When purchasing a new boat, various choices must be made
based on how the vessel is to be used. Plans to embark on
long distance voyages which take the vessel offshore for
more than an occasional overnight passage, make the
parameters of design quite clear.
Likewise, when planning for mostly coastal cruising with
only limited passages, yet with the security of a true
passagemaker, those same set of parameters will apply. One
thing is certain: A true offshore passagemaker can make
coastal cruising more comfortable and enjoyable, but a boat
designed only for coastal cruising is inadequate for long
distance voyaging.
Among other certainties are the seakeeping characteristics
and efficiency of a full displacement hull. A displacement
hull is designed to remain fully in the water throughout its
entire range of speed. It does not climb out of its bow wave
and plane on the water's surface. Requiring a fraction of
the horsepower that semidisplacement or planing hulls
require to attain optimum speed, a displacement hull in the
40 to 60 foot size will be capable of maintaining speeds in
the 7 to l2 knot range while burning a minimum of fuel.
Beyond these speeds, a boat in this size range must get up
on plane.
With entirely different seakeeping characteristics, a
planing boat requires tremendous horsepower, which leads to
enormous fuel consumption, not to mention a disproportionate
increase in noise and vibration. As the noted designer
William Garden once said, "A planing hull can't carry enough
fuel to get out of sight." The laws of hydrodynamics clearly
recommend the displacement hull for efficiency and
seakindliness if long range cruising plans are on the
horizon. A study of other basic principles separates true
offshore passagemakers from the ever popular "trawlers" or
semidisplacement "seagoing" yachts. Understanding how these
fundamental rules affect performance and safety will help in
determining whether or not a particular boat will suit the
owner's needs.
The principles are:
Displacement/Length Ratio (D/L)
Speed/Length Ratio (S/L)
Above Water/Below Water Ratio (A/B)
The
first ratio, displacement to length, or D/L, is a function
of heft or weight. Generally speaking, a heavier boat will
have more room for accommodation, fuel, stores, equipment
and cruising gear. This is a good ratio for separating the
serious passagemakers from the wannabes. Too low a D/L, and
the boat will simply not have sufficient volume to carry
what it needs for selfsufficient, longrange cruising. A
general rule is that a boat in the 50 foot range should have
no less than a D/L of 270. The D/L for the Nordhavn 46 is
383. The shorter the vessel, the higher the D/L must be in
order to carry a sufficient load. The D/L should be
calculated with the boat fully loaded. An added advantage to
designing a hefty boat is that it can be built to heavyduty
scantlings and without concern for weight saving
construction techniques. In the longrun, a more heavily
built boat will be more comfortable, can carry more supplies
and equipment, and will more readily endure the wear and
tear of the sea.
The
speed/length ratio is best understood by knowing that
theoretical hull speed equates to an S/L of l.34. This is
the speed at which the hull makes a wave as long as its
waterline, and it is the speed which cannot be exceeded
without applying great amounts of additional power. The
longer the boat, the higher its hull speed. Using l.34 as
your boat's S/L, one can determine its hull speed by
multiplying l.34 by the square root of its waterline.
Looking at a typical speed/power/range curve, we see that
the region between S/L ratios of l.l and l.2 offers the
greatest efficiency relative to horsepower and speed. These
speeds, slightly lower than hull speed, will provide the
greatest range in distance. It is important to note that
very small changes in speed make large changes in fuel
consumption. If a boat has a waterline of 49 feet, its hull
speed will be 9.38 knots (l.34 x square root of 49) and it
will operate most efficiently in the 7.7 to 8.4 knot range.
Another
point of differentiation between coastal cruisers and true
passagemakers can be readily seen by the side view showing
the area (A) above the water to that of (B) below the water.
The lower the ratio, A/B, the better. While true oceangoing
fishing trawlers have A/B ratios under 2, it is difficult
for a yacht with adequate accommodation to achieve this. It
is P.A.E.'s opinion that oceangoing passagemakers should
strive for A/B ratios of between 2.l and 2.7. It has been
found that many popular "trawler styled" boats on the market
today have A/B ratios in excess of 4, making them
inadequate, if not dangerous, for offshore work.
Stability and comfort in rough seas are a major concern for
those venturing offshore, and P.A.E. has given a great
amount of attention to determining the right combination of
ultimate stability and roll period with the Nordhavns. Too
much initial stability, and the boat will feel like it's
"snapping" from one side to the other once it encounters
rough conditions. Too little, and the boat will roll easily
even in small seas. Finding the right roll period, or the
frequency with which it rolls from side to side, is as much
an art as it is a science, and it is important that a
comfortable rate of roll is achieved through proper hull
shape and location of the ship's centre of gravity. Tank
testing for ideal hull shape and careful attention to weight
distribution are crucial to the seakeeping characteristics
of each Nordhavn. To minimise the danger of a knockdown and
enhance their righting ability, all Nordhavns are ballasted
with an amount that is approximately l0% of their unloaded
displacement.
Through years of development, a number of stabilizing
systems have been found to be highly effective on Nordhavns.
For utmost simplicity and reliability, nothing beats the
passive paravane systems sometimes referred to as "flopper
stoppers." They will also reduce roll while at anchor and
require virtually no maintenance. P.A.E. has extensive
experience in engineering and fitting paravanes to each of
its models. For convenience, today's new active fin
stabilizers have become very popular. Especially effective
at the typical cruising speeds of a full displacement hull,
these new systems are more reliable than ever and will
dramatically improve the level of comfort even in mild sea
conditions.
