Outrigger trawlers use outriggers, or booms, to tow the trawl. The outrigger hulls are "floats" or "outriggers". Boats in South East Asia and Polynesia centred on canoes, outriggers and multihull boats. In many tropical artisanal fisheries, trolling is done with sailing canoes with outriggers for stability. This chassis used outriggers and a steel bulkhead to carry mounting points for doors. The boom capacity depends on boom extension and placements of truck outriggers and boom jacks. The first outriggers evolved from the more primitive double-hulled catamarans. The tail, instead of being supported by outriggers, is attached directly to the hull. In the case of two outriggers, one is mounted on either side of the hull. The northern method is a removable pair of outriggers, lashed across the stern deck. It was a single-engine, two-seat biplane with a central main-float and underwing outriggers. A single stationary tower is attached to a base, or portable trailer with outriggers. The Velle derrick is quite similar to the Hallen but without use of outriggers. Outriggers and a rotating mount allowed all-around fire. Design concepts for vessels with two pair of outriggers have been referred to as pentamarans.