Converting a Plastic Model Boat to RC

Is your kit a good candidate?

Many plastic ship kits can be made to float and run under radio control, but some are much easier than others. Before you buy or start cutting, think about:

Size. Bigger is easier. Hulls around 50 cm and above give room for a battery, ESC, receiver and servo, plus the ballast needed to bring the model down to its waterline. Very small kits can be converted with micro electronics, but every gram counts.
Hull shape. Fuller hulls with some beam carry weight better and are more stable. Long, narrow warships can be tender (prone to rolling) and need careful ballasting.
Waterline vs full hull. You need a full-hull kit. Waterline kits, such as most 1/700 warship kits, only model the part above the water.
Hull construction. Kits with a one-piece or two-piece lower hull are simpler to seal than hulls made from many small panels.

Popular conversion subjects among hobbyists include larger-scale fast attack craft and PT boats (for example 1:72 and 1:35 kits) and big 1:200 warships, where there is plenty of internal volume. Our guides to PT boat model kits and building a 1/35 Schnellboot cover two of the most popular candidates. Searching for build logs of your chosen kit is worthwhile: someone has often converted it already and found the pitfalls.

Work out a weight budget first

A static kit is light; a working model has to float at its scale waterline, which means its total weight has to match how much water the hull displaces at that line. Too little weight and it floats high and looks wrong; too much and it sinks below the line or swamps.

A rough way to estimate displacement:

Seal the bare hull temporarily (tape over openings).
Mark the waterline.
Float it in a bath or tank and add known weights until it sits at the line.

The total weight it takes is your budget for everything: the hull and superstructure, the electronics, the battery and any ballast. In fresh water, each litre of hull below the waterline supports about 1 kg. Keep notes as you go — it is easier to plan weight than to remove it later.

Strengthening and sealing the hull

Styrene kit hulls are thin and designed to sit on a shelf, not to take knocks at the pond or the flexing of a battery being pushed in and out.

Joining hull halves

If the hull is in two halves, join them with plastic cement and let it cure thoroughly. The seam is the most likely leak point, so many builders reinforce it from the inside with a strip of styrene sheet or a layer of glass cloth and resin over the joint.

Internal reinforcement

Common approaches include:

Styrene frames and stringers glued inside the hull, which add stiffness without much weight.
Light glass cloth laid in with epoxy resin over the inside of the hull, which adds strength and seals the joint. Use thin layers; resin adds weight quickly. The technique is the same as in our guide to waterproofing a wooden hull with epoxy and glass cloth.
Plywood or styrene bearers for the motor, battery tray and servo mounts, so loads are spread over the hull rather than concentrated on one spot.

Check that your adhesive or resin is compatible with styrene before committing; some solvents and resins can attack or soften plastic. Test on a scrap piece from the sprue or a spare part.

Leak test

Before fitting any electronics, seal the openings and float the hull for several hours. Better still, fill it with water from the inside and look for drips along the seams. It is much easier to fix a leak now than after the deck is on.

Planning access

You will need to reach the battery and switch at every session, and the electronics occasionally. Plan a removable section before you build the deck:

Many builders make a large hatch from part of the deck and superstructure, held on with magnets, small screws or a locating lip.
Choose a split line that follows a natural feature, such as a deckhouse edge or a planking line, so the hatch is hard to see.
Add a coaming (a raised lip) around the opening below decks to keep splashes out.

Fitting the running gear

Stern tubes and props

Kit props and shafts are moulded plastic and usually won’t work. Replace them with a working brass stern tube, steel shaft and metal prop of a similar size. You will need to drill or file through the hull where the shaft exits, set the tube at the correct angle, and bond it securely — epoxy is common. Reinforce the area around the exit, because it takes load and is a common leak point. Our guide to fitting and sealing a stern tube covers this step in detail.

For props, shafts, couplings and choosing a suitable motor, see scale model ship propulsion.

Rudders

Plastic kit rudders are usually too weak and have no working stock. Replace them with a rudder built on a metal rudder stock running in a tube bonded into the hull. The top of the rudder tube should sit above the waterline, or be greased and sealed, so water doesn’t come in through it. Our guide to setting up rudder servos and linkages covers the linkage.

Choosing small electronics

Plastic conversions often need the smallest, lightest parts that will do the job:

Receiver: a compact 2.4 GHz receiver with enough channels.
Servo: a micro or sub-micro servo is usually enough for a small rudder.
ESC: a small brushed or brushless ESC with reverse, sized to the motor’s current; see choosing a brushed or brushless ESC.
Motor: small can motors are common; slow, efficient running suits most scale subjects.
Battery: LiPo gives the most energy for its weight, but NiMH is also used, and its extra weight can sometimes replace ballast. Our guide to model boat batteries compares the options.

For the overall installation, including binding, failsafe and antenna placement, see our RC setup guide for model boats.

Ballast, stability and trim

Plastic kits often end up with a high centre of gravity: thin hulls with heavy masts, guns, superstructure and fittings on top. To keep the model stable:

Put the battery as low as possible, close to the centreline.
Add ballast low in the hull, ideally along the keel.
Keep the superstructure light — leave out internal details nobody will see.
Test stability by tilting the model gently in the water and checking that it rights itself without hesitation.

Our guide to ballasting an RC model boat down to its waterline covers the process step by step.

Finishing and detailing with RC in mind

A working model gets handled and splashed. Some habits that help:

Use a durable paint system on the hull and seal decals well.
Attach fragile parts such as railings, aerials and davits so they can be removed for transport, or leave them off until the model is proven.
Avoid painting over the hatch joint so it still opens cleanly.
If you want working lights or a smoke unit, plan the wiring before the superstructure is assembled.

Common problems

Rolls over or feels tippy: centre of gravity too high; move weight lower and lighten the top hamper.
Sits too high: add ballast low in the hull until it reaches the waterline.
Water in the hull: check the hull seam, stern tube, rudder tube and hatch in that order.
Steering is weak: the rudder may be too small for a working model. Some builders fit a slightly larger rudder than scale, accepting a small loss of accuracy.
Runs too fast: reduce the prop size or pitch, or use a lower-voltage battery, to bring it closer to scale speed.

A converted plastic kit takes patience, but it can make a convincing working model at a fraction of the size of a typical RC kit.