It looks like your catamaran is coming along nicely!

Probably somewhere along the line you mirrored one surface and joined, creating 2 surfaces with an edge between them, then possibly that assembly got trimmed with the center line not arranged right in the center between the trimming objects.

It looks like that in-between piece is a straight connection (sometimes called a "ruled" surface).

So to tune this up what you can do is select both of those surfaces in between your hulls and delete them. Then select the 2 inside edges at the top of the hulls, and then do a Construct / Loft between them.

That will build a new single surface connecting those pieces - that will not have any interior edge anymore. Then you can join those hulls and the connecting surface together into one object.

Let me know if you need more detailed info on these steps - if you post your model or e-mail it to me, I can show you some step-by-steps images on your model if you want.

- Michael