When asked most seafarers say that a bow’s job is to enable a vessel to pierce through waves, slides over waves, keep water off the foredeck, or even that it’s just a place to put the anchor and chain. But few realise that the bow shapes the waves that flow alongside the vessel. A badly designed bow will create unnecessary drag; while a good one will cut resistance and increase comfort.
Half-angle of entry
In most cases, the criteria for the design of the bow begins with the half-angle of entry. This is the naval architect’s term for the more commonly known ‘horizontal bow angle at the waterline’.
A yacht with a large half-angle of entry will slam into, or at least fight, waves often throwing a lot of spray and green water ahead of it. A vessel with a fine entry and narrow half-angle will slide right through a wave with little resistance.
As a yacht’s half-angle of entry increases, the bow becomes more prone to slamming into waves, which in turn, requires more power to keep the yacht moving steadily into the sea and tends to increase pitching.
The angle of entry at the bow is defined by the yacht’s speed and function. In a sailing yacht, the half-angle might be between 10 and 20 degrees, with 10 degrees being a fine entry and 20 degrees being more suited for a slower displacement yacht.
On a motor yacht a fine angle of entry, say 12 degrees, is suited to high-speed semi-displacement style yachts, whereas a normal half-angle is between 18 to 24 degrees.
A very fine half-angle – less than 10 degrees – is to be avoided. It makes the bow narrow and reduces the forward volume of the hull forcing lockers, gear and equipment aft. That said, a long, narrow vessel will have a finer angle of entry than a short, wide vessel, so a specific half-angle of entry should only be used as a comparison factor on vessels of similar length and beam.
The widest half-angles of entry – from 30 degrees to more than 40 degrees – are rarely found on yachts, but can be seen on scow-type barges that move at very slow speeds and throw a lot of water ahead of the bow. These cargo carrying vessels don’t have passengers and move slowly, so wave impacts and slamming can be accepted.
However, if the wave is very large and the yacht has a narrow half-angle, the wave might simply rise up and wash across the deck. To eliminate this designers often flare the upper bow so the rising wave crest is turned back into the ocean.
The most extreme example is what has become known as the ‘Carolina flare’ on convertible sportfishermen built on the Outer Banks
Types of bows
Designed originally to combat the waves off the Carolina inlets, the so-called Carolina flared bow is often attributed to sportfishing boats built by Buddy Davis and the others of The Outer Banks. The idea is that a fine angle of entry drives into the waves, but as wave size increases the flare rises up over them, throwing the water back into the ocean without getting the deck wet.
Typically, a flared bow will often have a chine or two low on the profile to 'break' the flow of water up the sides of the flare and help direct water away from the bow. In terms of propulsion, the gradual increase in buoyancy from a flared bow ensures that a wave does not impact the bow with a hard crash, but is gently turned aside while the bow lifts to the wave.
This type of bow usually has a chine or lifting strakes carried well forward which also helps throw water to one side and provides additional buoyancy as the bow dives into a wave. The force of the bounce increases with depth and flare angle.
The biggest drawback of this type of the Carolina Flared bow is that, as the yacht slams into a wave, the gradual immersion of the flare causes the bow to pitch upward (vertical acceleration), making the entire yacht pitch, plus the vessel slows down as it pitches requiring more power (read higher fuel consumption) as it drives ahead.
Another drawback is that should the bow submerge, it acts like a giant scoop to dig into the water and throw it across the deck. For this reason, some builders incorporate considerable camber to the foredeck.
When designing such a bow, the designer needs to have a pretty good idea of the height of the waves the vessel is likely to encounter.
Bulbous bows are generally only found on displacement hulled yachts – that is, yachts that will not exceed about 1.5 x √LWL, or the Froude number for that hull. Marlow yachts, which are semi-displacement, have an option for a small, delta-shaped bulb that is flat on top and V-shaped below to both break the water and offer some resistance to pitching when performing above displacement speeds. (Bulbous bows work best when the yacht is moving at .9 to 1.2 x √LWL.)
The idea of the bulbous bow is that the wave generated by the bulb reduces the size of the bow wave and hence lowers the resistance of the entire hull. The size of the bulb is most often determined by tank testing when the shape of the bow wave and the bulb’s cancelling effect can be clearly demonstrated, but in general terms, the larger the bulb is, the greater the reduction in resistance as long as the yacht is moving in a relatively flat sea. When the vessel is pitching, the bulb can actually increase hull resistance.
However, a designer needs to be aware of the interplay between the size of the bulb and the anchor handling gear. It would not do to bounce the anchor off the bulb every time the yacht is anchored.
Japanese researchers have found that a bulbous bow along with a slight reduction in the hull waterline beam just aft of the bow, will reduce hull resistance even farther, but at the cost of a reduction in cargo carrying ability and more complexity in the vessel’s construction.
Types of bows (continued)
The opposite of the flared bow is the Axe bow, such as the scimitar bow on the Amel 199. Here where instead of increasing the flare above the water, a very narrow half-angle of entry is maintained from hull bottom to the deck, but extra buoyancy is built in below the waterline with a deeper forefoot, and the sheerline forward is raised against green water on deck.
This type of hull has with lower resistance and creates less pitching in a seaway than a flared bow. Although this shape of bow cleaves waves, it is wet in a seaway.
Experiments in The Netherlands have shown that instead of increasing buoyancy by flaring the bow above the water surface, axe bows increases buoyancy by bringing the underwater bow profile downwards and raising the sheer at the bow.
Not only do these features lengthen the vessel considerably, but they also make it easier to drive into head seas, requiring less power. In addition, it has been suggested that up to 20 per cent lower fuel consumption in head seas can be achieved because the bow does not have the vertical accelerations of a flared bow.
A relatively new trend for larger craft is the scow bow. It has been a fixture on Great Lakes A and C class sailing scows for many years, but its potential was suddenly realised when a Mini-Transat boat with a scow bow handily won the race. Now, designers Reichel-Pugh have designed a 27.4m sailing yacht with a scow bow.
This type of bow carries beam well forward with the intent that the waterline length is increased as the boat heels. The major drawback of scow bows is they slam when upright and on a large yacht, that might be more than the owner is willing to accept.
The totally opposite look to the Apple Cheek bow is the Wave Piercing bow as used by Craig Loomes Design of New Zealand and others on several superyacht and fast ferry designs.
The idea behind this bow is that the extended sponsons on each side of the catamaran or trimaran hull pierce the waves to reduce pitching in heavy seas. In this style of yacht, the main hulls have less buoyancy forward to allow it to slide through the wave rather than slam into it. By sliding through the waves, less engine power is required and the pitching of the yacht is lower.
The bow is an essential feature of any modern yacht. Elongated bows such as the wave-piercing bow, reverse or axe bow lengthen the waterline and make the angle of entry finer, decreasing the hull resistance of faster vessels and reducing pitching in a seaway. However, the longer waterline can make it harder for the vessel to turn. Meanwhile, bulbous bows decrease the size of the bow wave and consequently reduce hull resistance for vessels that operate at a set displacement speed and load.
A designer should pick the bow shape that is best suited for the desired speed, shape and pitching characteristics in a seaway.Originally published: MegaYachts Volume 14 (2013)