Balance - Everyone Needs it
The following guide from Towergate
Insurance, specialist providers of boat
insurance will help the sailor find balance. It can
make sailing safer, more comfortable and cut maintenance time and
costs.
Let’s start with a quick sailing experiment that can be done in
a few minutes, it just needs a light to medium breeze and no other
boats around – with the boat sailing upwind in a straight line, let
go of the wheel or tiller. Does the boat still go in a straight
line? No? Ok, grab that wheel back quickly!
The conclusion from the experiment is that the boat lacks
balance - the balance of the two forces that make a sailboat work.
It’s clear enough that the sails are the boat’s motive force or
power source, without them full of wind the boat isn’t going
anywhere. The sails push the boat along. It’s less obvious (because
we can’t see it) that the keel, hull and rudder are also exerting a
force. This is a resistance force, a resistance to the boat being
pushed through the water.
There is a complex physics involved in these two forces (except
when the boat is sailing dead downwind), because the air is flowing
at an angle across the sails they are acting as an aerofoil, just
like an aeroplane wing. Anyone who has ever taken off in a 747
jetliner knows how much force a wing can produce. Similarly,
because the water is flowing at an angle across the hull, rudder
and keel, then they are collectively acting as hydrofoils – just
the same as the foils that lift some high-speed ferries out of the
water.
We don’t really need to worry about the detail; we just need to
understand that there are two sets of forces at work; the aerofoil
action of the sails, and the hydrofoil action of the hull, rudder
and keel. The thing we do need to grasp is that each of these
forces can be regarded as acting through a single point -- much as
the weight of a pencil acts through a single point when balanced on
your finger. The aerofoil force acts through a point on the sail
plan called the centre of effort and is pushing to leeward. While
the hydrofoil force acts through a point in the keel, usually
called the centre of lateral resistance, and opposes the centre of
effort by pushing to windward.
When we talk about balance in a sailboat we are talking about
the fore and aft distance or displacement between these two forces.
So for instance; if the centre of lateral resistance of the keel is
acting through a point in line with the shrouds, then when the
centre of effort of the sails is also acting in a line through the
shrouds, the boat is balanced and will sail straight all on its
own. When they are not lined up, the boat is out of balance and it
will try to turn.
Let’s look at another simple experiment. In the same conditions
as before, sailing upwind in a straight line in a light to moderate
breeze, let the headsail flap. This removes its contribution to the
centre of effort, and so the centre of effort will move aft,
putting it firmly behind the centre of lateral resistance. The boat
will (should!) then try to turn into the wind. This is called
weather helm, and when the boat has it, if the wheel or tiller is
let go the boat will automatically start to turn up into the
wind.
The opposite occurs if the mainsail is allowed to flap or flog.
In this case the centre of effort will move forward, ahead of the
centre of lateral resistance, and that means that the bow will
start to turn away from the wind. This is called lee helm. Both lee
helm and weather helm can usually be resisted with the rudder. And
that is the most important point – the way the sails are trimmed
and set can either help or hinder steering the boat; both in a
straight line and around corners.
So having these forces in balance is much kinder on both the
boat and the helmsman; the latter doesn’t have to fight with the
wheel or tiller for control, all the steering gear is less heavily
loaded and less likely to wear and break. The best scenario is
actually a slight amount of weather helm. Then if someone
accidentally releases the tiller or wheel, the boat will gently
point into the wind and slow down. If you have lee helm it will
bear away and pick up speed - perhaps even go into an accidental
gybe, and that could be expensive and dangerous.
The final stage is how do we achieve this
perfectly balanced boat? The experiment where we let the headsail
flap is the clue. To change the balance we increase or decrease the
amount of power coming from either the headsail or the mainsail –
or both. A more powerful headsail will try to turn the bow away
from the wind (lee helm). And vice versa, a more powerful mainsail
will try to turn the bow into the wind (weather helm).
Similarly, a less powerful headsail will turn the bow into the
wind (weather helm), while a less powerful mainsail will try to
turn the bow away from the wind (lee helm).
How do we make those sails more or less powerful? The easiest
way is to change the size – by furling or unfurling some mainsail
or headsail, it ought to be possible to balance the boat nicely. It
is possible to do it by changing the shape through sail trim as
well, but that’s a whole new story - and maybe this short article
will have provided an incentive for you to look into that topic.
Good sail trim should not just be for racing boats; it really can
make sailing safer and more comfortable as well!
