Background
NavCad's drive line efficiency model distinguishes between propeller open-water efficiency, relative rotative efficiency, hull efficiency, and mechanical transmission efficiency (shaft bearings, gearbox, seals). Each component is entered or estimated separately in the Drive ata form. When the total predicted delivered power disagrees with an independent estimate, the cause in how one or more of these components is defined.
Drive Data Efficiency Fields
The Drive Data form contains the following efficiency inputs relevant to the shaft-gearbox system:
- Gear/transmission efficiency: The fractional efficiency of the gearbox. For a direct-drive or reduction gearbox, typical values range from 0.96 to 0.99. This efficiency multiplies the shaft power delivered to the propeller to give the engine brake power required.
- Shaft/bearing efficiency: The efficiency of the shaft line including stern tube bearings. Combined with gear efficiency to give total mechanical transmission efficiency.
- Relative rotative efficiency: A hull-propulsor interaction term reflecting the difference between propeller efficiency in the wake field behind the ship and in open water. This is not a mechanical loss and should not be conflated with shaft or gearbox efficiency.
Common Cause of Discrepancy
When comparing NavCad output to a back-of-envelope or spreadsheet calculation, common source of disagreement is whether the gearbox efficiency and shaft efficiency are entered as separate terms in NavCad but combined into a single factor in the comparison calculation. Check that the product of NavCad's gear and shaft efficiency terms equals the single mechanical efficiency factor used in the comparison.
Also check that the analysis is comparing equivalent power quantities NavCad reports brake power (at the engine output), power (at the ), and power (). These differ by the mechanical efficiency and propulsive respectively.
Estimating Efficiency Values
NavCad provides estimate buttons for several efficiency fields. For preliminary design, typical ranges are:
- Gearbox efficiency: 0.97–0.99 for single-reduction marine gearboxes.
- Shaft/bearing efficiency: 0.98–0.99 for conventional stern tube arrangements.
- Relative rotative efficiency: 0.96–1.04 depending on hull form and propeller type; typically close to 1.0 for single-screw ships.
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