EE go for it lad. I made brass circle propellors many (35?) years ago when I converted the Titan tug to (Mamod) steam. I just soft soldered the disc on to a brass shaft that happened to fit in the prop tube. With more power, silver solder is the way to go, and if you can cast aluminium I'm sure you can silver solder. You could start with a brass hexagon nut for the boss, which will give you three (or six!) flats to hacksaw slots for the blades in. Cut from corney to corner of the flats which should give a root pitch of about 45 derrees and bend until the motor is revving right. An aluminium hull sounds fun. I think there was a Nigerian pusher tug called "Conakry" - it might even have been a Vic Smeed plan. It had a square bow and stern, quite like the tug in your avatar and the one in the picture on this thread. I've never tried the Halfords ally welding stuff, but if the boat is quite large I'd fold a flange on the sides, and pop rivet the bottom on, with some car underseal or other black akky spread on to seal the joint. You should be able to borrow a pop riveter and scrounge a couple of handfuls of rivets. Remember that the bow angle can be a lot steeper than the stern, cod head and mackerel tail pushes the water out of the way of the boat and lets it back onto the props and rudders. Make a couple of card or paper models first - you could always put them up on the site and we'll all throw helpful or otherwise suggestions!!
Good luck.