Boat Propellers – Types and Technical Characteristics

Boat propellers are the driving force of the vessel. But how do they actually work? Learn the different types of Technical Recommendations for Choosing Engines for a Boatsboat propellers and understand how they work with our detailed guide.

Learn about fixed-pitch, variable-pitch and feathering propellers, among other things, and their unique features and benefits. Increase your boat’s performance and efficiency by choosing the right propeller for your needs.

It may seem out of place to discuss propellers in a book concerning buying a hull, but this is one topic always mentioned by owners, and with deserved importance. Propellers always seem somewhat mysterious but everyone realizes they move the boat in an efficient manner if they have the correct number of blades and are of the correct size. We will confine this brief discussion to average recreational powerboats as there are many types of specialized propellers used for commercial, slow speed hulls, and for very high speed racing boats.

Propellers are always getting bent in too-shallow water and the repair yards are kept busy with replacements. On most boats, the propeller is deeper than the hull and is damaged very easily. To prevent this damage, there should be a short skeg or a long keel to protect the propeller, both of which can be installed after the boat has been built. The lowest point of the keel should be about 6 inches below the bottom tip of the propeller. In this manner, the keel may squeeze into the sand or mud before the propeller touches. Whether a skeg or keel is installed, the propeller shaft strut should be in the shape of a «Y» so the lower leg provides some protection for the propeller.

Propeller Function

The purpose of the propeller is to convert the rotating force (torsion) of the engine and shaft to thrust that moves the boat forward. This is accomplished by the lift produced at each propeller blade, which is curved in cross section. There is a definite high and low pressure side to each propeller blade, similar to the pressure distribution on an aircraft wing or propeller. There are many variables that influence propeller efficiency, and projections from the bottom of the hull are some of the worst offenders. They produce disturbed water flow and adversely affect propeller operation.

On inboard engine installations, there must be:

• a shaft;
• strut;
• with a bearing;
• to support the propeller.

These produce detrimental turbulent water, that are part of the losses experienced in the design of boats. In an attempt to reduce these losses, some unique and specialized propeller drive systems have been developed such as the Ameson surface drive and water jets, both of which use very different propellers.

In propeller design, many factors are taken into consideration, and one of the most important is limiting blade pressure to enhance efficiency. This is accomplished by having a large total blade area and a large diameter propeller, but not so large that the maximum engine rated RPM cannot be reached. You can’t get the horsepower out of an engine if it cannot reach its rated RPM. A particular displacement hull requires a certain amount of thrust to move it efficiently, and a larger blade area means a lower blade pressure, as measured in pounds per square inch. This should not be interpreted to mean a greater number of wide blades, or a larger diameter are the prime factors to consider, as a number of other items enter into the decision for a propeller.

Experience with medium speed and fast recreational powerboats has been that three blades on propellers work efficiently on hulls to 40 feet in length and four blades are used on longer hulls. Racing boats may have custom propellers of unusual design, while sailboats may use two bladed propellers to reduce resistance while sailing. Many sailboats or motorsailers on long cruises change to a three bladed propeller if they expect to be under power for a good part of the trip.

Propeller Size

Before the hull lines are drawn, the propeller diameter is determined so there is adequate clearance between the top of the propeller and the hull (20 % of diameter) and so the total area is sufficient for efficient operation. In order to arrive at a successful size propeller, the boat designer must take many factors into account, and finally decide on a reduction gear ratio that literally brings all of these items together. This ratio is the maximum engine rated RPM divided by the actual propeller RPM and may be 1,5:1, 2,0:1, or 2,5:1 in an average powerboat. For example, if the engine RPM maximum is 2 800 RPM and there is a 2,0:1 reduction gear, the propeller would turn a maximum of 1 400 RPM. The reduction gear is located in the same gear case as the reverse gear and is bolted to the engine just forward of the output shaft.

The propeller manufacturers publish charts of correct propeller diameters for various engine horsepower and revolutions per minute (RPM). These data should be followed without exception and the manufacturers should be consulted for each hull and engine combination. Your local propeller repair shop can provide these data. I emphasize we have been discussing a conventional inboard installation with shaft, strut, and rudder. If you have an inboard-outboard I/O stem drive or an outboard motor, you should purchase a propeller from the engine manufacturer and strictly follow the recommendations.

The propeller diameter on these stem mounted engines is strictly limited by the height of the cast housing and trim plate. Often, the propeller hub mounting is proprietary with each manufacturer, and substitute propellers are not interchangeable. Because of this limitation on diameter, there is usually not sufficient blade area to produce successful operation for larger hulls. This is why you don’t see outboard motors used on cruising hulls over 30 feet in length. Exceptions do occur, and there have been some very lightweight racing hulls that have used four or six outboard motors. Other exceptions may include small boats in harbors or rivers that use Boat Outboard Motorsoutboard motors for pushing boats, logs, or barges at slow speeds for short periods of time, while making every attempt to reduce their cost of operation.

Propeller diameter can only be selected by carefully considering the engine horsepower, engine RPM, reduction gear, and whether the boat is a commercial fisherman or a lightweight recreational sport fisherman. The same engine may be used in both types of hulls, with a great difference in maximum speed. Propeller diameter may vary from 12 inches in a 25 foot boat to 15 inches in a 30 foot hull, 20 inches in a 35 foot boat, to 30 inches in a 60 foot hull.

Propeller diameter is always selected first and is the most important size factor. Propeller pitch is then selected secondarily so the maximum engine RPM can be achieved. There has always been a great controversy about propeller pitch and it is a constant subject of comment among boat owners and yard workers. The prime misconception in these conversations is higher propeller pitch produces higher speed. Nothing could be further from the truth. Fast boats use higher pitch propellers than slower boats, but this is trying to compare two entirely different sets of facts without adequate reasoning. Pitch does not produce higher speed. It is only higher engine horsepower and lighter boat weight that produce higher speeds.

This misconception probably resulted from the naming of an invention, the screw propeller in 1844. Some people have misinterpreted this to mean the action of a screw moving in a solid block where a higher pitch screw moves further in the block than a smaller pitch screw, using the same driving force. It is a shame this erroneous thinking has persisted as the propeller inventor was truly a brilliant engineer. John Ericsson (1803-1889) was born in Sweden, but moved to London and then the USA. He became a US citizen in 1848 and was the designer of the famous boat MONITOR that had a propeller and a revolving gun turret when engaging the MERRIMACK in 1862. Before John Ericsson, all of the steamships used paddlewheels in one shape or another.

Pitch for the propeller of an average boat can be estimated from the formula:

Pitch in inches equals boat speed in knots multiplied by 1 800 and divided by the propeller RPM.

Propeller pitch for slow, displacement speed boats is 20 % greater and propeller pitch on fast boats is 20 % less.

For example, if a 20 knot boat has a propeller RPM of 1 200, the correct pitch is 20 × 1 800/1 200, which equals 30 inches.

The reason for the variation of 20 % from average to fast boats is the total efficiency is greater on fast hulls (less slip). The total efficiency is less in slow boats (more slip) and thus the pitch has to be greater. The maximum propeller RPM is used in determining both diameter and pitch.

It is common for the ratio of propeller pitch to diameter to be 0,5 for displacement hulls. On moderate speed, average, hulls, this ratio can be 0,6 to 0,8. On planing hulls, the pitch to diameter ratio may be 0,9 to 1,2. It is good practice to check with the propeller manufacturer for each installation. When going on a long cruise, it is good to have:

• a spare set of propellers on board;
• along with propeller nuts;
• cotter pins;
• and spare keys to fit the keyway on the propeller hub.

The aft end of the propeller shaft and the inside of the propeller hub are closely machined to the Society Of Automotive Engineers (SAE) standard dimensions for shafts and keyways. A portion of the shaft and the inside of the propeller hub are tapered and each have a keyway to prevent the shaft from rotating on the propeller. After the taper, there is a straight, threaded section that takes a plain nut and a jamb nut, to keep the propeller from sliding aft. Aft of this threaded portion, there is a short stub of shaft with a hole in it for a cotter pin. Normally, the propeller, key, and nuts are silicon bronze, but the shaft is a precipitation hardened (PH) stainless steel alloy.

Read also: Technical Recommendations for Choosing Engines for a Boats

People with SCUBA gear can sometimes change propellers with the boat in the water, being very careful not to drop any of the parts. Propellers tend to get tightly stuck on the shaft taper, and a device called a propeller puller is a very useful product to have if you plan to do your own propeller replacement. It is better to plan any propeller repair or replacement at the same time you have the boat hauled for repainting of the bottom antifouling paint. It is always easier to check the condition of the underwater gear, replace the zincs, and replace the shaft bearing during the annual overhaul.

If you see spots of pink color on a bronze propeller, you will see they are pits where the zinc has been removed from the bronze alloy of copper, tin, and zinc. This is caused by stray electrical currents in the water and may sometimes be repaired by a propeller shop. Protection is provided by blocks of zinc located on the hull near the:

• propeller;
• propeller shaft;
• struts;
• rudders, etc.

Author

Olga Nesvetailova

Freelancer

Literature

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