Here is the shortened description of evolution of my thoughts when determining the hull type of the boat that we are designing – centreboard sailboat, dériveur intégral. All begins of course with defining requirements, because there is not a perfect solution that is suitable for all! So there is not absolutely right or wrong solutions.
With a fin keeled yacht I was rather satisfied with the performance. Even though the boat was only 22 ft, I wasn’t happy with the draft of the hull and vulnerability of the keel and rudder. That prevented many sheltered anchorages and interesting routes. Also the propeller had no protection. On hard during wintertime, the boat was difficult to transport and support.
The obvious solution was a long keeler metal boat. Now the draft was less with solid keel and rudder. But the performance is, how I can say it with diplomatic words to our boat, let me say classical.
The next evolution of my thoughts was a boat with longish fin keel. But a low aspect ratio fin keel is not so effective related of generating lift. Then I considered lifting or pivoting ballasted keel, but the forces that the construction is faced of are high and there is a risk of real troubles.
Then I started to study centreboard yachts that are known in France with name dériveur intégral. As many of us, I have been suspicious about their stability, but then I began to understand what was it all about. AVS (angle of vanishing stability) is probably still the best known and easy to understand value to describe the stability of the vessel. It is simply the tilt angle where the vessel loses its stability when RM (righting moment) falls to zero. Beyond that angle RM becomes negative and causes the vessel to turn upside down. Of course internally ballasted centreboarder has lower AVS and their ability to carry sails when struggling close hauled in hard blow is limited. The boat has higher centre of gravity, which has also a positive effect that makes her easier for the crew, because you are closer of the roll axis of the hull, not in the long arm of pendulum. And combined with above moderate form stability of chined hull, the boat is also more restful to live at anchor. But you can’t have all strengths in a same package!
When I asked our designer Jean-François about the centre of the gravity at a beginning of the design process he explained in the words of one syllable that centreboarder that is made of aluminium has lower centre of gravity than steel boat with full keel.
AVS describes only a static stability situation, but I believe that in real life the dynamic stability of the boat is more important related to the safety in extreme conditions, because the forces involved are never static. The thinking is based on the law of the force and counterforce. When hit by a breaker abeam the underwater body of the hull and its appendices generates counterforce for thrust of the wave in the form of lateral grip. Under the force of the wave the boat with enough lateral resistance sideways starts to tilt. And if the situation gets worse then the boat might invert in the procedure which is described as “tripping over the keel”. Eric Tabarly illustrated the effect in his book “Guide pratique de manoeuvre” (Practical yacht handling). The boat with long keel has a huge lateral resistance but some designs with short keel might also have enough surface area to produce enough lateral resistance for tripping.
Centreboard boat with centreboard hoisted has less counterforce by lateral resistance of the underwater body and it gives the boat the ability to absorb the power of breaker to sideslip movement maintaining its upright position. And because centre of lateral resistance is well aft the boat turns its bow away of wave crest.
If ever getting inverted despite of what is described above the design of the deck and superstructure has an important meaning of reducing inverted stability. The long and rather narrow cabin trunk and the extended pilothouse with watertight door will make the boat less stable if inverted. Please see the picture below, our boat has a relatively high conical shaped superstructure cross section for a reason. Low volume enclosed cockpit also adds buoyancy aft and helps the boat float higher in an inverted position. These things have an important role in internally ballasted boat that have a lower centre of gravity in inverted position compared to a fin keeler. When the next wave tilts the inverted boat, the centre of buoyancy moves quickly outwards of the centerline and helps highly floating boat to turn back upright again.
For me balancing the boat under sail has always been one of the greatest satisfactions when sailing. Because the water is thousand times denser than the air I have always wondered why balancing of the boat is usually done with sails only. Centreboard yacht will have much more ability in terms of adjusting balance in different conditions. If stern daggerboards are added the weather helm can be reduced even more effectively when close hauled. In running or broad reach conditions the centreboard is hoisted and the centre of the lateral resistance moves aft making the boat very course stabile. The boat flies straight like an arrow, and that is the most valuable feature if you want to sail far destinations surrounded by blue water.