Directional Stability

Along with the optimum coefficients of Form, A/B ratio, Intact Stability and Severe Wind & Rolling criteria the Directional Stability of a vessel built for ocean passages should be a fundamental property of the hull form and considered early in the design. Any designer worth his salt will ensure that all these Hydrostatic and Hydrodynamic properties take precedence over all other elements of hull design. Only in inshore and coastal boats should any of these be compromised.

Of all the important design elements this is the one that we consider to be of utmost importance. Of the vessels reported missing without trace, and we are not talking only motor yachts, it is our view that many are more likely to have broached and been overwhelmed than any other possible reason. Certainly this is a characteristic, or rather lack of, in some designs we have personally observed that have been lost. The argument of “collision with a semi-submersed” or “neutrally buoyed shipping container” does not stand up to any logical examination.

Directional stability is normally defined as “controls fixed directional stability” which means with rudder and any other control surfaces fixed amidships and in the absence of any disturbing forces of wind or sea the vessel, when underway, will maintain a fixed heading.

Directional “instability” manifests itself in the inability to leave the wheel unattended for even a few seconds without the vessel altering course. This occurs even in calm conditions and in a seaway continuous changes of course have to be coped with by the helmsman.

Under normal conditions the argument is that autopilots cope and this is true, albeit with considerable wear and tear on steering gear.

Where instability becomes dangerous is running in a heavy following sea, particularly at night, where the autopilot and often the helmsman cannot cope with a tendency to broach to.

Running with the sea

Directional stability can only be ensured by a proper distribution of the underwater lateral plane about the longitudinal centre of gravity of the ship.

To have any degree of stability the longitudinal centre of gravity (L.C.G.) of the ship must be ahead of the longitudinal centroid of the lateral plane (C.L.P).

The nearest analogy to this is the feathered arrow. Try firing an arrow without feathers and weighted point.

Achieving a degree of directional stability is difficult, usually for two reasons; the optimum L.C.G is already aft of amidships and the restricted draft designers are often working to. The only way is to have considerable rake of keel, deeper aft than forward, even if this conflicts with draft considerations.

Often the question of directional stability is dismissed by designers and builders as “easily calculated” or that it is simply a function of BL: beam to length ratio, or bow shape. Contrary to that, we have found, after more than 60 years of power vessel design, that the amount of separation between the C.L.P and L.C.G is critical to achieving satisfactory directional stability.

The best is the report back we had from the skipper of one of our Expedition Yachts where he found he could leave the helm unattended in reasonably heavy conditions for up to ten minutes at a time.

After the research we have done and personal experiences we have in many thousands of miles of voyaging, there is no doubt in our view that good directional stability is an absolute requirement. The tendency to broach is a serious risk that needs to be eliminated. It goes without saying that vessels that maintain a constant draft forward and aft; restricted draft, could not possess any satisfactory degree of Directional Stability.

Last Updated (Monday, 06 September 2010 12:08)