Technical » Bow Bulbs

Release Date: 11/28/2006

Bow Bulbs

Most ocean-going cargo vessels are built with a large bulb protruding forward from the hull, below the waterline. The purpose of these bulbs is to reduce or eliminate the bow wave and thereby increase speed or reduce power required. The applicability of these bow bulbs to multihulls is an intriguing topic.

Tom Jones, in his book Multihull Voyaging, writes that the late Lock Crowther tried them to reduce pitching and thereby achieve greater speed and tried them on a number of boats. Although Jones says Crowther gave up on them, son Brett Crowther’s Multihulls plans book includes the 37’ Cruising/Racing Catamaran and the 49’ Cruising Catamaran which have a hint of a bow bulb. It appears that the windward hull could benefit from the extra waterline length if the bulb were at the surface and the leeward hull could benefit from the immersed bulb in bow-wave reduction.

Recently, Lucas Criens asked that his catamaran “Lambada” be put on the Multihull Dynamics, Inc. website. Lambada is a modified Roger Simpson designed 13.7 meter, extended to 14.7 meters LOA. A feature of the extension is a pair of bow bulbs. We questioned Lucas about the purpose of the bow bulbs, because he indicated initially that they were intended to help prevent bow burying. He replied as follows (some editing done):

“…the main objective of the bow bulbs was to change the wave pattern [between the hulls] in moving the two bow waves were they come together approximately 3 to 4 feet aft [of the bows], resulting in the fact that the so-called slam takes place under the main bulkhead which is stronger. This at a boat speed of approximately 7 knots. The second objective when running with the wind at high speed, 12 to 15 knots, was to prevent the bows from burying under water. This is improved as each bulb gives approximately 75 kg lift and the bows stayed one foot above…. We created a one foot longer water line, a positive experience despite the bigger wetted surface.

"Mind you the boat is heavily overloaded as a live aboard with all spares required for our trip to Cape Horn, our home. There is often no shop at sea to buy spares or groceries. Also, the windows are 5mm Perspex in the design which is a no-no for blue water. We are now fitting 10mm Gebo Aluminum framed windows (as per EC law for ocean class sailing vessels) resulting in extra weight again. There are so many more points of difference between blue water live aboard and small trip French Tupperware (joke) cats."

This experience suggests that bow bulbs should be considered, especially for heavier catamarans. Roger Simpson's 15 meter design displaces 8.23 tons in comparison to Lambada's 12.3 tons. Benefits could be accrued in speed, pitching and bow burying.

Cal Markwood

Engineering Analyst

Multihull Dynamics, Inc.