The whale-watching boat that capsized near Tofino, B.C., had undergone major modifications that may have caused it to become less stable, CBC News has learned.

Last Sunday, six people were killed when the MV Leviathan II tipped over. The Transportation Safety Board said most of the passengers were crowded on one side of the outside deck, which may have destabilized the vessel when it was hit by a wave from the opposite side.

The Leviathan II was originally built by Vancouver-based RivTow Industries in 1981 as a tug boat, and was named the Crown Forest 72-112.

Nautical Training Instructor Capt. Ivan Oxford says modifications to the MV Leviathan II raise concerns about its stability. (CBC)

In 1996, it was converted into a whale-watching vessel and renamed. The major change to its design was a new deck added to the top of the vessel. It was also lengthened, according to Transport Canada.

Capt. Ivan Oxford, a nautical training instructor, says that kind of redesign raises concerns about stability.

"You run the risk of moving weight from lower down to higher up, as well as adding weight on the higher location," he said.

More stability tests not required

The refitted Leviathan ll didn't have to undergo more stringent stability tests outlined by Transport Canada that are reserved for larger vessels.

Small vessels are subject to a basic roll stability test where the ship is tipped, then righted. But larger vessels — or those that fail the roll test — also undergo an incline experiment to double-check the effects of weight shifting inside or on top of the ship.

That test is mandatory for BC Ferries, for example, with complex calculations that also take into account average passenger weight and what crowding on one side of a ferry would do to stability.

Transport Canada said on Thursday that the MV Leviathan II did undergo both a stability assessment and an incline test when it was refitted in 1996.

Modifications to the MV Leviathan II included refitting it with a passenger deck. (Jamie's Whale Watching/YouTube)

Vessels capsize after modifications

It's not the first time a refitted vessel has flipped in B.C. waters.

In 1997 the fishboat Pacific Charmer capsized near Nanaimo, killing two people. And in 2002, the Cap Rouge ll rolled near the mouth of the Fraser River, killing five people, including two children.

In both cases, investigators found modifications — including the addition of heavy equipment high up on deck — caused the vessels to flip.

"It's very important that we go back and reassess," said Oxford. "That will be a big lesson that might come out of this."

The TSB's Marc-André Poisson has said the investigation into the Leviathan ll tragedy is expected to takes months.

But, he added, preliminary conclusions may be presented earlier if investigators uncover any evidence that could immediately save lives.