Vessel Hull Measurement and Lines Generation

An example of the actual vessel, lines generation and the surface rendering.

Method

Measuring an existing vessel and producing a good set of lines is not easy, however, a method developed by the Wolfson unit has proven to work extremely well. The change from previous practice is to devolve the process into separate phases of: measurement; lines generation and verification. The measurement is undertaken by a land surveyor using a laser theodolite under the direction of a naval architect. The lines are then generated by the same naval architect using ‘ShipShape’, the Wolfson Unit lines generation program, and the verification is done by a second naval architect using a surface rendering program. Once the lines plan is complete the data may be imported directly into ‘HST’, The Wolfson Unit hydrostatics and stability program.

Measurement

A laser theodolite is used to measure the angle and distance of the hull surface by reflection. When the hull does not reflect well enough or specific points on the hull are required then reflective tape targets may be employed. A series of intervisible theodolite stations are set up at the ends and sides of the vessel. Each time the theodolite is moved the position of the new station with respect to the old and/or base or other station is measured as a check. Points on sections, longitudinals or other features on the hull are measured at suitable spacings. The sections and longitudinals may be at any angle to the vertical or horizontal as convenient. As many surface cuts or individual points may be surveyed as required to define the vessel. Once the measurement is complete a CAD file of all the points in 3d space coordinates relative to the base station is produced. This file and associated sketches are used as the basis for the lines generation.

Lines Generation

‘ShipShape’ is used to create the lines. A grid of sections and longitudinals is set up. Measured sections and/or longitudinals are read in by ‘ShipShape’ and mapped onto the grid interpolating where necessary using parametric cubic splines. The generated lines lie on the surface of the hull and may be manipulated by changing the end conditions, local curvature or the location of the points. As part of the fairing process the hull can be rotated about any of the three fundamental axes to take account of list, trim and yaw of the vessel on the blocks. Similarly the location of zero in the coordinate system may be redefined. The result of the fairing process provides a set of orthogonal sections, waterlines and buttocks which are used as the basis for the verification.

Verification

Using the output of ShipShape a surface rendering of the hull is generated. All the measurement points are then superimposed upon this rendering, including those not used in the lines generation process. The offset of each measurement point normal to the hull is determined. Any errors are used as the basis for refairing the lines until all inaccuracies are minimised.