in its basic geometry. The culprit turned out to be a very poorly designed rudder and simply replacing the blade on the existing stock solved all the problems. The boat even went on to win some races.
The key to all this is to be aware that balance is a key performance issue and not one that you should ignore. Put it into the front line and with a little effort you could get a big result.