unit itself, however it is essential to get the correct mass flow rate. It is also necessary to re-inject the water into the tunnel, usually upstream of the pump (to mix the flow), as shown in figure 3.3. This is a fairly specialised requirement, and tunnels fitted with a ‘waterjet loop’ are not particularly common.


to even predict the running sinkage and trim correctly. As the movement of the apparent metacentre will depend greatly on the running sinkage and trim the difficulties can be easily seen. (Renilson, et al, 2001).


Hence, it is necessary to resort to physical model experiments.


The trouble with conducting an


investigation into the loss of transverse stability at speed using physical model experiments is that to determine the change in the position of the metacentre it is necessary to conduct an inclining experiment underway. In principle, this is fairly straightforward, as either the change in heel angle for a constant heeling moment, or the change in moment for a constant heel angle, can be determined easily.


The problem, however, is that the asymmetric underwater shape will cause a transverse force to be generated, see figure 4.1.


Figure 3.3 schematic of waterjet loop in the Australian Maritime College’s tunnel (courtesy of Paul Brander)


4. DYNAMIC INSTABILITY


It is well known that high speed craft can suffer from two important forms of instability, even in calm water:


1. 2.


loss of transverse stability at speed; and porpoising.


4.1 LOSS OF TRANSVERSE STABILITY AT SPEED


Loss of transverse stability at speed manifests itself as the effective movement of the metacentre due to the dynamic component of the upward force.


For many


craft, particularly single chine vessels, this is movement is upward, increasing their stability. However, for semi- displacement craft, and some single chine vessels, the movement is downwards with increasing speed, resulting in the possibility of the effective GM becoming negative.


A negative GM will cause a loll angle, which will result in an asymmetric underwater shape and a yawing moment. It is usual for this yawing moment to be out of the turn, thereby increasing the heel. The whole process can be very violent, and of great concern. (Suhrbier, 1978).


Predicting whether this will occur or not for a particular hull shape is not easy. It is well known that the use of Computational Fluid Dynamics (CFD) to predict


the


pressure on the hull of a high speed craft is not satisfactory, partly due to the inability of CFD techniques


Figure 4.1 Roll moment caused by side force


This transverse force will cause an additional heeling moment, dependent on its vertical


location, and the


vertical location of the towing arrangement, making interpretation of the results difficult. If the magnitude of the force can be measured, then at least the likely extent of the problem can be estimated.


Of course, an unconstrained full scale vessel will also be subject to this transverse force. If the force were applied instantaneously the rolling moment it would generate would be caused by the vertical


lever between the


application of the force and the centre of gravity. On the other hand, if the force were to be relatively steady, then a drift angle would be set up and an equal and opposite force acting at the position of the vertical centre of lateral resistance would arise. In this case the rolling moment would be caused by the lever between these two opposing forces. In reality, the situation would be somewhere between these two.


Thus, all that can be achieved with a semi-captive model is to attempt to locate the towing constraint somewhere close to the vertical centre of lateral resistance and the vertical centre of gravity of the vessel, and to hope that the effect of the force due to the asymmetric underwater hull shape is negligible.


Another alternative is to conduct a free running model experiment. One way of doing this would be to set the


B-16


© 2007: Royal Institution of Naval Architects


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