When I think back to my first year of racing boats, I am amazed of how naïve I was. While I spent a considerable amount of time fussing over minor details on the boat, I clearly missed three of the most critical elements needed to improve performance – run with the least amount of weight, use the highest horsepower and utilize the most appropriate propeller. While technology in boats and engines has improved dramatically since then, these three key factors have not changed. But family boaters are not likely to modify a new boat to reduce weight or buy a costly racing engine just to heighten speed. That leaves the propeller as the most important item to modify. Lets look at how to choose, understand and use propellers to increase performance for your pleasure boat.
Talk the Talk
Learning basic propeller language is a prerequisite for finding the right prop. Understanding the terminology is really very simple, plus it’s a lot of fun to impress your friends by dropping a few terms like “rake” and “diameter area ratio”.
Propeller sizes are identified with two numbers. The first number is the diameter. If your prop has two or four blades, just measure the distance from one blade tip to the opposite blade tip. If your prop has three or five blades, measure the distance from the centre of the hub to the tip of the blade, then multiply that number by two. The second number, or pitch, is the theoretical distance (in inches) that a prop moves forward in one full revolution. So, if you have a prop with a diameter of 14 inches and a pitch of 21 inches, the prop configuration would be: “14×21”.
The center part of the propeller is called the “hub”. This is the part that centers the prop on the drive shaft. On propellers where the engine exhaust flows through the propeller, as is the case with most of today’s outboards and sterndrives, there is a barrel around the hub where the blades are attached. To learn more about prop terminology, read on.
How props work
Propeller blades push water in one direction and the boat moves in the opposite direction (“For every action there is an equal and opposite reaction”). As the blades spin and push water back, they also create a vacuum on the forward side of each blade. This vacuum is so strong that the extreme low-pressure area created can explode surface air bubbles with a force that can chip the paint off the prop. This occurs when air is introduced through either cavitation or ventilation. The difference between cavitation and ventilation is often misunderstood. Ventilation occurs when air bubbles from the bottom of the boat or transom find their way into the propeller and vent it. It can also occur when a prop is pulled in by surfacing blade tips.
Cavitation occurs when the propeller spins on its own accord (like a car tire on a slippery road) and produces air on the forward side of the blade. The unmistakable symptom of ventilation and cavitation is rapid over-revving. To correct this, you must reduce throttle until the prop reconnects with the water.
What a difference shape makes
Blades can be shaped in several different ways. The most common shape is the standard round ear or elliptical blade. These props deliver an optimum balance of thrust and speed.
Other propellers are tapered, which are designed to provide less drag and are usually more high-speed oriented. Props are also available with what is called a cup, a lip featured on the trailing edge of the blade. These cups help flick water off the blade, improve thrust and grip the water to reduce slipping – the amount of ineffectual spinning usually measured by a percentage. For example, if a prop with a 25-inch pitch is rotated four complete times it should (in theory) move the boat forward 100 inches. In practice, it may only move 90 inches, which would indicate the prop has a 10 per cent slip factor.
If a blade sticks straight out of the hub, or even perpendicular, the prop has zero rake. Blades can be configured with zero rake to offer optimum stern lift for boats that squat too much when running. If the blade leans back towards the trailing edge of the prop, it has rake. If it leans way back, then it’s called a high rake prop. Such rake can be measured in degrees and as a rule of thumb, the higher degree of rake, the greater the bow lift.
Cleavers or semi-cleaver props can be identified by their straight trailing edge. This shape keeps the water thrust low, allowing the blade tips to run at the surface without ventilating. The reduced drag of surfacing props allows them to turn higher rpm with the given amount of horsepower.
A prop with blades that sweep in a curve and follow its rotation is called a skew. This shape is ideal for running through thick underwater growth, as they are less likely to tangle in weeds.
Bronze, aluminum and stainless steel props
Early propellers were made of bronze and are still used today with hundreds of different sizes available to accommodate the wide variety of inboard applications. Over the last few years, nickel has become a popular additive for strength and where used, these props are called Nibral. Keep in mind, inboard props are very specialized and not always easy to find in the middle of a busy boating season.
For outboards and sterndrives prop manufactures commonly use aluminum since they are cheaper, faster (than bronze) and lighter. Recent modification, upgrades to aluminum designs, and manufacturing techniques have resulted in some excellent results when comparing performance to cost. As a result, most small boats still come standard with aluminum propellers.
Marine-grade stainless steel has become the preferred choice where strength and performance are mandatory. Since marine stainless steel is seven times stronger than aluminum, manufacturers are able to design their props thinner without sacrificing stiffness and strength. Incidentally, unless you’re running a solid hub-racing prop, stainless is not as damaging to your drives when you hit something. This is primarily due to the rubber hubs that spin or break if you do hit something, like their aluminum counterparts. Two styles of stainless propellers are available, polished and (the not so polished) satin-finished. Contrary to popular belief, there is no performance gain in polishing your prop. Stainless props are also twice the amount of aluminum props.
The most recent material used to make propellers is high tech composites. With today’s advanced resins, nylon and carbon fibers, we also see extensive use of composites used throughout the marine industry. In addition to having greater strength than aluminum, composite propellers won’t corrode or blister and come with lifetime hub warranties and even replaceable blades. Most are also priced very close to aluminum.
How many blades do I need?
One of the most frequent questions I am asked is “Do I need to use a three- or four-blade prop?” While there is no fixed rule, this explanation will help. As you increase blade size or add blades, you increase what is called the diameter area ratio. While more blade area adds to surface area that is pushing your boat, it also creates drag. Think of wider tires on a car and you’ll have a good comparison. Since blades create drag, your starting point should be as few blades as possible (with the minimum of course being two).
Over the past several years horsepower in boats has increased and boat manufacturers have created new ways to reduce wetted surface drag by using lighter materials, composites and incorporating “steps” in the bottom of hulls. These techniques opened the door for four bladed propellers.
If your boat and horsepower can handle a four-bladed prop, you’ll enjoy several benefits. A four-bladed prop has the same number of blades pointing up as it does down, providing smoother operation, quicker acceleration from idle, slower minimum on-plane speeds and even fuel savings while traveling at cruising rpm levels. Several boaters have switched to four-bladed props for this feature alone. Keep in mind, top speed will generally not increase and can even decrease slightly.
So, from my experience, the bottom line on the optimum number of blades is this: Boats over 23 feet such as light cruisers, will generally perform best with a four-bladed prop. Aside from these situations, stay with a three-bladed prop and save your money.
In the second installment of this feature we’ll take a look at a comprehensive test using propellers from many different manufacturers and report how each manufacturer faired. We’ll also provide a step-by-step guide of how you can calculate the best size of prop for your boat.