Dario's Lab

A Class Catamarans – A Look at the State of the Art Part 5

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 trimarans progressed to curved foils in order to take advantage of a bow up moment resulting from the forward positioning of their outboard foils – Foils that were already present with the primary purpose of providing sideforce in reaction to the side component of the sail force.
Because of narrower beam, aft rig positioning, and non-canting rig, A cat foils are further aft. However, in all but the lightest conditions, they are still forward of the centre of gravity (CG) of the boat (keep in mind that the position of the skipper has a big influence on total CG of boat + sailor).
Moving a foil aft has the effect of reducing the leverage it has to push the bow up.
But it also means that more upward force can be generated by the foil for a given bow-up trimming moment.
This can be understood by considering that the trimming moment is given by force X distance from the CG.
As a thought experiment, consider that if a foil were extremely far forward, a much smaller force multiplied by the much longer distance to the CG would give the same moment as a larger force applied further aft (i.e. at a shorter distance to the CG).
The extreme opposite case would be placing the foils at the CG so that the distance would be zero. In that case the force would impart no trimming moment. Its effect would just be to push the hull up without changing its attitude in pitch.
Practicing beach cat sailors can relate to this by recalling that lifting a boat right at the bow requires less force than lifting the same boat from the front beam.
Section through lower (more horizontal) part of curved foil shown in red.
With the skipper forward in the boat, the foil has no leverage to exert a trimming moment
The modern A Cat uses curved boards to support some of the weight of the boat and at the same time help keep the bows up.
Since the foils are placed close to the CG, they can provide a large amount of vertical lift for a given trimming moment.
As speed rises, they take more and more of the weight of the boat, making the hulls more and more efficient.
The added hull efficiency comes with a small foil drag penalty, but the net effect is less overall drag. This is because the foil has negligible extra drag compared to a ‘conventional’ straight/uncanted foil.
Looking at the components of foil drag, there is little added lift induced drag because the extra lift is small compared to the side force that has to be generated anyway (remember that at low speeds no attempt is made to produce enough lift to support large percentages of the weight of the boat so the foils are not sized to do so). There is little extra frontal area and little extra foil wetted area so the remaining components of additional foil drag are modest.
Intermediate CG position with crew approximately at aft beam.
Now the foils have some leverage to exert a trimming moment.
Note that changes in trim due to movements of the CG are ignored here.
They will be considered in later posts
At very high speeds the foils may be taking a large percentage of the weight. Remember that the ratio of side force to vertical force is determined by foil dihedral. Since side force is a reaction to sail force, vertical force is effectively also proportional to sail force. At high speeds, when more side force is required, more vertical force is produced automatically.
Extreme aft CG (crew trapezing off the transom).
The same force as the previous figure generates a greater moment
So foil assistance by means of angled boards is great in many respects.
Curved foils are even better because they have the same effect as angled foils but with less interference drag. A constant radius is a good compromise as it makes the foils and the housings easy to build.
Both solutions are preferable to ‘J’ foils since they couple vertical force to automatically adjust with side force, with minimal additional drag.
But how do the foils affect stability in pitch, ride height and righting moment?
Part 6 is coming next week.