Building a Shallow-Water RC Jet Boat: A Step-by-Step Build Log

Building a Shallow-Water RC Jet Boat: A Step-by-Step Build Log

This build log documents my recent RC jet boat project with a focus on impeller design, shallow water running, 3D printed hulls and cooling strategies for sustained runs. The brief was simple: a compact, light boat that will operate reliably in canals and shallow ponds while still delivering sensible speed and control. I started with sketches in Fusion 360 and a list of constraints including maximum draft, motor size and battery capacity. The aim was a tunnel-style hull to protect the intake while keeping the centre of gravity low and the overall weight under 1.2 kilograms.

Design and planning began with hull geometry and intake placement because shallow-water performance is largely determined by how the boat breathes. I designed a shallow tunnel with a recessed intake forward of the jet nozzle to reduce the chance of sucking air over waves and to limit weed intake when running in taped-off canals. The intake uses gentle bevels and a short converging duct to increase flow velocity into the impeller at low depths. I selected a brushless motor with moderate KV that matched a 28 mm impeller diameter at expected battery voltages, and planned for a nozzle to improve thrust while protecting the blades.

Impeller design took the most iterations and testing time because jet efficiency in shallow water is unforgiving. I started with a 3-blade, semi-axial profile printed in PETG for rapid prototyping, then moved to a 4-blade with a higher chord near the hub to handle debris strike loads. Balance matters more with small diameters, so every printed prototype was statically and dynamically balanced on a simple jig before testing. Smoothing printed surfaces and applying a thin epoxy coat reduced surface roughness and improved cavitation resistance. Testing on a bench with a tachometer and thrust scale allowed me to adjust pitch, blade skew and shroud clearance to find a compromise between low-speed thrust and top-end efficiency.

For the hull I embraced 3D printing for rapid iteration and complex internal geometry that would be difficult to form traditionally. I printed the hull in PETG with 0.6 mm nozzle lines for faster prints and set 3–4 mm walls with 40 percent infill to keep strength without excess weight. Key areas such as motor bulkhead and shaft bearings were reinforced with carbon rod and epoxy fillets after printing to avoid flex under load. Sealing is critical, so I used a liberal internal epoxy coat and added a removable hatch with an O-ring for battery and ESC access. Positioning the motor pod and nozzle as a removable unit made maintenance straightforward and allowed me to test different impeller/nozzle combinations without reprinting the whole hull.

Cooling in a jet boat is tricky because the craft does not have a propeller shaft passing through the hull that can be easily used for pickups, so I designed an internal water jacket that uses diverted jet flow to carry heat away. A small pickup in the intake duct feeds a narrow channel beside the ESC and motor housing, and returns the warmed water back into the tunnel where it mixes with the main flow. Using thermally conductive adhesives and aluminium heat spreaders on the ESC helped lower peak temperatures during sustained runs. For shallow water the pickup sits slightly higher than usual and is protected by a mesh trap to keep gravel and weeds out while still allowing sufficient coolant flow.

Shakedown and tuning came last, and I ran a checklist that started with bench rpm and thrust tests, then short slow runs to verify cooling and waterproofing, followed by incremental speed runs while logging temperatures and battery draw. Trim was adjusted with small pods and the nozzle angle to prevent ventilation and to keep the boat settled in a shallow run. In practice I found that running at slightly higher rpm with a larger nozzle helped keep the flow steady in weedy water, and routine clearing of the intake mesh reduced heating issues. If you want the CAD files and a photo series from this build you can find them on my site at https://watdafeck.uk for download and reference.

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