In Depth: Damen Shipyard’s First Full Axe-bow Patrol Vessel Delivered to Cape Verdean Coast Guard

Located some 570 kilometres off the Western African shores of Senegal is the Republic of Cape Verde, an archipelago consisting of ten islands. With an exclusive economic zone 180 times larger than the country’s land territory, and positioned on the cross-roads of various major shipping routes, the Cape Verdean Coast Guard is presented with quite a challenge. An equipment upgrade was needed for its mission to combat trafficking and illegal fishing, and to carry out search and rescue missions and disaster relief.

Stan Patrol

Originally, the client approached Damen Shipyards for the purchase of a 42 metre patrol boat of the yard’s successful series of enlarged-ship-concept patrol boats, which corresponds to the interior volume of a standard 35 metre patrol boat. The customer however wanted many functionalities which could not be fitted in the Damen Stan Patrol 4207. The solution preferred by the customer was to switch to the Damen Stan Patrol 4708, the design which was selected by US Coast Guard for their ‘Sentinel Class’ shortly before. Convinced by the superiority of the axe-bow concept (proven in the offshore market) and looking at the operational profile of the patrol vessel, Damen presented a concept for a 50 metre axe-bow patrol boat instead.

Its type designation ‘Stan Patrol 5009’ stands for Standard Patrol vessel with a length of 50 metre and a beam of nine metre.

Budget

The construction of the Guardião was funded by three parties: the Cape Verdean government, the port management company Enapor and the Facility for Infrastructure Development (ORET), a fund from the Dutch Ministry of Foreign Affairs to encourage public infrastructure development in developing countries. Even so, the 50 metre boat exceeded the available budget, which was originally calculated for a 42 metre. To remain within the provided budget, the top speed requirement was lowered from 26 to 20+ knots so that four much smaller main engines were needed. As a result, also the exhaust systems, gearboxes, shaftlines and even engine room ventilation could be trimmed down.

Resistance

In the end, Guardião’s four Caterpillar C32 main engines give her a top speed of 23 knots in trial conditions, but more importantly, the vessel is capable of maintaining that speed in heavy seas. In flat water, the resistance of the axe-bow hull type is about 10% lower than a conventional high speed hull form, but for example in a seastate 4, the resistance is 22% lower. The axe-bow hull type was originally developed with patrol vessels in mind, but knowing the long acquisition processes of governments, Damen Shipyards first marketed the new hull form for fast crew suppliers in the offshore industry. And with success: since the launch of the first one in 2006, 65 axe-bow ships have been sold, of which 36 are delivered. In the High Speed Craft department of Damen, the axe bow is solely responsible for an impressive growth curve in the last few years, which was only slightly dented due to the economic crisis.

Accident at sea

Jaap Gelling, product director of the High Speed Craft section at Damen, sketches the history of the axe-bow concept: “In the early eighties, Lex Keuning from the Technical University in Delft, was involved in an accident which had a profound impact on his further academic career. When measuring vertical accelerations on board a conventional fast crew supplier for a seasickness study, he was badly injured when the boat slammed extremely hard in steep waves at high speed. In this particular event of bottom slamming, the accelerometers onboard recorded almost ten G at the bow and 6.5 G on the bridge.”

Gelling continues: ”When recovering from this incident, Keuning decided to focus his scientific research on the improvement of the seakeeping behaviour of high speed vessels. His research – in co-operation with Damen – initially resulted in the Damen Stan Patrol 2600, of which more than 110 have been built worldwide. At the end of the nineties, the innovative Enlarged Ship Concept was launched, marking a clear improvement by lengthening the bow with void spaces. This concept was very successfully applied in the Damen Stan Patrol 4207 and 4708. In the new millennium this evolved into the more extreme and even more effective hull concept called axe bow, because of the resemblance to an axe of the keel, sheerline and stem line in profile.” The axe-bow research project was funded by the TU Delft, the US Coast Guard, the Royal Netherlands Navy, Marin and Damen.

Soft suspension

The axe-bow principle does not work by reducing the ship’s motions. These ships may even have marginally larger pitching motions. What it accomplishes is reducing the accelerations, which are ultimately the governing factor for seasickness and operability, thus limiting the speed of ships in rough seas. One can compare it with a Citroën car with air suspension on a bumpy road, whereas a normal fast boat with a planing hull would behave like a sports car. As the sea more often resembles a dirt road than a racetrack, the operability and speed of axe-bow ships is a lot higher than that of conventional fast hull forms.

The axe-bow hull is characterised by having its deepest point all the way at the bow, combined with a very slender foreship with almost no bow flair above the waterline. The bottom sections warp from near-vertical in the bow to a more classical V-bottom with hard chines in the stern. To prevent green water on deck, the freeboard at the bow is relatively high. Through the combination of these design elements, the bow acts as a soft damper for pitching motions, contrary to the very hard braking effect of a widely flared bow or a flat bottom section. Hard slamming is completely non- existent on axe-bow ships.

Fins

To combat bow steering in stern quartering seas, two fixed fins were added in the aft of the ship. The same effect could be achieved with increased rudder sizes, but the fins have the advantage that they are not subject to human error. Another pair of active fins is located around midships. These are stabilizer fins from Quantum to eliminate the rolling motions underway. Guardião’s two 100 kW bowthrusters are hydraulically powered through PTOs on the main engines. Two rudders are placed behind the outboard propellers. Because of the launching ramp in the aft ship, both rudders are not mechanically connected.

Propulsion

While the Guardião is equipped with four compact and economic Caterpillar C32 engines giving a total of 4,324 kW, the Stan Patrol 5009 can also be equipped with engines up to a total of 12,000 kW, to achieve a top speed around 35 knots. Each of the engines is connected to fixed pitch propellers through a Reintjes WVS 730 gearbox. For low speed trolling during patrol missions, the most economic way is to use all four engines, but as prolonged low-loading is not so good for them, trolling on two engines is also possible.

Contrary to most of the crewboat versions where noise is often not a consideration, the Guardião features exhaust silencers from Axces on her main engines. For the sake of simplicity and noise reduction, all of the main engines’ cooling water is injected into the exhausts before they go overboard above the waterline, resulting in a very visible cloud of steam wherever the ship sails.

Client

Jaap Gelling compliments the Cape Verdean Coast Guard as an excellent partner: “Being a relatively small organisation, they really were very open to all the advice we gave, but at the same time they were very knowledgeable and have given a valuable contribution during the whole project. Once the design was finalized, there were very few changes.” In the final stages of production and upon completion, the crew of the Guardião received extensive training at Damen’s headquarters in Gorinchem.

On deck

In the stern, an integrated slipway is provided for the quick launching of a 7.5 metre rigid inflatable boat. In a bid to keep things simple, the slipway consists of a V-shaped section clad with Hakorit, a high density poly-ethylene with excellent abrasion properties. To land onboard, the waterjet-powered daughter craft is motored as far up on the ramp as possible and then towed in using a standard vertical capstan on a pedestal. A hydraulically operated door then hinges down to close the opening in the stern and keep backwash from the tender bay.

Very noticeable on deck is the large number of liferafts. Six 50-person throw-overboard rafts allow the Guardião to quickly provide lifesaving capacity when it comes to assist a ship in need. A large fire monitor on the foredeck coupled to a 120 m3/h firepump allows for firefighting on other ships or on the shore. The fixed engineroom firefighting system consists of a Novec installation.

Crow’s nest

The accommodation is fully air-conditioned and includes an officer’s mess, a crew mess, a galley, a small hospital, a large room with seats for 76 survivors, seven double and four single cabins. The wheelhouse features a navigation desk with three helmsman’s seats and desks for the chief engineer and the navigation and communication equipment. Alphatron Marine integrated the propulsion and steering controls in the arm rests of the central helmsman’s seat. The wheelhouse is positioned where the vertical accelerations are low, resulting in less crew fatigue. Particularly useful for search-and-rescue missions is the crow’s nest on top of the wheelhouse roof, granting a 360-degree view from a higher yet sheltered position. The Guardião can also be steered from this position.

Fatigue strength

As with most prototypes of a new Damen model, the Guardião was built entirely in the Netherlands. The steel hull and aluminium wheelhouse were built at shipyard Made, then moved to Gorinchem for outfitting at Damen’s outfitting berths. Due to fatigue strength considerations, steel is a far more suitable hull material than aluminium in this size, even if weight is always of prime importance in fast boats. Although class rules do not take fatigue strength into account, Damen Shipyards includes it in the analysis of all their high speed craft.

Operability at sea

Since the early seventies, Damen have delivered over 1,000 high speed craft, over half of which are patrol vessels. So while the axe bow may still be a new and unfamiliar sight for many in the marine industry, they are bound to get used to it. The days are over that oceangoing ships were only designed for trial conditions, with no wind or waves. Nowadays, ‘operability at sea’ is perhaps the principal design parameter for many types of ship. There is no reason to doubt that what worked so well for fast crew suppliers and for windfarm maintenance vessels (in catamaran form) will not work equally well for fast patrol ships. The Cape Verdean Coast Guard deserves a tip of the hat for being the first to embrace this evolution. Damen Shipyards report that two very similar patrol vessels have been sold recently and another 14 Stan Patrol 5009s are expected to be ordered soon.

The patent of axe-bow hull form is owned by the TU Delft and licensed by Damen. The patent licensing fees are used exclusively to fund research into the seakeeping behaviour of ships, perhaps paving the way for yet smoother sailing in the future.

Bruno Bouckaert

 

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