How to Build an RC Boat with a Weedeater Motor (Trimmer Engine Conversion)
A string trimmer (weedeater) engine is a small two-stroke, usually 25–33 cc, with a centrifugal clutch, and it is the same type of engine that powers many gas RC boats. Converting one means mounting it in a strong hull, adding water cooling (trimmer engines are air-cooled by a fan the boat can’t use well), connecting it to a flex shaft and propeller through a stuffing tube, and controlling the throttle and an ignition kill switch from the radio. It’s a big, fast and loud project best suited to builders with some engine experience.
By Kyle HilsonUpdated: October 20264 min read
What you need
| Part | Notes |
|---|---|
| Trimmer engine 25–33 cc | Running well, with clutch |
| Hull 1.2 m or longer | Fibreglass or strong plywood; deep-V or catamaran |
| Water-cooling jacket or head | Marine conversions sold for common engines |
| Flex shaft, stuffing tube, strut, prop | Size matched to the engine (often 1/4″ or 0.25″ cable) |
| Coupler and engine mount | Aligns the engine with the shaft |
| Radio, receiver, 2 servos, receiver battery | Throttle and steering; waterproof radio box |
| Kill switch | Radio-operated ignition cutoff |
| Rudder with water pickup | Feeds the cooling line |
The hull
A 25–30 cc engine needs a hull of roughly 1.2–1.5 m to carry its weight and torque. Ready-made fibreglass gas hulls save a lot of time and are designed for the loads; a scratch-built plywood hull must be glassed inside and out. Reinforce the engine mount area with hardwood rails and leave space for the radio box behind the engine. See large-scale RC boats for ready hulls.
Converting the engine
- Remove the trimmer’s shaft, handle, guard and throttle cable.
- Fit a water-cooled head or cooling jacket, or a marine flywheel; trimmer fans give little cooling in an enclosed hull.
- Replace the exhaust with a marine tuned pipe or water-cooled muffler; route exhaust away from the radio box.
- Fit the clutch bell to a coupler that connects to the flex shaft.
- Adapt the carburettor’s throttle arm for a servo linkage.
Drive line and cooling
The flex shaft runs inside a stuffing tube through the hull to a strut and the propeller. Line the engine up so the shaft runs straight without sharp bends; misalignment breaks cables. Grease the cable before every session. Cooling water comes from a pickup on the rudder, flows through the jacket and exits through the side of the hull, where you can see the stream to confirm it works.
Radio and controls
- Use a waterproof radio box with sealed servo pushrod exits.
- A strong steering servo; gas boats load the rudder heavily.
- The throttle servo returns to idle when the radio fails (set failsafe).
- A radio-controlled kill switch so you can stop the engine on the water.
- Keep the receiver antenna away from the ignition.
Choosing engines and motors in general is covered in RC boat motors.
Testing
- Run the engine on the bench with water cooling connected from a hose.
- Check the failsafe: turn off the transmitter, the throttle must close and the kill switch should trigger.
- First run on calm water at low speed; check balance (CG) and that the hull doesn’t take water.
- Adjust the strut angle and prop for top speed once it runs straight.
Safety
A 1.2 m gas boat at full speed can seriously injure a swimmer. Run only where boats are allowed and nobody swims, use a retrieval boat or line, carry fuel in an approved can, and check local rules for gas-powered models; some lakes ban internal combustion engines.
FAQ
How fast is a weedeater boat?
It depends on hull, prop and tuning; well-sorted gas boats are fast enough to need a big lake.
Do I need water cooling?
Strongly recommended; an enclosed hull traps heat.
What size engine?
25–33 cc trimmer engines are the common choice.
Can I use a 4-stroke trimmer?
Possible but heavier and harder to convert; two-strokes are standard.
More on this topic: the complete RC Boats guide covers the basics behind this article.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
