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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 semi-displacement 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 semi-displacement "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 self-sufficient, long-range 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 heavy-duty scantlings and without concern for weight saving construction techniques. In the long-run, 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.
 

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