Introduction
The B-Series propeller is one of the most widely used marine propeller series in the world. Developed
by the Netherlands Ship Model Basin (MARIN) through extensive model testing, B-Series propellers
provide reliable, well-documented performance data that makes propeller selection predictable and accurate.
🎯 What You'll Learn:
- What makes B-Series propellers special
- Propeller geometry and terminology
- Diameter selection and optimization
- Pitch calculation and advance coefficient
- Cavitation analysis and prevention
- Blade area ratio considerations
- Practical optimization tips
1. What is B-Series?
1.1 Historical Background
The B-Series (Wageningen B-Screw) was developed starting in the 1950s by MARIN (formerly NSMB).
Through systematic model testing of hundreds of propeller configurations, they created standardized
propeller geometries with published performance data in open-water charts.
1.2 Why B-Series is Popular
- Extensive Testing Data: Decades of model test results
- Predictable Performance: Published KT, KQ, and ηo charts
- Wide Application: Works for most commercial and recreational vessels
- Reliable: Proven in thousands of real-world installations
- Well-Understood: Design methods mature and documented
1.3 B-Series Configurations
| Model |
Number of Blades |
Blade Area Ratio (BAR) |
Typical Application |
| B3-50 |
3 |
0.50 |
Sailboats, small craft |
| B4-40 |
4 |
0.40 |
Pleasure boats, tugs |
| B4-70 |
4 |
0.70 |
Trawlers, workboats |
| B5-60 |
5 |
0.60 |
Commercial vessels |
| B5-80 |
5 |
0.80 |
High-loaded applications |
2. Propeller Geometry
2.1 Key Dimensions
Propeller Parameters:
Diameter (D): Distance from tip to tip
Pitch (P): Theoretical forward distance per revolution
Pitch Ratio (P/D): Pitch ÷ Diameter
Blade Area (AE): Projected area of all blades
Expanded Area Ratio (EAR): AE / (πD²/4)
Number of Blades (Z): Typically 3-6
Rake: Axial tilt of blades (0-15°)
Skew: Blade offset from radial (reduces vibration)
2.2 Pitch Explained
Pitch is the distance a propeller would move forward in one revolution if it were screwing through
a solid (like a wood screw). In water, the actual advance is less due to slip.
Pitch and Slip:
Theoretical Speed:
Vtheoretical = RPM × Pitch / 60
Actual Speed:
Vactual = Vtheoretical × (1 - Slip)
Slip:
Slip = (Vtheoretical - Vactual) / Vtheoretical
Typical slip values: 10-20% for displacement hulls, 5-15% for planing hulls
3. Diameter Selection
3.1 Maximum Diameter Constraints
The ideal propeller is the largest one that fits. Larger diameter = higher efficiency. However,
practical constraints limit diameter:
- Hull Clearance: Tip must clear hull by 15-20% of diameter
- Aperture Size: Space available in hull aperture
- Shaft Angle: Inclined shafts require smaller diameter
- Draft: Deeper boats can accommodate larger propellers
3.2 Optimal Diameter Calculation
Approximate Optimal Diameter:
D = 15.2 × (Pdelivered0.2) / (RPM0.6)
Where:
• D = Diameter (inches)
• Pdelivered = Delivered power (HP)
• RPM = Propeller RPM
Example:
Engine: 300 HP @ 4000 RPM
Gear ratio: 2:1 → Prop RPM = 2000
D = 15.2 × (3000.2) / (20000.6)
D = 15.2 × 3.1 / 71.5 = 0.66 ft = 7.9 inches
💡 Pro Tip: If your calculated diameter doesn't fit, adjust pitch ratio. Higher pitch
ratios can recover some efficiency lost from smaller diameter. Generally, increase P/D by 0.1-0.2
for every 10% reduction in diameter.
4. Pitch Calculation
4.1 Required Thrust
Pitch must be selected to provide enough thrust to overcome hull resistance at design speed.
Thrust Requirement:
T = R / ηprop
Where:
• T = Required thrust (N)
• R = Hull resistance at design speed (N)
• ηprop = Propulsive efficiency (0.60-0.70 typical)
4.2 Advance Coefficient (J)
Advance Coefficient:
J = Va / (n × D)
Where:
• Va = Advance speed (m/s) = Vboat × (1 - wake)
• n = Propeller rotational speed (rev/s)
• D = Diameter (m)
Typical J Values:
• Displacement hulls: J = 0.3-0.5
• Planing hulls: J = 0.6-0.9
4.3 Using B-Series Charts
B-Series charts provide KT (thrust coefficient), KQ (torque coefficient),
and ηo (open water efficiency) as functions of J for various P/D ratios.
- Calculate required advance coefficient J
- Estimate target P/D based on application (0.7-1.2 typical)
- Look up KT and ηo from B-Series charts
- Calculate thrust: T = KT × ρ × n² × D⁴
- Check if thrust meets requirement
- Iterate P/D if needed
5. Cavitation Analysis
5.1 What is Cavitation?
Cavitation occurs when local pressure on blade surfaces falls below water vapor pressure, causing
bubbles to form. When bubbles collapse, they cause:
- Erosion: Pitting and damage to blade surface
- Noise: Loud "gravel" noise
- Vibration: Increased vibration levels
- Efficiency Loss: Reduced thrust
5.2 Cavitation Number
Cavitation Number:
σ = (p - pv) / (0.5 × ρ × V²)
Where:
• p = Local pressure (Pa)
• pv = Vapor pressure (Pa)
• ρ = Water density (kg/m³)
• V = Local velocity (m/s)
Rule of Thumb:
• σ > 2.0: No cavitation
• 1.0 < σ < 2.0: Possible cavitation
• σ < 1.0: Severe cavitation likely
5.3 Blade Area Ratio
To prevent cavitation, increase blade area ratio (EAR). More blade area = lower loading per area
= less cavitation risk.
Minimum EAR for Cavitation-Free Operation:
EARmin = (T/D) / (K × p)
Where:
• T = Thrust (N)
• D = Diameter (m)
• p = Ambient pressure (Pa)
• K = Empirical coefficient (typically 15-25 kN/m²)
If required EAR > available, increase diameter or reduce power.
6. Blade Area Ratio Selection
| EAR Range |
Application |
Cavitation Risk |
| 0.30-0.45 |
Sailboats, low power |
Low (light loading) |
| 0.45-0.60 |
Pleasure boats, fishing boats |
Low-Moderate |
| 0.60-0.80 |
Workboats, trawlers |
Moderate |
| 0.80-1.00 |
High-speed craft, tugs |
Moderate-High |
| 1.00-1.50 |
Naval, patrol boats |
High (heavy loading) |
7. Optimization Tips
7.1 Efficiency Optimization
- Maximize Diameter: Larger D = higher efficiency (within constraints)
- Optimal P/D: Usually 0.8-1.0 for best efficiency
- Minimize Blade Number: Fewer blades = higher efficiency (but more vibration)
- Match RPM: Select gear ratio to optimize propeller RPM
7.2 Number of Blades
| Blades |
Pros |
Cons |
Best For |
| 3 |
High efficiency, simple |
Vibration, less thrust |
Sailboats |
| 4 |
Good balance |
Slightly less efficient |
Most applications |
| 5 |
Smooth, low vibration |
Lower efficiency |
Commercial vessels |
| 6+ |
Very smooth |
Low efficiency, expensive |
Yachts, navy |
7.3 Gear Ratio Selection
Propeller RPM has major impact on diameter and efficiency:
Optimal Propeller RPM:
RPMprop = RPMengine / Gear Ratio
Guidelines:
• High-speed planing hulls: 800-1200 RPM
• Semi-planing hulls: 600-900 RPM
• Displacement hulls: 400-700 RPM
Lower RPM = larger diameter = higher efficiency (but larger size)
8. Practical Design Example
Design a Propeller for a 12m Patrol Boat
Given:
Engine: 2x 300 HP @ 4000 RPM
Gear ratio: 2.0:1
Design speed: 30 knots
Resistance at 30 kn: 18 kN
Shaft depth: 0.6m below waterline
Step 1: Propeller RPM
RPM = 4000 / 2.0 = 2000 RPM
Step 2: Diameter (use formula)
D = 15.2 × (3000.2) / (20000.6) = 7.9 in (use 14" for hull clearance)
Step 3: Required Thrust
T = 18,000 / 0.65 = 27,700 N per propeller
Step 4: Select B-Series
Choose B4-70 (4 blades, EAR=0.70) for cavitation resistance
Step 5: Calculate Pitch
Using B4-70 charts with J=0.75, P/D=0.95
Pitch = 0.95 × 14 = 13.3 in (use 13")
Final Specification:
• 14" diameter × 13" pitch
• 4 blades, EAR=0.70
• B4-70 series