How To Improve Your Prop Performance

Check other factors before you blame the prop. Here’s how to improve your prop performance.

When a boat runs poorly, propellers often get blamed. If the engines are in good shape, it’s only natural to get “propeller tunnel vision” and start indicting your blades for not doing their job. Before you yank out the prop puller and start trying new wheels, remember that props must work in harmony with the rest of the boat. If other parts of the vessel are not in top shape, then the prop can’t save it.

“Some people seem to view the propellers as isolated and independent from other important factors, such as the power and condition of the engine, or the weight, or the cleanliness and capability of the hull,” explains Jim Thelen, sales engineer for Acme Marine in Big Rapids, Michigan. “I’ve seen people buy numerous new propellers as if to assume that sooner or later they will find the one prop in existence which will ‘fix’ all other possible concerns and transform their performance by something outlandish.”

More than just a proppy face

There are some boat owners who take the oversimplified view of propellers merely in terms of diameter, pitch and number of blades. That’s like defining all automobile tires only in terms of diameter and width. Propeller design and manufacturing differences affect engine load, speed, power, plane time, smoothness, and efficiency.

“Some propeller factors which should be accounted for by a professional are the size of the blades or the amount of surface area in the propeller blades (often indicated as DAR or EAR), Cup (which affects load/engine RPM much in the same manner as pitch) and pitch distribution (varied, constant, cambered),” says Thelen. “The point is that one manufacturer’s twenty-by-twenty four blades may not be equivalent to another
manufacturer’s twenty-by-twenty four blades.”

What’s amiss?

The hull’s condition can also significantly affect performance. Thelen describes how one boat’s power and speed relative to published or reported performance was down. “He tested the new propeller I recommended and still, his RPM and speeds were down significantly.”

Thelen went through his usual litany of questions about boat weight, engine service, cleanliness and condition of the hull, possible modifications to the boat, and altitude. Nothing revealed itself until he dug deep enough to get the owner to admit that the boat had been moored in warm ocean water for a long period which resulted in a rough bottom. “It wasn’t until he had the bottom stripped and refinished that he gained the speed he was looking for,” says Thelen. “We had him into the correct propeller from
the start.”

Extra Load

An overweight vessel can also be a problem because, over the years, boats usually get heavier, not lighter. Stuff goes on board every weekend, but not all of it comes off, and it can really add up. When add-ons and enclosures increase wind resistance, or a dirty hull increases flow resistance or when engines get tired, the boat slows down, and the original props can become overpitched. Sometimes you can improve your prop performance by simply chucking some stuff.

“The original props may not be achieving sufficient RPM relative to the engine manufacturer’s RPM ratings,” says Thelen. “The loss of RPM and potential overloading can cause power loss, speed loss, slower plane time, and potential engine damage, especially in terms of long-term use. The most common approach is to reduce propeller pitch in order to achieve appropriate wide-open-throttle RPM.”

To determine if the propeller is working its best, “It’s important to keep an accurate log of one’s boat model and weight, engine model number, manufacturer’s RPM specifications, transmission or V-drive gear ratio, current prop size, etc.,” says Thelen. “Most engine manufacturers place an ID tag on their engines which indicate s horsepower and RPM specifications.

Determining appropriate propellers basically involves running the boat (with typical or average load) at full and verifying that the engines are achieving the expected or recommended RPM at WOT [wide open throttle] and that the boat is achieving reasonable, expected speeds.”

Thelen suggests that if RPM readings are below the manufacturer’s WOT recommendations, then the propellers are likely overloading the engines. If the actual WOT RPM reading is too high, then the propeller is likely underloading the engine and, perhaps, a different propeller can improve speeds.

Data is King

Data and proper care of all facets of your engine can help to improve your prop performance.

But before you start swapping out props, verify all the data: boat weight, engines, gears, current props, speeds achieved. It’s nearly impossible to get a precise propeller answer if the reported data and specifications for the boat are off the cuff, guesstimated or vaguely recalled from several years ago.

“Complete data, accurate numbers, and double-verified specifications can produce very accurate propeller recommendations,” says Thelen. “I urge people to verify all their boat, engine, gear, and current propeller data with their own eyes and to write everything down.”

Thelen advises that having a spare propeller is a good idea. “Most people don’t go very far with their automobile unless they have a spare tire,” he says. “And most people aren’t very happy being without a boat on the first day of their vacation because of a damaged prop.

Furthermore, there are so many different types and sizes of propellers in existence today that no one can carry them all, which means that the waiting time for a replacement, for some props, can be months rather than weeks. So if you like your boat and if you want to be able to continue using it when the surprise moment comes, then I suggest carrying a spare.”

acmemarine.com

By Doug Thompson, Southern Boating May 2019

Bow Thruster Maintenance

Proper bow thruster maintenance will ensure the best performance from your bow thruster

Bow thrusters are part of what I refer to as the “silent crewmember’s union.” Always willing to help out when needed, thrusters—along with other union members (autopilots, anchor windlasses, etc.)—make our time on the water both safer and more enjoyable. As with any boating equipment, proper bow thruster maintenance is key when it comes to reliability. Here’s how to make sure your thruster is always ready to lend a hand.

Electric vs Hydraulic

A bow thruster is simply a propulsion device that provides lateral (port and starboard) thrust, making the vessel more maneuverable. It is electric or hydraulic and will be either a traditional tunnel- or tube-mounted drive or an externally mounted unit.

Electric units can be further divided into 12- or 24-volt DC types or even the occasional
AC-powered unit. Hydraulic thrusters are commonly found on larger vessels, particularly those that have additional hydraulic systems on board to power anchor windlass or dingy davits.

A typical electric thruster installation involves either running cables of sufficient size to minimize voltage drop from an existing battery bank or installing a dedicated battery in the vicinity of the thruster along with a method of charging it. A third option is piggybacking off of an electric windlass circuit.

Bow Thruster Maintenance

Bow thruster maintenance requirements are determined by the type of thruster you have. Different models have different needs, so the first place to look when compiling your maintenance list is the owner’s manual. Schedule and follow the specific requisite provided there, but here are some other things to consider.

Access the thruster space at least twice a year and inspect the thruster compartment for excessive moisture levels, standing water and leaks. Examine the thruster tube ends (where glassed to the hull) for cracks in the gelcoat or laminate. Another place to check for leaking is the gasket at the thruster saddle, the bracket where the motor mounts to the thruster tube. Inspect the motor and all thruster components for corrosion. Even if there are no leaks, many thrusters are installed in compartments near the anchor locker, where wet rodes and chain create a moisture-rich environment. Running rust, flaking paint or a white, powdery substance on aluminum components are all indications of corrosion

Inspect the motor and all thruster components for corrosion. Even if there are no leaks, many thrusters are installed in compartments near the anchor locker, where wet rodes and chain create a moisture-rich environment. Running rust, flaking paint or a white, powdery substance on aluminum components are all indications of corrosion
and should be addressed. Many manufacturers recommend removal of the motor annually
for examination. This provides an opportunity to check motor brushes, grease couplers, inspect shear pins, etc. It also allows you to view the condition of the sealing gasket
and ensure all mounting hardware is tight.

Secure power to the thruster, then look over and clean all electrical connections. Check both ends of the battery cables for loose hardware, corrosion and other issues; remove and clean any terminals where decay is noted.

Moisture can wick under shrink tubing, so remove it, inspect and clean connectors as required, then recover them. Go over all other thruster system electrical connections and components (battery switches, fuse holders, solenoid connections, etc.).

Check the gear oil level. Some units are completely sealed (meaning their oil level never needs evaluation); however, others require periodic topping-off and changing at regular
intervals. Most units use 90-weight gear oil, but confirm whatever is called for in your owner’s manual.

If your thruster is powered by a dedicated battery, check its installation and condition at least twice a year. It is a good idea to put this on your calendar for the spring and fall months. Check the battery performance with a battery tester. If the battery is not holding a charge, replace it.

Don’t forget the helm control panel and joystick during your inspection, examining each for looseness, deterioration, corrosion, and UV damage. Verify proper operation of the
controls at each helm position, and if you have a wireless remote, now is a good time to install new batteries.

During Haulout

Clean the thruster drive, propeller(s) and inside of the tunnel of marine growth, and check each for cracks or dings. Inspect the drive for corrosion or other problems and the propeller for damage, looseness, etc. Oil in the tunnel beneath the propeller indicates a leaking seal, which will need to be replaced.

Check the condition of the sacrificial anodes (zincs). Although some units have composite gear assemblies and don’t require them, most thrusters will have at least one zinc installed. These should be looked at and renewed annually at a minimum—sometimes more often depending on your vessel’s location and other factors. Always check thruster zincs when you have your other zincs inspected (propeller shaft, rudder, trim tabs, etc.); those zincs can typically be replaced by a diver. Some thruster zincs are specialty items and may not be readily available locally, so carry plenty of spares should they need replacing. In fact, this is a good idea for any extra parts required for your thruster (shear pins, brushes, saddle gaskets, oil seals, spare propellers, etc.).

Apply appropriate antifoulant to the tunnel and drive. Metal drives may require a specific type of antifoulant paint, so follow whatever is recommended in your owner’s manual.

By Frank Lanier, Southern Boating October 2017

Hull-Prop Relationship

The prop you select must synch with both your engine and your hull.

Do you feel a vibration when your boat is running at speed? Does the vibration occur at high speeds and go away at low speeds? Or does it occur at low speeds and disappear at higher speeds? If it only appears at high speeds, your propeller may be too close to the hull, or the prop may have a bent or damaged blade. If the problem appears at low speeds it might be that the shape of the hull is causing a change in pressure in the area where the propeller is operating.

There are many factors that affect the interaction of a propeller and the hull. They include the hull shape, the proximity of the hull to the propeller, or whether the propeller is operating directly behind the hull (in the case of a single prop) or to one side of the hull in the case of a twin screw vessel. In addition, the speed of the vessel must be taken into account. Slow-speed boats generally have different problems than do vessels that get on plane and run at planing speeds. Boats that run at extremely high speeds have even more complex issues resulting from hull/propeller interactions.

If your hull and propeller are not optimized, they could be costing you money. For example, your small fishing boat’s skeg/keel is large enough to protect the single propeller should you go aground. However, it might also be reducing the efficiency of the propeller, meaning that replacing a dinged or bent propeller could be less expensive than having a skeg to protect it.

Inboard Installations

Having a hull, keel or other obstruction in front of the propeller causes loss of efficiency. The actual loss varies with the size of the obstruction. For example, the rather extreme example shown in Figure 1 pictures a two-blade propeller behind a thick skeg or keel. Every time the propeller is vertical the water flow is blocked by the skeg producing vibrations that could reduce the efficiency of the propeller by as much as 50 percent.

Figure 2 shows a better solution. Here, a three-bladed propeller is used and moved farther away from the keel. Another solution would be to taper the keel where the water flows toward the keel, but this is not always possible when the keel is laminated during the building process. (When the boat is being built, the person laying the fiberglass into the hull might have to get their hand down into the skeg area to be sure it is sufficiently reinforced.) On this boat, the turning radius would be enhanced by extending the rudder up to the hull at the top to get more “end plate” effect from the hull.

Figure 3 is an even better solution in that the skeg/keel is cut away in front of the propeller, allowing the water streamlines to flow more efficiently toward the prop. The rudder is taken up to the hull, and the skeg is deep enough that the propeller is protected. From a design standpoint, it would be smart to have a strut from the bottom of the keel to support and protect the bottom of the rudder, but you can’t have everything.

A far better solution—but one that uses more fuel—is the twin engine installation shown in Figure 4 (see opening header). Even though twin engines use more fuel, the efficiency of the exposed propellers is greater. Notice the keel is slightly deeper than the bottom of the propeller and only the prop shaft interrupts the flow of water to the propeller blades. The rudder is directly in the path of the propeller blade and operates at high efficiency.

All of these solutions, however, increase the boat’s draft. To reduce draft, many builders install a tunnel in the hull as shown in Figure 5. (Keep in mind that to reduce vibration and hull/propeller interaction, there should be at least 10 percent of the propeller diameter clearance between the hull and the blade tip.) By locating the prop in a tunnel, draft is reduced and some efficiency from the end plate effect of the hull is gained. However, Figure 6 shows how not to install a prop in a tunnel.

It would appear that the tunnel was built into the mold and either the designer did not take into account the width of the engine, or larger (and wider) engines were installed, pushing the shafts farther outboard. The propeller appears to be much smaller and farther away from the tunnel thus reducing its efficiency and increasing fuel consumption. The prop and rudder are both deeper than the hull and will be the first parts to hit should the boat go aground.

Finally, we come to Figure 7. The boat builder has taken great pains to ensure moderately clean water gets to the propeller but has then added an additional strut and inverted U-bar protection for the propeller and rudder. The rudder has several horizontal fins to either redirect water flow across the rudder or structurally strengthen the vertical portion of the rudder blade. All of these additional fins add resistance and increase fuel consumption.

Outboard Installations

The beauty of an outboard installation is that the engine can be installed at any height to locate the prop below the hull. Figure 8 shows a pair of outboards installed with the anti-ventilation plate about level with the boat bottom. This puts the propellers in water flow that’s only obstructed by the lower gear case. The anti-ventilation plate is designed to prevent air from being sucked down the lower unit and into the propeller where it can cause loss of thrust. The outboard in Figure 8 is at exactly the right height for a moderate speed. Figure 9 shows a similar set up on a catamaran hull with the anti-ventilation plates right at the hull bottom.

On faster vessels, the anti-ventilation plate is often located two to three inches above the boat bottom, but putting the engine higher may result in poor cooling water intake and an overheated engine, so care must be taken to locate the engine correctly. Figure 10 shows a faster speed hull with triple outboards. Notice how the anti-ventilation plates are about three inches higher than the hull bottom. On faster hulls, the strut/propeller/rudder drag (or resistance) is the largest single factor holding the boat back. For this reason, the shafts might pass through the transom to reduce drag, with the propellers some distance astern of the hull.

Another common case occurs when the propellers are supercavitating props with a straight trailing edge to reduce cavitation and loss of thrust. Arneson drives are typical of this drive train. To reduce drag on the fastest of hulls, only the lower half of the propeller is in the water. The shafts are then turned to steer the boat, eliminating rudder drag.

Roger Marshall, Southern Boating Magazine June 2016