CHOOSING & TUNING PROPS
Saltwater Magazine

Propellers may be the most important - yet least understood - component in an inboard boat's drive train. When you hear boat owners complain of poor performance, the props almost always get the blame. Given that general assumption, what can you do to make your props perform at peak efficiency?  Actually, quite a lot, but the first step is choosing your props wisely.

Your first choice is material - bronze or Nibral. Bronze propellers provide outstanding general service, while Nibral is simply a high-strength bronze alloy made of nickel, bronze, and aluminum. It provides greater strength and durability than plain bronze props, but costs roughly twice as much. Still, you may need Nibral if you have high-torque, high horsepower diesel engines, and many new boats come standard with Nibral props these days. Performance with either bronze or Nibral props should be about the same.

Whether you are buying boat or just want to tweak the performance of your existing boat, it is important to know how efficiently the boat runs. Boats are propped so that the props, or "wheels," carry enough pitch to allow the engine or engines to reach their maximum operating rpm full throttle. Pitch is the theoretical distance in inches that a propeller would move forward in one revolution if there was no slip, like a screw in wood.

Having the right propeller(s) can make a huge difference in your boat's performance, whether you seek extra top-end speed or greater fuel efficiency.

But with our apparent obsession with speed, boats are often propped to reach high speeds at full throttle while they are very lightly loaded.  Also, a brand-new boat that has fresh engines, clean fuel filters, a slick bottom, and which hasn't been burdened with a ton or more of gear and a tower may turn the right number of revs right off the showroom floor - but what about several years down the road?

Use the Formula

Bill Knowles of the Knowles Boat Company, (561) 286-5663, is a boat builder and marine consultant in Stuart, Florida, who sees a great deal of frustration among his clients regarding props. "Many people come to us with performance problems that are directly related to the boat's propellers," Knowles says.  "We help people establish whether they have the right wheels under the boat by first determining if the wheels, and the engines, are overloaded. Many boats simply don't have enough wheel under them."

Knowles evaluates the props through the use of simple formula. He divides the boat's known displacement (weight) by the total amount of propeller blade surface (stated on the propeller's specifications in square inches) to get a blade-loading number in pounds per square inch (psi). Ideally, this number should be 35 to 40 psi, but Knowles says many boats load the props as high as 75 psi, inviting inefficiency.

To measure efficiency, Knowles will make a series of runs in the boat at a specific rpm, as measured with a photo-tach in the engine room for accuracy, both with and against the current.  He averages the speed, then compares this real-world number with the boat's theoretical speed, derived from the following formula: rpm divided by gear reduction, multiplied by inches of propeller pitch, divided by 1,216, which equals theoretical speed in nautical miles per hour. (Note: 1,216 is a constant used to convert inches to nautical miles).

By comparing the difference between real speed and theoretical speed, Knowles can determine slip and therefore efficiency. Let's try an example. My 35-foot Bertram swings 24-inch-pitch wheels through a 1.5:1 reduction gear, so  at 2400 rpm the formula tells me I should be traveling at 31.6 knots (2,400/1.5 X 24"/1,216 = 31.6). In reality, my boat travels at about 25.5 knots at 2400 rpm, a difference of 6.1 knots. Divide 6.1 by 31.6 and you get 19 percent, the rate of slip, which means that am getting 81-percent efficiency 2400 rpm.

That's pretty good.  Many boat builders shoot for an efficiency rate of around 80 percent. But what do you do if your boat, or the one you want to buy, falls far short of that number? What if your blade-loading calculations indicate that the wheels are grossly overloaded?  Can the problem be fixed?

The answer is almost always yes, but the solution may not be easy or cheap.  If there isn't enough blade surface to carry the boat's weight, you must add more wheel.  Sometimes you can add props with more blades, but often you must consider a wheel with a larger diameter to carry the load, and larger diameters often mean a transmission change

Tip speed is crucial in props. Exceed a certain tip speed and the prop will begin to cavitate badly, compromising efficiency. Therefore, larger diameter wheels are usually higher in pitch and swing through bigger gear reductions to slow tip speed and also ensure that the engine isn't working too hard to turn them. For example, you may need to go from a 1:5:1 gear to a 2:1 gear to turn a bigger wheel, necessitating a gear rebuild or the purchase of new gears.

Kevin Mitchell is a naval architect who works as the vice-president of New Product Development for the Michigan Wheel Corporation, (616) 452-6941. He says the key to correct propeller selection lies in how you plan to use the boat. "We see a lot of boats that are over-propped to achieve top-end speed when the boat is light." Mitchell explains. "But in the real world, no one travels at full speed all the time. Our biggest challenge is, educating people to realize that there are a lot of factors which affect a boat's performance."

While many boats are equipped with three-blade props for speed at the top end, four-blades may be a better choice for boats that often carry a heavy load (like most sport fishing boats do), because four-blades may run more efficiently at cruising speeds. While you might lose a knot or two from your top end, you might gain a knot in the midrange. Some larger sport fishermen now feature expensive five-blade props. Their owners searching for that magic blend of efficiency and speed.

"All boats run differently," Mitchell says, "and none of them ever get back to ground-zero, like when they were new. We try to get people to prop the boat to run at the right rpm with a full, real-world load." It's all a balancing act, according to Mitchell, between cavitation and drag. If your props are cavitating because they are overloaded, you are losing efficiency and may need more blade surface. But if you add too much blade surface you create drag that may slow you down.

Vibration is another common propeller complaint. Excess cavitation can cause vibration as the wheels slip excessively in the water, but bent blades and mismatched props can cause vibration and lack of performance, too.  When the wheels are out of whack, you normally take them to a prop shop to be banged back into shape, but now there's a new, more precise method.

High Tech Balancing

Terry Ryan, an Australian inventor, has come up with a computerized method of measuring and balancing propellers. His company, Propeller Dynamics, markets the Prop Scan® system, which measures the exact propeller pitch and feeds the propeller's entire geometry into a computer.  It reads the pitch of each blade to see if they match or if they exhibit the desired pitch. If a different pitch is desired, the computer can tell the prop shop exactly where the blade needs to be hammered.  Once a single blade has been hammered into the perfect pitch, as measured by the computer, the prop shop can use the computer to help them match the rest of the blades to the first one. The result is a perfectly balanced propeller.

Mark Daily is the General Manager of Coastal Prop Technology, Inc., (800) 677-3926, in Fort Walton Beach, Florida, a propeller shop on the cutting edge of this new technology.  "We start with one blade and get it perfect," Daily explains, "then the computer captures the design and we shape every blade to the original Spec so that all blades are a mirror image of one another."

Critics of the Prop Scan® system say that even after the computer scans the prop, somebody still has to hit it with a hammer to bend it into shape, and that's an imprecise science.  But Daily says that his workers do all of their repairs freehand, rather than on pitch blocks, and with the aid of the computer they know precisely where to hit the blade.  The Prop Scan® system allows the shops that use it to make minute changes in pitch and camber (the degree of curve across the surface of the blade), so the prop can be fine-tuned to an individual boat.  They can also finely regulate cupping, the process through which the blade's edge is curled to increase thrust.

Daily claims that substantial improvements in both efficiency and fuel economy are possible due to this fine-tuning, and Coastal's initial computer analysis of your propellers is free.  It gives you a full evaluation, after which you can decide if you want the props balanced and how far to take the fine-tuning process.  Coastal doesn't quote prices, but basic balancing starts at a minimum of $150 per prop, and it can get expensive in a hurry if you want to keep tweaking the props to find your boat's ultimate performance.  It all depends on how far you want to go.

The good news is that propeller buyers have many more options today than they did ten years ago.  High-tech prop companies promising enhanced performance have sprung up everywhere, but by doing some basic homework and applying some commonsense reasoning to the propeller-selection process, you may find that a basic set of wheels work just fine after all.