Technical Propeller Info

Sea trials
determine performance

A fair amount has been written about marine propeller interaction on vessels of varying design. Nevertheless, in practice it becomes apparent that misinformation, incorrect assumptions and overlooked variables are rather common. This alone should warrant reservations in dealing with theoretical performance predictions. Though important for calculation purposes, such predictions have limitations and can be misleading or misinterpreted. No two vessels are exactly alike. Therefore, over-generalizing performance expectations carries with it a good deal of risk. Add to this the fact that no two hand-made/hand-finished props are the same, and things can become even more confusing (another good reason for using Acme fully CNC'd Propellers).

The surest way to determine the best propeller for a particular vessel is through sea trials. Again, it should be emphasized that during sea trials — whether it be testing a small ski boat on an inland lake or a large sport fishing yacht on the open ocean — numerous variables do influence the outcome. And since most vessels draw power from internal combustion engines, reliable engine performance data is critical to final analysis. In addition, internal combustion engines operate on an air/fuel mixture, which means air temperature, humidity and elevation also impact the final results. Some other factors that affect the outcome of sea trials are vessel load and distribution, appendages (i.e., rudders, struts, nozzles, shafts and towers, etc.), shaft angle, and wetted surface area of the hull, etc.. For sea trials to be valuable and conclusive it is vital to isolate variables, and to maintain as much consistency as possible in surrounding factors.

The Propeller

Figure 1.
Propeller anatomy looking
from aft (behind boat)

click on image to enlarge

Figure 2.
Propeller blade section cut

click on image to enlarge
 
Figure 3.
Blade terminology

click on image to enlarge

Figure 1 illustrates some common prop terminology. The prop shown is left rotation (L or LH), because in viewing from behind the boat the leading edge of the 12 o'clock blade (or top blade) is left of the trailing edge. This prop must rotate counter-clockwise, or to the left, in order to propel the boat forward.

Figure 2 illustrates a prop blade section (halfway between the root of the blade and the tip of the blade). Notice the difference in shape between the top and bottom of the section. The bottom side has a more pronounced curvature. (See figure 3 for a detailed view of this type of section.)

During forward operation the curvature in the suction face of each blade creates a low-pressure area in front of the prop, inducing "lift", much like the wing on an airplane. Of course, the resulting “lift” in most boat prop applications is generally perceived in horizontal terms, not vertical.

For illustration purposes imagine a prop moving through water as a screw moves through a piece of wood. The amount of forward or lateral travel depends largely on the pitch of the prop blade(s). Pitch is defined as the theoretical distance the prop (or screw) travels in a single revolution assuming zero slip.