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Dave Pope Design - Commercial & Pleasure Design
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Design
Philosophy - Why the displacement cat performs:
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All
boats operate as displacement craft. i.e. they will displace
exactly the same amount of sea water as the total weight of
the vessel. In order for the vessel to move forward, it must
push the sea water aside.
Each time the vessel moves forward one boat length, it must
push this amount of water aside. This water will form a wave
which is visible down the length of the vessel, and is referred
to the displacement wave.
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As
the vessel increases speed, the displacement wave will also
increase until a point is reached when the vessel must climb
up over the wave to travel faster.
The traditional displacement launch is unable to achieve this
is any extra energy is entirely consumed by the wave. The speed
at which this point is reached is called the hull speed of the
craft and is primarily determined by the hulls length to beam
ratio.
A typical monohull craft will have a length : beam ratio of
3 or 4:1, which will give a hull speed of no more than 8-9 knots,
but requires little power to achieve this.
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A
modern planing monohull uses its hull form, angled to the surface
of the water, to convert forward thrust into lift. This raises
the vessel out of the water and the vessel no longer pushes
the full equivalent of its weight aside into the displacement
wave, thereby eliminating the hull speed restriction.
Speeds of 25-30+ knots can then be achieved. This unfortunately
comes at a cost of a huge increase in power as a large proportion
of the power used for forward thrust is consumed to generate
the lift.
A semi-displacement launch has a hull form unable to produce
enough lift to raise the vessel sufficiently to eliminate the
displacement wave entirely, but manages to reduce the amount
of water pushed into this wave, which then consumes less energy,
and allows for a higher hull speed, frequently in the 12-20
knot range.
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The
Displacement Catamaran operates exactly as the traditional
displacement launch but because the long narrow hulls have
a length : beam ratio in excess of 12:1 the displacement hull
speeds are frequently over 30 knots, but still requires very
little power to achieve these speeds.
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These
long, narrow, round bilge hulls slide effortlessly through the
water, but this is only the first factor in performance and
sea-keeping ability.
As with the traditional displacement craft, the catamaran hulls
push a displacement wave down each side, which on the catamaran
means 50% outboard & 50% inboard.
The two waves pushed inboard will meet partway down the vessel
and join together forming one wave twice as high. if the cabin
structure (wingdeck) between the hulls is too low, this wave
will constantly pound the wingdeck, causing drag, noise and
making the ride uncomfortable.
When travelling in a seaway the catamarans bows will also direct
the extra water from each wave down between the hulls. When
this water is added to the displacement wave there can be quite
a large amount of water to be accommodated.
Insufficient wingdeck height for this is the second factor which
will greatly affect performance.
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Thirdly,
in a heavy sea state, the lack of reserve buoyancy of the narrow
hulls means that they will drive deeply into the face of the
wave before providing sufficient lift to raise the bows through
the wave.
If the cabin or wingdeck structure is too far forward relative
to the hulls, then the vessel will pound the wingdeck into the
face of the wave, which will greatly affect the ride and performance
as well as providing a stern test on the structural integrity
of the vessel.
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Finally,
any displacement craft operates most efficiently when trimmed
level. The narrow hulls lack of reserve buoyancy mean they are
susceptible to loss of for and aft trim, especially in a reasonable
seaway.
To overcome this, it is necessary to gain buoyancy with length
in the hulls. |
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will find therefore, that all my designs are characterised
by: |
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Long narrow displacement hulls |
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High wingdeck clearance |
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Hulls extended fore & aft beyond the cabin structure |
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A total package for exceptional performance, economy and
sea keeping. |
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