To go 20-25mph, I dont think the z-drvie or outboard will do it. In fact theyre probably pretty close in performance. The strudder setups probably have more potential, however 20-25mph is pretty fast. I would think you''d get roughly 15mph with either the ob or z-drive, it''s just a guess, but I havent seen a "fast" stock outboard yet. You''re really limited by the combination of 20" long and scale. To go quick and that small, you''d need ideally a surface drive mono(strut and rudder), a hydro, or a rigger. The rigger wouldnt work for your project, since you could just make one out of wood a lot easier, a hydro would just be tough to shape without experience. I''d either go with a surface drive or sacrifice some speed for the OB or z-drive. A strudder would work but I''m not too hot on them.