Marine Propeller Shaft Diameter for Torque

Torque follows from power and SHAFT speed at the customary 63,025 constant, and the torsional stress in a solid shaft follows from that -- but a propeller shaft is not loaded in torsion alone. The propeller hangs on the end of an overhung shaft supported at the strut, and its weight plus the hydrodynamic side loads put bending into the shaft that a torsion-only calculation misses entirely. That is why classification societies give a rule diameter of the form d = F x cube root of (hp / rpm), whose constant embeds an allowance for that bending and for corrosion, and why the rule figure is almost always larger. Both are reported here, named, so a reader who computed the torsion number elsewhere can see what it leaves out. It is also why the tail shaft -- the outboard portion -- is sized larger than the section inside the boat. The cube root is what makes a repower interesting. Diameter scales with the cube root of power over shaft speed, so a large power increase calls for a small proportional diameter increase -- which sounds negligible and is often a whole nominal size, at which point the coupling, the stern tube, the cutless bearings and the stuffing box all change with it. That is the difference between a repower that drops an engine in and one that rebuilds the running gear. Material choice moves the answer as much as the power does: aluminium bronze, the Aquamet grades and the stainless steels have substantially different allowable stresses and very different corrosion and fatigue behaviour in seawater, which is why the rule factor is entered from the society's own table for the material rather than assumed. This is a screening calculation: it does not compute shaft whirling or critical speed, size the bearing spacing that sets them, evaluate thrust and its bearing, check the coupling or the keyway, address shaft alignment, or account for a shaft's unsupported overhang beyond the strut. ABYC P-6, the classification society's rules, the shaft manufacturer, and a marine engineer govern.

torque T = 63,025 hp / rpm; torsional stress tau = 16 T / (pi d^3) and the diameter that satisfies an allowable is its cube-root inverse; the classification rule diameter is d = F x cube root(hp / rpm), and the repower ratio is the cube root of the power ratio.

Shaft torque and torsional stress as machinery practice writes them, against the classification-society rule form whose constant embeds an allowance for the propeller's BENDING and for corrosion -- which is why the rule diameter governs and torsion alone under-calls it. The rule factor is ENTERED from the society's table for the shaft material. It does not compute shaft whirling or critical speed, bearing spacing, thrust and its bearing, the coupling or keyway, or alignment.

One torque relation and one published rule form.

Estimate. AHJ and licensed professional govern.

Field names used by the API: engine_hp, shaft_rpm, shaft_diameter_in, allowable_stress_psi, rule_factor, repower_hp, torque_inlb, torsional_stress_psi, torsion_diameter_in, rule_diameter_in, diameter_ratio, repower_diameter_in

• The rule factor is entered bronze, Aquamet and stainless allowables differ substantiallythe classification society's table
• Torsion is shown, not used it cannot see the propeller's bending or the corrosion allowanceABYC P-6 and the society's rules
• No whirling or critical speed bearing spacing sets those and is a separate calculationa marine engineer