A Fast Offshore Cruiser - 12. Stability and flooding.

It is important to have knowledge of the stability and flooding characteristic of all yachts. The stability data allows the correct calculation of the rigging and it’s associated structures, it provides the knowledge of how the yacht will react in the capsize situation. To know that the yacht will stay afloat even when a compartment is flooded will give the owner and crew great comfort.

The computer program used to produce the stability and flooding data was "Hydromax". For the program to produce the data the hull was first exported from "Multisurf" as an IGES file which was changed by "Maxsurf" to a Maxsurf file. The deck and superstructure was not included in the calculations because of difficulties in exporting from one program to another. This will therefore not show the correct curves at high angles of heel, but what is shown is a worse case so the actual values like the limit of positive stability (LPS) will be greater.

The data produced by "Hydromax" has not been included here in its original form, due to it being in a Macintosh file and is therefore unable to be manipulated into a presentable form in Windows. The numeric data was inserted into Excel that also allows the individual curves to be seen on top of each other for better comparison.

Five combinations of loadings and water ballast arrangements were entered and data produced for. These conditions were light ship, half load and full load all with no water ballast effect (0° -180° ) and half and full load with full water ballast effect (0° -180° -360° ). No lightship with full ballast tanks condition was completed due this being an unobtainable condition.

Both plots of the no ballast effect and full ballast effect show only a very limited change in GZ values between the different loadings. This is a good characteristic for a cruising boat, the sail changes (reefing) wont happen at greatly varying wind strengths as the boat lightens during a voyage.

The LPS values are also steady at about 130° -135° , this is lower than you would ultimately want, the ideal being at 180° , but it is a compromise between form and total stability that must be reached. Production boats of this size have a typical range of 120° - 140° . As mentioned before the deck and superstructure was not included, this means that the actual LPS values will probably be about the 140° mark, so as good as the best typical production cruiser.

Taking the heel and trim data from "Hydromax" the heeled waterlines can be produced, showing if there will be any unfairness as the water plane changes shape. These will only be for a static heel situation, completely different from a dynamic one were there will be a hollow amidships, reducing the stability, but is a good guide.

The water lines produced show that the lines do change smoothly. There is an amount of roll forward, due to the stern, which lifts the rudder, stopping its seal and reducing efficiency. This should be counteracted by the stern wave though. Up to 30° of heel there is no banana effect to the waterlines, a problem with narrow entry boats.

When the yacht has full water ballast effect the hull becomes asymmetric. In a static situation with full ballast tank effect the induced heel angle is about 5° . This is enough to make sailing noticeable more comfortable or faster; this is a worth while use of the fresh water supply. The righting moment at 30° is increased. The GZ values at full load increases from 0.751m to 0.825m, a 10% increase, which has been included in the rig calculations.

The flooding is limited to single compartment flooding; realistically usable size compartments on small craft are too large to allow multiple compartments to flood with safe hull trim, e.g. with a 75mm margin line still above the water. 7seas 1524 will float with the forepeak and the forward cabin flooded but at 45° bow down.

The results of the single compartment flooding show that the worse case situation is the saloon etc and the middle cabins flooded. In this situation the hull has no significant trim and still has a substantial freeboard. The flooding of the compartments in the ends of the hull does not have any noticeable effect. The flooding of the crash compartment would not happen as shown because the volume that is indicated flooded is full of sacrificial foam.

The most noticeable effect of flooding is the forward cabin. A trim angle of 4° , bow down, is adopted with no heel.

In the unfortunate event of flooding this boat will provide a safe platform until rescue arrives or safety is reached.