How to choose the right prop
When it comes to the selection of a propeller for your rig one must take many factors into consideration.
Fisher FXDV18I have had dozens of people come to me and request a new prop for their boat motor. However, choosing the right prop is not always that easy. There are a great deal of factors that influence the proper propeller for your rig. First, it’s important to understand some basic propeller terminology. The overall diameter of the propeller is determined by the projected RPM’s and the power at the prop shaft. Basically the overall propeller diameter will increase as the need for power increases. A great example of this is a slow vessel such as a pontoon boat, which typically requires a rather large diameter prop. On the other hand if you look at a walleye rig such as my Fisher FXDV18 and 200hp OPTIMAX, the need for greater RPM’s will require a smaller diameter propeller.
The other important factor to the overall propeller choice is its pitch. Basically the propellers pitch is the distance that a propeller would move your boat in one revolution. The 14×21 propeller has a diameter of 14 inches and a pitch of 21 inches. This prop will move forward 21 inches in one revolution (theoretically). Pitch is also broken down into a constant and a progressive pitch. The constant pitch means that you will have a same pitch from the leading edge to the trailing edge of the prop. Progressive pitch begins at a low pitch on the leading edge and increases to the trailing edge. The pitch rating for this type propeller is the average pitch over the entire blade. Progressive pitch is very common in props designed for mid to higher horsepower engines.
The selection of a propeller for most boaters is a compromise. One must select a propeller, which will offer a reasonable hole shot (the ability to get the boat on plane) and high-speed operation. For instance, if your rig is primarily used for skiing you will give up the top end speed for the low-end torque (faster on plane). Although, this type of setup requires the very close monitoring of the tachometer to avoid going over the manufactures recommend RPM’s. Great damage and stress can be placed on your engine and its gears if too small of a pitch is used to get better hole-shot. On the other hand if you select a propeller with a too high of pitch you will lug the engine at low RPM’s (below the manufactures recommended range) which will also be damaging.
Knowing all that about the basic principles of propeller choice, one must consider the type that best suits their needs. There are a large variety propellers on the market from stainless steel, aluminum and composite construction just to name a few. For the last 20 years I have ran stainless steel propellers on all of my main engines. On the other hand my kicker propellers are constructed from aluminum. Each propeller construction has its advantages and disadvantages.
The aluminum propeller offers a much lower price point compared to the stainless version. This version can be repaired a cost typically of half the original cost. When underwater obstruction is hit with the aluminum propeller the propeller will absorb a great deal of the impact thus typically not harming the internal gears of your engine. The disadvantage to the aluminum propeller is its inability to hold a true pitch. Aluminum tends to “flex” under high torque. This is much more evident in the mid to higher horsepower ranges. For smaller horsepower engines however, the aluminum propeller is the ticket.
Now let’s take a look at those stainless steel propellers with 3, 4 or as many as 5 blades found on many mid to higher horsepower engines. Stainless steel propellers Stainless Steel Prop offer the greatest selection of pitches, blade design, blade numbers and ventilation variations. The stainless props will typically hold their true design specs from the low end to the high-end operation (no “flexing”). This is very important in conditions such as high winds and rough seas, because, believe it or not, your propeller will have the greatest impact on your hulls performance in those conditions. I have had the opportunity to visit the world leading propeller-testing grounds at Mercury Marine. The true propeller test is to run dozens of different props with various pitches and diameters in the same conditions with one constant (the same boat and motor). It is truly amazing the difference in the overall boat handing with the various propellers. Please take note however, not all manufactures of stainless steel props adhere to as strict a set of standards as others. Therefore, when you’re going to lay down your hard-earned cash for a new prop, consider a quality brand such as Mercury Marine.
When you’re looking to test props in order to find the right one for your boat and motor, always, as in any motor operation, follow the manufactures recommended RPM ratings. The tachometer on your rig is a vital indicator of the overall performance of your motor. Know the recommended RPM rating for your engine, and find a prop that gives you good hole shot, as well as a good top-end speed, while staying within your engine’s recommended RPM range.
New technology in the design of stainless steel props can also offer other features that positively affect your boats performance, such as Mercury’s Tundra Running PERFORMANCE VENT SYSYEM (PVS) and its FLO-TORQ II universal hub system. The PVS system offers the ability to tune the venting of your props blades for optimal performance. By simply removing/changing/adding the PVS rubber venting plugs you can fine-tune your rig’s optimum performance. The other major prop innovation, FLO-TORQ II, is a system designed to resist any slippage and cushion the lower drive train during any sudden underwater collision. This system also has the advantage of allowing owners of other manufacture’s engines the ability to use Mercury performance propellers on their engines. This is done by simply installing a hub that will adapt to the other manufacture’s prop shaft.
I hope the above will offer some insight to the propeller operation and makeup. For any further information regarding Mercury propellers or selecting the right prop for your boat and motor, you can go to mercurymarine.com and click on the propeller selection icon.