I brought my rib from new from Ribtec in 2001. The spec and layout was left up to me. Part of the original spec was for a 150 ltr fuel tank. When Ribtec started to make the rib up they found that the 150 ltr tank was too deep to fit into the space available and offered me two 100 ltr tanks instead. This was agreed. After a few months of ownership I found out that the most fuel that I could ever fill up with was aprox 150 ltrs even though the engine was hunting for fuel and would only pick up with all passangers forward. This leaves me to think that the fuel pick up was sighted either in the wrong place or the tanks when linked together left a void under the link point. I am therefore carrying around aprox 50 ltrs of fuel for the sole benefit of ballast!! Any ideas of how to pick up this missing amount without cutting into the fabric of the floor and distroying its strength?
May I suggest that you talk to ribtec directly on this, as they built it. They might also be able to help if not with the answers with where to cut to put a hatch in so you can get to the problem. I know you said you didn't want to cut the deck and can understand why - but I have a horible feeling this might be the only way to check.
Ribtec has joined forces with Ribeye and no longer seem to be too interested in products sold before the getting together. I will however give them another ring and press my case. Many thanks for your interest.
There are quite a few people with the ribtec hull inc the camel boats ( these have three tanks two large forward I think 150Lt-ish and a single aft ~75 Lts I think been a while since I used one of those hulls) I'm sure one of them has investigated. If you look on the Rib gallery TUE is rebuilding one of these hulls and posing pictures.