Thursday, May 31, 2012

Why build it upside-down?

I'm puzzled by this video clip I found on YouTube.  It shows the hull of what's clearly designed to be a shallow-draft vessel (said to be a ferry, although that's the strangest-looking ferry I've ever seen!) being launched upside-down, then towed down-river to be righted by being hoisted out of the water by a crane.

It was apparently built by Conrad Industries down on the Gulf Coast.  However, I haven't been able to find any reference to it on their Web site.

Can any marine experts tell us why such a vessel would be launched upside-down like that?  It's so shallow from top to bottom in its present form that I don't understand why it couldn't have been built right-side-up from the get-go.  Surely the engines, rudders, propellers, etc. could have been installed easily enough, irrespective of its orientation?

Hopefully some readers can enlighten the rest of us.



Thornharp said...

Move it downriver with the flat surface down = less chance of stuff getting snagged? That prop shroud looks especially vulnerable without the prop, shaft, and any other bracing present.

Once it's at the outfitting site, it has to go into dry dock, I suppose.

DISCLAIMER: NOT A MARINE EXPERT (but my daughter is one).

Will Brown said...

At a guess (also not a marine engineer), the presumed lack of superstructure and internal physical structure gives the extremely shallow keel hull insufficient stability as well as an excess of free board to contend with during maneuvering by a tug or tow boat.

As the video makes clear (especially the latter half showing the hull being righted), a tug can tow/push the vessel-to-be much as it would any other type of flat-bottomed barge without need for special modification to the tug/tow boat.

Dan said...
This comment has been removed by the author.
Dan said...

I agree with Will. Given how little it displaces, especially obvious after it is right side up, I imagine that it is much more stable while it is inverted. As a bonus, it looks like it would have no issues if it ran into a high wind situation while being towed.


Anonymous said...

Simple. It's easier to work from above than overhead. Rollover is usually accomplished ashore and can be tricky. Lots of lines and carefully maintaining control at the tipping point are the order of the day.


Chris said...

The company my mother works for builds barges. Every single one is initially constructed upside down, and then later flipped over. Though none of them are ever launched upside down.

trailbee said...

What a great way to launch the shallow bottom.

Paul said...

Will and Dan, the lack of displacement is because the engines et al aren't installed, there's no fuel in the tanks-diesel weighs over 8 pounds to the gallon, and there's no topside structure.

Will, there's lots of internal structure, otherwise the barge would have the same rigidity as a paper grocery bag. The bottom isn't truly flat, the hull design on this craft is better than a craft designed with a flat bottom, which obviously would have a square hull design.

I was a Machinist Mate in the Navy for several years, most of that was on ships, and while I'm not a ship designer I do have some knowledge. lol


Hunt Johnsen said...

This looks like a ferry designed to carry vehicles, and consequently the deck is the major structural element. laying out the deck and then framing "up" from there is a logical way to build this sort of thing. As Anon said, it's much easier to work on something from above - welding overhead is a bitch. Most of the boats I've built have started out upside down - even the little ones.
Flipping a hull is much easier than planking or glassing upside down - gravity helps a lot.

Johann Visagie said...

I ran into a British Pathé video today of a ship being built upside-down all the way back in 1943:

There wasn't much information accompanying the video, so I did a Google search (which is how I came across this blog post).

Among the other things I found was this page on WW2 destroyer escorts which describes the process of building ships upside down (about ⅔ down the page):

I'm assuming that the British Pathé video depicted one of these destroyers.

At any rate, the above page seems to indicate that this is done simply because it's easier to weld downwards (as others have mentioned). Building upside-down eliminates 90% of overhead welding, thereby saving time (and, presumably, money). All this according to the page linked above.