It is easy to see the attraction of using stainless steel as a building material for boats because of the lack of corrosion problems and reduced maintenance during the life of the boat. However there are downsides, one of which of course is the higher cost of the material.

When they were developing a new high performance patrol boat design, Swedish Steel Yachts (SSY) decided to build the vessel’s hull from stainless steel. This was a major step forward in patrol boat construction and now with the boat completed Rickard Levin of SSY has looked at the benefits and the problems that developed from using this innovative material. Using stainless steel for hull construction is not new but to use it for a high performance craft is a step into the future.

Project P16 is Swedish Steel Yachts’ prototype of an offshore patrol vessel which has a hull constructed entirely of stainless steel for demanding use. The stainless steel used was supplied by Outokumpu, based in Finland and is their Forta SAF 2507 type which was selected because it is a material noted for its durability and strength.

The hull design is based on a deep vee hull with a short vertical stem that helps to reduce the overall length without compromising the performance. It features a low set pilot house with the bottom of the forward sloping windows flush with the top of the coachroof. There is a low aft cockpit over the engine compartment.

Levin says that the stainless steel for the vessel was a small part of the overall cost, around 2% of the total cost although the sheet metal itself is probably twice the cost of matching aluminium. “The SAF 2507 alloy is strong and has extraordinary properties regarding resistance to corrosion in all maritime environments”, said Levin.

Comparing aluminium to the stainless steel, Levin commented, “First the amount of weld. As the weld is generally the weakest part of aluminium construction the thickness of the weld has to be large. This is due to the material in the weld going back to its elemental properties after welding. Using a thick weld for aluminium takes longer than a thin one. Secondly the aluminium is way weaker than the stainless steel which means that a denser framework of sections and longitudinal stiffeners has to be used. This increases the amount of welding.”

When welding the stainless steel used for the P16 it requires a ‘root gas’ during the welding process. This means that for some areas there has to be a welding assistant applying the gas from the back which increases the cost. Polishing the weld can be done mechanically for both types of material and overall the welding crew considered that the cost of welding the stainless steel structure was comparable to that for a similar aluminium hull.

There were some challenges in the fitting out of a stainless steel structure. Drilling is difficult, requiring special drill bits and constant cooling and lubrication so even a single hole takes some time. As many holes as possible were pre-placed so that they could be cut when the stainless sheets were laser cut. The same problems were found with grinding and cutting. Bending of the plates is also difficult and larger machines were required to handle the loads. The design also had to be developed to eliminate welding on flat topside surfaces as far as possible as these are made from thin plate that easily buckles from the heat of welding.

SSY has patented this method of construction and aims to offer this patrol boat design as a standard for use by customs, the military, law enforcement and similar high speed patrol duties. The big benefit of using stainless steel for the construction is that the maintenance is greatly reduced and it is claimed that the hull does not require anti-fouling paint although the above water sections may require paint for cosmetic purposes.

The overall length of the P16 is 17.1 metres and the beam is 3.99 metres with the lightship weight being 16 tonnes. The hull deadrise is 24° to give a well cushioned ride.

The prototype is powered by a pair of 858 kW DI16 XPI 076M Scania diesels which are close couple to the MJP water jets. These jets are of type DRB 350 and the combination makes for a compact engine room installation. The top speed is quoted as in excess of 47 knots which was the contract speed and the boat reached a speed of 53 knots when on trials at a weight of 18 tonnes.

The fuel consumption is 200 litres per hour when cruising at 37 knots and at this speed the P16 has a range of 500 miles. Humphree interceptors are fitted at the transom and these can be set in automatic mode to trim the hull to suit the conditions, to reduce the pitching motions and to provide a coordinated turn.

The pilothouse is constructed from carbon fibre to reduce weight and it is rubber mounted to improve the comfort levels for the crew. The boat is designed to operate with a two man crew with the helmsman’s sprung seat located on the centre line. Further seating can be arranged in the pilothouse to suit requirements. The electronic outfit has been supplied by Raytheon and can be specified to meet requirements.

Fuel is carried in two tanks under the pilothouse. The rest of the hull interior forward can be fitted out as required with accommodation or storage. An inflatable tender can be carried on the aft deck with a sloping section incorporated into the aft deck area to facilitate launch and recovery.

This new design in stainless steel will be watched with considerable interest in the commercial and military sectors as this construction material could offer significant long term benefits.

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