Why did DIVE CAT sink?

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On Waitangi Day, February 6th, half way across the Hauraki Gulf, heading back from a successful day’s diving at Rosalie Bay in Great Barrier Island, at about 3.30 pm, Dive Cat began to sink.

By Gilbert Peterson

Owner for less than a year, Norman Holtzhausen was on board his pride and joy with the skipper and 11 divers made up of family, friends and clients. It had been a pleasing day out. Everyone had had two dives, with lots of crayfish to show for it. The return trip was to be about two and half hours. The wind was getting up more than forecast, to perhaps 25 knots with a one metre swell’ not uncomfortable. Dive Cat was sweeping along at 9 to 10 knots. Passing across the top of the Coromandel and Norman notices the steering has become sloppy.

They pull in at Port Jackson to take a closer look in calmer water. It turns out there has been a loss of hydraulic fluid somewhere inside the hull, but the leak cannot be identified. This is a two engine boat and of itself, the issue is not a major problem. They open the portside rear compartment hatch to get some aluminium with which to lock down the steering gear for the rest of the trip. The compartment is dry. From then on the boat will be steered by alternating motor speeds. After testing she responds well, and they head down past Colville towards the tip of Waiheke Island.

The going is slower now. Using the throttles to steer the boat demands this, as does the swell right on the nose. After making about a third of the distance across to Waiheke Norman sees the boat is pulling to one side so they stop to adjust the position of one of the motors. It was then that Norman notices the stern is lower in the water than it should be. When they open the port side stern compartment hatch they now find it half full of water.

Coastguard called

Norman said. “I knew then we were in trouble. We called the Coastguard. It was about 5 pm. “We quickly moved as much weight as we could as far forward as possible, and we got everyone into lifejackets. We turned on the manual switch for the electric bilge pump but as the outlet was now below water we could not see if it was pumping effectively.”

The question right away was: Why did the auto bilge pump alarm not come on as soon as water entered the compartment? The float switch controlling it must have become stuck.

Norman said when we opened the hatch cover in the deck to inspect what was going on waves started breaking over the stern walk-through into the cockpit so we had to close it straight away. “By now people were working the manual pumps, and with the motors still running we kept our nose into the swell, heading towards Waiheke doing about five knots. But the stern was still going down.”

Eventually the motors washed out and died. They were just over half way to Waiheke, in about 45 metres depth.

Norman Holtzhausen’s Dive Cat photographed shortly after survey

Mayday?

“I was just about to upgrade our call to Mayday,” Norman said, “when the Coastguard appeared on the horizon.”

The Coastguard had been attending a fire on board an old tugboat at Kauri Point north east of the bridge in the Waitemata Harbour and when they got the call they immediately left the site – no one was on board the tug – to head for Dive Cat.

The Coastguard quickly took everyone off the boat, initially not allowing anyone to take any gear with them at all. A discussion ensued then as to whether they might tow it since it appeared not in imminent danger of sinking. But the decision was made that the risk was too high.

As the owner, Norman and his dive master were then permitted to return to the boat to offload any personal gear that could be easily removed, to put down the anchor, and turn on the anchor lights. None of the heavy dive gear – tanks, weights or regulators – were allowed to be taken off. Dive Cat remained afloat for at least another hour. Then its onboard GPS tracker went dead, indicating it had switched off. The boat had sunk.

Everyone on board was taken to Tamaki to go ashore. No one had been injured in any way, though several people were certainly shaken by the experience.

The abandoned Dive Cat begins to lest heavily to port

Why did the boat sink?

Dive Cat had been in survey for more than 30 years. Her last inspection was just six weeks prior. After much reflection, and a lot of sleepless nights wondering what they might have done differently, Norman thinks multiple factors contributed to the accident.

First, the standing wave rolling up against the stern became an issue when the steering problem resulted in the boat being unable to make its usual headway speed. The resulting water pressure initially forced water back into the bilge outlet, and later through the engine cable ducts as well.

Crucially the bilge pump outlets in the stern were too low. Once the stern sank by 20 cm they were below the waterline. This design flaw was not so important at higher speeds when the planning effect lifted the stern of the boat clear of the water. Also the outboard legs created a pressure wave which pressed up against the transom at low speeds. In addition the moved significant weight off the stern so she sat lower at the transom than originally designed.

Then there was the failure of the bilge alarm which prevented the pumps from clearing the stern compartments. This meant that the skipper was unaware of the problem until there was sufficient water, and weight, on board to affect the boat’s handling. And there was no way to close off the transom at the stern; it should have had a drop-in gate for this.

The heavy dive gear was left onboard

Question remains

But the question still remains: Dive Cat had three watertight compartments, three sections lengthwise. Shouldn’t she have stayed afloat regardless of how much water was on her?

The clinching explanation why this catalogue of circumstances compounded to cause the accident is, perhaps, that Dive Cat was originally designed for inboard engines. When she was converted to outboards in 1999 the centre of gravity of the boat was moved significantly towards the stern. Almost 400kg of each engine was located out where there was no direct buoyancy support. The result was that the rear compartment of just over a third of her length was supporting about half her weight.

Norman thinks perhaps that once her port-side compartment filled with water she would have stood on her stern with her starboard bow high before the pressure became too much, and the remaining compartments also filled. Then she would have gone down stern first.

What now?

Yes the boat was insured, and Norman thinks the insurance should cover most of what is still owed to the bank. It won’t cover all the costs invested in finding the boat, fitting her out including the latest electronics and safety gear, the compliance and inspection costs, survey certificate, or any of the valuable dive gear that went down with her. Some individuals on board lost up to $15,000 worth of gear each. Dive Cat probably lies in the deepest part of the Firth of Thames which, at around 45 metres, is well beyond the depth any recreational diver could safely attempt. Otherwise it’s too early to speculate on what attempt may be made, if any, of salvage.

A future?

In one sentence Norman says he is unsure if he has enough energy to get his nascent dive trip operation up and running again with a new boat. Then, almost in the same breath he starts talking about an electric powered catamaran being built in Tauranga that he has been invited to inspect. … All may not yet be lost.

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