I will remeasure with the formula ( bow to tip of sponsons behind transom), is this the front of bow?
No, the transom is at the stern (rear end) where the engine clamps on,
the transom would be the extent with most hard boats, but with ribs the tubes /sponsons travel further back past the transom, for extra stability.
"also is the width measured from the outside of the tubes at the widest point?."
Yes, usually getting wider toward the back / stern,
I will get some pics posted this weekend, Maybe someone will know what it is .
That's a great idea, I'll bet someone on here will know it,
All the best,