Proper Propellers

Proper Props

Ensure your propellers are the right size and pitch for fuel efficiency and comfort. 

Utilizing correctly tuned propellers is crucial for a boat’s optimum fuel efficiency and speed. Unless your boat is brand new, it’s possible that over time, added weight and reduced engine power has affected your propeller’s performance.

Almost all new boats are delivered with properly matching props to ensure they run in that sweet spot of performance. However, we all pack on boat weight with extra goodies. You’ve added a little here, a little there, and now the lockers are bursting.

Does that mean Weight Watchers for your boat? Maybe a diet wouldn’t hurt, a cleanse of the compartments, a purge of all the things you stuffed away. But if you have an older vessel, it’s simply gotten heavier by just being in the water. Equipment you added just might be necessary and useful.

That weight gain means the original propeller or propellers may no longer be the correct configuration for peak performance. Determining the weight of your boat, the condition of the hull bottom, and the power of your engine are three factors that can help you pick the perfect prop.

“When a motor gets weaker, the owner can compensate by lowering the pitch of the prop so it will turn a higher rpm,” explains Jimmie Harrison, owner of Frank and Jimmie’s Propeller (FJ Prop) in South Florida. Harrison’s father, Jimmie Sr., founded FJ Prop in 1947 with Frank Baron in Fort Lauderdale. “Call the manufacturer with the hull number and
ask them what the original props were on the boat. Find out the optimum rpm range for
the engines. It’s that easy. Then compare that to the props that are on the used boat now. A motor that has problems can be ‘fixed’ by changing the propeller.”

A big challenge today are supply chain issues involving propeller inventories. Some center console boat owners have been waiting a year or longer for stainless steel propellers in the most common sizes. Motoryacht owners who damage a propeller can’t get a replacement right away or have a damaged propeller fixed in time to save a trip or valuable charter.

Needless to say, having a spare set of props is invaluable. Charter boat operators especially should always have a spare set available; otherwise, they could lose big bucks in failed charter excursions.

“When you do the math, having a spare set of propellers is a really good tip that can save you money, especially if you do a lot of boating in shallow areas,” Harrison says. “You can bolt them onto the boat, or if you want to save weight, have the propellers stored somewhere where they can be easily accessed and delivered.”

While replacement props are important, how does a boat operator know the propellers are performing at their best? Aside from the obvious signs, such as vibrations or noise, the way to tell if your props are in good shape is by a sea trial. Start with a half load, a clean bottom, and clean fuel filters, and find out if the boat can achieve the engine’s rated rpm level at wide-open throttle.

A properly matched propeller should govern the motor at its rated rpm, so when the throttle is wide open, the propeller won’t under or over rev. “Every engine has a power curve with an rpm range that is ideal. You want to be able to achieve that rpm always,” Harrison says. “If you are overloaded, you are burning too much fuel and wearing out your engine prematurely. If you are underloaded, you are not getting the speed you should get and burning too much fuel. If you want to raise the rpms by X percent, lower the pitch of the propeller by X percent. It’s a simple formula that works.”

Today’s outboard propellers are primarily made of stainless steel, while inboard props are most likely composed of either bronze or nibral, a combination of nickel, bronze, and aluminum. “Most inboard propellers today are CNC milled to an S Class tolerance, the highest level of accuracy,” Harrison says. “Nibral is the preferred material and has many of the same characteristics of stainless steel. The advantage of nibral over bronze is strength. With too much horsepower, a bronze prop’s blades will eventually fall off.”

Buying new propellers for a used boat can also be an expensive money trap. For example, building new propellers for a used 80-foot sportfishing boat can be a $50,000 proposition.

“The cost of propellers can be a huge shock,” Harrison says. “Someone may have gotten a great deal on a used boat and paid five hundred thousand dollars, only to find out the propellers are twenty-five thousand dollars each.” Frank & Jimmies Propeller Shop

-by Doug Thompson

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