If you’re running on the stock prop that came with your boat, trying a new prop may gain you some speed and/or efficiency.
Your propeller is a critical element of your fishing boat, the element that turns your engine’s power into thrust, thus moving your boat through the water – which is why props earned an entire section in our Spring Boat Checklist, and why ultimately the propeller you choose has a huge impact on your boat’s performance. Wait a sec – you did choose your prop, didn’t you? We certainly hope so, though we also doubt it. Unfortunately, the vast majority of the boaters out there get a boat with the stock prop or buy a used boat and never think twice about what, exactly, is spinning on the end of that shaft.
First, let’s make sure everyone has a basic understanding of exactly how a boat propeller works. The easiest way to get a grip on the concept is to picture a screw creeping forward as it turns through wood, converting rotational force into a pushing force. Boat props create forward thrust through the water in a similar way. Yeah, there’s a lot of technical jargon you can dwell on like pressure differentials and coefficients, but we don’t need to dive into all that stuff. Bottom line: the prop spins, and your boat goes. We do, however, need to define the important propeller variables that have a huge impact on how your boat performs.
Propeller Pitch and Diameter
Pitch and diameter are measurements. Diameter is simple: it’s merely a measurement of how large your propeller is. Pitch is a bit more confusing, because it’s a theoretical measurement. Think back to that screw running into the wood. With a single rotation, it will creep forward a specific distance. This, essentially, is the pitch measurement of a boat’s prop – how far forward it will move with a single rotation, measured in inches. The problem here is that water is not a solid. Now add in a bunch of variables like drag, ventilation, and cavitation, which create what’s called “slip.” Slip is the difference between a propeller’s theoretical pitch, and the distance it travels through the water in the real world.
When it comes to deciding which is the best prop for your specific boat, diameter doesn’t usually come into play. Since it’s much easier to find the ideal diameter for a rig, that’s usually predetermined by the manufacturer. Pitch, on the other hand, is a huge variable. As a general rule of thumb, changes of one inch of pitch will result in engine rpm changing, inversely, by about 200 (note: most props are available in two-inch increments). Go down an inch in pitch, and rpms will go up approximately by 200. Increase pitch by an inch, and engine rpm drops the same amount.
So, how do you know if your propeller’s pitch is correct? The easiest way to judge it is to take your boat out on the water, open the throttles all the way, trim up until you’re going as fast as possible, and then look at rpm. If it’s below or very low in the manufacturer’s recommended wide-open throttle range, dropping the pitch a bit will be beneficial. If it’s revving above the range, an increase in pitch is in order.
Why is pitch harder to predetermine than diameter?
Because there’s a huge difference between the boat that leaves the factory floor, and the one sitting in your slip right now. The manufacturer doesn’t know how much gear and people you’ll load aboard, if you’ll paint the bottom, or if you’ll add a tower. And changes like these will have a big impact on how easily the boat will get on plane and how fast the engine will be able to spin when you pour on the coals. You can usually get into the right neighborhood with help from things like the Quicksilver Prop Selector or the Yamaha Propeller Selector, but on-the-water fine-tuning is almost always beneficial.
In most cases when the average recreational boat owner is presented with a choice in propeller material, you’ll be picking between aluminum and stainless-steel. The benefits and deficits of each are straightforward and easy to understand: aluminum is less expensive, but the blades flex more and as a result the boat loses a couple-few MPH as compared to stainless-steel. Since stainless-steel is stronger it’s also more resistant to damage, but on the other hand, if you strike something hard with a prop made of this material, internal lower unit damage is more likely.
Number of Propeller Blades
The fewer blades a prop has the less drag it creates, but more blades also mean fewer vibrations and more bite. In the case of high-revving outboards three blades are usually ideal, but you may want to consider a four-bladed prop if your boat has trouble getting onto plane or struggles with a heavy load. As a general rule of thumb that extra blade will help with these problems significantly. The down-side? Going from a three-blade to a four-blade also usually results in clipping a couple-few mph off your boat’s top-end.
In the case of larger inboard diesel boats, four or more blades are common. These props are usually so specific to the rig, however, that choosing the ideal prop and blade number is best left to professionals.
Beyond these major variables, there are a few subtler differences that may come into play. Cupping, for example, adds a downward curve on the lip of the blade. It can increase a prop’s bite and assist with hole-shot, but it also can cause a drop of 100 to 150 rpm. Rake, the degree of the slant of the propeller blades as they leave the hub, is another to keep in mind. A more aggressive rake can affect lift, and in some cases improve bite in aerated water. Then, there are some (very) oddballs to consider: surface-piercing propellers, variable-pitch props, cleavers, and more, but these are outliers. On top of all that, now add into the mix twin-prop rigs like that found on the Suzuki DF350A.
Put all these variables together, and there’s a good chance your head is spinning just as fast as all those propellers. The good news? For the average boater, it’ll all boil down to pitch, material, and possibly number of blades. Play with these three factors, try several different props on your boat, and there’s a good chance you can boost your performance, efficiency, or both.
More boating tips from Lenny Rudow.