Into the Wind
by Jeff Williams
http://skycraft.net

A Question of Balance

I was talking to a fellow the other day who wanted to reduce the size and complexity of the sails on his two masted sailboat. His solution is to remove the smaller mast near the stem, and to change the gaff rigged sail for a modem triangular or Marconi sail. Unfortunately, if he follows through on this plan, the boat won't sail towards the wind at all.

The sideways force of sails is offset by a keel or centreboard that projects down below the boat. The yacht designer ensures that the size and shape of the keel is adequate to keep the boat from skidding sideways when going towards the wind. Designers call this quality lateral resistance. If you were to find the mid point of the underwater area of the boat's profile, then that would be the centre of lateral resistance (CLR). If you were to hook a towline onto a boat's CLR, it would be pulled sideways.

The boat's sail plan also has a centre of area called the centre of effort (CE). In theory, the centre of effort (CE) should line up exactly over the CLR but there is another factor at work here. Sailboats are usually heeled (tilted) sideways as they sail. This positions the sails over the water and therefore the power transmitted to the boat is off centre, forcing the boat to turn towards the wind. The boat's 'footprint' on the water also changes shape as the boat heels over, which also causes a turning effect, In practice, the CE is always positioned ahead of the CLR to compensate for these turning forces. This offset is called 'lead' and varies with the type of boat.

Wind surfers use this principle to steer. The CLR is constant (sort of, let's keep this simple), but the sailor can tilt the sail forward or aft. This changes the position of the sail's CE. Tilting the sail forward brings the sail's force in front of the hull's centre of resistance. The board turns away from the wind. Tilting the sail aft has an opposite effect, and the board will turn towards the wind. A yacht designer doesn't have this degree of flexibility however, and has to come very close to getting it all right on paper before the boat is built.

Sailboats with poor balance can be fixed. The base of the mast can be moved forward or aft on most small boats with wire rigging. Remember though, that moving the base of the mast forward will cause it to rake (lean) aft resulting in sail area moving back. Altering the length of the supporting wires can do the same.

The position of the CLR of the boat itself can also be changed, often unintentionally. Heavy weights such as extra fuel or water tanks, a diesel engine, or extra crew sitting in the cockpit can cause the stern (back end) to settle low and change the balance. Boat's with a swing keel can change the underwater profile by raising the board on its pivot a little.

It can get a little confusing, so here's a quick guide to sail balance:

1. Changing the rigging or size of sails will affect the handling of your boat. Consult a designer if you are going to make expensive and/or irreversible changes.

2. Boats that veer up into the wind excessively have weather helm? Sail the boat with less heel (lean) if practical. Move sail area forward by reducing area aft and/or increasing sail area forward. The same can be done by raking the mast forward. Make sure there isn't excessive weight in the bow of the boat. Anchor chain is a common offender. If you have a swing keel or centreboard, raise it slightly to change the underwater profile. You should retain some weather helm. It increases windward performance, and allows the boat to head into the wind and stop should you fall overboard.

3. Boats that swing away from the wind have lee helm. This should never happen except in very light air. Move the sail's area back by raking the mast aft. Too much weight in the stem from extra crew, stores, engines, etc. is a common cause of this problem.

Keeping the sails in balance with the hull will make steering easier and let the boat go faster.

Jeff Williams
http://skycraft.net