The Importance of Mast Rake for an Optimist
For most sailing boats the ability to control the power is made by bending or straightening the mast, as the mast bends it flattens the fabric of the sail, moving the center of effort down and opening the leech, the opposite happens when straightening the mast.
The optimist is a special boat, the ability of the mast to bend is very limited as the mast is very short and there are no controls to assist with bending of the mast, over the years we encountered many Optimist sailors and coaches who used to play with their sail ties to alter the shape of the sail, as much as this is in the correct direction it is again a very limited method and creates a lot of deformities in the sail shape.
We can depower the sail by loosening the sprit which again will deform the sail, but can we power it up?
The question arises why do we need to control power at all in an optimist?
The answer is pretty simple, if we find it hard to keep the boat flat while sailing upwind we have to depower the boat in order to keep sailing fast by preventing the need for corrective rudder use and prevent the boat from making too much leeway, When the wind is light we want more power as more power teamed with correct sail flow will be worth more speed. It can be impossible to gain correct flow when overpowered.
The optimist is a slow boat and uses a low aspect ratio sail, than means that the width and height of the sail are almost equal, the longer the foot of the sail is the more power it creates. High aspect ratio sails are fast sails, that means that the width of the sail is shorter than the height of the sail, these sails creates a faster airflow and less drag, the higher the mast for sails of the same area, the faster the sail will be, but the less acceleration will be available.
Back to the Optimist, How can we control power? The answer will be by changing the rake of the mast. In an Optimist when we rake forward, that means our rake becomes bigger, we are changing the flow angle on the sail, what we are actually doing is allowing air to flow for longer over the sail (making the sail effectively wider) this will generate more power. When we rake backwards that means our chord length (width of sail) becomes smaller, we are again changing the flow on the sail and allowing the air to flow for a shorter distance over the sail (like making our sail narrow) which reduces the amount of power the sail will produce.
There are limits. When we change the rake we also move the center of effort around, when we rake forward we naturally move the center of effort forward, we are basically able to power up until we raked forward enough that the boat develops a tendency to bear away (since we move the center of effort forward enough), so when powering up it is necessary to feel the boat and know if it develops such a tendency, that would usually be your top limit and you will be unable to power up more. We combat the effects of the centre of effort moving forward by sitting further forward ourselves, making the boat feel “in tune”.
When we rake backwards we also move the center of effort backwards and when moved enough the boat will develop a tendency to luff up since we moved the center of effort backwards enough. Again, as the centre of effort moves backwards, we sit with our weight further aft, and again keep the boat in tune with the use of our own bodyweight. Again the need to feel when the boat is developing that tendency to go upwind will enable you to know your limits, after that if you need to depower more you will have to use the sprit (but that is usually in survival mode as it costs height).
Over the years we encountered many sailors and coaches who were unaware of these reasons to rake. During our training at Toplevel Sailing we ensure to teach the reasons behind set-up knowledge, and develop the sailors abilities to know their personal limits dictated by height, weight sail design and fitness.