Marine steam sizing tool
Model Boat Steam Engine Size Calculator
Input your model boat hull dimensions and craft style to calculate the optimal live-steam engine displacement, torque profile, marine propeller diameter, and boiler pressure requirement.
Hull Parameters
RW-V2
90° Twin-Cylinder Marine Steam Engine
For an 85 cm scale cruiser hull, a 6 cc twin-cylinder engine provides the optimal balance of starting reliability and continuous propeller authority. The 90° cylinder layout ensures instant self-starting in forward or reverse via Stephenson link motion.
Engineering methodology
Key Principles of Marine Steam Sizing
Sizing a live-steam powerplant differs fundamentally from sizing electric or internal combustion motors. Review the thermodynamic and hydrodynamic relationships below.
1. Torque vs. Rotational Speed
Scale steam engines develop peak torque at low rotational speeds (600–1,500 RPM). A model steam engine should swing a large-diameter, high-pitch propeller directly without reduction gearing to maximize hydrodynamic efficiency in scale hulls.
2. Displacement & Dead Centres
Single-cylinder engines (such as the BL1K) are light and fuel-efficient for hulls under 80 cm, but have dead centres. Twin- and four-cylinder engines (V2, L2, V4) phased at 90° guarantee instant self-starting under radio control from any stopped position.
3. Boiler Steam Evaporation Balance
Match boiler output to the engine and follow the matched boiler documentation. The documented normal engine working range is 0.5–2 bar; keep within the lower operating limit of the complete installation. Maximum pressure and safety-valve settings are not continuous operating targets.
Technical FAQ
Frequently Asked Questions
How does the model boat steam engine calculator determine the recommended engine?
The calculator correlates hull waterline length, displacement class (light launch, medium cabin cruiser, or heavy workboat), and water resistance against the continuous torque curves and displacement of Rocking Whale live-steam engines. Single-cylinder plants (RW-BL1K, 3 cc) suit light hulls under 80 cm; 6 cc twin-cylinder engines (RW-V2 and RW-L2) provide self-starting torque for 70–130 cm hulls; and the 12 cc four-cylinder RW-V4 delivers high bollard pull for 90–150 cm heavy-displacement craft.
What propeller size should I use with a miniature model steam engine?
Unlike high-RPM electric motors that swing small propellers at 10,000+ RPM, reciprocating steam engines produce high low-end torque and run best at 600–1,500 RPM swinging larger scale propellers. Single-cylinder engines match 35–45 mm propellers; twin-cylinder engines match 45–60 mm propellers; and multi-cylinder engines like the RW-V4 swing large 60–80 mm multi-blade marine propellers.
Can I reverse a model steam boat using radio control?
Yes. Models equipped with full Stephenson link motion valve gear (such as the RW-V2, RW-L2, and RW-V4) can be reversed directly by connecting a standard RC micro servo to the reversing lever. This alters the valve eccentric lead, reversing propeller rotation smoothly without requiring a mechanical gearbox.
What boiler pressure is required for RC model boat operation?
The documented normal engine working range for RW-BL1K, RW-V2, RW-L2 and RW-V4 is 0.5–2 bar. Always follow the matched boiler documentation and the lower limit of the complete installation. A maximum engine pressure or safety-valve setting is not a normal operating target; request current specifications for a separate boiler.