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Thursday, April 18, 2013

FAQ: Optimum Altitude

Q: "You seem to be just 'skimming' above the surface. Why not fly higher?"

A: Altitude control in Martin Fischer's foil concept is supposed to come from the change in foil curvature just below the hull exit point. As the boat rises, the radius of the part of the foil immediately under the water surface changes progressively so that the immersed portion of the foil gets more vertical. This gradual reduction in dihedral of the wet part of the foil reduces the vertical lift component and encourages ride height to settle.

The position of this change in foil curvature determines ride height.
It would be possible to position it further below the hull. However the span under the inflection would have to remain the same as it is sized to provide sideforce when foiling. Therefore the whole foil would have to be longer. This would make the span excessive at sub-foiling speeds so would bring a drag penalty upwind and in light winds.

Actually, to be technically correct, such additional span at the top (aimed only at increasing ride height when foiling) would need to be 'washed out' to stop it making significant sideforce. If the additional top part did provide sideforce, it would effectively reduce overall dihedral angle: Its sideforce would subtract from the contribution to sideforce by the rest of the foil. So the vertical component of the remaining span would also shrink...

Q: "Would flying higher have any advantages?"

A: Flying higher would require some additional foil span that would add only drag at sub-foiling speeds. In a racing context this penalty would be present more than 50% of the time.

Spray drag is an interesting consideration: Though it looks messy, the spray being thrown up by the foils does not cause additional drag when it hits the hulls. The reason is that energy had already been transferred to the water in the spray when it was directed upward by the foils. If anything, redirecting the spray down and back returns some energy to the hulls.
Think of the exchange of energy in terms of equal and opposite reactions: When you push water up and forward it pushes you down and back which slows you down. When you push it down and back it pushes you up and forward, a form of energy recovery.
So spray only adds drag if it strikes the front half of the boat while moving back. If it strikes the back part of the boat while moving forward it can be ignored...
The ideal solution would be to fence the foils to suppress/redirect the spray in the first place, but this is not practical since the foils must pass through the bearings at the hull surfaces.

It may be that in future it will pay to foil all the time as rigs get more powerful, sailing techniques develop, materials get stiffer and our understanding of hydrodynamics evolves.
If that happens then considerations such as wave clearance and amplified shifts in the CG due to heel will come into play.

It should be noted that, as long as two foils are being used, the dynamics of heeling to windward are not analogous to those on a Moth. If it were possible to fly on the leeward foil only (as the AC72s are doing) then flying higher might allow some windward heel which may have some advantages. If that is the case then foiling higher still could amplify those advantages.

Finally there would be a tradeoff between raising the rig into better wind and losing some end-plate effect from the water surface.

But in the A class, with current technology and within the present rule restrictions, it appears that it does not pay to foil all the time. The long slender hull combined with a low displacement is very efficient at low speed and even more so when foil assisted. The limited sail area and constrained foil horizontal span also contribute to make foil assisted sailing the most attractive option at intermediate speeds before stability becomes an issue.

The initial solution chosen for Paradox is therefore to make the necessary compromises for what is effectively a foil assisted boat that can transition fully onto the foils and become dynamically stable when certain conditions are encountered. With the initial Fischer S foil solution full foiling is proving just too expensive in terms of drag.

This is an example of how a clear brief is vital in guiding the assessment process during development: The brief called for a boat that could be pushed hard through being dynamically stable as the foils begin to generate enough lift to support 100% of mass.

Regardless of whether that goal has been achieved (still being evalusted), the bottom line is that overall drag around the course is what matters.

We will continue to experiment until this crucial value has been reduced below that of other designs.
At the same time the drag reductions must be exploitable: the boat must be simple and intuitive to use so the single-handed skipper can look 'out of the boat' and concentrate on the race.

Now we are working to establish the best settings to get the most out of the first generation concept.
The next step will be to assess whether the configuration is in fact faster around the course in a wide range of conditions.
This includes straight line speed, maneuverability and ergonomic aspects such as ease of handling and making adjustments.
After that we will play with different concepts and draw some informed conclusions.

We will continue to be open and honest about our findings and to share the process as we learn more along the way.

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