Upthrust due to propeller
Upthrust from trim tab
Figure 5.2 Simulation of up-thrust from propeller
As the operation of these craft, and the range of the significant motion frequency, is different to conventional craft, interpretation of the effects of motions on human performance is not simply an extrapolation of
the
techniques developed for conventional craft. This is not particularly well understood at present.
5.2 STERN QUARTERING/FOLLOWING SEAS
It is difficult to predict the performance of any vessel in stern quartering, or
following seas, as the surging
behaviour can have a strong influence, and all current theories have difficulty in predicting the longitudinal force, and hence surging motion.
With high speed craft the surging motion is even larger, and it is impossible to use models constrained in surge to predict full scale behaviour.
Of particular interest is the prediction of broaching, and deck diving.
5.2 (a) Broaching
Broaching occurs when a vessel is operating in following or stern quartering seas, and is violently yawed away from its heading despite application of maximum rudder. Often, this
is preceded by surfriding behaviour, and
usually occurs when the vessel is being carried along by the wave at, or close to wave speed (Renilson, 1980).
Broaching is often thought of as a stability issue, whereas it is better considered to be a case of the balance of the moment from the wave increasing the heading angle being greater than the moment from the rudder trying to get the vessel back onto its heading.
Unless the information is being used to obtain forces and moments as input to a numerical simulation code, it is necessary to make use of a self-propelled free running model. Such codes have been developed by the author, and others, but
they tend to be only appropriate for
conventional craft, not high speed vessels. (Renilson & Driscoll, 1982, Hamamoto & Matsuda, 1994, Tuite & Renilson, 1998, Umeda & Renilson, 1994)
© 2007: Royal Institution of Naval Architects
Y v (Y -m)v (Yr m)r 0
v Y
v g r (Y mx )r (1)
In order to predict broaching behaviour it is essential that the model be fitted with a representative autopilot, and correctly simulated rudder rate, as these can have a significant influence on the behaviour in stern quartering and following seas.
Equally, the vertical centre of gravity must be correctly represented as the coupling between roll and yaw can dominate the behaviour (Tuite & Renilson, 1999, 2000).
5.2.(b) Deck diving
Deck-diving occurs when a vessel is following or
operating in stern quartering seas, and overtakes the
wave in front, burying its bow into the back of the wave. Typically the vessel will decelerate rapidly, and damage to the forward superstructure is possible (Jullumstroe, 1990).
To date, it has not been possible to reliably predict whether a vessel will deck dive or not numerically. A mathematical model has been developed for following seas, which requires input from semi-captive model tests, however
this requires more validation (Renilson &
Anderson, 1997, Hannon et al, 1999, Renilson et al, 2000a, 2000b).
At present the best way of predicting the threshold of deck diving is by using free running model experiments, however as there is some evidence that deck diving is much less likely in stern quartering seas, than in following seas, it may be that short crested waves are required to obtain this boundary accurately. A lot more work is required to correlate the results from model tests with reported full scale incidents.
6. MANOEUVRING
The coupled linear equations of motion in sway and yaw for a ship manoeuvring in calm water are as follows:
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