Start with the full-size ship: scale speed
The most common complaint about scale models on the water is that they go too fast. A 1:50 cargo ship tearing across the pond with a bow wave up to its deck rarely looks right. Before choosing a motor, it helps to know how fast the model should go.
Model boat builders usually use Froude scaling: a model looks right on the water when its speed is the full-size speed divided by the square root of the scale factor. For example:
- A 1:36 model: divide the full-size speed by 6.
- A 1:49 model: divide by 7.
- A 1:100 model: divide by 10.
So a ship with a service speed of 12 knots modelled at 1:36 should look realistic at roughly 2 knots. This is a useful target rather than a hard rule, and you will want some extra power in reserve for wind, current and getting out of trouble. But it points most scale models toward low-revving, efficient drive setups rather than raw power.
Choosing a motor
Brushed can motors
Brushed motors are still common in scale models. They are cheap, simple to wire (two wires), run happily on a basic brushed ESC, and are easy to reverse. They are often described by “can” size, such as 380, 400, 540 or 600, though exact dimensions and performance vary by maker. Smaller cans suit small, light hulls; larger and multi-pole motors, such as those sold for scale boat use, tend to suit bigger, heavier models that need torque at low revs.
Many brushed motors spin faster than a scale prop wants. A gearbox or belt reduction lowers the shaft speed and increases torque, letting the motor run in a more efficient range while the prop turns slowly. This is a common approach in tugs, trawlers and other models with large props, such as the Clyde puffers and beam trawlers in our working boats guide.
Brushless motors and gearboxes
Brushless motors are more efficient, have no brushes to wear or cause radio interference, and are increasingly used in scale models. For scale use, choose a low-kV (low revs per volt) motor, sometimes called an outrunner, or pair a higher-kV motor with a gearbox. Brushless motors need a brushless ESC with reverse, which not every boat ESC offers — fast-electric ESCs are often forward-only. See our guide to choosing a brushed or brushless ESC for the details.
Whatever motor you choose, mount it firmly, in line with the shaft, on a mount that can be removed later.
Choosing a propeller
Diameter, pitch and blade count
Three numbers describe a model prop:
- Diameter — the overall size. Scale models usually use a prop close to the scale size of the real one, limited by the space between hull and rudder.
- Pitch — how far the prop would move forward in one turn if it were screwing through a solid. Higher pitch means more thrust per revolution and more load on the motor.
- Blade count — 2, 3, 4 or 5 blades. Scale models often match the real ship. More blades spread the load and can run more smoothly at a given diameter, though a two- or three-blade prop is usually more efficient for fast boats.
A larger diameter, slower-turning prop is generally more efficient for heavy scale hulls, which is why gearing is popular. If a motor runs hot or the current is high, a smaller or lower-pitch prop is often the first change to try.
Handedness and twin-screw rotation
Props are left-handed or right-handed, depending on which way they turn to push the boat forward. The prop must match the motor’s direction of rotation in forward; a prop turning the wrong way will still push, but weakly.
On single-screw models, prop torque can pull the stern to one side, which you correct with a little rudder trim. On twin-screw models, builders usually fit one prop of each hand so the torque effects cancel out. Many full- size twin-screw ships are arranged with the props turning outward at the top when going forward, and modellers often copy that, but check the drawings for your ship.
You set each motor’s direction in the wiring, as described in our RC setup guide.
Shafts and couplings
The prop shaft runs through a stern tube that keeps water out of the hull. Between the motor and the shaft there is usually a coupling that tolerates small misalignments:
- Universal joint couplings handle some angle between the motor and shaft.
- Oldham-type couplings handle small offsets.
- Flexible tube couplings are simple and forgiving but limited in torque.
Even with a flexible coupling, aim for the motor and shaft to be as close to in line as you can get. A misaligned drivetrain draws more current, makes more noise and wears faster. Our guide to fitting and sealing a stern tube and prop shaft covers installation and alignment.
Kort nozzles for model boats
What a Kort nozzle does
A Kort nozzle is a ducted ring fitted around the propeller. It is common on tugs, trawlers, pushboats and some workboats. On full-size ships, the duct is used to improve thrust at low speeds and high loads — for example, when a tug is pulling hard but moving slowly. On a model it can do something similar, and on tug models it is often fitted for scale appearance as much as performance. If you’re scratch-building a tug, our roundup of free model tug boat plans is a good place to start.
A Kort nozzle generally helps most when a boat is doing heavy work at low speed. On faster, lightly loaded hulls it may add drag without much benefit.
Fixed vs steerable nozzles
- Fixed nozzles are attached to the hull, with a rudder (or rudders) behind them.
- Steerable nozzles pivot on a rudder stock, so the whole duct turns and directs the propwash. This can give very tight turns, which is why it suits some tug and trawler models.
Follow the arrangement on the real vessel if you are building to scale.
Fitting a Kort nozzle
Some practical points from common workshop practice:
- Match the prop to the nozzle. Props used inside nozzles typically have broad, squared-off blade tips (often described as Kaplan-style), so the tips run close to the duct wall. A standard rounded-tip prop leaves a larger gap.
- Keep tip clearance small and even. The gap between blade tips and the inner wall should be small but never touching. Centre the nozzle carefully on the shaft, and check clearance at several points as you turn the prop by hand.
- Mount the nozzle firmly. Nozzles take load in use and can be knocked by debris or the bank; a flimsy mount lets it move and foul the prop.
- Check the shaft is true. A bent shaft or worn stern tube bushes can let the prop wobble and strike the nozzle.
- Allow for the extra load. A nozzle can change how much the prop loads the motor. Watch motor current and temperature on the first runs.
Ready-made nozzles in plastic or brass are sold in a range of sizes to suit standard prop diameters. If you are building from a kit that includes one, check that your chosen prop fits it before fixing the nozzle in place.
Other propulsion types
Scale models sometimes need something other than a conventional prop:
- Paddle wheels — side or stern wheels driven at low revs, usually through heavy gearing. The wheels need to sit at the right depth to work well, so ballasting matters. See paddle steamer model kits and the Waverley for building fixed and feathering wheels.
- Voith-Schneider propellers — vertical blades on a rotating disc that can push in any direction. Working models exist, but they are mechanically complex.
- Azimuth (Z-drive) thrusters — a prop in a pod that rotates to steer the boat, common on modern tugs. Model versions are available, typically needing an extra servo or a continuous-rotation mechanism.
- Bow and stern thrusters — a small motor and prop in a tube across the hull, giving sideways thrust for berthing. They need their own channel and usually a small ESC.
- Water jets — used on some fast patrol and rescue craft; model units are available for these types. Our Shannon class lifeboat article walks through a twin-jet installation.
Steam is another option for those who want real working machinery; see our guide to building a model steam boat.
Testing and tuning
Once everything is installed:
- Run the motor on the bench with the prop fitted and the boat supported, to check rotation and listen for vibration.
- Do the first runs at low throttle and check current draw if you have a wattmeter.
- Watch the boat at its scale speed. If it squats at the stern or throws a large bow wave, it is probably going too fast or sitting out of trim.
- After a few minutes, feel the motor, ESC and battery. If anything is hot, try a smaller or lower-pitch prop, or more reduction.
- Make sure the model sits on its waterline before judging performance; a boat that is too light or too heavy will behave differently. Our guide to ballasting an RC model boat covers this.
Changing the prop is usually the cheapest and quickest way to tune a scale model. Keep notes on each prop you try, and you will soon find the combination that looks right on the water.