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Test Breaking A Curved Lifting Foil

A few F-32SR curved lifting foils have now been broken, and usually while the foil is being used in heavy conditions, to where it could reportedly fully fly the boat at times. The foils were never intended to do this, having been designed and developed prior to the possibility of larger multihulls being able to fly, as was demonstrated in the 2013 Americas Cup. The fact that they have broken is thus no surprise, but any failure like this should still be investigated to see if anything can be improved.. These foils use the exact same 'one piece' construction method as is used for F-22/F-32 rudders or daggerboards, not one of which has ever broken, so it looked like something else may be a factor.

F-22 rudder - infused in one piece, same as the curved lifting foils, and none of these or the
similar F-32 rudders have ever broken
.

Thus it was time to do some testing to find out exactly how these foils were breaking, and to see if they could be strengthened.

Test breaking jig set up with test foil in position and being loaded up. A digital scale gives the load.

Thus a test foil was setup in the rig as shown above, and put under load until something gave. The manner of breaking was interesting, this being initially a small localized failure/crack on inner skin, and one can also see this on the various broken sections that have been sent back for examination. The below photo shows the various cracks we were able to induce, while testing fully extended and then retracted 500mm.

These are mostly surface cracks, and the foil still retained significant strength, but it has been weakened.

The foil started to fail at less than what was predicted by theory, but it appears that the curve is intensifying the load on the inner compression side. Thus it starts to fail a little earlier than it should and the failure begins in small steps - no spectacular big bang or shower of shards, more like a starter pistol shot, and a bit of an anti-climax in reality. Had we proceeded on to a full break, then we would have seen some exploded carbon shards, but this also tends to hide how the foil initially failed

What seems to be happening is a few outer (surface) layers fracture, due it seems to the relatively tight curve, but foil does not in itself break, so it appears to be a progressive thing. Thus it would be would be wise for any foil owner to frequently check the underside (inner curved side) of any existing foil when retracted on the boat to see if any cracks have developed. If found, they could possibly be repaired before foil degenerates further and breaks.

We then cut the test foil open, and the infused laminate always looked excellent. It seems like a surface failure initially on the inner curved side, and the carbon further into the foil seems mostly unaffected as per photos below. Foil as pictured above, had failed several times, but was still capable of carrying a good load, as the next unaffected layer seems to take over (but the foil will be weaker).






On closer examination, it is possible to see on a few samples some very fine interlaminar shear cracks 4 - 5mm in, and it appears that an outer layer of the compression layer has failed, or separated from the inner layers. There is no sign of any core shear. Thus it looks like increasing the amount of carbon with a smaller core will not do anything, only make foil heavier and more expensive. Even if we made it all solid carbon it seems it will still break at around the same load. 

The maximum load it took before the first crack was 1436kg (3160lbs), which means it will break should the whole boat be flying on the foil (load on the foil is basically equal to the boat weight). After the initial crack we then loaded it up again to 1136kg (2500lbs) and it still held together, so the remaining inner compression layer was still doing an effective job. Did not want to trigger a second break as this would likely have been a catastrophic failure, and we still wanted foil in one piece in order to test how it stood up when retracted 500mm (photo below). This then took 2136kg (4700lbs) before the first crack. Thus it will not break in heavy winds when used retracted, as expected, and has always been recommended.

Foil retracted 500mm and under load

The only way to make the foils stronger is a better resin, or stronger carbon, or technique improvements, or just a much bigger section foil. Our vinylester-epoxy infusion resin has tested as well as epoxy in the past, so I doubt we will pick up much there. However, doing some more tests now. The only other option is a stronger higher modulus carbon but this gets very expensive, plus foil will be more brittle. Such high modulus carbon was tried on ORMA 60s a few years ago, and it resulted in a rash of broken beams and floats. 

Regardless, I have investigated supply, but it is only available in a pre-preg form, which would require a freezer and an oven to handle, which we do not have, and they would put costs through the roof. Apparently even Team NZ cannot obtain high mod carbon in a dry hand laid form, as the order has to be very large, making cost horrendous. 

A Team NZ foil engineer (previously with Oracle) also came by just after the test, to pick up some carbon mast parts for his F-22. We attended Canterbury University together, and he is currently building an F-22. We brainstormed on the foils, and he confirmed that the curve does seem to intensify the load on the inner compression side compared to a straight foil. In fact they had to use steel reinforcing inside the ‘elbows' on their first foils to make them strong enough. However, their foils are becoming more curved, and they have found one limiting factor to be interlaminar shear on the compression side. 

To overcome this, their compression side laminates are now made up in a special way to help prevent interlaminar shear. We already do something similar with the foil core, but not the outer laminate itself. Doing this is easy to do on their foils as they still hand make, only using a mold for one side, the other side being hand faired, one reason for their six figure expense.  The original F-27 foils were also hand made in this manner, but it is inefficient and expensive. My friend was impressed at how we can make them in one piece, and took away samples of our special core material that is used to hold the carbon against the outer mold surface, and also allow the resin to infuse through. 


Foil section as it currently is

After our discussion, I can see ways that we can make the foils stronger, the main one being reformatting uni on the compression side, which is awkward in the mold but doable. However, at this time I can only see a small increase in strength as being possible, and the foils will still not be strong enough for full flying.

The foils as they are will be strong enough to fly an F-22, which offers some interesting possibilities in the future. But they would need to be a much larger section to safely fly an F-32SR. That would need a new larger case, and we are not able to develop anything like this at present.  Less of a curve would also help, but the foils would then have to be removed every time the boat was trailered, which is not easy considering their weight and the difficulty in handling. The tight curve also means there is no excess windage when foil is raised on windward side, nor is a crane needed to lift them, a big plus.

Foil in raised position neatly behind forward beam, with no windage, and can be left in place
when folded or while being trailered. Foils in Use

I first started designing building these curved foils back in 1985 with the original F-27, and it was even obvious back then that it would be very hard to make them strong enough to fly the whole boat. The full boat weight will be riding on that foil should it be good enough to lift the whole boat.

The prototype F-27 in 1985, with the original curved foils in place just behind the beams.

However, this was never expected to happen, and it was just a intriguing possibility. The F-27 was heavier and slower, plus the foil section was not as good, and we had no way of varying the angle of attack, so we never got close to flying. Thus breaking these foils never became an issue. The foils were also larger and it was very noticeable how awkward and heavy they were, this being one of the reasons I discontinued their use at that time.

The bottom line is that we can only make the existing F-32SR foils so strong, and as such they were never intended to be suitable for flying the whole boat. They are what they are. This whole flying aspect was not even a realistic possibility in 2010 when the foils were originally designed, and foil buyers were always advised to start retracting the foils in 10 knots and above, and that they should be retracted at least 500mm (20") in winds over 25 knots, as foil strength would then be getting close to borderline.

It has however been both interesting and a surprise to hear that F-32SRs have been able to fly with the foils on occasion. But it will always be at a big risk to these foils, and not surprisingly they have broken. 

So in summary:

1. The F-32SR foils were never designed or intended for full flying.

2. The F-32SR foils were always intended as an option only and the boat does not need them to perform well. The F-32SR floats are 11.6 % larger than the F-32R floats (comparison drawing below), and proportionally much larger then the F-25C floats, my only other racing orientated boat, and one with a great race record. However, used as intended, the foils will safely provide another 1000kg (2200lbs) of lift to a float, which is a significant further 35% increase in buoyancy. 


F-32 to F-32SR float comparison - the F-32SR floats will be faster, but will be more uncomfortable, in large waves,
and harder on beams (more shock loads). Such a shape will also be inferior to windward and downwind if the fullness
was carried all the way through to stern. However, the F-32SR tapers off at the stern, allowing sterns to sink into waves,
to avoid the pitching and scary nose down attitude that can result from too full sterns. F-32SR main hull is also narrower
than F-32 to allow it to fold up to a legally trailerable width.

3. The foils were also intended to improve light weather and windward performance (as proven possible on Kim Alfred's F-31 Cheekee Monkee) and hence their size was based on the Monkee’s foils. They were a similar small and more easily handled section to minimize drag, but were made curved for more convenience, and more vertical lift. 

Thus they could thus provide significantly more lift at higher speeds due to their length and curve, but if not retracted they will not be strong enough to safely provide such extra lift. Cheekee Monkee broke several foils while pushing beyond the limits, on the verge of flying, and owner eventually purchased the molds so that he could make his own as they broke. But otherwise CM was significantly faster than F-31s without foils, and this was what convinced me to offer foils again.

The laminate on every foil broken so far has looked excellent, and every future foil will now be proof tested before it leaves the factory. Thus any breakage at full extension will have to be assumed due to the foil not being retracted as recommended, and thus cannot be covered. The only way we can offer a full warranty (as they are) would be to reduce length of foil by 500mm so that it is impossible to be overloaded. However, light weather performance will then be inferior, and it would be a pity to have to do this.

We are now doing more material tests, to try and find better materials, plus some technique improvements, and expect future foils to be better than ever, but they will still remain unsuitable for fully flying the boat except for an F-22, or a very light F-85.

The fact that these foils have been able to fly an F-32 has been somewhat of a surprise, and even a success, but one that also causes their own demise.

The Next Test Project:

F-25, F-27, and F-31 beams have been test broken in the past, and it is time to break an F-22 beam.
The foil test rig is strong enough, and will be able to do this after some modifications.

F-22 beams have performed very well so far, but there is always room for improvement, particularly in the glued join flanges. These are usually the first area to show any signs of stress in break tests, and finding a good enough glue to handle the loads has always been a problem. Thus backup bolts have been used from the early days, and finding a reliable tough glue to where such bolts can be eliminated has always been a target. May now have such a glue, and this, coupled with the 50% more gluing area on F-22 and F-32 beams (compared to earlier designs) will make it an interesting test. Will need a larger digital scale to handle the loads, but this has now been ordered.

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