Picking up where the last A Cat post left off, we were contemplating the relative merits of foam core and Nomex style aramid paper honeycomb. Honeycomb is a very efficient structural solution because it concentrates material in effective load paths between the skins.  Each cell is braced at the interface with other cells, and there...Read More

As mentioned previously, the choice of tooling material and shape depends on the construction process of the parts to be moulded. To decide on construction method we look at the desired properties of the finished product.   The hull can be thought of as a box girder that has to resist global bending loads and...Read More

Having chosen a hull and foil geometry, the next task is to execute the carefully optimised shapes accurately and efficiently. Class rules mandate a minimum overall weight of 75Kg for the complete boat with no other restrictions on material and shape above the waterline.  Keeping weight at rule minimum is very important for performance as...Read More

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