>>1539867Anon mentioned it briefly. You can only put so much power into a propeller before it stalls and produces no thrust. Since your prop must work from standstill to top speed you have a limited pitch that's going to work for all of these.
If you had variable pitched blades you would be able to push more power through a single blade.
If you make the blades longer then the tip speed increases so the rotational speed is limited. Also larger blades mean the centerline must be deeper in the water causing more drag and reducing clearance.
More motors generally means more acceleration and not more top speed.
