J Mar Sci Technol (2005) 10:82–95 DOI 10.1007/s00773-005-0191-4 Sail–sail and sail–hull interaction effects of hybrid-sail assisted bulk carrier Toshifumi Fujiwara 1 , Grant E. Hearn 2 , Fumitoshi Kitamura 1 , and Michio Ueno 1 1 National Maritime Research Institute (NMRI), 6-38-1 Shinkawa, Mitaka, Tokyo 181-0004, Japan 2 School of Engineering Sciences, University of Southampton, Highfield, Southampton, UK The idea of sail-assisted motor ships 1–4 was first ad- dressed when fuel prices soared in an oil crisis some 20 years ago. In this case, rectangular rigid sails were formed into a circular arc and rigidly connected to the masts that were installed on the deck of a ship, see Figs. 1 and 2. The tanker form 2 in Fig. 1 has an L PP of 66 m and two rectangular rigid sails with a total sail area of 194.4 m 2 . The larger bulk carrier 4 in Fig. 2 has an L PP of 152 m with two rectangular rigid sails of 640 m 2 total area. The effectiveness of the sail-assisted ships was confirmed in Japan 1–3 by model and full scale ship ex- periments. In the 1980s, about 14 sail-assisted ships were constructed in Japan. However, once the high fuel prices fell significantly, the advantages of the sail- assisted ships decreased as sail construction, mainte- nance, and operation were now seen as an unnecessary cost rather than a fuel cost saving device. Currently, global warming is a political and an envi- ronmental issue, and therefore it is very important to reduce the CO 2 content of ship exhaust. New types of rigid sail have recently been examined by Rosander and Bloch 5 in Denmark. However, the construction of the Danish sail presented in Fig. 3 is very complex. The authors are of the opinion that the geometric and con- structional complexity of these designs led their design- ers to conclude that the new rigid sails lacked financial viability. On the other hand, Nojiri et al. 6 proposed a more practical hybrid-sail that consists of a slat, a rigid wing sail and a triangular soft sail. This hybrid-sail is simpler in construction than the Danish sail and has a higher lift coefficient that the rectangular rigid sails of Ishihara et al., 1 Matsumoto et al. 2 and Hamada. 3 In National Maritime Research Institute (NMRI), the aerodynamic characteristics of hybrid-sails consist- ing of a slat, a rigid wing sail and a rectangular soft sail, see Fig. 4, have been investigated using wind tunnel experiments 7,8 . Fujiwara et al. 8 have confirmed that the rectangular soft sail based hybrid-sail is more effective than the triangular soft sail based hybrid-sail of Nojiri et Abstract In a previously reported study, wind tunnel experi- ments were undertaken to investigate the aerodynamic char- acteristics of hybrid-sails in isolation. Such sails are seen as providing a worthwhile reduction in the delivered power to the propeller and hence the engine generated thrust, with a corresponding reduction in the CO 2 production of diesel engine exhaust. In this paper, wind tunnel testing is used to investigate sail–sail interaction effects for two sets of four identical hybrid-sails, and the sail–hull interaction effects for the same two sets of four identical sails in the presence of a bulk carrier hullform. The analysis presented suggests that to build a sail-assisted ship requires an appreciation of the sail–sail and sail–hull interaction effects. Key words Global warming control · Wind tunnel experi- ments · Aerodynamic sail characteristics · Sail–sail and sail- hullform interaction · Sail-assisted ship Introduction The environmental impact of global warming is of concern to the marine industry because of the large amounts of fossil fuels used and the associated releases of CO 2 into the atmosphere. It is, therefore, very impor- tant to promote the adoption of using natural energy to reduce the required in-service delivered power of a ship. Whilst reverting completely to sailing ships may be seen as regressive, new technologies can be used to design effective sail-assisted ships. Whilst reduced CO 2 emissions can be achieved, there is a need to balance economic viability and environmental advantages. The thrust derived from sails may be viewed as applying natural energy with little negative environmental impact. Address correspondence to: T. Fujiwara (fujiwara@nmri.go.jp) Received: May 6, 2004 / Accepted: January 13, 2005