Apart from the diameter and the pitch, also the number of the blades of a propeller, the designing interferences of the blades, as well as other variations such as the thickness of the blades, the weight of the propeller, the material are made of etc. have their own share of "responsibility" in performance of a propeller.
Let's see what the Cup is and which its role is.
A widely used technique for improving the efficiency of the propeller, and therefore our boat, is the cup's configuration to the propeller.
It is about a curvature that is running along the trailing edge of the blade, and often extends across the blade tip. The concave surface of this curvature faces towards the side of the blade which "bites" and accelerates the water rearward.
The size of this curvature varies from propeller to propeller and is usually in the range of ½ to ¾ of an inch.
This curved lip on the trailing edge of the blade improves the performance of a propeller, because it allows it to get a better "bite" on the water whence all the advantages derive.
When we change a propeller without cup to a cupped one (both with the same design and all the other features), the second one behaves as if it has a higher pitch than the first. This is due to the fact that the area the cup is added has the effect of increasing the angle of the 'pitch' which implies an increase in pitch.
The second one gives us higher speed than its nominal pitch because of the cup and therefore "works" as a longer pitch propeller.
The application of the cup in a propeller increases its pitch by 1 or 2 inches. The degree of the pitch's increase depends on how intense (more concave) the curvature of the cup is, to which height it will reach, what area will occupy, and of course, how well-designed it is.
However, the cup of the propeller will increase the load applied to our engine and it is therefore reasonable to expect a drop in rpm of about 150-400, depending on the size of the cup.
The advantages for the application of cup do not stop here.
Cup helps the propeller to have a better bite in the water and therefore better grip at high angles of trim and turns.
Because the cup is applied to the trailing edge of the blade (the edge from which the water leaves the propeller) the propeller holds more water to the surface of its blades, which reduces its ventilation and slipping, thereby allowing higher angles of trim and higher engine mountings.
Our propeller therefore is able to run closer to the sea surface reducing the drag at high speeds.
This is another reason, apart from the "increase" of the nominal pitch, that cup will give us higher final speed.
When the cup is added to the trailing edge of the blade it increases the pitch of the propeller, and when it is added to the blade tip it increases its rake.
Accordingly, when the cup is added along the trailing edge of the blade and extends to the blade tip it will result in the increase both of the pitch and the rake of the propeller.
Another primary purpose of the cup is to "control" the propeller's cavitation. This slight interference to the trailing edge of the blade (cup) changes the pressure distribution along the blade increasing the buoyancy in front of the trailing edge.
This change in buoyancy is very helpful and especially "critical" in controlling and limiting the phenomenon of cavitation.