How to Ballast an RC Model Boat Down to Its Waterline

Why model boats need ballast

Most model boats, once built and fitted out, float higher than they should. The full-size ship carried engines, fuel, cargo and stores; the model’s hull and electronics weigh a fraction of that in proportion. Adding ballast brings it down to its scale waterline, and it does more than improve the look:

• Stability. Weight placed low lowers the centre of gravity, which makes the model less likely to roll or capsize.
• Handling. A boat at the right depth puts its prop and rudder where they were designed to work. A boat floating high may ventilate the prop or skid in turns.
• Realism. A correctly ballasted model moves through the water more like the real thing, especially in a breeze.

Ballast is best done once the model is close to complete, with the motor, battery and radio gear installed, because they all contribute weight. Our RC setup guide for model boats covers installing those parts.

Marking the waterline

You need a reference line to ballast to:

• Kit or plans: most kits and plans mark the design waterline. Transfer it to the hull with a scribing block, a height gauge, or a pencil taped to a block at the right height, with the hull set level on a flat surface.
• Painted boot-topping: many models have a painted band at the waterline. The water should sit along this line, often at its lower edge. See how to mask and paint a straight waterline.
• No marking: use photographs or drawings of the real ship to estimate it.

Mark it lightly at the bow, stern and midships on both sides if you haven’t painted it yet. These marks are your targets in the tank.

Ballast materials

• Lead — dense, so it takes up little space. Common forms include sheet, flashing offcuts, shot and cast blocks. Handle it with care: wear gloves or wash your hands after handling, avoid making lead dust, and don’t eat or drink while working with it. Check local guidance on handling and disposal.
• Steel shot or steel weights — less dense than lead but easier to handle, and widely available.
• Batteries — the heaviest component in many models. Using a heavier battery, such as a lead-acid pack, can replace some ballast and give longer run times. See our comparison of LiPo, NiMH and lead-acid batteries.
• Stick-on weights — adhesive balance weights are convenient for fine trim.

Avoid anything that rusts or absorbs water, and avoid loose ballast that can shift.

Step-by-step: ballasting in a test tank

You can use a bath, a large storage box or a water butt — anything deep and long enough for the hull to float freely.

1. Install everything that will be in the boat when it sails: motor, ESC, battery, receiver, servos and superstructure. If anything is missing, add a stand-in of the same weight.
2. Float the model and look at the waterline marks. Note how far above or below the water each mark is.
3. Add ballast in small amounts, placed low and roughly where you plan to fix it. Bags of lead or steel shot are handy for this stage because you can move them around easily.
4. Bring it down evenly. Watch the bow and stern marks; move weight forward or aft until both reach the water at the same time.
5. Check side to side. If the model lists, move weight to the high side, or move the battery or other heavy components.
6. Stop slightly short. Leave a small margin for paint, fittings and the odd splash of water in the hull. Many modellers aim to sit just at or slightly above the line.
7. Weigh the ballast you’ve used, so you know the final amount.

Where to place ballast

• Low. Put ballast as close to the keel as you can. The lower the weight, the more it adds to stability.
• Central. Keep it near the centreline and in the middle third of the hull where possible. Weight far forward or aft makes the model pitch more and turn more sluggishly.
• Balanced around the centre of buoyancy. Ballast doesn’t have to be in one lump; it can be split fore and aft as long as the model trims level.
• Clear of moving parts. Keep ballast away from the prop shaft, coupling and rudder linkage.

If you can, put the battery where it does some of the ballasting work, low and central, and use fixed ballast to fine-tune.

Fixing ballast in place

Loose ballast can shift when the boat heels or is carried, which can change trim suddenly. Common methods:

• Shot set in resin — pour lead or steel shot into a box or along the keel and bind it with epoxy or polyester resin. Test the resin’s heat on a small batch; some resins get hot while curing, which can affect plastic hulls.
• Glued blocks or sheet — fix lead sheet or blocks with epoxy or a strong adhesive.
• Removable ballast — screw or strap ballast to a tray, so you can adjust it later or remove it for transport.

Many builders prefer to keep at least some ballast removable, especially if they might change batteries or add features later.

Checking stability

Once it’s at the waterline, test that the model will stay upright:

• Roll test: push one side down gently in the tank and let go. A stable boat rolls back upright quickly and settles; a tender one rolls slowly and may hang over.
• Wind and turns: on the water, try tight turns and sail in a breeze. Watch for excessive heel.
• Top weight: if it’s tender, look at removing weight high up (masts, superstructure, fittings) and moving ballast lower. Tall-superstructure subjects such as RC lifeboat models need particular care here.

For the special case of plastic kit conversions, which are often top-heavy, see our guide to converting a plastic model boat to RC.

Common problems and fixes

• Bow down, stern high: move ballast or the battery aft.
• Stern squats under power: the model may simply be running too fast; see our guide to scale model ship propulsion for prop and speed choices. Moving weight forward can help.
• Permanent list: check that the battery and ballast are centred, and that water isn’t collecting on one side of the hull.
• Sits too low even without ballast: the model is heavier than its displacement. Look for weight to remove, such as a heavy battery or over-thick resin.
• Trim changes during a run: look for water coming in — often through the stern tube. See how to fit and seal a stern tube.

Write down your final ballast weight and battery position. If you ever change components, you’ll know where you started.