Custom Sportfish Yacht Stability Analysis

Naval Architecture: Stability Analysis

  • Inclining Experiments & Tests
  • Intact Stability Analysis
  • Trim & Stability Booklets for Master
  • Damage Stability Analysis
  • Probabilistic Methods

Ocean5 has extensive experience in preparing stability analysis to ensure the compliance of designs and existing vessels with various domestic and international regulatory, statutory and classification body requirements. Our team of naval architects is experienced in planning, conducting and analyzing the results of inclining experiments. We’ll prepare stability analysis, and trim and stability booklets for use by the vessel master, for submittal and approval by classification bodies.

When a ship or boat is designed, stability calculations are performed for the intact and damaged states of the vessel. Ocean5 can perform these calculations for a new design, existing vessel, or refitted vessel, to ensure compliance with stability requirements. We can identify the various applicable requirements for analysis, and help the owner to work with the various regulatory and classification bodies to demonstrate compliance.

Inclining Experiment

All stability calculations are based on the use of the inclining experiment or inclining to determine the craft actual vertical center of gravity. Procedures for the Inclining are specified by ASTM, IMO and other standards bodies. Typically, inclinings must be witnessed by the regulatory body. Ocean5 typically prepares the test plan, attends and conducts the inclining using shipyard or vessel labor, and prepares the resulting report for submittal to the regulatory body.

The inclining experiment consists of several relatively simple steps. First the weight of a vessel is readily determined by reading drafts and comparing with the known properties.

Second the inclining test is usually done inshore in calm weather, in still water, and free of mooring restraints to achieve accuracy. The GM position is determined by moving weights transversely to produce a known overturning moment in the range of 1-4 degrees if possible. Knowing the restoring properties (buoyancy) of the vessel from its dimensions and floating position and measuring the equilibrium angle of the weighted vessel, the GM can be calculated.

Once the GM of the ship is determined, the vertical center of gravity (VCG) of the vessel can be calculated. This final value is then adjusted to calculate the lightship center of the vessel. This lightship center can then be used to calculate the state of the vessel in any new loading.

Using this inclining information, there are generally two types of stability we are concerned with for ships and boats, intact or undamaged stability of the craft, and damaged stability.

Intact Stability

Intact stability calculations are conducted for the case where the vessel is undamaged, and in working order as designed. These calculations are used to determine if the vessel has sufficient stability for the intended use. Many considerations must be included, including cargo arrangements and loadings, crane operations, and the design sea states. An inclining experiment is typically first conducted to determine the location of the ship’s vertical center of gravity. All of the other calculations are based on this important characteristic.

Specialized software is used to calculate the stability of craft in various loading states. Ocean5 employs GHS to quickly and accurately determine the intact stability of the craft. We create plain English reports of our findings for submittal to the owner and classification bodies. With years of experience in working with the ABS, Lloyds, DNV, BV, USCG and IMCI, we can quickly prepare a stability analysis that is fully compliant and rapidly reviewed and approved by these organizations.

Damaged Stability

Damaged stability calculations are much more complicated than intact stability. This analysis, required for larger craft, and specialized types of craft, involves the consideration of damage in each of the watertight compartments, and calculates the resultant stability of the craft. Due to the typically large number of watertight compartments, and loading conditions, this type of analysis can add up to a large number of individual cases or permutations that must be checked.

In the process of damaged stability analysis, we must consider many factors, including the loss of stability from flooding may be due in part to the free surface effect, water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and actually increasing the metacentric height (GMt).

For merchant vessels, and increasingly for passenger vessels, the damage stability calculations are of a probabilistic nature. This is a concept in which the chance that a compartment is damaged is combined with the consequences for the ship, resulting in a damage stability index number that has to comply with certain regulations.

Damaged stability analysis is a complicated and involved affair, and requires expert attention. Ocean5 has the extensive experience and comprehensive engineering tools to provide these difficult and complex calculations.