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Monday, March 31, 2014

Honing In

We are continuing our tests of L/V foils. The constraints imposed for this series are:
- Foils made of straight segments with minimum (hydrodynamically clean) transition radius.
- Total foil horizontal projection not more than 400mm to respect the inboard exclusion zone (tip-to-tip measurement) in the A Class Rule.

The two main variables to explore are:
-Foil immersion (draft).
-Tip-up angle.

Findings so far have been very interesting.
In short, the two variables listed above interact in a way very difficult to quantify but with a pronounced effect on handling.

Deep foil partially retracted.
Notice tip just beginning to breach the surface
As you can see in the above picture, we have made the verticals very deep so that we can explore a range of draft values. The goal being to fine-tune foil depth to one that gives an immersed area such that the range of values of lateral resistance yields the desired coupling between leeway angle and lift.

To obtain the desired range of values of lateral resistance, foil area has to be such that it gives enough hydro sideforce at full immersion (hull in the water) to counter maximum aero sideforce (driven by righting moment) with a relatively low Cl.
Meaning that when 'lowriding' the vertical has to produce enough sideforce to keep leeway angle small.
But the area must also be small enough that it will require a high leeway value (large Cl but before the stall) to generate the same sideforce at optimum ride height. Thus permitting some sideslip to unload the horizontal before it breaches the surface.

However, if foil chord is such that draft must be very large to immerse enough vertical foil area, then leeway coupling will become too dominant as a means of heave control.
Meaning heave equilibrium will be reached long before the inboard tip of the horizontal foil reaches the surface.
Therefore, in situations where the speed is high but leeway is small, the boat will tend to fly too high. This in turn could lead to loss of 'grip' by the rudders/elevators.

So one of the interesting realisations to come from this testing is that chord is actually a critical value in the design of stable L/V foils. Our iterative approach, combining a range of vertical and horizontal foil designs at different angles, is proving to be a quick, economical and fun way to explore this new design space.

Check out the following video to see an unedited run on one foil in barely enough wind to be on the wire upwind. The acceleration is addictive.


Sunday, March 16, 2014

Mythbusting

Some thoughts on our recent testing with heave-stable 'acute L' foils (L/V for short).
This experiment had one aim: To prove that a simple cheap upgrade is possible to convert existing A Class catamarans to stable foiling without major structural modifications.

The story so far

We knew from previous testing that L/V foils give stable foiling. Our conclusion was that the crossover speed was relatively high so the overall advantage of this configuration for racing would be marginal.
The complexity of 'tacking' the L/V foils tipped the scales in favour of adopting our 'comma' foils for production and racing. These proved competitive giving skimming flight with neutral stability when required and minimising drag when in foil-assisted mode.

After the Worlds we revisited the crossover numbers armed with new knowledge about kinetics and the tactical options made accessible by foiling. It is now beyond doubt that foiling will pay.
L/V foils maximise righting moment, are inherently stable and can be made to work within the rule.

While it is tempting to wonder whether there is some lateral breakthrough design somewhere within the 'four point' design space, all the evidence right now points to the fact that 'three point' foiling offers the best performance with consistent stability (and hence safety).

An objective application of existing A Class rules allows simple, cheap conversion. More tortured interpretations may require workarounds such as hinged foils or large cassettes.

Stretching Rule 8 to serve the interests of the 'no foiling' constituency adds no value but instead imposes complexity and compromises efficiency. It creates costs that bring no benefit.

A growing majority of Class members is asking whether a simple retrofit foiling solution would be feasible.
Any lingering doubts revolve around ease of handling and, especially, the difficulties of converting existing boats that may otherwise become obsolete.

Modifications

The foils in these videos are old Marstrom C boards with new horizontal legs bonded on.
The horizontal legs are a proprietary shape. Critical features such as the area, tip-up angle, twist and angle of incidence were determined in light of the work carried out over the course of our Paradox development programme. However the Marstrom verticals were not modified.

For some runs the foils were installed in the original cases of a Melvin A3.
For other tests they were mounted in the existing foil cases of a Paradox test platform. In the latter boat the rotating bearings at the hull and deck were replaced with simple plastic blocks.

In both instances the foils are held down by a rope led to a deck cleat. They are retracted using pre-tensioned bungee that pulls them up when the down-line is released.

Rudders are standard Paradox with some transom reinforcement added to the older boat.

Longitudinal placement of the foils on the Paradox is further forward than usual because that happened to be the arrangement on this particular boat.
Previous testing has already shown that, within limits, keeping the foils further aft gives a maneuverability advantage without adversely affecting foiling stability. At any rate, installing a simple foil case further forward is cheaper than putting in a cassette.

Findings

In short the transition is surprisingly gradual with drag falling away as the hulls rise.
Once 'unstuck', the boat rises more rapidly until the heave control features of the L/V foils come into play.
At this point the normal instincts of a cat sailor remain applicable. However there are a few interesting differences:

The most important lesson is to do less rather than more.

Pulling away aggressively in response to building wind strength can force a reduction in ride height, especially if it is done after the boat begins to heel in response to the gust. The foils automatically go to work to restore level flight but the momentary reduction in ride height does sap energy.

Once the coupling between steering and heel is noted (if heeled to leeward, steering into the wind will make the bow come up. Pulling away will make it go down), then one quickly learns to anticipate. The usual response of letting the windward hull rise then bearing away is somewhat modified:

As pressure increases, the best technique seems to be to ease the sheet a tiny amount, let the boat heel to windward ever so slightly, then pull away as normal. This results in addictive, exhilarating acceleration in total safety. Unlike a displacement cat where forward buoyancy gradually runs out, the foils provide more lift as pressure from the rig increases. The feeling is one of total immunity to nosediving (so far!).

It is easy enough to become accustomed to just trusting the foils and keeping steering inputs to a minimum. Obviously the boat will spin on a dime when foiling so the key is to be subtle with the tiller.

Heeling to windward a tiny bit helps to increase ride height when bearing away. This really boosts VMG downwind. Interestingly the same technique works upwind because luffing up to depower helps lower the bow and settle the ride height. But more on upwind foiling later.

For reasons I do not yet fully understand, Keeping the sail more open works better than strapping the sheet on. Letting the traveler down slightly (say to the hiking strap) and allowing a few degrees of twist causes the boat to fly higher. Closing the leech seems to lock the foils up so the boat settles closer to the water. More investigation is needed here because as speeds rise and the apparent wind goes forward, sheeting in will become necessary.

My best theory at this stage is that increasing sideforce causes the equilibrium ride height to decrease. This is based on the coupling between leeway and lift built into the L/V foil geometry.
Another factor may be that, since drag when foiling is so much smaller, it is not necessary to load up the boat with maximum sail CL. Instead the goal is to have the greatest drive force component exploiting fore-and-aft righting moment rather than resistance to heel...

Sailing downwind with both foils down could lower the crossover speed significantly.
Having both sides down effectively gives a pair of V foils. The reduction in foil area due to the inboard tips breaching the surface becomes the dominant heave-control mechanism instead of leeway-coupling. This is an advantage because stable foiling becomes possible at small sideforce values. It is important to note that this arrangement is draggy at higher speeds and definitely unstable as soon as sideforce becomes significant.

Raising the windward foil and relying on leeway-coupling effectively doubles righting moment whilst halving foil area. This is good when sail power is 'excessive' and speeds are very high.
In lighter winds the leeward foil would have to be bigger (if used alone) for the same takeoff speed. More importantly you would have to sail much higher to generate enough sideforce to fly a hull while trapezing.

In other words you would have to generate enough sideforce to lift the windward hull and enough speed to takeoff on one foil only. This is achievable at a much lower windspeed upwind than it is downwind. Having both foils down instead allows a very early takeoff while sailing deeper because righting moment is effectively halved and foil lift is doubled.

I suspect that top level sailors will gradually be able to bring down the critical windspeed for 'downwind windward foil raising'. But for now this option allows mere mortals to foil safely in as little as 6 knots TWS.

Conclusion

We have now proven objectively that it is feasible to convert an existing A Cat platform to stable foiling with minimum fuss and expense. The boat remains practical and exploitable but it offers a whole new level of performance.

On the emotional side, the feeling of foiling is just fantastic. I am sure that hundreds of Moth sailors already knew this. But it must also be said that foiling on an 18' cat while on the wire, and with no mechanical control systems is special in a wholly unique way.

It really is simple to learn and no outstanding athletic ability is required. I have no doubt that this will be the future. Many top level sailors agree. The only question for the Class is this: Will we be permitted to do it in a way that is simple and cheap or will we have to use complex and expensive workarounds?

After the thrill of the 'magic carpet ride', touching down feels like sailing through honey. It becomes frustratingly restrictive. This really must be tried to be understood. Hopefully many A Cat sailors are about to do just that.

Saturday, March 8, 2014

A World(s) of Learning - Part 2

One more post on appendages. Then we can look at other areas of development such as aerodynamic tailoring and control systems/ergonomics.

Glenn Ashby balancing nicely on J foils and Paradox rudders.
Photo by Rhenny Fermor of www.sailingshots.com.au
Steering system

The new kinetic techniques used to promote early flight, combined with much higher top speeds, really put our cassette and gudgeon fittings to the test.

When foiling high, immersed rudder area  is much smaller but, since speeds are higher, the equilibrium sideforce generated by the rudder is the same. Dynamic forces are bigger.
Because these forces are exerted at a greater distance from the hull (further down), the moments on the cassette assembly, gudgeons and transom are considerably greater than previously measured.

When retrofitted to the transoms of existing boats, some reinforcement may be required to create a stiff connection between gudgeons and hull. This can be done internally or externally.
Applying extra carbon reinforcement internally is ideal. But simply bonding on some carbon plate to the outer skin of the transom is an easy and quick solution.
Care must be taken that any external bonded-on reinforcement remains within the hull overall length limit. If this is exceeded then some material must be sanded off the bows.
Some filling of the core may be needed in way of the gudgeon fastening bolts if the area around the new bolt positions is not already 'cored out' with carbon or filler.

External transom 'C-plate' reinforcement. Plus stiffening added to sideplates.
Initial observations were that our cassettes were too flexible when subjected to the higher loads. They were not in danger of failing but their flexibility impinged on the 'crispness' of the feel/feedback through the tiller extension when flying fast.
It should be noted that these cassettes evolved through our in-house testing on Paradox platforms fitted with our more stable foil setups. The inherent stability of our foiling geometries did not require the aggressive inputs needed to correct unstable configurations. Therefore we progressively reduced the gauge of the sideplates connecting the top and bottom machined 'stocks' that hold the rudders and swivel pins. The earlier versions (round holes) had thicker sideplates than the later (triangular holes) design.

Since the flex was in the sideplates only (the machined stocks are robust and fit the rudders snugly) it was easy to reinforce them by adding carbon plate or box section to the sides of the cassettes.

Latest cassettes developed on stable Paradox platform had very thin gauge sideplates.
These were flexing on other boats when steering aggressively to stay 'on top of' unstable foils.
The production cassettes have been revised to incorporate stiffening ribs in the sideplates.
Aluminium remains the better material choice for production when weight and cost are considered together.
Aside from stiffness, strength seemed adequate. None had broken after the equivalent of three seasons on our various prototypes.
The more surprising failure we had at the Worlds was shearing of the rod ends (off-the-shelf spherical bearings) connecting the gudgeons to the cassettes. We had been warned early on by experienced Moth sailors about the possibility of fatigue failure in these components when they are loaded cyclically in bending over long periods of time. Therefore we inspected them often and monitored the hours each unit had sailed so that if and when one failed, we could know the expected fatigue life.
It seemed the parts (8mm SS) were conservatively over specified.
Even so we replaced them before the regatta.

Brittle failure of 8mm SS rod end fitting.
In our application the fittings are loaded almost exclusively in sheer, with bending moment minimised: They are not used to adjust the steering geometry so they are always wound all the way into the gudgeons.
The rod ends that broke (one in practice and one during a race) exhibited brittle failure with no sign of fatigue.
The instantaneous load simply exceeded the strength of the part.
We have since sourced a higher-spec equivalent rod end made with a forged/rolled process using a stronger steel.
These were used for the remainder of the series and held up well.

Looking at failures of rudder fittings on other designs (and there were a few), especially on boats where conventional gudgeons were adapted to take some vertical lift, it is obvious that the engineering has to take into account much greater moments. The transoms and fittings must now be engineered accordingly.

As always the learning curve is absorbing and the weakest link is constantly exposed as it is chased around the system. Fascinating times!

Applied

Here is the updated design to emerge from the experience.
More details later but, as you can see, the spherical bearings have been replaced in favour of machined sliders. Existing systems can be updated with higher spec rod ends or swapped for the new system as mounting hole spacings and rudder housings are compatible.