Barry Hayes, owner of UK Sailmakers Ireland, explains what Lead is and why it is so important to sailmaking.

In sailmaking terms, the "Lead" is the distance between the sail plan’s Center of Effort (usually the working sail area which is the main and fore triangle "CE") and the Center of Lateral Plane of the hull/foils (CLP). The relationship between the two effects a boat’s balance. Both the CE and CLP are approximate positions for the centers of pressure for the sail area and the hull underbody, respectively, and are used by yacht designers at the drafting stages to ensure that the keel and rig positions are well balanced. CE is always forward of CLP by a small amount when in equilibrium mode. This is because when the yacht heels, the CLP usually moves forward due to the new heeled hull form until it lies almost under the CE. The result: helm balance.

There is an amount of Lead (pronounced "leed") that is acceptable, but if the design falls outside this area, the boat will have either too much weather helm (‘Lead’ is too short) or too much lee helm (‘Lead’ is too long). Too much weather helm results in drag (and in some cases helm exhaustion) and too much lee helm can result in dangerous situations such as crash gybes or worse, particularly if self-steering or helm issues come into play.

Determining just how much Lead is required depends on what type of boat you have. There is a rule of thumb that compares the Lead to the static waterline length. You divide the Lead by the waterline length and the result is: Percent of Lead (POL). Depending on the methods you are using to work out CLP, the allowable percentage can vary from 4% up to 17% after incorporating a number of factors. Hull shape is a key factor affecting Lead. A wide hull with hard bilges will require more Lead in static mode to compensate for the additional helm generated by the heeled hull shape. A narrow hull needs less Lead. Similarly, a tall rig creates more weather helm than a shorter one so the POL needs adjusting accordingly.