The raising of Divecat

She was in the worst possible location with absolutely no shelter from the wind in any direction, in the middle of one of the strongest tidal stream zones in the Hauraki Gulf, and at a depth that required prohibitive safety protocols. The commercial guys declined to take up the challenge. It looked like the end of her.

When my boat sank in 43m of water in the middle of the Firth of Thames I was devastated. (see Dive Pacific Issue 174 April/May 2020 for the full account.)
We were returning from a charter trip out to Great Barrier Island in February, 2020 and about an hour after all our crew and guests were safely on board the Coastguard rescue boat, Divecat sank.

I was determined to get her back.

Research and planning

During the first Covid lockdown, which followed the sinking by less than two months, I had lots of time to research ways to achieve this. Thanks to advice from a couple of experts I knew, very-heavy-duty lift bags were the only viable option. ‘Lift her off the bottom and float her into shallow sheltered water’ was the gist of the advice. Once she was in less than 20m it would be an entirely different prospect to get her back to the surface.
I began assembling the gear needed and started to plan. There are major challenges diving to that depth and location. To start with for me the biggest one was how to maximise my bottom time on open circuit equipment. The first time we got down to her we breathed normal air and stayed within no-decompression limits. It meant a maximum of eight minutes bottom time. Hardly enough to do anything besides have a brief look around. If I was going to achieve anything I needed to delve into serious decompression diving.

Tec dive challenge

I am a fairly heavy gas user, with a SAC rate above 25 litres/minute, which means at 40m even a 15-litre tank only gave me enough gas for an 18 minute bottom time, allowing the usual safety margin. On my first drop-down-and-look dive I used a side-mount configuration to provide a completely redundant air delivery system along with an ample gas reserve.

Being Nitrox certified, the next step was to optimise my gas mix. I could safely increase the bottom gas to EAN26 since 43 metres was the deepest we could go unless we started digging. In that case the staging gas could have a higher concentration of oxygen to reduce the deco time. Eventually we switched to a final staging tank of pure oxygen for the last sub-5m deco stop.

More control of everything

The second big challenge became clear when planning for the lift. A runaway ascent of any submerged object is a huge danger, and from viewing a couple of salvage videos online I could see how terrifying this prospect could be. A chat to some supposed Tec diving experts revealed how little knowledge most people have of this phenomenon. One ‘expert’ thought it was realistic to be able to stop the ascent by shutting down valves or manually dumping air from the lift bags. I ran a few tests of the bags and realised I needed a plan that meant the bags would not be touched once initially inflated. During those tests we discovered how dangerous a bag breaking free could be.

The only safe plan was for the divers ALWAYS to be above the hull, and below the lift bags. And I had to make sure the boat would only rise by a small amount at a time. A main concern was for the bags as they neared the surface since the gas in them doubles in volume just in the last 10m. Even with two dump valves per bag I had to try and limit the speed and range of each stage of the lift.

"One ‘expert’ thought we could stop the ascent by shutting down valves or manually dumping air from the lift bags

More planning

My eventual plan called for lift straps to be attached to the boat with a 40m rope tied to each and with shackles pre-tied every 5 metres. Initially we would fit bags at 5m, 10m and 15m as the boat rose. As the boat came up the bags at 5m would reach the surface and stop, but the remaining bags lower down would be insufficient to lift the hull any further. We would then fit more bags at 15m to lift her another 5m. We repeated this process, deflating and re-using the bags as they reached the surface and took no further weight.
This meant for the bulk of the salvage operation our divers would be required to operate at between 10 and 15 metres. With appropriate Nitrox that meant we could have periods of up to three hours work time. At surface intervals we used pure oxygen for our safety stop and surfaced only briefly to change tanks as required, while also taking on refreshments during those brief intervals. In the end fatigue became our main limiting factor.

Entanglement

The third technical challenge was entanglement. The boat had a commercial fishing net wrapped around her, which was in fact how I located her final resting place. A fisherman had snagged his net around an object that should not be there! A serious risk. Not from the main net which was wrapped tightly around the hull and predictable, but from those loose sections of it that became hazardous as we cut them away!

Then we discovered a further entanglement risk of our own making. The first lifting plan involved numerous, relatively thin ropes attached to various points on the hull - I used polypropylene rope for its stretch capabilities. However, this type of rope is positively buoyant in the water and resulted in quantities of soft rope floating around as we tried to attach it. It soon became clear we needed rope more predictable when handled underwater. We upgraded to fewer, but heavier, sinking polyester ropes which would always hang or flop on the cabin roof when released.

There was also a danger the ropes would damage the hull or worse, if the hull cut through the ropes. So I sourced 11-tonne straps which the 4x4 fraternity use. Wide and flat to avoid point-loading and with some stretch to counteract the relatively inflexible polyester. The last part of the equipment needed were heavy steel shackles that were easy to manipulate while wearing gloves. They added to the ropes weight too which helped pull the sets of bags down with them.

First lifts

Determination can overcome just about anything. On the 30th of January this year we were finally ready to attempt the lift. The straps were pre-fitted to the hull and all the gear ready. We were on site before slack tide and headed down with ropes and the first lift bags.
My plan fell apart slightly because I grossly under-estimated the time it would take to get the ropes shackled to the straps, then get the first set of bags fitted and inflated. Fortunately we were able to get her just slightly off the bottom. And once her drift had started we were able to drag her about a nautical mile closer to the Coromandel. Then she again snagged the bottom.

A second attempt two weeks later was much more successful. Right away we had her closer to the surface, and a slow, steady tow saw us put her down in 17 metres just inside Elephant Cove in the Happy Jacks Islands of the Coromandel.

A couple more trips out there followed, to clean off that net and remove extraneous ropes in readiness for her to taste the air again. But though working in sheltered and relatively shallow water, visibility was even worse in the enclosed cove than in the middle of the Firth.

Frustration

A first attempt at lifting her above water was confounded – we ran out of time. Small issues meant we had her horizontal and floating just below the surface before the tide turned. Frustratingly we had to put her down again, this time in 7 metres. After a week of frantic preparations we scheduled the final lift for Easter Friday.
Finally. We achieved the seemingly impossible. Using bags attached directly to the hull this time we lifted her roof to the surface, then, by putting bags inside the cabin and between the twin hulls we managed to lift the boat fractionally higher. Exactly at high tide we pulled her to shore to place her hull firmly on the bottom. Then we took our first real rest for the day while waiting for the tide to recede by 2.1 metre that night.

"By putting bags inside the cabin and between the twin hulls we managed to lift the boat fractionally higher and exactly at high tide we pulled her to shore.

Pumped

Close to midnight we started our range of pumps to clear the water out of the hulls, and about 2am she was starting to float. We left the battery-powered pumps on while grabbing a few hours of sleep.
At dawn we found her hulls as dry as we had left them. No leaks! After a few more hours preparing we began the slow tow back to Half Moon Bay Marina where, just after 5pm, she was finally lifted out of the water. Job done.

Divecat was underwater for 2 years 2 months and 10 days. Every deep dive we did was just myself and buddy Jordan. One other diver joined us on four of the later trips. The main lifts were also with just the two of us under water - we both knew exactly what had to be done without the need for communication. The final surfacing effort was a team effort with four of us alternating between surface support duties and diving.

What’s next?
Now I face a year of weekends stripping the boat down to raw aluminium and rebuilding her. I also have numerous heavy lift bags and associated hardware in storage I hope never to use again!