Fundamentals

Perspective on A True Passagemaker

Those who own a Nordhavn, or aspire to own one, know the score: There is a big difference between a recreational “trawler” and a true oceangoing passagemaker like a Nordhavn. The ability to power with confidence across any sea or ocean in safety and comfort is chief among reasons experienced boaters go to Nordhavn whether their adventures are coastal, global or anywhere in between.

These boaters know their history: In 1989 with the N46 Nordhavn became the first to design and develop a production deep-sea passagemaker – and ultimately sold more than 80 of these groundbreaking hulls, most of which are still actively taking their current owners to all corners of the globe.

In 2001 Nordhavn became the first builder to take one of its own boats, a smaller N40, around the world – a feat that was accomplished in 27 weeks and has yet to be matched – or even attempted – by any other builder. At present Nordhavn offers the fullest line of true deep-sea passagemakers with more than 15 models ranging from 40 feet to 120 feet in length.

It’s no surprise that other builders have entered the market, many with offerings that fail to pass muster with knowledgeable boaters when compared to the level of durability, reliability and luxury that Nordhavn’s 30+ years of experience bring to the water. These boaters know that Nordhavn’s commitment to solid, proven construction fundamentals will deliver them and their crew safely to their destination, near or distant, every time while offering the level of comfort and enjoyment that Nordhavn owners and dreamers have come to expect.

Full-Displacement

Plans to embark on long-distance voyages that 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, the 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 wholly inadequate for long-distance voyaging.

Among other certainties are the seakeeping characteristics and efficiency of a full-displacement hull. A full-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 full-displacement hull in the 40- to 120-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 as well as 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 full-displacement hull for efficiency and seakindliness if long-range cruising plans are on the horizon.

By The Numbers

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.

Stability is Key

One of Nordhavn’s principle guarantees to owners is the ability to cross any sea or ocean in safety AND comfort. Thanks to proven design techniques and continuous advances in stabilization technologies, Nordhavn is able to offer boats that go way beyond taking the edge off of rough conditions, which makes the experience of transiting an ocean, or even circumnavigating the globe, accessible to those who might normally shun such an adventure due to motion issues.

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 initial stability, 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 center of gravity.

Tank testing for ideal hull shape and careful attention to weight distribution are crucial to the seakeeping characteristics of each Nordhavn. To minimize 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. Active fin stabilizers are the current method of choice for most Nordhavn owners. They are especially effective at the typical cruising speeds of a full-displacement hull and they dramatically improve the level of comfort even in mild sea conditions. After several years of use on Nordhavns, they have proven themselves reliable and ready to the task.

Passive paravane systems, sometimes referred to as “flopper stoppers,” are also popular thanks to their ability to reduce roll while at anchor and the fact they require virtually no maintenance – and, when stabilization is not required, they can be pulled out of the water to prevent unnecessary drag.
P.A.E. has extensive experience in engineering and fitting paravanes to the N47, N43 and N40.

Bulbous Bow Benefits

With the development of the 62, P.A.E. began extensive tank testing of new models in order to verify hydrodynamic characteristics of hulls, with emphasis placed on resistance and “seakeeping” qualities. Most Nordhavn displacement hull forms are tested with both conventional as well as bulbous bows. Bulbous bows have been used for years on most tankers, freighters and large fishing vessels because the bulb reduces hydrodynamic drag at the speeds ships travel resulting in a considerable fuel savings over the lifetime of the vessel.

P.A.E.’s interest in the bulbous bow was originally focused on reducing resistance, improving efficiency and thereby increasing range. By affecting the bow wave, tank testing has shown that a bulbous bow reduces resistance in the range of 3 to 15 percent depending on the model and speed. This reduction in wave making resistance allows less horsepower to be used for a given speed, thereby increasing range and reducing engine rpm with a corresponding decrease in engine noise. While the primary intention of bulbous bows was to increase efficiency, it was noted that that they also contributed to decreases in pitch motion on some models. Pitch amplitude on the Nordhavn 62 was reduced by 20% and pitch acceleration was reduced by l8%, contributing to the “seakindliness” of the boat in most sea conditions.

At present the bulbous bow is offered as an option on all the larger Nordhavns, starting with the N76 (and it is standard on the new N120, where tank testing points to a wave reduction of at least 10%). Specific cruising habits and locations might influence a buyer’s decision to include a bulbous bow, and most of the larger boats do end up leaving the factory with a bulb attached. In fact, every N86 to date has been built with a bow bulb.

On the smaller yachts it was found that the bulb can cause what some might consider to be an unwelcome amount of noisy pounding or slapping while going to weather in rougher seas. In order to make the ride as quiet as possible, bow bulbs are not fitted on the smaller Nordhavns. Even though a bow bulb would certainly bring an improvement to the lifetime fuel economy of the N55, N47 and other smaller boats, it was decided that the benefits were not worth building a boat that some might find noisier than they’d like.

Powerful Choices

Obviously, the motor-powered passagemaker depends entirely on its engine for propulsion, and a great amount of attention must be paid to selecting, installing and maintaining the power plant. The marine diesel engine has proven to be one of the world’s most dependable machines ever made, and today’s diesels are better and more reliable than ever. In fact, the undeniable reliability and resilience of these engines is key to Nordhavn’s promise to safely cross any ocean with absolute confidence.

All of the smaller Nordhavns are designed with a single engine in mind, although twin engine installations are available on most of the larger models. For the smaller yachts, the advantages of a single engine include greater economy and reduced noise and vibration. And, of course, maintenance is half of what a twin-engine installation requires. Most Nordhavns are ordered with a single main engine plus a smaller wing engine for “get home” power (Nordhavn’s mega-yachts, the N86 and N120, are designed with twin engines primarily because the diameter of the propeller required for a single-engine application would result in excessive underwater draft).

Because of the inherent efficiency of a full-displacement hull, the amount of horsepower required to drive the boat at hull speed is far less than what typical “trawler-like” semi-displacement boats require. When specifying horsepower, P.A.E. also uses the far more conservative “continuous horsepower rating” figure provided by the engine manufacturer, which is often half of what other boat builders will claim for the same engine. Be sure to compare horsepower ratings carefully, and remember that full-displacement hulls require far less power than other types of hull designs.

All Nordhavn designs call for extremely reliable and durable engines built to run for virtually weeks- or months-on-end and to be serviceable worldwide. Large, slow-turning props are best, so P.A.E. specifies gear ratios of at least 3-to-l or more. All engines are de-rated to ensure trouble-free, continuous use. In order to further reduce noise, wear and vibration, engines that develop their rated horsepower at low RPM are specified.

Owners sometimes have specific engine preferences, and because P.A.E. provides custom capabilities, it is able to accommodate the requests. Caterpillar, Cummins, MTU, John Deere, Lugger, Mann and Yanmar diesels have all been successfully installed on Nordhavns.

Construction Details

The middle of the ocean is no place to worry about your boat, which is one reason why Nordhavns are built to extremely heavy scantlings and construction details. The in-water portion of the hull structure is a solid laminate of fiberglass using vinylester resin with a network of full-length, longitudinal and transverse stringers. P.A.E. believes a series of solid laminates in the hull bottom is superior to cored construction, especially because reduced weight is not a requirement with full-displacement designs. The heavier scantlings to which Nordhavns are built not only enhance the overall quality and feel of the boat, they also contribute to its long-term life and resale value. Forward sections feature extra laminates for collision protection, and watertight bulkheads and doors are provided where specifications require.

The main decks are a continuous fiberglass laminate, stem to stern, adding to the strength and level of finish. The wheelhouse, Portuguese bridge, salon and bulwarks separate from an advanced multi-part mold in one piece, ensuring a strong structure free of joints and potential leaks and cracks. All exterior horizontal walking surfaces have been finished with a fine diamond-patterned non-skid. Great care has gone into the detailing of the deck tooling with relieved waterways, hardware bases, hatch bosses, hawseholes and more. When one has the opportunity of seeing a Nordhavn up close, careful attention should be given to its hull and cabin sides. Because all female molds are l00% tooled and highly polished, one will see a beautiful, glossy, smooth surface that represents the very finest in fiberglass yacht building.

Much of the hardware used on Nordhavns is custom made to P.A.E. specifications, and is of the highest-grade stainless steel. Hatches, doors, windows, deck boxes, storage bins and lazarettes are all built to take decades of strenuous use. There are no compromises in quality, no shoddy workmanship, no poor finishes and no cutting of corners to save money. Over the years, P.A.E. has found that whenever there is a choice between doing it right and saving a few dollars, doing it right is the only option. P.A.E. knows its customers will be depending on their boat to take them across oceans, and there can be no room for second best in materials or workmanship.

Dry Exhaust Systems

Years of development and painstaking effort to design a clean, quiet and foolproof exhaust system have resulted in the system found on most of today’s Nordhavns. In lieu of a conventional wet exhaust, all of the single-engine Nordhavns are designed with a dry exhaust system and recessed keel coolers. This is a closed system where the engine’s coolant supply is circulated through a captive series of tubes recessed in the vessel’s hull. And, even though it meant the addition of more under water keel coolers, some twin-engine Nordhavns have been successfully designed using dry exhaust systems.

No salt water enters the ship, and the engine-mounted raw water pump, heat exchanger, anti-siphon valves and seawater exhaust are eliminated. With no strainers to clog, no impellers to break and no belts to fail, reliability is greatly increased. Designed with special noise attenuators and installed as an isolated, free-floating system, gas and noise are virtually eliminated. The exhaust goes largely unnoticed by all as it is vented far above the top deck.

Noise Control

Controlling noise and vibration on a long-distance passagemaker is extremely important as the crew will be exposed to ambient noise while underway for long periods of time. Aggressive steps are taken to reduce noise, including the installation of special sound-deadening materials throughout the engine room, and the use of resilient mounts wherever required. Exhaust systems are free-floating, with their hoses and metal piping mechanically isolated from any hull material with specially designed mounting hardware. On a Nordhavn, owners and guests will appreciate being able to hear the gentle sounds of water passing along the hull.

Accommodations

The interior accommodations of Nordhavn yachts exhibit the exquisite craftsmanship that P.A.E. has become known for starting with its beautiful Mason sailboats. The quality of joinerwork is among the best in the world, the choices of woods and materials are second to none and the overall level of fit and finish rival the best of today’s mega-yachts.

In fact, with the addition of the N86 and N120 to the line, Nordhavn has officially entered the mega-yacht and super-yacht market – a transition that was seamless thanks to its existing ability to design and build attractive and luxurious showboat interiors that will last every bit as long as the durable and hard-wearing Nordhavn hull. Each model is available with a number of recommended layouts, and there is a wide variety of customization that can be done.

Nordhavn interiors are designed to be comfortable and practical at sea. These are oceangoing vessels that are equipped to treat the crew and guests to a high level of luxury while providing a safe, and comfortable haven from the elements.

Electrical/Mechanical Systems

Because Nordhavns must function for long periods of time while at sea or at anchor, a great amount of attention has been given to their electrical and mechanical systems. There is an old truism that electricity and salt water don’t mix, and only through careful planning, meticulous execution and quality components will a boat’s electrical and mechanical system perform flawlessly. To begin with, all electrical and mechanical work is done to comply with applicable regulations and published boat-building standards. In many cases the degree of engineering design and quality workmanship that goes into the working mechanisms and systems of each Nordhavn is far above the level required by established boat-building codes and guidelines.

On all Nordhavns only the highest grade tinned wiring is used throughout, and each model’s electrical panel is custom made to suit that boat’s requirements. Separate circuit breakers are specified for all AC and DC powered equipment, and thoughtful, practical approaches to power generation and distribution have been taken based on years of experience in building offshore vessels. Where required, backup systems or parallel circuits are specified, as Nordhavns are expected to be self-sufficient as they cruise in all parts of the world.

P.A.E. has also become an expert in the world of marine hydraulics, as the power requirements of added accessory equipment have become more demanding. Hydraulic power is often utilized for the windlass, bow and stern thrusters, active fin stabilizers and high-capacity bilge pump systems.

Each Nordhavn model has an extensive list of standard equipment, and P.A.E. provides expert engineering recommendations on the installation of any accessory that may be desirable in outfitting the ideal passagemaker.

Nordhavn – A New Era

As mentioned earlier, with the N86 and N120 Nordhavn has officially entered the mega-yacht and super-yacht market. Nordhavn received its first order for an N86 in 2005 and, by early 2011, was in the process of delivering Hull No. 9 of the N86 and well underway on the construction of Hull No. 1 of the N120.

Being able to cross an ocean in comfort and safety is simply a basic requirement for yachts of this grand scale. Thanks to its long and successful history of building smaller mostly owner-operated vessels capable of global passage, Nordhavn’s learning curve for stepping into the mega-yacht and super-yacht market was short indeed. The knowledge, competency and ability were already in place, and the Nordhavn crew was ready, willing and eager to take it to the next level – and in a little more than five years Nordhavn has established itself as a leading builder of luxury mega-yachts and super-yachts as well as the first name in true oceangoing passagemakers.

From the N40 to the N120 and all points in between, solid construction fundamentals, a commitment to safety and comfort, reliable mechanical and electrical systems, classy and long-lasting interiors, and a do-it-right instead of do-it-cheap attitude are at the heart and soul of every Nordhavn.