By: Bill Jennings
“The Prop on the Boat Goes Round and Round”
Propellers are the final gear in your boat’s driveline. Having the correct prop on your boat can make the difference between happy boating and costly floating. Because props come in a huge variety of shapes and sizes it can be a tricky task to identify the one that works best on your boat. Here are the steps that I recommend to both find your propeller and understand how it works.
Information gathering is the first step in finding your best prop. This includes the make, model and year of your boat and engine. Also note the make and size of your present prop (those are the two numbers found on the hub that are separated by an “X” sign). Then consider how you use your boat. Is it for family day trips, long cruises, or tow sports? Finally, record the maximum RPMs that you can achieve with your existing prop at wide open throttle.
For outboard motors and sterndrive applications, you must first choose a prop material- between aluminum, composite and stainless steel. Composites have come a long way recently and make a practical choice. They are also excellent as a spare prop because they are light and will not do much damage if carried loose in a boat locker. Aluminum props cost less and are standard on lower horsepower motors, but of course they damage easily. Prop shops everywhere make good money repairing damaged aluminum props, but here’s what they don’t tell you: when aluminum props are heated to hammer back into shape, the molecular structure is compromised. So while the prop looks good with its fresh paint, it is not as strong.
Because stainless prop blades are seven times stronger than aluminum blades, they do not have to be as thick. Thinner blades slice more easily through water, so the stainless prop has a clear performance edge. Blade “flex” is less with stainless, but while sometimes quoted as a problem, in truth it can be a slight benefit when accelerating. The objection I hear most about stainless props is that if you hit something you can damage your lower unit. While the obvious answer is don’t hit something, I would point out that most stainless props will chop through sand and bottom growth without damage and even if you bottom hard, the hub on a stainless prop will often just ‘spin.’ Some are even designed to hook up again after a spin, which enables you to keep going. For all these reasons I would recommend you look for a propeller made with stainless steel. The stainless you choose will come in either a dull ‘satin’ finish, or a high polish finish. Satin finish will cost less and perform just as well. Highly polished looks best but is more susceptible to theft.
Ordering the right size for your prop is more difficult. Prop sizes are identified by two numbers, usually etched into the hub. These numbers represent 'diameter' and 'pitch.’ Remember which is which by noting that in the alphabet, “d” (for diameter) comes before “p” (for pitch). Diameter is the distance in inches from the center of the hub to the tip of a blade, times two. Bigger boats call for larger blades and/or larger diameter. The second number, Pitch, is defined as the theoretical distance in inches the propeller will carve itself forward with one revolution. As horsepower increases, you increase the ability of your prop to power higher RPM’s, which will accommodate a higher pitch. For example, if you have a 19" pitch and want more top speed (or go further with every turn of the prop), move up to a 21" pitch or more. But since a propeller does not add horsepower, your engine must have the available power to still achieve the recommended RPM’s that you reached with the 19".
Looking closer at this example, let’s say your owner’s manual specifies 4,800 as the recommended maximum engine RPM (or 'red line'), and your present prop is labelled 15x19. Find some flat water and run your boat to full throttle. If your engine RPMs were to read 5,400, this is 600 RPM above the engine 'redline.’ This indicates that you have power to spare, and that extra power is overrevving your engine. You can convert this extra power into turning a propeller with a higher pitch, such as 21" (i.e., using a 15x21 prop.) Conversely, if when at full throttle the RPM reads below the redline, say 4,200 RPM, the engine is telling you it doesn't have the power to achieve its recommended RPM, so you should reduce workload by going to a less aggressive prop with a lower pitch, such as a 17" pitch (i.e., using a 15x17 prop). This procedure applies to all prop tests, but as you might expect, there are other factors to consider.
Larger propellers may also talk about their “DAR” or Diameter Area Ratio. If you look at your prop from behind, you can picture the circle that its tip cuts through the water. Some props have fat or numerous blades, that effectively fill in that circle by overlapping each other. Others have narrower or fewer blades and leave much of that circle open. The amount of space within the prop blade circle that the blades cover, is measured as a percentage and called "Diameter Area Ratio.” The higher the "DAR" number the more blade area is being used to move your boat. You can relate this to tires on a car. More blade area is like having wider tires. You get more traction, but you will need more horsepower to turn them. Heavy boats and boats used for tow sports logically work better with a higher DAR, or more blade area.
Most pleasure boats and all performance boats today can benefit from a curl, or lip on the trailing edge of the blade that is called a “Cup.” Imagine pitching a baseball and adding that flick of the wrist to increase the ball’s velocity. A cup on a prop blade both increases the velocity of water as it runs off the blade and focuses it into a tighter cone of water thrust. I would specify a cup on the prop you order.
One prop factor that few boaters consider is the “X-dimension.” This is the position of the propeller relative to the bottom of the boat and varies with the height of the motor. As your boat moves forward, the water leaving the stern of your boat will begin to rise back to the surface. If your propeller is located close to your transom, the water does not have much time to rise before being struck by the propeller. If your outboard is mounted on a set-back bracket, or your sterndrive has a spacer, the water will have a little more time to rise higher before reaching the prop. A set-back bracket allows you to mount your drive higher and still have sufficient water enter the prop to provide solid thrust. With a drive mounted higher, there are less parts of the lower unit in the water to create drag. This ability to change your drives X-dimension is good, so you may want to consider the position of the drive on your boat.
While almost all outboard and sterndrive propellers have a tubular hub designed to keep exhaust gases out of the blades for a solid water connection, there are some that intentionally have no hub and allow exhaust to flow through the blades. Props that allow air and gases into the blades have more slip and therefore less resistance. This permits a higher pitch prop which can translate to higher speed at the expense of all-around performance. Unless you are seriously racing, stay with standard hub prop types.
The final step in prop selection is testing. Even the best calculations can result in a disappointing performance. Anticipating this I would suggest you speak with your prop source about their prop exchange policy before buying. If you can’t find a local dealer with a trial program, call the professional help desk at PowerTechPropellers (PTProp.com). They will refer you to one, or service you directly.
If you are looking for a prop for a shaft drive inboard boat, such as a yacht, your selection process can be easier. Sometimes it is obvious when you need a new inboard prop, but not always. It may just have become brittle with age. A simple test is to flick your fingernail against the edge of a blade. A good inboard prop will return a “ping” sound, whereas a worn or brittle prop will just go “thunk.” When you are offered a material choice between Bronze and Nibral, I choose Nibral for its extra strength. With this decision and the make and model of your boat and engine, I would view the websites for the two giants in the industry, GeneralPropeller.com and PropShopInc.com. They will review the complex and custom choices for your boat and ship from their enormous inventory the following day. These shops also provide affordable propeller service options, such as “prop-scan.”
Of course there are other considerations to detailed prop selection, such as altitude, water temperature and new prop designs with hooped blades, but you may go blind trying to consider all of them. I believe that if you follow each of the above prop selection steps, you will be incredibly happy with the improved performance of your boat.
*Bill Jennings owned Canada Propeller for 10 years, distributing props nationwide.