Hunt for the missing foil

Useful lesson gained from foiler catamaran's appendage loss

The autumn trials and sailing programme were progressing well when they were dramatically curtailed by an unexpected event at sea, but one that demonstrates the versatility and safety of the C-FLY concept: Leaving Portland harbour, C-FLY headed out to sea at over 25 knots. The wind was SW F4 with a 1 metre sea, ideal conditions for a fast ‘cross country’ sail. Riding high over the waves, the crew sheeted in and headed south at a pace similar to the high-speed cross channel ferry. Far offshore we tacked around and headed back towards Weymouth.

On the return, a mile and a half offshore, C-FLY’s leeward front canard foil burst from the front of a large wave and into the trough, before ploughing deep into the back of the next wave. It’s nothing unusual for the C-FLY foil system, working in such a dynamic offshore ‘washing machine’ environment and one that we regularly sail through at speed. But this time, although we remained on an even keel, the boat decelerated suddenly. So we rounded her into wind to look for any signs of ‘foreign objects’ snagged on the foil. It was soon obvious that the starboard canard foil had been lost. After filling the swear box with coins, we foiled back to port on three rather than four foils. Although this wasn’t the outcome we were expecting; C-FLY had yet again proved its offshore prowess. Although she had lost a foil, the boat was sailed back to port unaided. The crew were safe and uninjured and the boat uncompromised by the event.

Inspecting the boat on land, the engineers found the aluminium structural framework that supports the foil, had failed. The failure mechanism had compromised this framework and some of it had gone with the foil to the bottom of Weymouth Bay. The engineers were reckoning it was a fatigue failure. Obviously they were keen to get the foil back. It is not only an expensive and essential part of the boat but it should reveal the cause of the failure.

From the data provided by the on-board monitoring system, we were able to get a precise GPS position of the point where the foil had detached. The GPS is accurate to three metres and the boat was travelling at 27.6 knots on a NW course. We knew the point of detachment to within a few milliseconds, captured by the strain gauge outputs. Our data expert, Nigel Wright, provided a position, corrected to within a few metres radius of certainty. After all if he can navigate a spacecraft to Jupiter, he should be able to pinpoint our missing foil.

Given an accurate position and around 30m depth of water, it looked as if it would be pretty easy finding the foil. However, we were to find ourselves on a steep learning curve, having consulted with an underwater sonar and recovery expert. It became apparent that we were looking for a needle in the proverbial haystack. The key points being that you might find the foil with a sonar scanner if it hasn’t become buried, moved by the strong tides and if or when the divers go down they will encounter zero visibility and have only 30 minutes to find it, by touch alone.

And just to complicate matters more; how did the canard foil drop to the seabed? Did it descend erratically or did it spiral down or even more concerning, would it fly or glide tens of metres underwater? This uncertainty could be the ruin of us getting it back. So a tenth scale model of the foil was made and using the deep end of an Olympic swimming pool, we performed some ‘flight’ tests. It was discovered that the foil did indeed glide underwater. However, the glide slope was sufficiently steep that the real foil, offshore, would be no more than twenty metres from our GPS position.

With confidence building, we hired the expert in sonar scanning with his team, dive boat and tow fish sonar scanner. After four hours of detailed scanning of the seabed around our GPS position, they reviewed the sonar trace and found a likely target. It didn’t look like the foil but it had one straight edge and some other bright reflections. There is a sonar man’s rule of thumb that anything with a straight edge is man-made and so this object looked very promising. He transferred the object’s GPS position to the navigation computer and found the target to be within twenty metres of our GPS position. It looked as if we were in luck.

Recovering an object from the seabed could be likened to a coastguard helicopter trying to place a man on a wire, on the stern of a boat in zero visibility. The horizontal position is critical and you have to take account of the drift of the dive boat caused by the influences of both tide and wind. Bear in mind, if you are on the seabed, an arm’s length, in zero visibility is the difference between finding the target or not. The boat was positioned with great skill and precision and a shot (weight) on a line with buoy, dropped to mark the seabed. The pair of divers descended and feeling their way, started laying a reference line from the shot, heading north. We were very lucky for within five metres, they literally bumped into the foil. The foil was then raised to the surface by air bag.

The anodised aluminium foil was complete and undamaged. On inspection of the broken part of the framework, we found the cause of the failure to be a fractured weld.

C-FLY will be back on the water early next year and we are a little more knowledgeable about underwater sonar search and recovery!