Ask The Experts | Propeller Selection Basics
Article courtesy of Yamaha Motors
Propeller Selection Basics
No smoke and mirrors or high-level math – we promise
Choosing the best propeller for your boat isn’t nearly as complicated as it may seem. A boat’s weight, its hull style, how much horsepower is on the transom, and your performance goals (slow trolling, pulling kids on tubes, being the first to reach a hot fishing spot in a tournament, fuel conservation, etc.) influence the type and size of propeller for a given boat.
Just a Couple Questions
Propeller selection boils down to a few simple questions. The first question flags the props that could be suitable for your application and guides you toward appropriate propeller series (or “families”). The second question further narrows the field to specific types, and potential size ranges of propellers that could do the job. OK, here we go.
What Kind of Boat do You Have?
- Large boat (27' and longer) = heavy boat
- Multiple engines
- Express cruisers, walkarounds, deep-V center consoles (hardcore offshore fishing type)
- Medium Boat (20'-26' long) = middle-weight boat
- Single or twin engines
- Multi-species fishing boats, some center consoles, walk-around cuddy cabins, day cruisers
- Small boat (less than 20' long) = mid-weight to light boat
- Single engine
- Modified V hulls typically used for fishing, boats using tiller handle steering, and jon boats
- Single engine
- Fish 'n' skis, bowriders, runabouts, deck boats, and pontoons
- Bass/Bay/Flats boats = mid-weight to light boat
- Single engine
- High performance hull design
- Mission-specific machine for fishing in these specialized environments
Family and watersports boats = typically a middle-weight boat
How Do You Use Your Boat?
The way you use your boat plays a major role in which propeller will help you make the most of your time on the water. In other words, what do you want—or need—the most in terms of your boat’s performance?
- Acceleration/pulling power – Flats boats need to plane immediately; watersports enthusiasts crave the rush of a consistently strong pull; big boats require pushing power and torque to get over the hump and underway.
- Mid-range cruising/fuel economy – Owners of larger boats with big engines are usually concerned with going as far as possible on a gallon of fuel, so this an important part of their boating experience.
- Top Speed – Bass boats, as well as many flats boats, are obsessed with top speed; getting to the fish first is very important to these captains.
- Good overall performance – The propeller lets the boat do what it’s supposed to do: gets up on plane and stays on plane well, accelerates respectably, cruises comfortably and economically, and attains a decent full-throttle speed; all with an average-to-full load for your type of boat.
Great overall performance appears to be the logical choice, and is quite doable for boaters who own general-purpose boats and just want to have fun on the water.
However, for those who consider themselves more-than-casual boaters, propeller selection is more often than not a compromise. Rarely can you find a prop that will deliver intense acceleration and blistering top speed – you can have one or the other, but seldom both.
A Few Propeller Insights
1. A propeller’s "blade surface area" directly affects a boat's acceleration. For most props, achieving more blade surface area is done by increasing the propeller's diameter. Taking a bit of literary license, diameter carries the weight of the boat, and creates “push” by muscling huge volumes of water to transform the vessel from a static into a moving object. Large diameter propellers generate more pushing power – thrust – than smaller diameter props.
Heavy boats regularly use large diameter wheels to get on plane and stay on plane at midrange cruising speeds; middleweight and light boats tend to run props with less diameter in pursuit of ideal performance.
2. A propeller’s pitch relates directly to a boat’s speed. The higher pitch propeller an engine can turn (while maintaining the outboard’s proper wide-open throttle WOT rpm parameters), the faster the boat will go. Pitch is indicated in inches. The pitch number relates to the theoretical distance that propeller would travel forward in water in one revolution.
On most pleasure boats, a low pitch propeller may make the boat jump on plane and accelerate well, but could either exceed the engine’s maximum rpm and/or not be particularly fast at full throttle.
Conversely, a high pitch prop can be slow to plane and have lackluster acceleration; however, the big wheel might throw down some impressive top speeds (if the engine has the power to run within its max WOT rpm range). On paper, pitch=speed.
What About Four-Blade Propellers?
Remember “blade surface area” we just mentioned? Nearly all four-blade propellers have more blade area than three bladed props. Therefore, on most boats, four-blade props accelerate better than comparable three-blade wheels, because the fourth blade adds more area for better “grip.” This makes a four-blader an excellent choice when you’re looking for better acceleration and need to stay hooked up in tight turns and rough water – especially in following seas.
Four blade propellers use additional horsepower to do all these wonderful things, affecting fuel economy (better on some boats, less on others), and are usually slightly slower on the big end. For certain applications, the benefits can offset the potential drawbacks.
Any boat’s performance (or lack thereof) is directly related to its gross weight (completely loaded, including full fuel tanks, live wells, gear – everything) and the amount of horsepower available.
This is important: A big, heavy boat with a little engine has a low power-to-weight-ratio; a light boat with a big engine has a high power-to-weight-ratio.
In the real world, boats vary from one extreme to the other – and this is a major factor in the propeller selection process. That’s why you’ll be asked what kind of boat you have and how much power is on the transom – it’s a tactful way to determine if your boat has the intestinal fortitude for an appreciable performance improvement.
Thankfully, there has never been a better selection of propellers to meet almost any boater’s performance goals (within reason), on nearly every boat, no matter what the weight or horsepower.
[EDITOR’S NOTE] Find propellers for Yamaha or other brands of outboards or stern drives at iboats.com.
Article courtesy of Yamaha motors. For additional information on Yamaha boating, visit yamaha-motor.com.