Heliciel tutorial — Kt, Kq and J

The curve every marine propeller file expects

One checkbox of the multiple analysis produces the open water coefficients of your propeller, and places them on the advance coefficient axis.

Kt, Kq and J: reading a marine propeller in open water

Function tutorial. The option sits in the Multi Analysis window, in the Marine J analyse group, checkbox Marine: Kt Kq J. That group has kept its French labels in this version; they read the same either way.

A marine propeller file is always read the same way: three curves against the advance coefficient. Heliciel produces them for your propeller, the one you have just designed — not for a stock propeller.

1: The definitions, and one warning

J, the advance coefficient: it compares the speed of advance to the product of the rotation speed by the diameter. A low J means a heavily loaded propeller; a high J, a propeller that is "unscrewing";
K_T, the thrust coefficient, and K_Q, the torque coefficient: thrust and torque made dimensionless;
the open water efficiency follows from the three.

The warning: some publications use coefficients of the rotor kind, whose definition differs from that of the propeller kind. Heliciel uses the propeller definitions and says so in its displays. Mixing two sources without checking that convention is one of the most expensive mistakes in the propeller literature.

2: Turning the marine analysis on

Open the multiple analysis, tick Marine: Kt Kq J, then bound the range of J to be explored. The bounds are chosen around the operating point aimed at: there is no point sweeping up to values of J your boat will never reach.

Marine Kt Kq J checkbox and J min J max bounds
The option and its bounds

3: Reading the three curves

The classical reading: K_T decreases as J increases — the propeller pushes less as the boat moves faster; K_Q decreases too, but not at the same rate; and the efficiency goes through a maximum. That maximum is not necessarily your operating point, and this is the whole value of the curve: it shows the gap.

Kt Kq and efficiency curves against J
The three curves

operating point of the project placed on the J axis
Where your propeller works

4: Comparing with a reference propeller

The Wageningen B series provides the yardstick: at equal diameter and operating point, where does your propeller stand? A gap in your favour deserves an explanation — better suited sections, an optimised chord distribution; an unfavourable gap deserves one too.

comparison of the curves with a B series propeller
Your propeller, against the yardstick

Validation. Heliciel's BEM computation laid over the published curves of four propellers of the series, B4-70 at pitch ratios 0.60, 1.00 and 1.40 and B3-50 at unit pitch, over the whole J sweep: validating Heliciel against published data.

5: The torque of the propeller and that of the engine

The real matching criterion is not efficiency: it is where two torque curves meet. The multiple analysis lets you enter an engine torque curve and compare it with that of the propeller: their intersection gives the rotation speed at which the assembly will actually settle.

exported table of the multiple analysis
The table, for the file

Going further: the multiple analysis, propeller advance and the boat propeller.