Training at Port Revel
The Shiphandling Training Centre
Laws of similitude
On the first day of the course, before heading off to visit the lake (as we have just seen), the morning’s lecture is naturally devoted to a reminder of Froude’s law (named after British engineer William Froude (1810-1879) who introduced the idea of creating towing tanks), and its application to scale models and shiphandling.
Similitude of the scale models
Froude’s law, associated with the other physical laws that govern the magnitudes involved during ship movements, are explained in the appendix (photo: Port Revel). They are summarised below.
• Shape: all the dimensions of the real ship and the scale model are in the same ratio — the scale factor — which is 1:25 on the Port Revel model ships. So the scale model has exactly the same shape as the ship that it represents. The angles and the block coefficient(*) are the same. Of course, the surface areas and volumes are at ratios proportional to the square and the cube of the scale factor respectively.
(*) The block coefficient is the ratio of the immersed volume of the ship to the (theoretical) volume of the rectangular parallelepiped enclosing the immersed part of the ship (product of length by waterline width by moulded draught). It could be described as a "filling coefficient".
• Mass and displacement: these magnitudes follow the same law as the volumes (lake water and sea water have similar densities). The same applies for forces.
• Time: since Froude’s law induces that the time scale is the square root of the length scale, everything happens five times faster! So the students must react five times faster at the helm of their scale model than on the bridge of the real ship. Students must pay attention during the first "practical session" on the lake ... and be even more proactive. On the other hand, acceleration is identical. Lastly, the angular velocity variation is five times faster..
(photo Port Revel)
• • Power: the coefficient has been shown (cf. appendix) to be 1/78 125. Taking the example of the EUROPE (255 000 dwt), her propulsion power is 29 550 kW. Her scale model (19 dwt) only requires a 0.30 kW engine which, to use a "landlubber’s" image, is equivalent to running a 20 t truck on a moped engine!
Wind is obviously the only parameter that cannot be modelled... but the lake was chosen in order to minimise uncontrollable effects (region not subject to strong winds, lake surrounded by a "protective" forest). However, students must bear in mind the fact that a 10 kn puff of wind on the lake corresponds to a 50 kn gale!
To conclude, the scale models at Port Revel are 25 times smaller than the real ships, with five times slower speeds but, since the distances are 25 times shorter, time is five times faster. But the advantage is that 35 hrs of manoeuvres at Port Revel are equivalent to 175 hrs of real manoeuvres!
Manœuvres on the scale models are the same as in reality, except that the captain sends his orders to the helmsman five times faster!
|Click on the diagram opposite to enlarge it (photo. Port Revel)|
On to the first practical session and ... no mistakes allowed as all ship movements are tracked to within a centimetre by Port Revel’s DGPS(1)! This facility, fitted in 2000 and linked to the American and Russian satellite systems, has an accuracy of 25 cm in reality (i.e. 1 cm on the lake). Positions of ships and their main parameters (rudder angle, RPM, ship speed, wind speed and direction, etc.) are continuously recorded and sent to a central computer at the port for processing. The evening debriefing is the moment of truth ... the trajectory maps don’t miss a thing! This is a highly valuable teaching tool.
(1) DGPS : Differential Global Positioning System.
(1) Before the DGPS was fitted throughout the fleet, an IR theodolite fixed to the top of the tower (17 m above the lake) tracked the ships and sent their angular positions (as xy/yz coordinates) to the central computer, which reconstituted their trajectories.
Navigating on scale models over the five days of the course enables the students to "feel" and anticipate to the full the ships’ reactions to waves, currents and wind, taking account of obstacles and possible rudder or engine failure. It is also an opportunity to experiment with “extreme manoeuvres” that no captain or pilot would dare attempt in reality... although they could prove to be essential in a real emergency situation.
After getting to grips with the ships, the rest of day one is devoted to studying ship turning in deep and shallow waters, considering the role of the "pivot point" in ship movements. An initial turning exercise in deep water (equivalent to 80 m in real life) is followed by another in shallow water of just 23 m with under-keel clearance of no more than 2 m, with the same rudder angle and engine speed: we see that the turning radius (cf. trajectory map below) is twice as large in the second case as in the first. This is because the ship turns while "skidding" on the water, some of which passes under the keel. But if the under-keel clearance is reduced the water flow is impeded, slowing the turning speed down and increasing the turning radius.
Turning with different under-keel Docking and undocking manœuvres
The tugs left the port on day two. A former tug captain from Marseilles, Michel Vallette, joined us for a full day of manœuvres with escort tugs: conventional berthing and hauling, as well as zigzag manœuvres with the tug astern and engine or rudder failure, shiphandling in a channel with engine or rudder failure, using the tug to stay in the channel, etc. A few photos of these manœuvres are given below.
Centre de Port Revel 38870 Saint-Pierre de Bressieux (France) – www.portrevel.com
Tél. 33 (0)4 74 20 02 40 – Fax 33 (0)4 74 20 12 29 – firstname.lastname@example.org
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