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
https://skycraft.net
