SCIENCE AND SPORT
Rob Bowker has taught primary
school children to sail. Here, he gives an
outline of how the forces involved work
together, and shows how children can
design, make and test their own boats.
about forces, in a fun and exciting
way. Most children will intuitively
understand the idea of a boat
sailing with the wind behind it
(running before the wind).
Children can be encouraged to
experience this force by running in
the playground with the wind
behind them and holding an
umbrella, bedsheet or large piece of
cardboard to catch the wind. Whilst
some of the children experiment in
this way, you can discuss what is
happening with the rest of the class;
you can help them understand and
experience the force of the wind in
the same way that the historic tall
ships with their large square sails
excelled in such a wind.
How do you sail into the wind?
Sailing boats not only sail
downwind (running), they can sail
at right angles to the wind (beam
reach), and also into the wind (close
reach), and all points in between.
To understand how a sailing boat
can go upwind, some
understanding of physics is needed.
n the UK you are never more
A sail, especially the modern
than two hours drive away from Bermuda sail that would be used in
the sea, where many of us go to
an Olympic event, acts like an
spend our holidays and relax. The
aerofoil (i.e. the same as an
sea has inspired great artists,
aeroplane wing), creating lift. The
musicians, writers and thinkers.
air must flow faster on the outside
Historically, the British Empire was of the sail, because it has further to
created by sea trade and the vast
travel around the camber (curve) of
Royal Navy that defended it. Today the sail (see Figure 1). The stream of
many people use the sea for leisure air flowing the longer distance
and British sailors compete
around the camber of the sail
successfully with the best in the
creates a lower pressure than the
world. They did well in the last
air flow on the underside of the
Olympic Games in Sydney and
sail. The push force is created by
there are high expectations of
the difference in the two air
further successes in future
pressures from high to low. A
Olympics, especially here at
sailing boat can sail at about 40º to
Weymouth in 2012.
the wind and an efficient Olympic
Sailing can be a very good
dinghy can sail even closer to the
context in which to teach children
wind. By zig-zagging (tacking)
I
Photo Roger Orpin
PRIMARY SCIENCE 102 March/April 2008
9
SCIENCE AND SPORT
Figure 1 Flow
of air around
a sail
through the wind on a close reach,
a sailing boat can make progress
upwind.
Children can see the effect at first
hand simply by blowing across the
top of a strip of paper. The faster
flow of air creates an imbalance of
pressure and the paper moves
upwards. This can also be very well
illustrated by getting children to
take on the role of air molecules.
Draw a large sail in chalk on the
playground surface. Arrange the
class of children into pairs and ask
child A to run around the outside
camber of the sail, and child B to
run in a straight line on the
underside of the sail. Child A on
the outside has to run much faster
to keep up with child B, who has
the easier and shorter distance to
run. Continue this exercise with all
the children in their pairs, making
a continuous stream of air
molecules moving on the outside
and on the underside of the sail. At
a chosen moment, ‘freeze frame’ the
children on the sail.
Discuss with them how the
spaces between the children on the
outside of the sail are wider than
the spaces between the children on
the underside of the sail. Explain
how the bunching up of the
children (air molecules) on the
underside of the sail leads to higher
pressure and the wider gaps
between the children on the outside
of the sail lead to lower pressure.
This means that once you have such
a ‘drive’ from the sail the boat will
start moving through the water.
Resultant forces
At this stage, the sailor will have to
set the sails at the most efficient
angle to the wind. To work out how
a sailor manages this we first need
to know a little more about forces.
We can describe a force as a vector,
that is, it can have both direction
and value. Forces also do not work
10
the water, the ratio of the true wind
to the apparent wind is greatest,
but as the boat starts to increase
speed it generates more apparent
faster
air flow
wind. The sailor will experience the
resultant wind shifting increasingly
slower
to the front of the boat (Figure 3),
air flow
and sets the sail accordingly by
pulling (sheeting in) the sail
towards the centre line of the boat.
Watch Ben Ainsley in his Olympic
laser class dinghy (Figure 4)
pulling the sail hard towards him
alone, but in pairs or more to create as his dinghy accelerates in a good
a resultant force. Children can
wind. You will also notice him
experience a resultant force in the
leaning right out to try to stop the
following way. Ask two children to force of the wind from tipping
hold the opposite ends of a
(heeling) his dinghy over.
skipping rope with each standing at
By keeping his dinghy as flat as
possible on the water he will
increase his speed. Of course his
boat will have a dagger board
(centre board and keel in larger
boats), the part of the boat below
the water, which also helps the boat
keep upright and prevents it from
moving sideways through the
water (lee way). The forces acting
above and beneath the water are
shown in Figure 5. The sideways
Figure 2 Resultant force experienced by
components of the force of the
child C when two others (A and B) pull the
wind and water on the boat make
ends of a skipping rope
the boat heel away from the wind
but the heeling effect is resisted by
an angle away from a third child
the hull and keel. These two
holding the middle of the rope. If
horizontal components have equal
the two children at the ends of the
size but opposite direction and as
rope give an even pull on the rope
forces they cancel out. In short, a
at the same time, the third child
sailing boat can be thought of in
will move in a direction between
terms of aerofoils above the water
the two pulls. The force the third
and hydrofoils below the water. A
child experiences is the resultant
sailor will monitor and adjust the
force (Figure 2).
forces exerted by both in order to
A sailing boat moving through
sail a boat in the most efficient
the water will have the force of the way.
wind (true wind) blowing across
Making, designing and testing
the boat and also the force
The overall design and materials of
generated by the forward
the sails and hull will determine
movement of the boat (apparent
how fast a dinghy will sail.
wind). To experience this, you can
take children on to the playground Children can have a lot of fun
designing, making and testing their
on a still day and get them to run
around holding onto streamers. As own boats (Figure 6). Give children
a block of wood, perhaps balsa
they run the streamers will start to
stream horizontally, because of the wood (14 cm x 6 cm). First they
have to design the hull of their boat
wind generated by their running.
This is the apparent wind and is the on paper. Then, using some basic
same sensation as children will feel tools, a hacksaw, surform and
sandpaper, they can start to shape
on a still day when riding a bike
their boat. Overall dimensions need
downhill.
to be laid down: e.g. at its widest
The force a sailor experiences
point the hull needs to be at least 5
when a boat is moving is the
cm and its length has to be at least 12
resultant of the forces of the true
wind and the apparent wind. When cm. Within these parameters the
the boat first starts moving through children are free to design and shape
PRIMARY SCIENCE 102 March/April 2008
SCIENCE AND SPORT
their boat in any way they wish.
Your ‘test tank’ can be a length of
house guttering sealed with two
end-pieces and filled with water.
The children’s boats can be pulled
along the ‘test tank’ by attaching
fishing line to the boats and
running the other end to a weight.
Each of the children’s boats can be
timed between two points using a
stopwatch or, better still, datalogging equipment. Using light
gates to time the boats accurately to
a 100th of second gives the best
results (Figure 7). During the
Figure 4 Ben
Ainsley sailing
his Olympic laser
class dinghy
Figure 3 Wind forces on a sailing dinghy as
it moves through the water
3
(Travelling fastest.)
testing, you can discuss fair testing
with the children and the need for
multiple tests on each boat. The
children can put their results into
Excel and graph them. They can
then evaluate which hull design
features worked the best and
perhaps compare them to real
Olympic class dinghies. The
children will have a lot of fun
acting as boat designers, testing,
evaluating and redesigning their
boats using sound scientific testing.
It helps to know some of the
Figure 5 The forces acting on the boat above and
beneath the water
2
(Now travelling faster.)
1
(Travelling slowly.)
PRIMARY SCIENCE 102 March/April 2008
11
SCIENCE AND SPORT
Figure 6 Children time their boats in the test tank
theory behind sailing, but it is even
better to experience it yourself. As a
follow-up, if you wish to take the
children sailing and allow them to
experience the forces of the wind
and water themselves, there are
many good sailing centres around
which may be only too willing to
help. Although this article is
Figure 7 Using light gates and data-logging to time the
passage of the boats
principally about the science of
sailing, it would also be very
rewarding to expand and extend
this into other areas of the
curriculum: sailing has strong links
with art, literature, history and
geography. As inspiration in a
topic-based approach the
possibilities are endless.
Gatekeeper Educational
Rob Bowker is Senior Lecturer in
Primary Science Education at the
University of Exeter.
Email: [email protected]
www.gatekeeperel.co.uk
Illustrated materials for identifying and understanding plants and animals most likely to be
found in school grounds, gardens and open spaces. Key Stage 2 & 3.
Gatekeeper Guides. Keys which sort by observable features. Laminated A3. Species on both sides
Use as poster,
folded to A4 or
rolled for pocket
HALF PAGE AD
Ten titles: Key to Identifying Freshwater Invertebrates. Land Invertebrates. Butterflies. Wild Flowers of Grassland. Common Grasses. Dispersal of Fruits and Seeds.
Trees and Shrubs. Common Birds. Flowers of the Hedgerow. Common Fungi.
£2.50 each. £2.25 each for five. £2.00 each for 10 plus. OFFER: £18.00 for 10-title multi-pack
Tel:01435 862731. Fax:01435 865722. E-mail: [email protected]. Also from the ASE Bookshop. www.ase.org.uk
Copyright. L.Merrick
12
PRIMARY SCIENCE 102 March/April 2008