John......have you considered the hull dimensions?
For example, a model Narrow Boat hull [with blunt ends] 1000 mm long x 100 mm wide and having a draft of 25 mm....will provide [1000*100*25] = a displacement of 2.5 kg
...[or say 2.3 kg with a semi pointy bow & a rounded stern]
This means the complete hull + superstructure + engine power system + R/C gear cannot exceed this same 2.5 kg given the hull and draft dimensions
However lets think of displacement as being cubic or 3 dimensional...... so a hull twice the size [2000*200*50] will displace 20 kg
So from this, it would appear that the 1000 mm long hull could be built, but would have difficulty in maintaining buoyancy [if fitted with engine & R/C gear], however the 2000 mm long hull could support a far greater mass at the expense of transporting a longer model
Either way, I don't think you would need a drop down keel of a great amount of ballast.......we must also remember all vessels of quasi dimensions need a low centre of gravity [metacentric balance]
Good luck
......Derek