A study of 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 (D/L), is a function of heft or weight. Generally speaking, a heavier boat will have more room for accommodations, fuel, stores, equipment and cruising gear – and it will offer a more comfortable ride in rough conditions. 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 Nordhavn 55 has a D/L of 365. The shorter the vessel, the higher the D/L must be in order to carry a sufficient load – and the D/L should be calculated with the boat fully loaded.
To find D/L start by calculating a boat’s displacement in long tons (DLT). One (1) long ton equals 2,240 pounds. Next multiply the boat’s loaded water line (LWL) by 0.01 and cube the result. Finally, divide this result into the DLT. The final formula is D/L = DLT ÷ (0.01 x LWL)3.
For example, the Nordhavn 55 has a displacement of 115,000 pounds and a LWL of 50’ 10” (or 50.83 feet). DLT is 51.3 (115,000/2240), and (0.01 x 50.83)3 is 0.1313. So D/L is 51.3/.1313, which is 391.
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. Again, 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 to length ratio (S/L) 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 that 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 offer 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 50 feet, its hull speed will be 9.46 knots (l.34 x square root of 50) and it will operate most efficiently in the 7.8- to 8.5-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 and the area (B) below the water. The lower the ratio between the two (A/B) the better (in other words, increasing the area below the water will result in a lower, and more favorable, ratio). While true oceangoing fishing trawlers have A/B ratios under 2, it is difficult for a yacht with adequate topside accommodations 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.