Sales for Sea Axe offer encouragement
Dutch shipyard group Damen has developed a bow form that is said to improve seakeeping for fast craft with minimal detrimental effect on performance. Dag Pike explains.
Damen Shipyard in Holland is reporting booming sales of its Sea Axe designs. Developed in conjunction with the Delft University of Technology the Sea Axe concept has revolutionised the design thinking of fast vessel design to create a concept that has improved seakeeping without any loss of performance. Today Damen has built over 70 vessels using the Sea Axe concept and the orders continue to flow in from many sectors of the marine industry.
The theory behind the Sea Axe hull design is that the hull has a fine and deep entry that can act in a semi-wave piercing way, cutting through waves at speed rather than lifting over them as happens with a planing or a full displacement hull. Sceptics of the Sea Axe concept claimed that the design would be very susceptible to broaching in following seas and it would have poor directional stability because of the deep bow but the concept developed at Delft and by tested at the Maritime Research Institute Netherlands (MARIN) demonstrated that this was not the case.
Tank testing of the concept was followed by a commitment from Damen to build a full scale version. The initial application of the concept in 2006 was as a fast crew supply vessel for the offshore oil industry. This is a sector where performance and reliability are vital and where crew and passenger comfort can be critical factors in operations involved with servicing offshore oil platforms and rigs. This market was dominated by US designs using planing hulls powered by multiple engine installations.
Whilst the US designs worked well in slight and moderate sea conditions, the Sea Axe design has shown on trials that it can offer much improved capabilities in adverse conditions. This has allowed these crew boats to maintain schedules over a wider range of conditions. Damen claims that the safety of the Sea Axe vessels and their crew and personnel onboard is improved considerably, mainly due to the reductions in the vertical accelerations of the hull and in the reduced slamming. Crews and passengers are less tired which not only reduces the operational risks but also increases their efficiency.
In their analysis of the Sea Axe design, Damen engineers concluded that when operating in all-year-round conditions in the North Sea, the operability of the ship is 100% at high speed compared with the 45% that was achieved by conventional designs. Course stability is said to be very good at all speeds investigated between 20 knots and 50 knots at full scale. The vertical accelerations are approximately 75% lower when compared with more conventional hulls and the calm water resistance is between 10% and 25% lower, which results in lower fuel consumption. The main negative aspect of the design is the reduced hull volume; this can impact on the available cargo and passenger space.
Damen has now developed a range of standard Sea Axe designs for the crew boat market. These range from 22.26m in length up to 65m in length with construction of the smaller designs in aluminium and the larger vessels in steel. The most popular size is the 33.5m design, of which there are 13 in service and a further 10 under construction. Also under construction are six of the larger 51.3m designs, adding to the six of this type already in service. The first vessel to the 65m designs is also under construction. Speeds up to 30 knots are available for most of these designs.
Damen has translated the concept into a catamaran version, having produced a design for a 25m vessel with aft wheelhouse, intended to offer significantly greater deck area for cargo, and aimed primarily at the wind farm service and installation market.
Damen has a strong presence in the patrol boat sector and in the 1990s the company developed its ‘Enlarged Ship Concept’, examples of which have been sold to the US Coast Guard, the Canadian Coast Guard and to the Mexican Navy. Now Damen has a 50m Sea Axe patrol craft design under construction, with a propulsion system comprising four diesel engines coupled to propellers to give flexibility and redundancy. A larger 66m version is also under construction; this will be powered by two engines. Damen claims that the installed power can vary between 5,000kW and 12,000kW depending on performance requirements, to give speeds of between 23 knots and 35 knots, which covers most modern patrol requirements.
A more recent application of the Sea Axe concept has come from the leisure sector, having been developed as a super yacht tender vessel, intended to accompany large superyachts as a service and support ship, carrying fuel, supplies, crew and the small tenders, helicopters and other craft, freeing up space onboard the mother vessel and helping to maintain the ‘luxury’ ambience that an owner may require. A 50m Sea Axe tender has been delivered and a second 67m variant is under construction. These vessels are being built by the Damen group’s superyacht yard Amels in Vlissingen.
In addition, the Sea Axe concept has been translated into a new lifeboat design that is being developed for the Dutch sea rescue organisation KMRN. This 19m Sea Axe version has been developed and tank tested and it is showing significantly superior rough sea performance compared to conventional planing lifeboat designs based on deep-vee hulls. Powered by a pair of 1,000bhp diesels, the lifeboat will be capable of speeds up to 35 knots and it is expected that the prototype will start construction soon with a contract recently placed by the Dutch KNRM. This design could also be marketed as a patrol boat and, according to Damen, could mark a potential revolution in small fast boat hull design.
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