This calculator uses the methods of the Heliciel propeller design software.

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Water-ski, wakeboard and wakesurf propeller calculator

A ski-boat propeller is chosen first by the need: the activity sets the speed, the loaded boat sets the drag, and the engine sets the available rpm and power. This water-ski, wakeboard and wakesurf propeller calculator starts from these three inputs, computes the required thrust and names the models that deliver it at the best efficiency.

Your boat, your activity, your engine

1. The activity

2. The loaded boat

Loaded mass: —

Width of the bottom between the two chines (the fold between the bottom and the side), smaller than the overall beam. Measure it with the boat out of the water, at the transom and at the middle of the wetted part; Savitsky uses the average. None of the manufacturers in our database of 45 boats publishes it.

Published values: 20° for the Centurion Ri230, Ri245 and Nv213 (manufacturer); 10° for the 2003 Malibu Response LXi (BoatTest review); 9.5° at the transom for a 24 ft MasterCraft inboard hull (US patent 5,046,439, 1991). Other manufacturers do not publish it.

No published value for these boats. Literature landmark: on the 21 models of Series 62 and 65 tested by the US Navy research centre, the centre of gravity ranged from one third to one half of the planing bottom length forward of the transom (Hubble, DTNSRDC/SPD-0840-01, 1978, revised 1981, after report NSRDC 4307). That is the test domain, not a value for your boat.

This is the input that sets the trim, hence the drag: ballast and passengers move it. The safest way is to measure it by a two-point weighing (front and rear of the trailer) with the boat loaded.

3. The rider

4. The engine and the shaft line

1 for direct drive; 1.29, 1.56, 2.00 or 2.48 for a ZF 63IV V-drive.

Share of the thrust lost to the depression the propeller creates under the hull.

Slowing of the water reaching the propeller behind the hull and the shaft strut.

Published landmark for planing hulls: small t and w, from 0 to 0.10, with t ≈ w (Hadler, SNAME Transactions 74, 1966, quoted by Ladd, Oregon State University, 1976; Schoenherr, 1941). No published value for a ballasted surf boat.

Fill in the four steps, then press the button.
Hull drag against speed; the dot marks the speed of the activity.

How the calculator chooses the propeller

The hull drag comes from Savitsky's method (1964) for planing hulls, the same as in Heliciel's hull resistance module, transcribed line for line: the equilibrium trim is searched speed by speed, and the drag combines the pressure component and the friction on the wetted bottom. Outside the method's domain, the page says so instead of returning a number.

The required thrust adds the rider's pull to this drag, and divides by 1 − t. The propeller works in water slowed by the hull: its speed of advance is V (1 − w).

Each propeller model is described by its open-water curve, computed by Heliciel (blade element method). For each one the calculator searches for the rpm that gives the required thrust, derives the engine rpm from the gear ratio and the power absorbed, discards the models that exceed the engine's rpm or power, and ranks the others by efficiency.

Sources