While an inboard boat prop performs a simple task, propelling your boat through the water, there are quite a few considerations to keep in mind when choosing the proper propeller for you particular boat and engine. Using the wrong propeller can be inefficient and waste gas as well as causing damage to your engine over time. Plus, you won't get the full enjoyment from your boat. The following is a basic guide to choosing the right inboard boat props for your boat and type of usage.
How Many Blades
Inboard boat props can come with 2, 3, 4 or even 5 blades and the general rule of thumb is the more blade surface area you put against the water the more thrust you develop. While simple in theory, it isn't really that simple in practice. In general though most pleasure boats will use a 3 blade prop, although once horsepower goes up a 4 blade prop can sometimes give a smoother ride and higher plane. It is really a function of your boats hull design and needs to be tested to see which you prefer. A 4 bladed prop will also help with slipping problems especially in rough water. 5 bladed inboard boat props are generally only used by trawlers, tug boats and workboats and then only when 4 bladed props give too much vibration.
Same Prop, Different Uses
Many boaters use their boats for a variety of things. Fishing, cruising, water skiing and trawling are just a few of the uses for a pleasure boat. If this is the case for you then you will have to make a decision on whether you want a prop that will give you more pulling power or a prop that will give you better speed. In general the pitch will control this with a higher pitch giving more speed and a lower pitch giving more power and acceleration. You will have to decide which is most important to you and choose your prop accordingly or else switch inboard boat props based on your intended usage. One thing you can do if you usually prefer power, but occasionally need speed is to have your blades cupped, which will increase your top speed by 3mph or so.
What Diameter Prop
Inboard boat props should have a clearance of at least 15% of the diameter of the prop between the prop tips and the bottom hull of the boat and ideally this will be 20% or more. If your inboard boat prop diameter is too large then you can increase the pitch to compensate for the smaller diameter as long as the difference is no greater than 1 or 2 inches. It can also help in some instances to switch from a 3 blade to a 4 blade propeller to put more blade area against the water and increase control.
As you can see there are many different considerations to keep in mind when choosing the proper inboard boat props. Using the guidelines can help you get good performance, but there is always room for improvement and the only way to know for sure if an increase in pitch will help or if you like the feel of 4 blades versus 3 is to test it out for yourself. If in doubt find a good inboard boat prop specialist and ask for guidance. It may take some time, but while testing different inboard boat props you will also learn more about your boat, how it handles and your own preferences when boating.