conditions. Differences need to be taken into account, because real life sea conditions could not properly modelled. It is basically up to the designer’s skills.
For vessels that operating in rough sea conditions such as search and rescue boats, prediction of characteristic behavior of the vessel is crucial. Cost of prototype is considerably high for rescue vessels, so before executing prototype it is essential to see how ship will handle severe sea conditions. Dynamic model testing is the method for estimating that behavior.
Traditional towing tank experiments are very similar to dynamic model testing method. In towing tank test, non-powered hull is pull in the water. Even so, it is a good way to examine bow wetness, model does not expose to any other wave headings.
Dynamic model testing carried out with self-propelled model in open sea environment. Unlike towing tank experiments, this time model experience several wave heading at once. Although superstructure of model geometrically not as important as it was in static model testing, in order to protect propulsion system, superstructure should be watertight. Also, superstructure weight should distribute as in real design. Then the model can be directed to encounter waves from all headings considering capsize, broach and knock-down.
- 4.2 Computer Modelling and Analysis
With the recent technological development computer programs are fairly helpful to a Naval Architect for stability analyze in early design process. Modelling software enables to test different effective hull forms and superstructure geometry. In order to properly model a self-righting craft, software must be able to analyze 180 degrees of roll to starboard and 180 degrees of roll to port. Additionally, software should manage asymmetrical superstructure geometry computation like many rescue boats have in their design. Also, modelling the true shape of design is critically important since any buoyancy location have large impact on self-righting ability on the vessel. Buoyancy sections that are not included on main superstructure body such as mast volume also should be computed on that software.
Computation of weight and centers are standard practice for any ship design. But, weight and weight distribution have utmost importance in any self-righting craft. Even a slight change in weight or weight centers could cause negative GZ curve. Computer software should calculate these weight centers on high accuracy. Any increase in Vertical center of gravity (VCG), also could change the stability curve and cause negative righting levers. Naval architect should have always checked and updated weight calculations as design and construction progress.
When construction is finished, an inclining test is carried out to determine the accurate center of gravity and verify the calculations.
- 5. Design Criteria
In general, design and outfit requirements for self-righting vessels differ little from those of conventional craft of similar size and function. There are, however, a few areas requiring special consideration.
