A jigsaw puzzle is made of many pieces, says Paul Davison QC, the lawyer who successfully prosecuted Scott Watson for the murders of Ben Smart and Olivia Hope.

In a speech to legal colleagues in 2004, five years after the sensational murder trial, he said: "When put together, the clear picture that was revealed was Watson's responsibility for these killings."

But what happens when the pieces don't fit any more? What happens when - as the Herald on Sunday has confirmed - a key section of the prosecution case is proved physically and scientifically impossible?

Davison told the jury in the 1999 trial that Watson was the killer and was seen on his yacht Blade in Cook Strait between 4.30pm and 4.45pm on the day Hope and Smart went missing.

"Good place to go if you had something to dump," he told the court. Speaking later, he said "... by employing Olivia's sleeping bag and weights, the bodies are likely to have been disposed of by him there".

The court heard he was next seen 18km to the west, at Erie Bay, between 5pm and 6pm. Davison told the court "it may have been around 5(pm) he arrived, it might have been a little later".

The Herald on Sunday has found the reality quite different. We took Blade on the same trip, recreating a voyage taken by author and campaigner Keith Hunter seven years ago. We rode the same tide in the same boat from the nearest position Watson was alleged to have travelled from. It took two hours and 35 minutes. Hunter's trip started farther out and took a comparable length of time.

The trip is impossible to achieve in the time stated by the prosecution if calculated mathematically using data from the prosecution's own files. The trip is also impossible using a scientific calculation that determines the maximum possible speed that yachts can travel at.

In fact, the evidence suggests that Scott Watson would have had a better chance of completing the trip if he had walked there across the water.

So why hasn't anyone said anything?

* * *

This is not a story about Scott Watson's innocence. Much has been written about the flaws which appear to have emerged in the case. Rather, this is about one key piece of the jigsaw that filled a hole in the prosecution case against Watson. The journey was the only explanation offered by the Crown as to how Watson might have dumped the bodies of Hope and Smart.

A full 5 per cent of Davison's 30,000-word closing argument was consumed by this section of the case.

And this is how it was described.

Two witnesses on board the Aratika Inter-Island ferry told police they watched a yacht in Cook Strait on January 1, 1998, estimating it was 4-5km from the mouth of Tory Channel. They marked the time of their observation by the ferry reducing speed _ about 4.30pm _ and said they watched it for 10-15 minutes.

The ferry log showed it was at reduced power at 4.28pm but placed the ferry further from the mouth of the channel. The statement from the ferry master puts the distance at "five miles" from Tory Channel _ although does not specify whether it is standard miles or nautical, which are a greater distance. In kilometres, the distance is 8km or 9.2km, depending on the measure.

The witnesses watched the boat, which they later identified as Blade, describing it as wallowing with its bow pointed out to sea.

That day in 1998, the tide was due to turn at 5.15pm. Any boat heading into Tory Channel before then would be facing a constant surge of outgoing water. Davison told the jury "he would have to wait, wouldn't he, off the entrance of Tory Channel until the tide started to change because if he was going to start making his way back up he wasn't going to be able to do that against an outgoing tide".

The next sighting of Blade presented in evidence was in Erie Bay between 5pm and 6pm. Those witnesses _ a man and his two children _ gave three statements to police over two months that shifted Watson's arrival time from between 10am-12 noon to 3pm-6pm.

There have been questions about the statements from those witnesses. Watson's arrival moved later in the day after police found 250 cannabis plants on the man's property. The man's recollection of Watson's visit changed before the cannabis charge was heard. He later received a comparatively light sentence for growing the plants.

But _ setting aside the suggestion of a conspiracy _ the statements relied on by the prosecution at trial put Watson's most likely arrival about 5.20pm. The prosecution alleged he now had time to dispose of the bodies.

So these are the numbers. If Blade was last seen at 4.40pm, then next seen about 5.20pm, how quickly would it have to go to cover 18km _ the distance between those Cook Strait and Erie Bay sightings?

The answer? A lot faster than any speed squeezed out of her by police tests. In fact, faster than Blade is capable of travelling.

* * *

In the rock-and-roll of the Cook Strait, I ask Scott Watson's father Chris why he keeps fighting, battling the system 12 years after the conviction of his son for the murders.

His answer is not the expected response, that a father would do anything to help his son. Sure, he says, there's the "emotional side". His wife Beverley, Scott Watson's mother, "takes care of that".

For Chris Watson, it is different. "Mostly because they offended me," he says. "They were supposed to be fair. It was supposed to be truth and justice. I'm really pissed off."

Blade is turned to face the mouth of the Tory Channel. We are about 6.5km out from the mouth of Tory Channel at (adjusted for daylight saving) 4.20pm and manoeuvred to make the run back to Erie Bay.

The depth here is 125m. Davison observed in a speech that "off the entrance to Tory Channel, there are deep undersea canyons ... the bodies are likely to have been disposed of there".

Scott Watson built Blade by hand. It is a 7.2m (23.75ft) single-masted sloop that owes more to practicality in the water than grace. The unpredictability of winds here and in the Tory Channel means power comes not from sail but the noisy motor, its low-speed guttural chugs underscoring everything that happens on the boat.

Blade is not quick. The speed achieved by police in their own testing was just over 9km/h (5 knots). With the push of the tide, it takes half an hour to cover the 3km to our starting point for the run to Erie Bay.

By police witnesses, the earliest Watson could have started out for Erie Bay was 35 minutes before the turn of the tide.

Here we are, at 4.20pm, about 45 minutes from the turn of the tide and about 6.6km from the mouth of Tory Channel.

It's a slow grind through the water against water pouring out of Tory Channel with the tide.

The engine is running at 2100rpm, just under the maximum rate in police tests. "If Scott knew I was running her that high, it'd give him the heebie-jeebies," says Chris Watson.

At 5.05pm, when the tide turns, Blade has yet to pass the mouth of the channel. It is not until 5.30pm that we pass between towers of rock on either side, the tidal surge under Blade hurrying the yacht and its passengers on towards Erie Bay.

Our progress is quicker now, as the water carries us forward and down Tory Channel. But it is not quick enough, could never have been quick enough, to have Blade in Erie Bay at the time she is meant to arrive.

A decommissioned whaling station glides by at 5.45pm. The speed of the yacht is better noticed with land closer. The green of the Sounds glides by.

Night has fallen when Blade calls on Erie Bay. It is 6.55pm. The journey _ in circumstances most favourable to police witnesses _ took 2 hours and 35 minutes.

The Herald on Sunday test took three times as long as the journey Davison described to the jury, at its most generous timing. Keith Hunter's test took the same. Incredibly, the police measured time and distance while motoring Blade about Marlborough Sounds _ but they never did this journey.

At Scott Watson's trial, Davison said of the Cook Strait-Erie Bay trip: "It doesn't matter quite how you do the arithmetic of the distance and the time and all the rest of it, because the times of these events in every instance have to be taken as being a little bit fluid."

* * *

And then there is the science. As long ago as 1861 English naval scientist William Froude devised an equation for working out the maximum speed at which a displacement-hulled vessel can travel.

Boats placed in water will shift their own weight in water out of the way to make room. When moving forward, boats must push that much water out of the way, making waves that move at the same speed as the boat.

University of Auckland professor Andy Philpott, who works in the Yacht Research Unit, explains that the wave crests get further apart as a boat travels faster.

"Resistance caused by making these waves goes up slowly until the boat speed approaches the critical Froude number," says Philpott, referring to the equation devised by William Froude, which still applies for traditional displacement-hulled vessels. At that speed _ called the hull speed _ the waves at bow and stern create a trough in which the boat becomes caught. It is unable to move any faster without planing, which would require a quantum leap in power.

The Froude number equation allows the hull speed to be calculated using the square root of the length in feet of the boat at the waterline, multiplied by 1.34. It gives the answer in knots.

Philpott _ a mathematician and engineer who worked on America's Cup boat designs _ says the hull speed is an approximation of the maximum achievable speed of a displacement vessel, which also depends on the hull shape and sea state.

In the case of Blade, the length at waterline is 9.06m. The equation gives the yacht's hull speed as 6.53 knots (12.1km/h).

Even at this speed, Blade would not move fast enough to get into Erie Bay to match the evidence given in court.

* * *

The boat carries no name. It never really did, says Chris Watson.

It wasn't registered the summer Ben and Olivia went missing. Scott Watson sailed the east coast of New Zealand trying out a number of names.

The name "Blade" was that used in the most recent log entries. Other names, according to the Erie Bay witness, included Caligula or Mad Dog.

Scott Watson has had no need of his yacht, having now spent 12 years in prison. He sold it to Chris Watson as part-payment for legal bills and the cost of hiring private investigators.

And Chris Watson has little use for the yacht. He hadn't taken it out recently, and he was scrubbing growth from the keel when the Herald on Sunday arrived for the experiment.

"Looks like bodies have been scraping up against it," he says, wryly, pointing at the boat lying clear of the water, smear marks down the waterline. He's referring to the marks found on the Blade which, the prosecution said, were made by the bodies of Smart and Hope, lashed to the side. It's another piece of the jigsaw he believes shows a different picture.

Others involved in the case have moved on. The two secret witnesses who testified to admissions they claimed Watson had made in prison have followed their own paths to restricted freedom.

Secret Witness A has since recanted and spent time in a mental health institution. Secret Witness B _ who police said was turning his back on crime _ is currently awaiting trial for offences which could see him sentenced to life in prison.

Deputy commissioner Rob Pope, who oversaw the case, has nothing to say on the impossible journey. A response from Police National Headquarters states: "The totality of all the evidence regarding this case has already been presented in court, where it was tested and properly considered.

"The case has also been before the Court of Appeal and the Privy Council. "Police will not be providing further discussion on this matter."

And Paul Davison QC, who steered almost 500 witnesses through three months of evidence, summing up in 30,000 words, says the evidence around the boat trip has been "advanced, considered and dealt with by the courts".

But the links to the Watson case haunt them all. Many of those ghosts arise from Picton where the town's most enduring self-taught bush lawyer Chris Watson studies the case for flaws, working to dismantle the prosecution jigsaw that convicted his son as a murderer.

Chris Watson has recently filed complaints about Davison after finding the names of witnesses whose identifying details were suppressed. A transcript of Davison's speech names the witnesses.

This is a minor skirmish in the battle. The main conflict at the moment is a petition for clemency to the Governor-General. Kristy McDonald QC has been hired by the Ministry of Justice to investigate "potentially fresh evidence"which could lead to a retrial, a pardon _ or nothing at all.

This all happens outside the walls of Christchurch Prison, where Chris Watson and Bev Watson visit their son regularly. Scott Watson is not involved in the detail of his father's efforts _ there's not enough room in a jail cell for the paperwork, says Chris Watson.

He says he has never asked his son if he is innocent, never posed the question most think would have been obvious. "They [police] said he was a liar, so I can't then say, `But he told me ...'

"His word is no good to me. And the whole thing just developed ... we never sat down to talk about it.

"My perception that he didn't is not from faith. "It's from living through this thing and knowing this case."

* * *
Murky waters: the questions that bedevil the conviction of Scott Watson
1. Watertaxi operator Guy Wallace was a key Crown witness. He dropped Hope, Watson and the "mystery man" believed to have killed them at a yacht he described in detail as a ketch. He has since retracted his identification of Watson as the "mystery man" but not his description of the ketch. This is significant because ...

2. There were a number of sightings of a 12m (40ft) ketch (a two-masted vessel) as described by Wallace. Police have been accused of ignoring the ketch angle and focusing on Watson and his 7.2m (23.75ft) single-masted sloop. The Crown relied on photographs taken at dusk on New Year's Eve and dawn of New Year's Day _ even though it was claimed the ketch came and went after dark.

3. Rozlyn McNeilly was also a key identification witness for the Crown in identifying Watson as being at Furneaux Lodge on New Year's Eve. She has also retracted her identification, saying she was tricked by police.

4. The description of the "mystery man" and the identikit issued based on those descriptions show an unshaven man with shoulder-length hair. Images taken on New Year's Eve show Watson as clean-shaven with tightly-cropped hair. The "mystery man" was said to have hooded eyes _ which witnesses found when shown a photograph of Watson by police. It emerged the photograph was taken when Watson was blinking and his eyes were half-shut.

5. In a media interview, Secret Witness A retracted evidence that Watson confessed in prison. The witness is being reinterviewed by QC Kristy McDonald in a fresh inquiry to see if Watson's case should be reviewed.

6. Secret Witness B's credibility has been under constant question after he received assistance from the police following his testimony. He has also gone on to commit increasingly serious offences and is currently facing the possibility of a life sentence after a fresh arrest last month.

7. Wallace now says he can identify the exact place in Endeavour Inlet where he saw the ketch he claims to have taken Smart, Hope and the "mystery man" to. The specific location is also believed to be information to be presented in the new inquiry.

8. Water taxi driver John Mullen now says he took someone resembling Watson to his boat on New Year's Day. Mullen matches the description of the water taxi driver who Watson always claimed returned him to his yacht.

9. Water taxi operator Donald Anderson claims he took Watson to Blade at 2am. The Crown used Wallace's now-retracted identification of taking Watson to Blade at 3.30am-4am. Both claims, along with evidence Watson was ashore at 3am led to a late claim by the prosecution that Watson somehow returned to shore between watertaxi rides. Witnesses on the boat moored next to Watson have given a version of events that compete with Anderson's evidence and Wallace's now-retracted testimony.

10. The only DNA evidence in the case were two blonde hairs matched to Olivia Hope from a blanket inside Blade. They were not present on the first search of 400 hairs but were found on the second search _ the same day Hope's hairbrush with her hair was present in the ESR lab. A 1cm long slit was later found in the evidence bag, prompting some to claim the evidence was planted.