Balancing your boat - by Ben Morris
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What does it mean 'balance your boat'? It simply means
in the end that once sailing and pointed in a particular direction, a yacht
will continue sailing in the same direction with little or no rudder input -
any turning forces are balanced. There are huge advantages in having a
radio yacht balanced firstly in reducing drag and maximising driving forces
and second in maintaining a consistent course where because of distances, it
may be difficult to steer effectively so if the yacht does it itself that's
a real bonus.
What are the forces acting and how are they balanced?
The two main forces are produced by the sails in air (aerodynamic forces)
and those produced by the hull and appendages in water (hydrodynamic
forces). In terms of balance, the important issue here is where these
forces are centred.
The centre of forces (often call 'Centre of Effort') produced
by the wind on the sails when sailing across the wind. is to the first
approximation the geometrical centre of the sail area. In reality it
will be some distance forward of this as the sail acting like an airfoil has
its centre of lift at approximately 30% of the chord (not 50%). The
sail produce a force partially forcing the yacht forwards and partially
sideways. The areas of the hull, fin and rudder act by producing a
force which counteracts the sideways component. The centre of this
force (often called the 'Centre of Lateral Resistance') is again
approximately the geometrical centre of those areas but in reality is also
some distance forward for the same reasons as the sail force is slightly
forward. See diagram below to see these two centres.
To be balanced then these two centres should be in line so
that there is no resulting twisting force trying to push the stem or stern
of the yacht away from the wind.
Should the Centre of Effort (C of E) be too far forward
compared to the Centre of Lateral Resistance (C of LR) then there will be a
twisting force pushing the stem of the yacht away from the wind. The
yacht is said to have 'lee helm' as the stem of the yacht is pushed to
leeward.
Should the C of E be too far back compared to the C of LR
then there will be a twisting force pushing the stern of the yacht away from
the wind and steering the stem up into the wind. The yacht is said to
have 'weather helm' as the stem of the yacht turns into the weather or wind.
The effect of lee helm is that the yacht does not appear to
point into the wind (falls off the wind) as it requires continual rudder
pressure to push the stem of the yacht into the correct direction.
The effect of weather helm is that the yacht continually
wants to point too high (screws up to windward) so that the jib leading edge
folds back (or luffs) and yacht eventually points directly into wind and
stops. This needs continual rudder pressure to push the stem of the
boat away from the wind
There is another issue affecting this as well. As the
yacht heels over the forward force generated by the sails is set to one side
of the yacht causing a turning force into the wind. In other words it
adds to the weather helm of the yacht. This is particularly apparent
when gusts hit and the yacht often can violently screw up to windward, stall
the sails and come to a stop.
The ideal type and amount of helm seems to be one where there
is a very small amount of weather helm when sailing in optimum wind strength
for the sail used. This will have the effect of causing the yacht to
slowly climb up to windward, eventually causing the jib to begin luffing.
Ideally this should take 5 - 10 metres to happen. An occasional minor
rudder adjustment will be needed to correct the swing to windward.
With these settings, the yacht will be sailing close to the wind by default
and climb to windward efficiently.
There are a few ways that a yacht can be altered to change
the nature of the helm. These require either the C of E to be moved
forward or aft or the C o LR to be moved forward or aft.
The C of E can be altered by moving the mast fore or
aft (may not be possible!) or tilting (raking) the mast fore or aft.
It can also be altered by twisting the mainsail and to a lesser degree the
jib as this alters the effective centre of the area of the sail.
Another may might be to alter the relative size of the main and jib sails
(OK for Marbleheads or Ten Raters but not allowed by some classes such as
IOM).
The C of LR can be altered by moving the fin fore or aft
(again may not be possible) or angling it fore or aft. Remember though
that any change to the fin is probably semi-permanent and will affect all
rigs and may change the angle of the bulb another job to do!
Hopefully though your designer has thought this all through
and tested it thoroughly so the boat is balanced reasonably well to begin
with and all you have to do is fine tune it.
To decrease lee helm and/or increase weather helm
-
Increase the rake of the mast by angling it aft
-
Reduce the twist in the main and jib
-
Move mast aft
-
Move or angle the fin forward
-
Make mainsail larger compared to jib
To decrease weather helm and/or increase lee helm
-
Decrease rake of mast by angling it less aft
-
Increase twist in main and jib
-
Move the mast forward
-
Move or angle the fin aft
-
Make jib larger compared to mainsail
A balanced yacht is generally a fast yacht simply because there is little
need for rudder movements when sailing. If you find you have to
continually adjust the rudder to maintain direction in steady breezes then
its time to adjust the balance.
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
