Tuesday, October 27, 2015

Tofino whale watching tragedy

Its a perverse contradiction of yacht design that passenger comfort or initial stability and ultimate vessel stability are fundamentally at odds with each other.  In the simplest analysis the most comfortable boat is arguably also the most dangerous boat to be onboard when the shit hits the fan.

Its been hard for us not to obsess about the tragedy off Tofino.  The image of the Leviathan bow sticking out of the water is sad and frightening to anyone who spends as much time on the ocean as we do.  The fact that the vessel went down so rapidly that the crew couldn’t even get off a Mayday call let alone launch any liferafts or suit the passengers in lifevests is also frightening.  We always assume that in the awful event of Gray Hawk sinking we would have at least a few minutes to get ready to disembark. 


Initially I thought that the boat must have struck a reef hard enough to open a huge hole in the hull but even in that circumstance it was hard to explain how rapidly it must have gone under.  One of the native rescuers mentioned hearing a passenger comment about how the boat seemed to suddenly turn over and that got me thinking about hull form and stability.  The initial reports of the sea state were 3 to 4 meter waves which isn’t huge for the west coast of the Island but its still a serious swell.

The pictures of the submerged hull and prior pictures of the vessel suggest a fairly flat bottom hull, similar to these Bayliner images.


As you can easily see, these hulls have a relatively flat bottom with a sharp turn at the bilge (where the bottom part turns into the sides).  This hull form tries to remain parallel to the surface of the water it is floating on.  If you imagine pushing down on one side of one of these boats you can see that the displaced water will increase rapidly which results in buoyancy that tends to return the vessel to sitting parallel to the surface.  Note that the vessel tends to remain parallel to the surface of the water which is not necessarily the same as level with the horizon. 


Contrast the Bayliner hulls with this shot of Gray Hawk in the slings.  What you can’t see is that aft of the maximum beam at midship the turn of the bilge almost completely disappears.  When you push down on one side of Gray Hawk her displacement doesn’t change appreciably – she’s not completely round like a wine bottle but not a whole lot different either.  On an average day on the water Gray Hawk bobs and rolls like a cork.  Which occasionally results in SWMBO getting seasick (and routinely resulted in Jorgito the Idiot Cat shatting himself). 

What keeps rounded hulls like Gray Hawk’s upright is massive amounts of weight deep in the hull.  We have lead in the keel and our engines, fuel tanks, water tanks and generator are all below the water line.  The hull may roll easily initially but as that weight moves and starts to lift in the water it naturally tries to pull the hull back to vertical.  Note that this effect is independent of the surface state of the water – Gray Hawk’s hull will always try to be vertical, regardless of what the waves are doing.  On an average day either hull form can give an acceptable ride.  The Bayliner hull form is better suited to going faster and it is pretty common on vessels that look superficially similar to Gray Hawk.  Grand Banks, Nordic Tugs, Bayliner/Meridian – probably over 3/4 of the recreational trawlers have this hull form. 

On a non average day however it all changes.  The Bayliner hull form still tries to remain parallel to the surface of the water, even when the surface of the water goes all out of whack with big waves rising and falling.  But its worse than that.  When any boat is on the side of a wave it will try to slide down the wave. Gravity doesn’t shut off just because the waves get up.  The problem with a sharp turn of bilge is that it can dig into the water and effectively “trip” the boat.  That appears to be exactly what happened to Leviathan.  The initial TSB report says that the passengers were all on the port side (presumably looking at whales) when a large wave approached from the starboard quarter. The combination of extra weight pushing the port side deeper in the water and raising the centre of gravity (because they were all on the top deck) put together with the flat bottom hull caused the boat to trip and capsize suddenly. 

The theory with a more rounded hull form is that in those extreme conditions the vessel is more likely to slide down the face of the wave.  I hope we never find out that it doesn’t work that way. 

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