How to Fit and Seal a Stern Tube and Prop Shaft on a Model Boat

Why a model boat stern tube leaks

A model stern tube is not a sealed gland. It keeps water out mainly through close-fitting bushes at each end and a filling of grease or oil in the tube. When water gets in, the cause is usually one of these:

The tube has run dry. Grease washes out over time. Once the tube is empty, water can work its way up the gap between the shaft and the bushes.
The inboard end sits below the waterline. If the top of the tube is under the outside water level, water will tend to find its way up and out of it. Raising the inboard end above the waterline is one of the most effective cures.
The bond between tube and hull has failed. Water comes in around the tube rather than through it. Look for wet glue joints, cracks, or a tube that moves when you push on it.
Worn bushes or a bent shaft. A shaft that wobbles opens up the clearances, and the bushes wear faster.
Misalignment. A motor that is out of line with the shaft loads one side of the bushes, wears them oval and often makes the drive noisy and power-hungry.

Find out which it is before you fix anything. Dry the hull completely, float the boat in a bath or tank with the electronics out, and watch the inboard end of the tube and the outside of the joint with a torch and a piece of tissue. The rest of this guide covers fitting a tube so these problems are less likely in the first place.

What a model stern tube is made of

A typical model stern tube assembly has:

The tube — usually brass, sometimes stainless or plastic.
Bushes or bearings — one at each end, often brass or bronze bushes; some higher-end tubes use small ball races.
The shaft — usually stainless steel, threaded at the prop end and sometimes at the inboard end.
An oiler tube or grease point — a small pipe or nipple soldered into the tube so you can top it up, fitted on many but not all tubes.
Prop nut, locknut and thrust washer — to hold the prop and take the forward push of the shaft.

Between the inboard end of the shaft and the motor sits a coupling. The coupling types (universal joint, Oldham-type, flexible tube) are described in our guide to scale model ship propulsion.

Choosing a stern tube and shaft

Shaft diameter and thread

Model shafts are commonly 3 mm, 4 mm or 5 mm diameter, often with M3, M4 or M5 threads at the prop end, though suppliers vary. Pick the shaft so it suits the prop’s thread or bore and the power going through it; the prop and coupling then need to match. Small plastic kits and light hulls often use the smaller sizes; larger tugs and working models usually use bigger ones. If you are buying a prop and tube separately, check the thread before ordering.

Tube length and the waterline

Choose a tube long enough that:

Its inboard end sits above the waterline if the hull shape allows it. On hulls with a steep shaft angle this is easy; on flat-bottomed or very shallow hulls it may not be possible.
It leaves enough room inside for the coupling and motor in line behind it.
The prop sits in the right position relative to the hull and rudder, with clearance all round.

If the inboard end has to sit below the waterline, plan to keep the tube well greased and expect to check it more often.

Fitting the stern tube

Marking the position and angle

Take the shaft line from the plans if you have them (our guide to reading a model ship lines plan explains the sheer, or profile, view). Mark where the shaft exits the hull, then check the angle from the side and from above. The prop needs clearance from the hull and the rudder, and the inboard end has to line up with somewhere you can mount the motor. Dry-fit the tube, prop and rudder together before cutting anything. On scale models, keep the prop position close to the real ship’s.

Cutting the hull

On a wooden or GRP hull, start with a small pilot hole and open it out gradually into a slot along the keel line, checking the tube angle as you go. Many builders aim for a little clearance around the tube rather than a tight push fit, so the tube can be aligned before bonding and there is room for the adhesive. If the hull has a deadwood or skeg, the tube often runs through it, so the slot may need to be extended.

Wear a dust mask when cutting or sanding GRP and work somewhere well ventilated.

Aligning with the motor before gluing

This is the step that most affects how the drive runs. Rather than gluing the tube in first and fitting the motor afterwards, many modellers fit the whole drivetrain loosely, align it, and only then bond the tube:

Fit the shaft into the tube and attach the coupling and motor.
Mount the motor on its mount, which should be adjustable or packable at this stage.
Turn the shaft by hand. It should spin freely with no tight spots through a full turn.
Adjust the motor and tube position until the shaft runs freely and the coupling is not visibly bent.
Hold everything in position with tape, blocks or a few spots of quick glue or hot glue.

The same idea appears in full-size boat work: treat the tube and shaft as one assembly, centre it, and only then bond it in place.

Bonding the tube in place

Slow-setting epoxy is the usual choice, because it gives time to make adjustments and fills gaps well. Thickening it with a filler such as microballoons or colloidal silica helps it stay in the joint rather than running away. Some general points:

Clean and roughen the tube and the hull where the glue will go.
Keep epoxy out of the tube ends and off the shaft. A smear of grease or some tape over the ends helps.
Build a fillet where the tube meets the hull, inside and out. On a GRP or wooden hull, a strip or two of glass cloth over the inside joint adds strength. Our guide to waterproofing a wooden model boat hull covers working with epoxy and cloth.
Leave the shaft in the tube while the epoxy cures and turn it by hand now and then to check it is still free.
Follow the epoxy maker’s curing time before running the motor.

With a plastic hull, check that your adhesive actually bonds to the plastic. Roughening the surface helps, and some builders add a doubler of plasticard or a block around the tube to give a larger bonding area. Our guide to converting a plastic model boat to RC covers working in thin plastic hulls.

Sealing and lubricating the shaft

Grease or oil

Most modellers fill the tube with a waterproof grease, which forms a plug that resists water creeping up the shaft. Some prefer a light oil, which reduces drag a little but washes out sooner and needs topping up more often. Fast electric boats sometimes use thinner lubricants to cut friction; scale models and slow-revving drives usually run happily on grease. Whichever you use, choose something sold as marine or waterproof grease rather than general-purpose grease, which may break down in water.

If the tube has no oiler, you can pack grease in from the inboard end before fitting the shaft, or push it through with a syringe.

Oiler tubes

An oiler tube soldered into the middle of the stern tube lets you top up without removing the shaft. Bring the open end up above the waterline and somewhere you can reach through a hatch. Fill it with a syringe of grease or oil until a little appears at both ends of the stern tube. Some builders cap the oiler with a short piece of silicone tube to keep dirt out.

Setting end float and fitting the prop

The shaft needs a small amount of fore-and-aft movement, but not too much:

Fit a thrust washer between the prop and the outer end of the tube. When the boat is going forward, the prop pushes the shaft forward and the washer takes the load.
At the inboard end, a collar or locknut stops the shaft sliding back out when going astern.
Leave a small amount of end float so the shaft does not bind; too much lets it knock back and forth on each change of direction.

Tighten the prop and its locknut securely. A prop that unscrews in reverse is a well-known way to lose one in the pond. Check the prop’s handedness before fitting; see our propulsion guide for how it matches motor direction.

Testing for leaks

Before installing the electronics:

Float the bare hull in a bath, tank or bucket and leave it for a while.
Check the inboard end of the tube, the oiler, and the joint between the tube and hull, using a dry tissue to pick up small drips.
Turn the shaft by hand while it is afloat and look again.
If you can, run the motor on a bench supply with the hull afloat and check once more. Some leaks only appear when the prop is turning.

A dry hull after this test does not mean water will never get in, but it does mean the tube and the joint are sound. Fixing a leak at this stage is much easier than after the deck is on.

Fixing a leaking stern tube in a finished model

Work through the likely causes in order, starting with the easiest:

Re-grease the tube. Remove the prop if needed, push fresh grease through until it appears at both ends, and test again. This cures many leaks on its own.
Raise the inboard end if you can. Sometimes the motor and coupling can be repositioned, or an extension fitted, so the open end of the tube sits higher.
Seal the joint. If water is coming in around the outside of the tube, dry the area thoroughly, clean and roughen it, and add a new fillet of epoxy inside the hull. A visible crack on the outside may need filling and repainting too.
Check alignment and wear. If the shaft has play in the tube, or the drive is noisy, the bushes may be worn or the shaft bent. Replacing the shaft is straightforward; a badly worn tube is usually replaced completely, which means cutting the old one free and fitting a new one as described above.

A bilge sponge, or a small absorbent pad under the motor, is worth having even with a well-sealed tube. Our RC setup guide covers keeping the receiver and other electronics high and dry.

Routine maintenance

After each session, open the hull and let it dry out. Note where any water collected.
Top up the grease or oil every few sessions, or whenever water starts appearing inside.
Check the prop nut and locknut are still tight.
Turn the shaft by hand from time to time. Rough spots, new noise or extra play are signs of wear or a bent shaft.
If the model has been stored for a long time, re-grease before the first run of the season.

The rudder tube can leak in the same way and is sealed using similar methods; see our guide to setting up rudder servos and linkages.