Detail
ES-TRIN, Article 19.03(6) - Specification of the conditions of full-scale tests, model tests or calculations
Question:
For the passenger vessels capable of achieving greater speeds than v=0,4 √gL (so called “planing boats”), the turning stability shall be determined from:
- the formula of Article 19.03(6) at speeds lower or equal to v=0,4 √gL; and
- full-scale or model tests or else from corresponding calculations at greater speeds than v=0,4 √gL.
What are the conditions of full-scale tests, model tests or calculations mentioned in Article 19.03(6), last sentence?
Answer:
The dynamic stability of passenger vessels at speeds greater than v=0,4 √gL may be pre-estimated in design phase by either calculations or model tests. The tests can be carried out with the application of a constant heeling moment simulating the passenger and wind moment.
Option 1: a free running model test can be applied. The procedure is used for surface vessels only, with Froude scaling law applied, where the smaller the model, the higher are the negative scale effects and the bigger is the challenge to create a realistic model with its own propulsion and steering. On the other hand, for larger models, negative scale effects decrease, so from the perspective of accuracy of results, the scale should be chosen as large as possible, meaning the model size should be as large as possible, but keeping in mind that the size of the actual test basin in relation to the required area for the tests to be carried out, as well as the capability of the test equipment are governing factors, which can make test impractical or too expensive to perform.
If the model shows a tendency to heel inwards in a turn or shows a decrease of heeling at increasing speeds in the towing tank test, the stability requirements are sufficiently fulfilled.
Option 2: A simpler alternative could be towing test of the model (without its own propulsion) free to move in the relevant degrees of freedom in calm water, which is commonly carried out to find the inception of or the region of advanced velocities, as well as conditions of vessel weight and location of centre of gravity, where vessel motions due to dynamic instability occur. An oblique towing test, with model inclined by applying the additional weights on one side (simulating the passenger and wind moment, as mentioned above), may provide dynamic (in)stability information on the model in calm water. Namely, if heeling of the model increases with increasing of speed, it indicates transverse instability of the vessel inherent in hull form (for specific conditions of vessel weight and location of centre of gravity), so further analyses of vessel behaviour, and/or further modification of hull form or weight characteristics, should be carried out. On the other hand, if heeling of the model decreases with increasing of speed, it indicates that there are no negative dynamic effects on transverse stability of the vessel, which should be confirmed by full-scale test. Also, the influence of the propulsor and appendages on the vessel motion due to dynamic instabilities can be significant, so the model used in the experiment should be with scaled propulsors and appendages or with suitably adapted towing procedures. Furthermore, the effects of cavitation and ventilation, which may be significant for small models, cannot be avoided in tests at atmospheric pressure, so experiments in cavitation facilities may be required when the effect is expected to be significant.
Option 3: If various negative effects cannot be avoided in the experiments, a computer simulation (calculation) using measured or predicted hydrodynamic coefficients would be more reliable than the experiments.
Regardless of model tests or calculations, in a full-scale test trial according to Chapter 5 a turning test on the range of high speeds (non-displacement speeds) must be carried out. If the passengers are to be seated during high-speed operation of the vessel, the passenger moment can be neglected. The application of the heeling moment due to wind can be also neglected in the full-scale test trials.
If the vessel inclines towards the centre of the turning circle, it is stable, with no negative effects of turning, so turning moment can be neglected when calculating overall heeling moment of the vessel. On the contrary, if vessel heels outward when turning on the advanced speed, this is unwanted and atypical situation which indicates possible inadequate stability of non-displacement vessel, so further analyses of vessel behaviour, and/or further modification of hull form or weight characteristics, should be carried out.
The results of the full-scale test, i.e. the turn manoeuvre, should be properly measured and documented, f.e. by an electronic inclinometer and related speed measurements. A full 360°-turn to each side should be performed.
CESNI/PT/Pax (20) 29 rev. 2, CESNI/PT/Pax (21)m 14, item 3.11
Working group for passenger vessels (CESNI/PT/Pax), turning moment, full-scale tests, stability
