Power Boats: Choosing the Propeller

Previous page: Sizing the Engine

Propeller diameter Propeller pitch

Propellers come in various shapes and sizes, as they have to match the engine power and RPMs, the boat displacement and the main type of usage expected (it takes a different propeller to pull a skier fast out of the water, than the one that should efficiently and economically push heavy load).

D: Propeller diameter
Pitch: Distance the propeller travels in a single turn (theoretical, as it ignores slip)

Engines are usually equipped with the manufacturer's selected propeller generally suitable for a wide range of boats. So, you got your engine, together with a propeller, and it all runs more or less fine, or so it seems.

Yet, taking that the engine power is adequate for your boat, you might occasionally get that question in the back of your mind: Am I really using the best propeller for my boat? Could I somehow squeeze a few knots more, or get the boat to a faster start? Could I achieve a better cruising economy with a different propeller?

Or, your propeller blades have been nicked or/and eroded after years of use, or damaged by hitting a submerged log or running aground, and you are considering a replacement.

Matching Propeller to Your Boat

A good match between your boat, the engine and the propeller is essential. It is a complex relation, with many inter-related factors. Only the basic parameters and very simplified practical methods of selecting the propeller size are given here to help you understand the subject better. It is always wise to discuss your final choice with an experienced professional before buying a new propeller.

The propeller is determined by the number of blades, its diameter and its pitch (definitions shown in the above drawings), and the direction of rotation (left or right). The 3-blade propeller is most often used, but the same definitions are valid for the 2-blade, 4-blade and multi-blade propellers. Typically the propeller size is shown as D x P, where P stands for pitch. Take as an example a 3-blade 13" x 19" propeller: The diameter is 13", and, you guessed it, the pitch is 19". Sometimes you will see the pitch listed as 19P (that should be read as "19 inch pitch").

The diameter is twice the distance measured from the tip of any blade to the center of the propeller hub. Generally, propellers with a bigger diameter are more effective than the smaller ones, especially with heavy displacement boats, as it is more effective to push slower a wider column of water, then to push fast a narrow one. But, with the high speed planing hulls this principle is reversed.

The quoted pitch is actually a theoretical value. By definition, a propeller with a 19" pitch should move 19" in a single revolution. In reality, the actual one revolution movement is some 10-50% shorter. This effect is called slip. The actual amount of slip depends primarily on the boat displacement and inertia (resistance to changing speed), the engine power, the current RPM (Revolutions-Per-Minute, the shape of the blades, the friction between the hull and the water, the presence of cavitation and/or ventilation, and a number of other factors.

A rotating propeller generates load (resistance) that has to match the engine torque. Using a propeller that is to small for the engine will not put the required load on the driving shaft. As a consequence, the engine may be easily driven into high RPMs beyond the recommended maximum, resulting in a permanent damage. On the other hand, a too big propeller would overload the engine, not allowing it to achieve its maximum power and optimal working regime.

The shape of the propeller blades greatly influences its performance. High speed engines are often fitted with propellers that have concave face blades, the so called cupped blades. The cupped shape gives a better grip on the water and delays or reduces cavitation at high speeds. The improved grip reduces the slip and increases the propeller effective pitch (as a result, propellers with cupped blades come with pitch reduced by 1 inch). The curvature of the blade also increases its strength, so a cupped blade can be made thinner, which additionally increases its efficiency at high speeds. At speeds over 35 knots (below 30 knots the cupped shape has no effect) the cupped blade effectively increases the speed by some 6-12%.

The Optimal Propeller Size

The propeller blades are set at an angle to its hub. As the propeller turns, the hydrofoil (same as aerofoil, only in water) shape of the blade generates high pressure on the back face, and a vacuum on the front. The difference in the pressure generates the force that pushes the blade, the propeller and the whole boat forward.

Cavitation occurs at high speeds (30-35 kn, depending on the shape and size of the propeller, the displacement and the design of the hull bottom) as the vacuum on the front face becomes so high that it starts forming imploding bubbles of low pressure. Over time, excessive cavitation can result in pitted blade face and permanent damage to the propeller.

A cavitating propeller may still generate plenty of trust. In contrast, ventilation causes sharp increase in slip, loss of trust at a sudden burst of the engine RPM.

The propeller size DxP has to match the torque generated by the engine. There are some rough guides in determining the optimal size:

Type of Boat Speed (kn) Slip
Auxiliary sailboats, barges under 9 45%
Heavy powerboats, workboats 9 - 15 26%
Lightweight powerboats, cruisers 15 - 30 24%
High-speed planing boats 30 - 45 20%
Planing race boats, vee-bottom 45 - 90 10%
Hydroplanes, catamarans over 90 7%
  • The slip can never be entirely avoided, but it should be kept as low as possible. Generally it varies between 20% and 50%. On the initial start up, while the propeller has to overcome the boat's inertia, the degree of slip will be very high. As the boat starts moving, the slip lessens. Pushing ahead in a head sea, the slip will be much higher then in calm water. In extreme conditions it may approach 100%, when the boat is hardly moving ahead. There is no precise way to determine the actual slip other than to put the selected propeller on and do some test runs under strictly controlled conditions. The table below lists typical slip values as dependent on the type of boat and its speed:

  • The same propeller can't deliver both high speed and maximum power. A propeller sized for high speed has a small diameter and maximum pitch. In contrast, propeller that has high trust has a large diameter and lower pitch. Both diameter and pitch absorb the torque generated by the engine. As a rough guide, 1 inch increase in diameter requires decrease of 2 - 3 inches in the pitch for the same load.

  • The higher the pitch that the engine can turn near top horsepower and RPM without excessive slip, the faster will the boat go. But there is a practical limit how far this can go. Increased pitch induces higher slip, and at some point the angle of attack of the propeller blades becomes so high, that the propeller stalls loosing all trust, much like an aeroplane that climbs too steep.

  • At a constant diameter, insufficient pitch can damage the engine by letting it race at RPMs beyond its design limit. If your engine easily reaches the top rated RPMs with more spare power, you should change the propeller with a larger size one. Never run your engine at RPMs beyond its design limit.

  • As a rule of thumb, every 2 inch increase in pitch will decrease the engine speed by 450 RPM.

  • The combination of the propeller diameter and pitch, and the reduction gear should be selected to allow the engine to operate at its full rated RPM under full load.
  • Unlike a car, which has a gearbox to match the engine to different loads, the boat's reduction gear and propeller are fixed (unless variable-pitch propeller is used, which we will discuss in another material). That inevitably means the engine and the propeller will operate at the optimum level only at one particular set of design circumstances, determined primarily by the intended type of use of the boat (sport-fishing, racing, water-skiing, passenger and cargo transport, heavy load), while in all other conditions the performance will be less than optimal.
Choosing the propeller

The diagram to the right shows a simple method of determining the propeller diameter that matches the engine BHP (brake horsepower) and the gearbox reduction.

Take the example of the Platypus Yamaha FourStroke 100 HP outboard engine with a built-in gearbox with a 2.31 reduction. The engine produces its 100 BHP at 5500 RPM. At this maximum engine power RPM, the propeller shaft rotates at 2381 RPM (5500/2.31, rounded). The engine design speed limit is 6000 RPM, and the shaft turns at 2597 RPM (6000/2.31). Following the rule that the propeller has to generate full load at the engine top RPM, a vertical line starting from the 2600 propeller RPM crosses the 100 BHP "diagonal" at roughly 14" diameter. As the actual propeller fitted by Yamaha is 13"x19", obviously the manufacturer has opted for 1" smaller diameter and a larger pitch, considering that the engine will be used mostly for medium to high speed boats where speed is more important than economically pushing heavy load.

Well, having the propeller design diameter is fine, but we also need to determine the pitch. If it was not for that always present slip, that would have been easy. Just take the maximum design speed of the boat (in knots), the maximum engine RPMmax (in revolutions/min) and the known reduction ratio:

pitch (inches) = 1 propeller revolution travel (inches)

speed (knots) = speed (nautical miles/hour) = speed/60 (nautical miles/min) = speed * 1852 / 60 (m/min)
                     = speed * 1852 * 39.37008 / 60 (inches/min) = 1215.223 * speed (inches/min)

In one minute the boat would travel (at no slip):

pitch (inches) * RPMmax (rev/min) * 1 min / reductionRatio = 1215.223 * speed (inches/min) * 1 min

from where we calculate the result as:

pitch (inches) = 1215.223 * speed * reductionRatio / RPMmax

where the value of speed is in knots.

Take the example of the same 100 HP Yamaha Four Stroke engine mentioned above. Its RPMmax is 6000, and the reduction ratio is 2.31. Let us say we wish to achieve the design speed of 40 knots (theoretical, at no slip). Then, we can calculate the propeller pitch as:

pitch (inches) = 1215.223 * 40 * 2.31 / 6000 = 18.71444 inches => 19 inches (rounded)

Recommended Pitch Ratio (pitch/diameter)
Type of Boat Pitch Ratio Range
Very heavy cruisers 0.55 - 0.8
Average cruisers 0.65 - 1.0
Medium and fast cruisers 0.8 - 1.2
High-speed cruisers and runabouts 0.9 - 1.5

Now, this is an excellent result. If we introduce a 20% slip as an average for high-speed planing boats (see the table above), that would mean our boat would lose about 20% in speed. So, instead of the theoretical 40 kn, it would run at about 32 kn. Good to know that, isn't it?

There is one more rule of thumb that relates the propeller pitch to the diameter depending on the type of boat, the pitch ratio (pitch/diameter), as shown in the table to the right.

If you review our example, we initially dimensioned the propeller for this 100 HP Yamaha as 14" x 19" (D x P). Yamaha has actually used a 13" x 19" stock propeller. We are pretty close, and we could be satisfied with our result. Considering that Yamaha has a great experience, they know why they went for a 13" diameter. This is a good example why it is always good to consult the professionals.

Together we know more. Together we can do more, and do it better.
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Last upd: 19-Aug-10 F150806