Hopefully those of you who had the patience to follow this series of posts now have a clearer understanding of the state of play in A Cat design. This will be the last instalment on geometry and dynamics. I will cover structures and detailing in the next post.We saw that the boats are powered by...Read More

When we looked at influences on hull shape we concluded that minimum wetted area is a priority. Minimum wetted area for a given prismatic coefficient is obtained by using semi-circular cross sections. Prismatic coefficient in turn is driven by resistance to bow down trimming moment and by operating speed. Both are essentially functions of the...Read More

We saw in the last post that current foil assisted A cats are inherently unstable in pitch. As sail force and hence bow down trim increase, the angle of attack (AoA) of the foils decreases resulting in less bow up trimming moment. Conversely, if drive force decreases and the bow comes up, the AoA increases...Read More

Let’s take another look at the figures in the previous post, this time considering stability in pitch. What happens when we introduce two real world factors: drive force and changes in pitch attitude? Stability in this context simply means the tendency to return to a level attitude when perturbed by some external force (in conventional...Read More

In previous posts we looked at the extremes: full ‘foiling’ and simple displacement modes. I touched on a middle way that I refer to as ‘foil assisted’. As is often the case, a compromise is preferable to either extreme. And is what the existing fleet seems to have settled on. I mentioned that ORMA 60...Read More

In the case of ORMA 60s and A cats, the hulls have such favorable drag characteristics that ‘foiling’ does not (yet) pay in the majority of conditions. Instead, boards already present to provide side force are modified and ‘double purposed’ to complement the buoyancy of the displacement hull. This is a ‘foil assisted’ mode. ORMA...Read More

With reference to the illustrations, I will attempt to roughly cover the progression to angled and then curved foils. Let’s start with a conventional centerboard dinghy. On any point of sailing except dead downwind, the sail force will have a component across the boat. This is the aerodynamic side force. For the boat to sail...Read More

   Understanding hull cross section shape independent of foils In the following discussion we will look at the principles influencing cross section shape.  As mentioned in Part 1, the extremely high length to beam (slenderness) ratio of each A Cat hull reduces the relative importance of wave making drag as a component of total drag....Read More

   Key dimensions   Length: 5.49m (18’) Beam: 2.3m (7’ 6½”) Weight fully rigged: 75Kg (165lb) Sail area: 13.94m2 (150ft2) Crew: One person      Rig and power management   Though relatively narrow for a catamaran, righting moment is good thanks to the skipper being on trapeze.  The wide staying base and lack of a...Read More

In previous posts I expressed my motivation (why) and approach to new projects (how). I stated the importance of clearly identifying the needs of the user, and talked about the effectiveness of working to a congruent brief. These beliefs drive quality. They will be shared by all who value passion applied with focus to create...Read More