Damen’s Sea Axe development
The road to unrivalled seakeeping behavior of high speed vessels
Since the year 1995, Damen’s High Speed Craft department has focused on making seakeeping behavior of high speed vessels safer and more comfortable. In cooperation with the reputable Dutch scientific institutes, TU Delft and MARIN as well as partners such as the Royal Netherlands Navy and the Unites States Coast Guard large steps were taken to reach unrivalled seakeeping behavior.
The first step in this process consisted of analysing the essentials of unsafe and uncomfortable behavior of high speed vessels in waves. It was observed during an extensive series of full scale testing that all crew imposed voluntary speed reductions after the occurrence of a single so-called ‘wave slam event’. Moreover, it was shown that the incentive for these voluntary speed reductions was not based on the prevailing magnitude of the significant acceleration values, as was (and still is) the common way of analysing seakeeping behavior.
Although the frequency of occurrence of these peak accelerations is limited, it was obvious that these extreme accelerations drive safety and comfort, as well as the feeling thereof, on high speed vessels. The realisation that almost all high speed operations at sea were limited by these vertical peak accelerations, in particular, resulted in the understanding that a different hull design philosophy was necessary to seriously improve the seakeeping behavior of high speed vessels.
With this in mind, the first large design step was initiated. Based on the fact that, in general, larger fast vessels have superior seakeeping behavior in waves over smaller vessels, studies and model tests were performed on lengthened versions of a 26m high speed vessel.
By only lengthening the bow, keeping all other dimensions and functionalists equal as much as possible (no modifications on superstructure, outfitting or equipment), it was concluded that with small extra initial investments large advantages could be gained with respect to operability (seakeeping) due to 50% lower vertical peak accelerations. This principle acquired the title ‘Enlarged Ship Concept’ (ESC).
The first vessel to be sold according to the ESC philosophy was a 35 metre patrol vessel lengthened to 42 metres. This vessel formed the base of the Damen Stan Patrol 4207 and 4708 series, of which more than 130 are in service for a variety of satisfied customers.
Encouraged by the achieved results, the focus on designing a better hull shape for seakeeping of high speed craft became even greater. Knowing that high peak accelerations are the key, the fundamental formulations that describe the nonlinear hydrodynamic lift were studied more intensively. The analysis of these formulation resulted in four characteristics that are of key importance to reduce vertical peak accelerations:
- Bow flare (less flare is favorable)
- Bow depth (deeper keel line at bow is favorable, postponing bow emergence)
- Change of waterline beam (less change is favorable: V-shape)
- Bow height (increased height to have equal reserve buoyancy at the bow)
Implementation of these four characteristics initiated the second big step in hull design: a vessel featuring a bow shape with close similarities of an axe, giving birth to the ‘Sea Axe’.
An extensive model test programme was executed substantiating the expected improvement: vertical peak accelerations were again brought down by more than 50% compared to the Enlarged Ship Concept, resulting in a reduction of peak accelerations of more than 75% in comparison with conventional vessels. Additional advantages of the Sea Axe were discovered: lower fuel consumption, steady speed in waves and no voluntary speed reduction, resulting in full power against the waves. Bow slamming did not occur, due to the deep, narrow bow, which directly results in safer and more comfortable behavior of the vessel in a seaway. The lower acceleration levels induce lower forces on the vessel’s construction, increasing the life span of the vessel. At the beginning of the model test programme it became clear that fixed fins in the aft ship were necessary for improving the course stability of the Sea Axe hull shape and reduce the potential risk of broaching in stern quartering seas. Extra tests were performed to optimise the location and size of these fins. In the year 2006, the Sea Axe was ready to serve the market, starting with a 33 metre Fast Crew Supplier.
A large amount of Damen high speed ship types are based on this successful hull shape. Sea Axe vessels of various types, between 22 and 67 metres support a wide variety of markets: Sea Axe Fast Crew Suppliers, Patrol Vessels and Fast Yacht Support vessels serve their owners safely and comfortably every day. A large number of catamaran Twin Axe vessels are active in the wind farm industry and across a wide range of maritime sectors.
The research efforts of Damen, together with its partners, created a futuristic Sea Axe hull shape with unrivalled seakeeping behavior in waves, resulting in a fast, safe and comfortable ride through the waves. Crew stay more alert and can fulfil their assignments better and in higher sea states.
Fast Crew Supplier 5009 Security