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One of the first
things a yacht designer learns when grappling with the design
of racing yachts is that there are many compromises to be made.
Generally, what makes a boat fast downwind will in all likelihood
make it slower upwind. The aspects that make a boat safe to sail
will ultimately make it slower. The comfort-factor, considered
in terms of accommodation, will add weight and size and slow the
boat down. Racing class rules will inevitably place constraints
on a design that will reduce potential speed and too much or too
little stability will slow a boat around a racecourse.
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Everything a yacht
designer incorporates compromises speed to achieve some other
design requirement.
Boats such as the eighteen-foot skiffs and the modern 49ers show
very little compromise and every design element that departs from
the skiff concept reduces the speed potential of any given design.
We often talk about sport boats being fun, fast and safe to sail.
By safe a boat is assumed to be self righting and unsinkable.
These requirements cost speed but are a necessary requirement
of the modern sport boat. But the really big compromise is the
addition of a lead bulb to make a boat self righting. This alone
sets the sport boat apart form the modern skiff.
The single biggest change in concept enjoyed by modern sport boats
is the fact that owners are willing to accept a 6-8 metre yacht
without cruising accommodation. Because of the reduction in these
facilities we can design lighter boats and reduce the structure
to provide for hull strength, rigging and keel loads only. This
in turn allows more ballast to be added to the lead bulb and stability
increased. |
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Foil
design has made a significant contribution to the progression
of on modern sport boat designs. The most limiting factor
of modern sport boat foils is the drag caused by the lead
bulb. While it is a necessary evil as far as the designer
is concerned, it represents a significant portion of the
overall drag. It is important also to get the lead bulb
as deep as is practical in the boat but this again has its
drawbacks as the extra draft adds extra wetted surface.
There is of course a balance as far as depth and drag is
concerned. Modern materials such as carbon fibre are often
used in keel foils to help reduce the overall size of the
fin. Carbon fibre allows a designer to produce a narrow
yet relatively stiff foil to support the lead bulb and often
the lateral area requirement of the fin is exceeded because
of its depth.
Rudders
tend to be rather large for the size of the boat. This is
generally due to the fact that oil design has made a significant
contribution to the progression of on modern sport boat
designs. The most limiting factor of modern sport boat foils
is the drag caused by the lead bulb. While it is a necessary
evil as far as the designer is concerned, it represents
a significant portion of the overall drag. It is important
also to get the lead bulb as deep as is practical in the
boat but this again has its drawbacks as the extra draft
adds extra wetted surface. There is of course a balance
as far as depth and drag is concerned. Modern materials
such as carbon fibre are often used in keel foils to help
reduce the overall size of the fin. Carbon fibre allows
a designer to produce a narrow yet relatively stiff foil
to support the lead bulb and often the lateral area requirement
of the fin is exceeded because of its depth. |
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Rudders
tend to be rather large for the size of the boat. This is generally
due to the fact that oil design has made a significant contribution
to the progression of on modern sport boat designs. The most limiting
factor of modern sport boat foils is the drag caused by the lead
bulb. While it is a necessary evil as far as the designer is concerned,
it represents a significant portion of the overall drag. It is
important also to get the lead bulb as deep as is practical in
the boat but this again has its drawbacks as the extra draft adds
extra wetted surface. There is of course a balance as far as depth
and drag is concerned. Modern materials such as carbon fibre are
often used in keel foils to help reduce the overall size of the
fin. Carbon fibre allows a designer to produce a narrow yet relatively
stiff foil to support the lead bulb and often the lateral area
requirement of the fin is exceeded because of its depth.
Rudders tend
to be rather large for the size of the boat. This is generally
due to the fact that very large asymmetric spinnakers tend to
push the boat around in the puffs of wind. Once the boat is up
on the plane the rudder size could probably be halved, as there
is generally very little force on the rudder while the yacht is
planing. Rudder sectional shape is therefore aimed at low drag
rather than high lift, as there is ample area to provide the lift
required.
In summary the modern sport boat design has evolved into a very
sophisticated package. It includes a holistic approach that includes
all of the design parameters of hull shape, foils, rig and sail
design. Gone are the days where a boat can be put together without
consideration for all of these elements and still remain competitive.
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Easily driven
boats with narrow waterlines allow a designer to add more sail
area, in particular to asymmetric spinnakers. This is because,
when a puff of wind hits a boat that is easily driven, it accelerates
and the added energy is transposed into forward motion rather
than healing moment.
Rig design has been simplified so that they automatically exhaust
additional air from the sails in the puffs. This is a very important
aspect of sport boat design as it is important to keep the boat
easy to sail. Careful attention is paid to the topmast and mainsail
roach profile so that they work together to power up when required
and exhaust in heavy air. The use of carbon fibre in the mast
has helped induce this effect. When the topmast bends in the
wind puffs the leach of the mainsail opens up to exhaust the
additional air not required.
A great deal of sail development has gone into providing sport
boats with sails that maximise forward motion and minimise healing
force. Part of the answer is in the shape of the mainsail that
provides a sail shape that is deeper downwind when the mast
is straighter and a lot flatter upwind as the mast bends under
load. The sailmaker’s art of getting the luff curve to
fit the mast is very important. Jibs often use full-length battens
with a positive roach in the top of the sail to help with twisting
off at the top to exhaust the wind puffs much like the mainsail.
Forestay tension is gained from the effect of mainsheet tension
and the rigging loads.
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 I mentioned that
too much stability can reduce speed. In particular this will
slow the boat downwind as stability is gained by increased weight
or increased hull form stability that increases wetted surface.
With this in mind, the hull drag is reduced by narrowing the
waterlines and trading hull form stability for righting moment
from an increased lead bulb weight.
The fact that crew weight is often carried outboard of the gunwale
when the crew use hiking straps, further increases this trend.
Modern sport boats often have a ballast ratio greater than 150%
when the crew weight is taken into consideration.
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