I have no idea of ship operating costs so don’t know what fraction of marginal operating cost is fuel related, but as oil has dropped there must be some impact on operating costs that could flow into the BDI.
That's true, but it is a second-order effect. The lack of demand for the goods covered by BDI (raw materials) dominates. Same with trucking rates. If no one wants the goods carried then no one wants the trucks, even if they can run more cheaply on lower priced fuel.