I have a sandwich / foam cored constructed hull. Will the system be effective?

Yes. The system does work on double skin GRP or cored hulls, for these hulls you would just need to remove a small section of the inner skin (liner) or core to accommodate the transducer allowing it to make contact with the outer skin of the hull. For cored hulls, the area will need to be filled and sealed solidly with laminate and epoxy resins before mounting the transducer.