Aluminum-water reaction based ramjet propulsion system shows higher theoretical performance when compared to the conventional solid propellant based propulsion system for high-speed underwater applications. In the present study, theoretical performance calculations and also the primary combustion characteristics of the water ramjet have been studied using window bomb test facility. The aluminized solid fuel-rich propellant compositions contain 40-50% of aluminum with the particle sizes of 110 nm, 250 nm and 18 µm which also comprises of 34.5-46% of Ammonium Perchlorate (AP) in a total of 15 formulations in the 0.2–3.1 MPa pressure range are considered in this study. Paraffin wax and HTPB based binders are used for the preparation of nano and micro aluminized propellants respectively. In comparison with Mg, Al-based water ramjet shows higher theoretical specific impulse for Rwf greater than 0.5. The stoichiometry of Al-based water ramjet occurs at Rwf of 0.5 and it shifts towards a slightly higher value of Rwf when the Al content increases to 50%. Combustion of 50% aluminized propellant compositions causes more than 40-50% of initial mass as residue which mainly contains unreacted aluminum and other condensed phase combustion products. Nano aluminized propellant compositions show 2-5 times the higher burning rate than the micro aluminized counterparts in the CO2 atmosphere. These propellants exhibit a wide range of pressure indices (n) from 0.23 to 0.56. © 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.