[QUOTE=RDD;31781]Sorry if I’m being dense on a minor point but I still don’t understand the sonar pic. Are we looking down on the rig or is it a horizontal view? I just don’t recognise the angled bracing unless that’s part of the deck or the bottom structure.[/QUOTE]
It’s truly upside down. What you are seeing are the two pontoons and supporting cross-members.
Here’s a photo of the Nautilus – you can see two of the major cross-members.
