Advanced Aerodynamics and Hydrodynamics for Powerboats

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AR© develops Drive Unit Drag Analysis with TBDP©/VBDP©
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Figure 1 - Lower Unit Drag design & dimensional inputs

Figure 2 - Lower Unit Drag coefficient vs Velocity graphic output

Figure 3 - Images of cavitation and ventilation patterns for gearcase geometry shows cavitation behind water inlet holes and ventilation from exhaust gas exit behind the propeller hub

Figure 4 - Lower Unit height affects drag and performance
(note comparison of different lower unit depth in water)

Figure 5 - Lower Unit Drag vs Velocity graphic output

Figure 6 - TBDP/VBDP input screen for Lower Unit Design info


TBDP©/VBDP© Drive Unit Drag Analysis calculate drag and coefficients for ANY drive design configuration throughout the entire hull operating velocity range

AR® has developed complex algorithms that calculate hydrodynamic drag of the outboard lower unit or I/O outdrive design & configuration.  This performance is different for each design, each setup arrangement and drag contributors are 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 (see below) that determine the overall drag of the lower unit. 


The fluid flow field around the lower unit components exists in all of water, water vapour (cavitation), air and exhaust gases.  All of these constituents and fluid phases are exposed to interaction with the lower unit.


 The TBDP©/VBDP© analyzes the specified design of the skeg, the leg, 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 50+ OEM drives, including NEW massive Yamaha and Mercury HD 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 and G2 gearcases, Alpha, Bravo, Volvo IO drives, Imco SCX, Ilmor Indy drives, even Arneson surface drives and RC outdrives.

How It Works:

Hydrodynamic drag is calculated for each of key drag contributors based on the conditions throughout the entire operating velocity range.  Motor height and trim angle affects how much of the drive unit is immersed in flow, generating drag.  The contributors to overall drive unit drag include components of friction drag, pressure drag, spray drag and induced drag from:

  • Torpedo drag - submerged and partially submerged blunt and streamlined bodies, with cavitation and ventilation
  • Skeg drag - thin plate (high L/d) in turbulent parallel flow
  • Leg drag - surface piercing streamlined bodies in turbulent flow, also exposed to spray drag
  • Induced Drag - from lifting and side forces generated
  • Spray drag - of surface piercing bodies, increased surface wetting

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/vee running surfaces. ('+ve' is positive or less submerged; '-ve' is negative or more immersed). This “height” (depth) or motor lift dimension affects MOTOR DRAG and the moment force contributing to dynamic stability.

TBDP©/VBDP© software generates accurate performance prediction: 

AR software generates accurate performance results throughout the full operating velocity range, showing the total hydrodynamic drags at Drive/Lower Unit AND the impact on overall perforamnce.  Compare quickly what the performance differences are for:

  • different lower unit selection
  • change in lower unit Height
  • Lower Unit modifications

Example: "What is my top speed difference if I change my engine Height from -3" (above) to 0" (even) depth to hull bottom?

Your results are unique to your hull design, engine power and boat setup - so just input the info to TBDP©/VBDP© and complete the 1-2-3 performance analysis to easily see the results differences with your before/after case:

  • Input your current engine height and complete the 1-2-3 performance analysis to easily see the detailed results.
  • Change engine height to your 'test' case setup and complete the performance analysis again to see the changed results.
  • View the Performance graphic charts to see the COMPARISON results showing your before and after comparison for all aspects of performance, through the full operating velocity range.




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

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


Research results now included in performance analysis by TBDP©/VBDP©

[more about AR's research  more about AR's publications and technical articles/papers]

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Material from this website may be not copied or used or redistributed, in whole or in part, without specific written consent of Jim Russell or AeroMarine Research®.