Boat Propeller 3/5: Optimum Rotational Speed and Ways to Add Thrust

Part 1 · Part 2 · Part 3 · Part 4 · Part 5

For a boat propeller, Heliciel finds the optimum rotational speed by searching for the speed that gives the best kinetic efficiency; the thrust is then adjusted with the speed, the blade area, the number of propellers or the profiles. In this third part, we build the two-blade propeller of part 2, we search for its optimum speed, then we compare the levers that can reach the 105 newtons of thrust of the design brief.

On this page:

Searching for the optimum rotational speed
Comparing the performance with the design brief
First lever: raising the rotational speed
Second lever: increasing the blade area
Other levers: a second propeller and profiles

1: Searching for the optimum rotational speed of the boat propeller

We set the rotational speed aside when we defined the operating point: it is time to deal with it. Here is the two-blade draft built in part 2.

3D model of the draft two-blade boat propeller in Heliciel before the search for the optimum speed

The two-blade draft propeller of part 2, whose speed we now search for.

We could test rotational speeds by changing this parameter and rebuilding, until we find a good efficiency. These iterations may be long and tedious. We therefore discover a Heliciel function: optimizing the rotational speed for the best efficiency. We select the 3: Optimize tab.

Optimize tab: button searching for the optimum rotational speed of a two-blade boat propeller

The button that searches for the optimum rotational speed of the two-blade propeller, in the "Automatic search speed rotation with maximum efficiency" group.

We click this button. Heliciel searches for the rotational speed that gives the maximum kinetic efficiency. The propulsive efficiency is not used as a reference in this procedure, because it would have no meaning for propellers whose fluid velocity is close to zero.

Here we obtain a kinetic efficiency of 0.86 at 200 revolutions per minute.

Efficiency curve of the two-blade boat propeller as a function of rotational speed, with a maximum of 0.861 at 200 revolutions per minute

Propeller efficiency as a function of rotational speed: 0.72 at 50 rpm, 0.83 at 100, 0.86 at 150 and at 200. The message at the bottom left gives the maximum, 0.8611814 at 200 rpm.

At the end of the search, Heliciel asks whether we want to rebuild the propeller in 3D. We click "Yes" to update the model. The propeller is rebuilt with a new twist, suited to its optimum rotational speed.

Two-blade boat propeller rebuilt at 200 revolutions per minute in the Heliciel 3D prototype, with a thrust of 28.5 newtons and a shaft power of 51.32 watts

The propeller rebuilt at 200 rpm: thrust of -28.5 N and shaft power of 51.32 W.

2: Comparing the propeller performance with the design brief

The propulsive efficiency of our propeller, (thrust × speed) / (shaft torque × rotational speed), is 0.861: that is close to the maximum. It is good, but a closer look at the performance shows something else.

Performance of the two-blade boat propeller: axial force of -28.5 newtons, torque of 2.5 N.m, shaft power of 51.32 watts

The Heliciel performance displays: axial force -28.5 N, torque 2.5 N.m around the axis, output power 44.21 W and shaft power 51.32 W.

The thrust of our propeller (axial force) is about -29 newtons. The negative sign means that the force is opposed to the direction of the fluid;
the shaft power is 51.32 W.

We are far from our basic design brief:

cruising speed: 3 knots;
thrust required: 105 newtons;
power available at the propeller shaft: 192 watts.

How do we raise the thrust to our 105 newtons? This question lets us look further into propeller optimization. Several solutions are open to us.

3: Raising the thrust with the rotational speed

This is the simplest solution: we change the speed until we get the thrust we want. Let us try right away, raising the speed until we reach our available shaft power. After a few trials of changing the speed and rebuilding , we obtain 206 W for 105 newtons of thrust at 460 rpm, with an efficiency of 0.79. We are getting close to our design brief.

Two-blade boat propeller at 460 revolutions per minute: axial force of 104.7 newtons and propulsive and kinetic efficiency of about 0.79

At 460 rpm (blade tip speed 34.7 km/h): axial force -104.7 N, the 105 N we aim for, and the histogram of propulsive and kinetic efficiencies.

4: Increasing the blade area (number of blades, width)

Two methods let us increase the blade area.

Increasing the number of blades.
- Advantage: when the diameter must not grow (often the case for boats that must stay navigable in shallow water), more blades raise the thrust.
- Drawback: the weight, the complexity and the price of the propeller rise quickly.
Increasing the width of the blades.
- Advantage: by widening the chords of the profiles, we increase the lift area of the profiles and the risk of cavitation decreases.
- Drawback: tip losses are lower the longer the blades are. By widening the blades without lengthening them, we increase the share of tip losses.

5: Other levers: a second propeller and better profiles

Increasing the number of propellers. We could add a contra-rotating propeller.
- Advantage: the aft propeller can straighten the tangential flow, and the overall efficiency of the two coupled propellers is better.
- Drawback: it is more complex and more expensive to build.
Improving the performance of the blade profiles. Finding the best profile shapes requires a comparative analysis of the performance of the profiles available to us. This solution takes us to the heart of Heliciel, which was developed to find the best profiles easily thanks to its database.

The blade root radius stays the one set in part 2 (20 mm), and so do the chords: in this part, only the speed, and hence the twist, has changed. To go further, we can choose the best blade profile, using the lift-to-drag ratio and the Reynolds number, in part 4 of the tutorial.

← Part 2: parameters and geometry · Part 4: selecting the profiles →