cavitation, determines the effectiveness of a propeller at a chosen speed of rotation... in effect if the propeller angle of attack is too severe, then one negates the action of turning it faster.. because vacuum bubbles form behind its rear faces - those vacuum bubbles - caused by the pitch of the prop and its rotational speed are - because they are a vacuum - the surrounding water has not had time to fill, are lighter than the water, since they contain nothing, so - they draw the water up toward the surface... because the water surrounds them.. in doing so. they negate the whole purpose of the prop, which is to push water out behind it in a horizontal plane, not a vertical or diffused one.. So the pitch of the prop blades and the speed they turn is important... there is an optimum based on the effort needed to turn the prop at a chosen speed, and its effect on cavitation at that speed.. i.e being able to turn at any speed does not guarantee efficiency... one wastes effort for no benefit.. and that is expensive. so the attack angles ( the pitch, the means of fuel supply and labour to make it possible, the load one wants to push along, and the speed one wants to achieve) , reach an optimum based on all those things.. its a mathematical computation and has a desired median based on input, and output.. for example, in a car, you can go very fast.. but your fuel economy goes down, and your cost goes up... in the Titanic the median really didnt have time to be known for certain.. even though some of the maths had been done.. they were still trying 3 and 4 bladed props.. they were still discovering how much fuel of the right type was needed, how many folks to use to deliver it... and what that all meant to cost and profit depending on load..