Technical Integration · REV C · Surface Drive Config + Reduction Drive · 2026-04-05
40 kW Surface Drive
Transom Pod Integration
6 m aluminium catamaran · Transom Drive 20 · Nibral 20″ prop · no rudder · near-zero draft appendage · speed-reduction drive
Surface-piercing prop
Steerable pod · no rudder
~150 mm operational draft
No stern tube · no shaft seal
Reduction drive · 3:1–5:1 ratio
40 kW
Transom Drive 20 · Nibral 20″
3-blade · 508 mm dia
Bus voltage
417 A continuous · 95–120 mm² HV feed
Motor power
40
kW continuous
Cont. current
417
A @ 96 V
Prop diameter
20“
508 mm · Nibral
Appendage draft
~150
mm (prop arc only)
Design speed
8–10
knots · full power
System architecture — power & signal flow · surface drive config
Transom Drive 20
Profile & transom arrangement — surface drive pod position
No rudder · no stern tube · no shaft seal
Propeller selection — Transom Drive 20 · Nibral options
| Option | Diameter | Blades | Material | Unit compatibility | Touring boat suitability | Notes |
|---|---|---|---|---|---|---|
| Selected | 20″ (508 mm) | 3-blade | Nibral | Transom Drive 20 only | Best for 40 kW · highest efficiency · lower RPM | Larger disc area → better low-speed thrust. Requires TD20 bracket. |
| Alternative | 17″ (432 mm) | 3-blade | Nibral | TD17 · SternPowr 107 · MCR III SSM | Suited to lower power (20–30 kW range) | Adequate for 40 kW but will require higher RPM for same thrust — less efficient. |
| Alternative | 17″ (432 mm) | 4-blade | Nibral | TD17 · SternPowr 107 · MCR III SSM | Better low-speed than 3-blade 17″ | Smoother torque delivery, less vibration — good for tour comfort if staying with TD17. |
Pitch selection note: Surface-piercing props are typically pitched for higher speed operation. At 8–10 knot touring speeds, pitch must be reduced compared to standard inboard props — typically a lower pitch-to-diameter ratio (0.8–1.0 P/D). Final pitch should be confirmed with the drive manufacturer at your motor’s rated RPM. Underpitching is always recoverable; overpitching will overload the motor at low speed.
Drive & propulsion
Drive unitTransom Drive 20
PropellerNibral 20″ · 3-blade · solid hub
Prop diameter508 mm
Prop positionSurface-piercing at waterline
Appendage draft~150 mm (prop arc below WL)
Bracket mountTransom face · external · adjustable
Reduction driveGearbox or toothed belt · see below
Reduction ratio3:1 – 5:1 (to be confirmed)
Motor RPM (typical)3,000–5,000 RPM
Output RPM (target)800–1,400 RPM at prop
Shaft typeTwo sections: HiSpd + LoSpd · inside hull
Stern tubeNone required
Shaft sealNone — no through-hull penetration
SteeringPod rotation · hydraulic actuator
RudderNot required
ReverseMotor reversal via VFD · no reversing gearbox
Input couplingFlexible bellows · motor to reduction input
Output couplingRigid or flexible · reduction output to pod
Design speed8–10 knots
Power electronics
Bus voltage96 V nominal
VFD continuous I417 A DC input
VFD peak I (30 s)~625 A
VFD efficiency95–97%
HV DC cable95–120 mm² dual-core flex
3φ AC cable (VFD→motor)35–50 mm² screened · short run
Main fuse500 A Class-J at battery +
Pre-chargeMandatory · 50–100 Ω resistor + relay
Main contactor500 A DC rated
IP rating — VFDIP54 · dry aft compartment
IP rating — motorIP67 min
Regen brakingYes — drag braking returns to battery
Aux DC-DC12 V / 50 A for ship systems
Battery
ChemistryLiFePO₄ prismatic
Voltage96 V
Cell config30s2p (3.2 V cells)
Capacity100–200 Ah
Energy10–19 kWh
Mass budget~280–320 kg
PlacementKeel · bridgedeck · low CG
BMSActive balance · CAN output
Shore charge rate1–2 C max per BMS
Steering & control
Helm inputWheel → hydraulic pump
Steer actuatorElectro-hydraulic · pod bracket
Steer angle±30° pod rotation
ThrottleSingle lever · 0–5 V / CAN
MFD displaySOC · speed · power · temp
E-stopKill switch + lanyard · helm
InterlockBMS fault → VFD inhibit
CAN busVCU ↔ VFD ↔ BMS ↔ MFD
Thermal & safety
Motor coolingWater-cooled housing · raw water
VFD coolingHeatsink + forced air · louvres
Battery thermalPassive · bilge ventilation
Over-temp cutoffBMS → contactor open
Bilge alarmFloat switch → helm alarm
HV interlockService hatch switch
Fire suppressionABC ext. · motor bay
Galvanic isolationShore charger isolator · mandatory
Cable schedule · surface drive config
| Type | From → To | Size @ 96 V | Route | Notes |
|---|---|---|---|---|
| HV DC | Battery → Contactor → VFD input | 95–120 mm² flex, dual-core | Keel duct, watertight, aluminium conduit, bonded | Pre-charge relay (50–100 Ω) in parallel with main contactor. Fused at battery +ve terminal. |
| 3φ AC | VFD output → Motor terminals | 35–50 mm² screened, 3-core | Rigid conduit, motor bay. Keep run <3 m. | Screen bonded at VFD chassis only. Separate conduit from HV DC. |
| HYD STEER | Helm pump → Pod bracket actuator | 6 mm OD hydraulic hose, 200 bar rated | Transom penetration · watertight gland | Separate from all electrical runs. Single lock-to-lock steer angle: ±30°. Use marine-grade hose fittings. |
| CAN | VCU → VFD → BMS → MFD (daisy chain) | 2× 0.5 mm² twisted pair, shielded | Separate conduit, min 100 mm from HV cables | 120 Ω termination each end. Shield bonded at one point only. |
| SHORE AC | Shore inlet → Galvanic isolator → Charger | 6 mm² / 32 A marine cable | Aft deck socket → charger below sole | Galvanic isolator mandatory. ELCB on AC inlet. Waterproof shore connector. |
| 12 V AUX | DC-DC → Nav / lights / bilge / helm | 4–6 mm² per circuit, fused individually | Helm console panel | Isolated from HV bus. Single-point hull bonding. |
Reduction drive options — gearbox vs toothed belt
A 40 kW PMSM typically delivers peak torque at 3,000–5,000 RPM. The Transom Drive 20 and Nibral 20″
propeller require 800–1,400 RPM at hull speeds of 8–10 knots. A reduction drive of approximately
3:1 to 5:1 is required between motor output and pod input shaft.
Two practical options are compared below.
| Parameter | Marine Gearbox | Toothed Belt Drive (HTD/GT) | Recommendation |
|---|---|---|---|
| Reduction ratio | Fixed · typically 2:1–5:1 | Very flexible · 1.5:1–10:1 via pulley sizing | Belt: easier to tune ratio exactly |
| Efficiency | 96–98% (helical gears) | 96–98% (toothed belt, no slip) | Comparable — both acceptable |
| Torque capacity at 40 kW | Easily handles 100+ Nm | Requires correct belt width / tension — calculate carefully | Gearbox: inherently more robust |
| Reverse capability | Reversing gearbox adds cost/complexity — not needed (VFD reverses motor) | Belt runs reverse freely with motor reversal | Belt: simpler for VFD-controlled reverse |
| Shaft alignment tolerance | Requires precise axial + radial alignment | Centre-distance flexible · minor misalignment tolerated | Belt: more forgiving in boat builds |
| Noise & vibration | Gear mesh noise — isolate from hull | Near-silent · no gear mesh · good vibration isolation | Belt: better for passenger vessel |
| Maintenance | Oil changes · seals · periodic inspection | Belt replacement every 2,000–4,000 hrs · check tension regularly | Both require periodic service |
| Ingress protection | Sealed housing · IP65+ typical | Open belt drive needs sealed enclosure (bilge moisture) | Gearbox: better in marine bilge environment |
| Mass (indicative) | 15–40 kg depending on ratio & make | 3–8 kg (pulleys + belt + tensioner) | Belt: significant weight advantage |
| Cost (indicative) | USD 800–3,000 (marine-rated gearbox) | USD 150–500 (industrial pulleys + belt) | Belt: much lower upfront cost |
Design recommendation: For this application a
toothed belt drive (HTD 8M or GT3 8M profile)
is the preferred option — lower mass, quieter operation, easy ratio adjustment, and significantly
lower cost. Use a minimum belt width of 30–50 mm and confirm the dynamic torque capacity at peak
motor output (40 kW ÷ [output RPM × 2π/60] Nm). Enclose the belt drive in a sealed aluminium
housing with drain plug. If the installation environment is particularly wet or space-constrained,
a compact helical marine gearbox is the more robust fallback.
RPM calculation note: Final reduction ratio = Motor RPM ÷ Target prop RPM.
Example: 4,000 RPM motor ÷ 1,000 RPM prop = 4:1 ratio. Confirm motor rated RPM at 40 kW with
the motor supplier, then calculate: prop RPM = motor RPM ÷ ratio. Cross-check against drive
manufacturer’s recommended prop RPM range for the Transom Drive 20 at your displacement and speed.
Surface drive advantages in this application: Eliminating the stern tube removes the single most common source of catamaran bilge flooding. No shaft seal to maintain, inspect, or replace. The external bracket allows the drive unit to be inspected, serviced, or swapped without drydocking — important for a commercial tour vessel. Prop replacement is above waterline.
Low-speed performance note: Surface drives are optimised for planing speeds. At displacement/semi-displacement touring speeds (8–10 kn), expect the prop to run partially ventilated rather than fully surface-piercing. This is acceptable — the efficiency is still good — but pitch selection is critical. Request the drive manufacturer’s pitch recommendation for your specific motor RPM at 8–10 knots and a ~1,400 kg displacement.
Transom structural note: The Transom Drive 20 bracket imposes significant thrust and vertical loads on the transom plate. The aluminium transom must be reinforced with a backing plate (minimum 8 mm 5083 plate, 400 × 600 mm) and the bracket bolts should pass through with large-area washers inside. Have the transom structural load verified before drilling.
