The diameter and pitch are the fundamental characteristics of a propeller that determine its size. They are in any case the ''identity'' of propeller and are usually stamped on its hub.
On the hub of many propellers for example the following numbers 16΄΄ X 23΄΄ are distinguished and are measured in inches. The first number refers to diameter and the second one to the pitch of the propeller.
As a diameter of a propeller is defined the diameter of the circle that the blade tips make when the propeller is rotating.
This diameter is very easy to be determined if, by observing the propeller, we measure the distance, multiplied two times, from the center of the hub to the edge of the blade, which corresponds to the radius of the circle made by the propeller when rotated.
Let's now think how the blades of a propeller work:
- They entrap a certain amount of water and push it rearward.
- The larger the surface of the blades is the more water is pushed back.
- The more water they push astern of the hull, the greater the thrust generated is and hence the greater the boat propulsion is.
Therefore, the larger a diameter is the thrust greater gets and in most cases the prop's efficiency too.
What a large diameter propeller offers to us:
- It gets the boat on plane more easily and more quickly because the large diameter overcomes easier the resistance from the boat and loads.
- It keeps the boat on plane at a lower rpm, compared with a smaller diameter propeller, which is very important when traveling in rough sea.
- We travel better on heavy seas, because the large surface of the propeller's blades overcomes the additional resistance from the big waves and strong wind, feeling at the same time that we have more torque and more driving control in bad weather. Imagine how much we need the great thrust at low speeds when the boat goes up in a wave-mountain.
- We travel faster at low and medium engine's rpm, because the large diameter propeller has less slippage in these revolutions related to a smaller diameter. We have higher cruise speeds at low and medium rpm.
- We have lower fuel consumption and therefore more fuel autonomy at low and medium speeds in most cases. In this range of speeds the engine usually operates at 90%.
- Our boat responds more efficiently to maneuvers in confined spaces (ports), because we have great thrust at low rpm.
On the other hand, a large diameter propeller:
- It increases the load on the engine and on the shaft. It has a greater resistance during its rotation in the water, due to the larger blades. This means that it is harder for the engine to rotate it.
- It has greater hydrodynamic resistances at high speeds.
- It reduces our final speed, since the drag at W.O.T. is further increasing. This is because the large-diameter propeller blades have large surface, while in addition, the gear case of the lower unit is bigger so as to fit the larger gear which is necessary to support the great torque that the large-diameter propeller demands to be able to rotate.
As a general conclusion, the large diameter propeller is more efficient at low and medium rpm and it will get the boat faster at high speed, but from that point and after its performance is limited concerning the achievement of maximum top speed.
When our goal is to achieve high speeds –over 40 knots or so- it has less efficiency because more surface of large diameter propeller is wet and therefore causes excessive drag.
Generally, deep-V boats with large hull resistance or overloaded boats will perform better with large diameter propellers. Obviously in these cases the top speed is not among our goals.
In contrary to that, boats with low resistance hulls that are certainly not intended for heavy loads will perform the same well with smaller diameter propellers, thus gaining in speeds. This is due to the reduced drag from both the smaller hydrodynamic resistances of the smaller surface of blades and the smaller gear case of the engine's lower unit.
In other words, if we are not interested in high speeds and our goal is to cruise at medium rpm or we usually have our boat loaded, then the use of a large diameter propeller is the best choice. On the other hand, if we are interested in high speeds the use of a smaller diameter propeller is the best choice.
For all-around handling and heavy-weather performance its better to use a large diameter propeller and we must accept the reduced top speed caused by its great drag at full throttle.
If we fit a larger diameter propeller on the same boat with the same engine we will have more thrust but we will have less top speed at WOT.