TBDP©/VBDP© Drive Unit Drag Analysis (click on screen to return)
TBDP©/VBDP© Drive Unit Drag Analysis calculates drag and coefficients for ANY drive design configuration throughout the entire hull operating velocity range

Lower Unit Drag design & dimensional inputs

Lower Unit height affects drag and performance

AR® has developed complex algorithms that calculate hydrodynamic drag of the outboard lower unit or I/O outdrive design & configuration.  This performance is, of course, different for each design, each setup arrangement and different at each operating velocity.

The Appendage drag of motor or outdrive lower unit is  difficult to calculate in a simple manner. There are many different designs of outboard lower units in use today; every boat will have the lower unit set up with respect to the hull differently; and there are several contributing drag components that determine the overall drag of the lower unit.

 The TBDP©/VBDP© analyzes the specified design of the skeg, the torpedo and the propeller, and calculates the overall hydrodynamic drag and drag coefficient based on configuration and relative velocities. Height of the lower unit relative to the water surface and hull planing surfaces is also included in the analysis.

TBDP©/VBDP© now calculates hydrodynamic drag of ANY lower unit design; includes standard design specs for 31 OEM drives, including NEW massive Yamaha and Mercury HD 300-350hp drives, Merc TorqueMaster, SportMaster, and FleetMaster drives; Merc M8 and drysump outdrives,, Merc Verado 350Sci HD, Merc SSM4, Merc SSM6, Nissan & Tohatsu SportC, Tohatsu 50HP D-Stock foot, Suzuki DF and Evinrude ETec HO gearcases, Alpha, Bravo, Volvo IO drives, even Arneson surface drives.

  Some of the design inputs include:
  • Drive Unit - Select from many OEM drive design defaults OR input your specific detail drive dimensions of ANY size/design
  • Drive Number - number of lower unit drives (no. of engines). This selection affects the total motor drag calculated for your setup.
  • Skeg Width -  average width of motor lower unit/outdrive skeg (leading edge of skeg to back of skeg).
  • Skeg Length -  length of motor lower unit/outdrive skeg (top of skeg to bottom of skeg).
  • Skeg Thickness -  thickness of motor lower unit/outdrive skeg (thickness of the skeg plate).
  • TorpedoLength -  length of motor lower unit/outdrive torpedo housing (leading edge of torpedo to aft edge of torpedo, at prop shaft).
  • TorpedoDiameter - diameter of motor lower unit/outdrive torpedo housing (in section).
  • Gear Ratio - Ratio of lower unit/drive unit gearing between propeller shaft and engine crankshaft.
  • Lower Unit Height - Height of LOWER UNIT bullet above/below the sponson running pad surface. ( is positive; - is negative). This “height” (depth) or motor lift dimension affects MOTOR DRAG and the moment force contributing to dynamic stability.
  Lower Unit Drag vs Velocity graphic output
(note comparison of different lower unit depth in water)
Lower Unit Drag coefficient vs Velocity graphic output

The Drive Unit hydrodynamic drag results are presented in regular
TBDP®  and VBDP® output and in graphic analysis format.

TBDP® makes it easy to see the performance and stability improvements that are achieved by design modifications and/or setup changes.