Section 6 Waterjet propulsion systems

6.1.1 The requirements for the construction and installation of water jet units apply irrespective of rated power.

6.1.2 Water jet ducts may be fabricated as an integral part of the hull structure, or as a bolted-in unit. In either case, detailed plans indicating dimensions, scantlings and materials of construction of the following are to be submitted in triplicate.

1. Arrangement of the system including intended method of attachment to the hull and building-in, geometry of tunnel, shell opening, method of stiffening, reinforcement, etc.

2. Shaft sealing arrangements.

3. Details of any shafting support or guide vanes used in the water jet system.

4. Details and arrangements of inspection ports, their closing appliances and sealing arrangement, etc.

5. Details and arrangements of protection gratings and their attachments.

6.1.3 When submitting the plans requested in 6.1.2, details of the designers’ loadings and their positions of application in the hull are to be submitted. These are to include maximum applied thrust, moments and tunnel pressures for which approval is sought.

6.1.4 All materials used in construction are to be manufactured and tested in accordance with the Rules for the Manufacture, Testing and Certification of Materials, July 2022.

6.1.5 Steels are to be of suitable grades in accordance with the requirements of Vol 1, Pt 6, Ch 6 Material and Welding Requirements.

6.1.6 Irrespective of the material used, the strength and supporting structure of all tunnels are to be examined by direct calculation procedures which are to be submitted. In no case are the scantlings to be taken as less than the Rule requirements for the surrounding structure. The strength of the hull structure in way of tunnels is to be maintained. The structure is to be adequately reinforced and compensated as necessary. All openings are to be suitably reinforced and have radiused corners.

6.1.7 Consideration is to be given to providing the inlet to the tunnel with a suitable guard to prevent the ingress of large objects into the rotodynamic machinery. The dimensions of the guard are to strike a balance between undue efficiency loss due to flow restriction and viscous losses, the size of object allowed to pass and susceptibility to clog with weed and other flow restricting matter.

6.1.8 The inlet profile of the tunnel is to be so designed as to provide a smooth uptake of water over the range of craft operating trims and avoid significant separation of the flow into the rotating machinery.

6.1.9 Single or multiple water jet unit installations having a total rated power in excess of 500 kW are to be contained within their own watertight compartment. Other arrangements for maintaining watertight integrity may be specially considered depending on the size and installation layout.

6.1.10 For details of machinery requirements, see Vol 2, Pt 4, Ch 2 Water Jet Systems.

6.2.1 Standard units built for ‘off the shelf’ supply and which include the duct are to be installed strictly in accordance with the manufacturer’s instructions, see also Vol 1, Pt 3, Ch 3, 6.1 Construction 6.1.4.

6.2.2 Integral water jet ducts are to be constructed in accordance with the manufacturer’s requirements and the relevant plans submitted as required by Vol 1, Pt 3, Ch 3, 6.1 Construction.

6.2.3 Where load is transmitted into the transom and/or bottom shell, the thickness of the plating adjacent to the jet unit is to be increased. The increase in thickness is to be not less than 50 per cent of the calculated transom and bottom plating thicknesses respectively or 8 mm, whichever is the greater. Such reinforcement is to extend beyond the surrounding stiffening structure.

6.2.4 For ‘bolted-in’ units, hull receiving rings are to be of a material compatible with the hull. Scantlings of the receiving rings are to be as required by the jet unit manufacturer and suitably edge prepared prior to welding in place. The receiving ring is to be installed using an approved welding procedure. Where a manufacturer’s specification is not provided, full details are to be submitted.

6.2.5 Bolt sizes and spacings are to be specified by the manufacturer, and are to be of suitable marine grade, insulated as appropriate and locked by suitable means.

6.2.6 Where studs are proposed for the receiving ring(s), the remaining thickness below the depth of blind tap is to be not less than the bottom shell plating thickness plus 2 mm. Bottoms of all blind taps are to be free of sharp corners.

6.2.7 The use of approved alignment resins may be considered where accurate seating and faying surfaces are required. Details are to be submitted for consideration and approval.

6.2.8 Where a water jet unit forms an integral part of the hull structure, such units are to be installed using an approved weld procedure and in accordance with the manufacturer’s instructions. Materials to be welded are to be of compatible specifications.

6.2.9 Water jet units transmitting thrust into the transom structure are to be supported by a system of radial, athwartship and vertical stiffening. Drawings are to be accompanied by a set of detailed structural calculations. Where complex installations are proposed, a finite element model may be submitted in lieu of direct calculations.

6.2.10 Water jet units transmitting thrust to a bottom shell connection or intermediate tunnel connection are to be supported by additional stiffening, the details of which are to be submitted.