Advanced Aerodynamics and Hydrodynamics for Powerboats

ADVANTAGE

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
(comparison of lower unit depth of -3" (below) water to 0")

Figure 5 - Torpedo diameter affects drag and performance
(comparison of lower unit Torpedo design of 3" diameter compared to 4.25" diameter

Figure 6 - Lower Unit Drag vs Velocity graphic output

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

Figure 7 - Multi-engine interference drag increases when lower units are located in close proximity

VBDP V8, "Lower Unit design" Performance Analysis video

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  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, direct (inboard) drives.

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
• Multi-engine Interference drag - when lower units are located in close proximity

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.
• Multi-engine spacing -center-to-center spacing between lower units when multi-engine installations.

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 performance.  Compare quickly what the performance differences are through the velocity range for:

• different lower unit selection
• varying lower unit Heights
• Lower Unit modifications
• Running trim angles
• different engine spacing (multiple engines)

Determine quickly effects of drive unit design/setup on:

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

All above research results included in performance analysis software by TBDP©/VBDP©

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