Optimization of Bow Shape for Large, Slow Ships
Traditionally ships have been optimized for minimizing the fuel consumption in calm water. For slow, large ships like tankers and bulk carriers this has resulted in very blunt bows with high added resistance due to waves. The objective of this thesis has been to investigate if the optimal bow shape, when realistic wave conditions are taken into account, should be more slender than the current blunt bows. The added resistance is also highly dependent on the actual wave conditions the vessel experiences. Thus a question has been if the optimal bow changes with the operational area, or route, of the vessel.
Five designs have been investigated representing a range of waterlines from blunt to sharp. They are based on the MOERI Tanker KVLCC2. KVLCC2A is the original design of the MOERI Tanker with no flare. KVLCC2B has the same water line curve as KVLCC2A, but with straight sides and small bilge radius in the bow. KVLCC2C has a more slender bow by moving volume from the shoulders to above the bulb. KVLCC2D is a blunter design than KVLCC2A and KVLCC2E has been elongated by 8 m compared to KVLCC2C to get a more slender bow.
Four routes have been chosen to represent trades and ocean areas. The routes are; Arabian Gulf (AG) to the Gulf of Mexico (GM), AG to Japan, Brazil to China and Norway to the East Coast of US.
Calm water resistance has been calculated and verified against experimental data. The wave resistance was calculated numerically using Shipflow. These calculations were not satisfying and should be taken a closer look at. Modification of the results had to be done.
The results show that KVLCC2A, KVLCC2C and KVLCC2E have very similar calm water resistance. They have slightly lower values than KVLCC2D. KVLCC2B has the greatest calm water resistance.
The added resistance was calculated by ShipX. The sharper bow designs have significantly lower resistance in the diffraction regime, as intended. KVLCC2E has slightly a slightly lower added resistance coefficient in the short wave regime than KVLCC2C.
The speed-loss calculations were performed by combining wave statistics for the routes, calm water resistance, added resistance and engine and propulsion characteristics in ShipX. The result is an attainable speed at a given power input, 27 000 kW.
The results show that KVLCC2C and KVLCC2E have the lowest speed-loss. The attainable speed is highest for KVLCC2C and it can thus be concluded that a sharper design is more optimal when realistic wave conditions are taken into account.
The relative speed loss on different routes between KVLCC2C and KVLCC2A shows that the speed loss of KVLCC2C is 14.2% lower for the AG to GM, 13.8%, 16.2 % and14.9 % for respectively AG to Chiba, Mongstad to East coast of US and Brazil to China. Thus, a small difference can be seen, but not enough to change the best design in this case.
A review of innovative bow shapes dealing with added resistance was performed and an evaluation based on working principles and applicability to a large, slow vessel was discussed. The designs reviewed were X-bow (Ulstein Design), a new bow from STX OSV and Beak-bow, Ax-bow and LEADGE-bow designed in Japan especially for larger ships.
The two first bows are designed primarily with offshore service vessels in mind and focus more on the long waves. The LEADGE-bow, which is based more or less on the same principles as KVLCC2C, shows that a simple sharpening of the bow is an easy and effective measure. This seems like the most promising bow for large, slow ships of those evaluated.
Place, publisher, year, edition, pages
Institutt for marin teknikk , 2012. , 82 p.
ntnudaim:7873, MTMART Marin teknikk, Marin hydrodynamikk
IdentifiersURN: urn:nbn:no:ntnu:diva-18564Local ID: ntnudaim:7873OAI: oai:DiVA.org:ntnu-18564DiVA: diva2:566082
Steen, Sverre, Professor