tbdp - drive unit analysis


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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 (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* *Figure 8 - Lower Unit thrust angle* *Figure 9 - Lower Unit Resolved Lift, Induced Drag* VBDP V8, Lower Unit design" Performance Analysis video [Note: video reflects VBDP Version 8.8.3, current software version is ]		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, Merc 'R-drive', Merc OB-SSMIV, 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 Onedrive, Indy drives, even Arneson surface drives and RC outdrives, direct (inboard) drives, and more. Of course, you can manually enter your drive dimensions yourself, so ANY drive sizing is possible! 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 Also accounted for... torpedo lift torpedo induced drag, torpedo resolved lift correction 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. Torpedo/shaft thrust angle 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 size/shape/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: lower unit drag drive unit spray drag lower unit resolved lift required HP hull attitude/trim angle max hull speed acceleration/elapsed time Example: "What is my top speed difference if I change my engine Height from -3" (below) to 0" (even) depth to hull bottom?" Example: "What is my performance difference if I change my lower unit design with Torpedo of 5" diameter to a Torpedo 4.25" diameter?" 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. Interference Drag - Research has also shown that interference drag occurs when lower units of multi-engine (outboard or sterndrive) installations are located in close proximity to each other. As lower units are closer together, interference drag can become significant. AR® has developed analysis techniques for calculating 'multi-engine interference drag' and TBDP©/VBDP© accounts for this drag based on lower unit configuration and multi-unit spacing. (see more on Multi-Engine Interference Drag). Drive/Torpedo Angle - Normal setups have the Drive/Torp Angle = NEUTRAL relative to water surface (Drive/Torp Angle = 0 degrees is NEUTRAL). This means that when the driver manually changes Trim UP or DOWN during operation to adjust the hull's Angle of Attack, the HULL angle changes, but the Drive/Torp Angle angle normalizes itself to NEUTRAL. Some setups require positive (+) or negative (-) Torpedo angle in order to keep the HULL at the desired Angle of Attack (WAngle). In these cases, the Torpedo angle in unable to 'normalize' itself to the neutral position when adjusting the hull Angle of Attack. This is usually less efficient, since it adds DRAG and reduces effective THRUST. Use this setup condition with CAUTION. Conditions resulting from Drive/Torpedo Angle NOT zero: Resolved Thrust - a Thrust Angle aiming UP (+ Lift) or DOWN (- Lift) adds lift or subtracts lift on hull, affecting required LTotal and Dynamic Stability. Reduced Thrust - due to thrust directed UP/DOWN, effective (forward) thrust is less than total THRUST Torpedo Lift - Lift from torpedo as lifting surface aiming UP (+ Lift) or DOWN (- Lift) adds lift or subtracts from total required Lift Induced Torpedo Drag - Induced Drag caused since torpedo generates Lift, adds to total Lower Unit Drag

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.

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]



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