AciD2Hash

You get two different water jet propulsion systems. The one that you find ina jet ski engine which basically sucks water in and then shoots it out the back at a high rate. The magneto hydrojet is different. It has moving parts but does not have a motor that turns it and makes the humming sound ur throb of an throb of prop. The sub will have two "tunnels" on either side. These tunnels will have props lined in a row all along the "tunnel". These props will not be driven by an engine but by charged coils. The coils will be coiled around the tunnel. The props will be magnetised. When a current passes though the coils the props will start to turn because of the magnets. A magnetic driven prop will increase speed very quickly and because there is no turbine or moving parts no sound will be emmited. The only niose that might admit will be from the electric charge but that can be easily eliminated by good insulation. Look a magnetic hydrojet is very simple and basic physics. Just think of it logically

You can through a layer of those little grey stones as a bed under the heat source coz it reflects heat. It should help. hope it does. Maybe if you can explain how ur furnace looks like and can get ideas.