How to make a better yacht bow
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.