In drawing a sketch of a hammerbeam into the dimensions you provide, I think the 14 foot center section is too much. However, I'm not an engineer....
It sounds as though you would like the center 14 ft. section to be around 12 feet high?
This is not easy(neither hamlet's frame nor a scissor truss wants to go that high)... could there be posts at the inner edges of the docks?
If this is possible, it would simplify joinery, lighten the timber sizes and raise the 14' center beam to 13 feet(you would, however, need and additional 10 peices of timber for the posts...) you could always stick frame the sidewalls to make up for it???
