Tech Talk - Bulbous Bows
An honest look at the lines of a Nordhavn will actually cause one to think more of a “ship” than a “boat” – especially with the larger models. No surprise here, since a Nordhavn really is the same as a deep-sea ship in terms of being asked to cross vast oceans in comfort and safety.
Full-displacement hulls; large, slow-turning propellers; large rudders; and some means of stabilization are all common elements shared by Nordhavns and deep-sea commercial vessels.
An additional design element that is almost always incorporated into the larger Nordhavns is a bulbous bow, which is nearly ubiquitous on any commercial ship of decent size. In a brochure or on a website you might see a simple line describing its benefits: “The addition of a bulbous bow improves ride comfort and extends cruising range.”
Behind the scenes, however, there is a lot more that can be said about how this little appendage delivers on its promise.
Interestingly, some of the first bow bulbs were actually created for a more offensive purpose. The Greek trireme, which had a bow bulb in the form of a battering ram intended to disable enemy vessels, was a decisive weapon in the Battle of Salamis in 480 BC against the Persians.
At 120 feet long and powered by 170 rowers, the trireme was built to be fast and maneuverable, capable of speeds of 7.5 knots under power and 14 knots under sail – and you can bet the underwater battering ram on the bow aided in the vessel’s performance.
Wartime vessels of the early 1900s began to incorporate a bulbous bow design specifically for speed, and soon thereafter scientific studies were conducted to really get into the dynamics of what was happening. By the 1930s bow bulbs similar to what we’re used to seeing today began to find their way onto passenger ships and commercial vessels – at first mostly for increased speed, and then later for improvements in efficiency and fuel consumption.
Today you would be hard pressed to find a tanker, container/cargo ship or passenger ferry not taking advantage of the bulbous-bow design, which typically results in a reduction of power of at least 5% while running at speed and a substantial corresponding reduction in fuel consumption over the lifetime of a vessel.
The basic purpose of the bow bulb is to reduce hydrodynamic drag and allow the boat (or ship) to move more easily through the water. This is achieved by designing the bulb to create an additional wave that will partially cancel out the influence of the bow wave created by the hull (“wave cancellation” or “destructive interference” for the textbook crowd).
In many cases, the bulb can also reduce a boat’s tendency to “squat” at the stern at higher speeds. Significant squatting can produce a large stern wave, which will result in increased drag. In addition to minimizing the tendency to squat, the bulb can also greatly reduce pitching (the rising and falling of the bow), which results in a much more comfortable ride.
Of course all of these bulbous bow benefits did not go unnoticed by passagemaker builders like Nordhavn – and not surprisingly the trawler type of vessel turns out to be well suited to the use of a bow bulb. Our first experience with a bulbous bow was with the first N62 ever built back in 1991.
The boat was conceived and designed from the ground up with using a cylindrical bow bulb in mind, and tank test results with the model were more than encouraging. While a designer will typically hope for a reduction in resistance of at most about 8-10% when using a bulbous bow, testing of the N62 revealed a 15% wave reduction and a 20% pitch deceleration at 9 knots, which is right at the sweet spot for that particular hull.
Moving forward, the N57 and N50 were both designed using the bulbous bow (cylindrical for the N57 and elliptical for the N50) with results as good as or better than with the N62.
However, the bow bulb is not without its detractors. The first, and most obvious, is anchor handling. A captain or owner who forgets about the bow bulb being down there while handling the anchor will likely be reminded of its presence in unforgettable terms.
In other cases, depending on overall hull shape and the many other factors that come into play, 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 – especially with the smaller yachts. In order to make the ride as quiet as possible, bow bulbs are not fitted on Nordhavn’s current smaller models.
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.
At present the bulbous bow is offered as an option on all the larger Nordhavns, stating 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.
While the advantages of a bulbous bow are evident and well documented, we really are just beginning to scratch the surface concerning how these appendages can be designed and used specifically for our kind of boat.
Computer modeling is being used by naval architects as a low-cost method to search out possible winning designs that can later be invested in for actual tank testing. Cross-sectional area, shape, length and other factors can all be tweaked to produce different results to bring out a bulbous bow’s advantages while minimizing its detractors.
And if you’re in a hurry, you can always take your current boat down the boat yard and have an aftermarket bow bulb fitted, which is not at all uncommon. Just remember that it’s down there before dropping anchor!