Horse power actually means very little when it comes to how much power an engine kicks out. By definition, one HP is how much force it takes to lift 33,000 pounds one foot in one minute.
Torque, on the other hand, is a measurement of force in a twisting or rotating motion. Since you want to spin a prop, the amount of torque is more important than how many HP you have. That is one reason aircraft engines don't turn the same amount of RPM that a boat or buggy engine does(not counting DF types in this), it takes more torque to spin an airplane prop than it does to spin a boat prop while a buggy engine turns the input on an reduction gearbox that multiplies torque while reducing RPM to usable levels. A second reason is that you can spin a prop too fast and it will either fail or it will go supersonic and stop giving thrust, but I digress here. Thus, if you compare three .21 sized engines, you will find the following:
Aircraft 21s will give you around 17KRPM
Boat 21s will give you around 35KRPM
Buggy 21s will give you around 50KRPM
Just for the record, these are approximates, with no load on them and based on manufacturer's literature
Last edited by Hydro Junkie; 02-16-2020 at 01:57 PM.