John Guest connectors for boats, yachts and marine use: most common sizes, tubes and fittings
Many customers with boats, yachts, sailing boats, pleasure craft, marinas and marine projects come to Install Beer looking for John Guest connectors to repair a leak, change a pump, connect a tap, adapt a heater, replace an elbow, install a filter or renew part of the freshwater circuit on board. In marine applications, the most common size is usually 15 mm outside diameter for hot and cold potable water, with 22 mm on main runs or higher-flow sections and adaptors to BSP, NPT or NPS threads depending on the equipment installed.
Quick summary
On boats and yachts, the most common standard in potable freshwater circuits is 15 mm outside-diameter tube with push-fit fittings. For main lines, manifolds, heaters, higher flow or distribution to several zones, 22 mm may also appear. Adaptors 15 mm x 1/2" BSP, 15 mm x 3/4" BSP, 22 mm x 3/4" BSP, tap connectors, elbows, Ts, shut-off valves, check valves, clips, inserts, through-bulkhead fittings and 22–15 mm or 15–10 mm reducers are also common. On imported vessels, imperial sizes such as 1/2" OD, 3/8" OD, NPT, NPS or NPTF may appear.
The key idea: 15 mm is not 1/2"
On push-fit fittings, tube size is identified by outside diameter. A 15 mm tube needs a 15 mm fitting. A tube with 1/2" outside diameter measures approximately 12.7 mm and is not compatible with a 15 mm metric fitting. In addition, 1/2" BSP or 1/2" NPT are nominal thread sizes, not tube diameters. Before buying, you must confirm whether you are measuring tube, thread or hose.
This article complements the John Guest collection
This guide is used to identify sizes, fitting families and common uses in boats, yachts and fresh water marine systems. To buy specific parts, check stock or search for connectors by format, see the John Guest fittings collection and the connectors and fittings category. If you have an old part, an imported pump or a mixed installation, the safest option is to measure and confirm before replacing.
Guide contents
- What is used in marine and shipbuilding
- Most common sizes on boats
- Why 15 mm pipe dominates
- When 22 mm pipe appears
- Imperial sizes and imported systems
- Which John Guest range to use
- List of the most common connectors
- Which fitting to use depending on the area of the boat
- Connection to pressure pumps
- Connection to taps, showers and kitchen
- Heaters, boilers and hot water
- How to install push-fit fittings on board
- Recommended marine spare parts kit
- Common mistakes
- What to buy at Install Beer
- Frequently asked questions
What is used in boating, yachts, sailboats and ships
In boating, compact, durable systems are used that are easy to maintain and suitable for tight spaces. A boat has hard-to-reach areas, vibration, humidity, ambient salinity, temperature changes and many connection points: pump, tank, heater, filters, galley, heads, showers, washbasins, transom showers, shore connections and auxiliary systems.
For this reason, push-fit fittings are common in fresh water circuits: they allow you to assemble, disassemble and inspect connections without soldering, without glues and without hot work. On European or UK-influenced yachts and boats, the most common tube outside diameter is 15 mm. On larger vessels or those with higher flow, 22 mm is used on main runs, manifolds or feeds to several areas.
Main size for cold and domestic hot water on boats, yachts, sailing boats and many European marine installations.
Common on main runs, manifolds, distribution to several heads or higher-flow lines.
Common threads on pumps, taps, heaters, filters, showers and imported equipment.
Most common sizes in marine plumbing
The following table summarises the most common sizes in customer enquiries and marine spare parts. Before buying, it is advisable to measure the outside of the pipe with callipers and check whether the connection is threaded, push-fit pipe or flexible hose.
| Size | Where it appears | Usual use | What to order |
|---|---|---|---|
| 15 mm outside diameter | Hot and cold potable water circuits on boats and yachts. | Freshwater main line, taps, showers, bathrooms, kitchen, pump and boiler. | John Guest / JG Speedfit fittings for 15 mm pipe. |
| 22 mm outside diameter | Distribution runs, manifolds, main lines or higher-flow installations. | Supply several areas or reduce pressure loss. | 22 mm fittings, 22–15 mm reducers and 22 x 3/4" BSP adapters. |
| 10 mm outside | Small branches, auxiliary equipment or compact connections. | Small taps, filters, specific equipment or flow reduction. | 10 mm fittings or 15–10 mm reducers. |
| 12 mm outside | More common in caravanning, but it can appear on small boats or mixed installations. | Branches, repairs or imported leisure parts. | 12 mm fittings only if the pipe actually measures 12 mm outside diameter. |
| 3/8" male | Water treatment, filters, dispensers, reverse osmosis, ice machines or beverage equipment. | Auxiliary potable or drinking water lines. | John Guest fittings in inches, usually PI/PP range depending on the application. |
| 1/2" male | SeaTech/AquaLock systems, US-style installations, imports or RV/marine spares. | Fresh water on imported boats or US equipment. | Imperial 1/2" OD fittings, not metric 15 mm fittings. |
| 1/2" BSP | Taps, pumps, filters, heaters and European/UK plumbing connections. | Adapt 15 mm pipe to the equipment thread. | 15 mm x 1/2" BSP adaptor, male, female, straight or elbow. |
| 3/4" BSP | Boilers, tanks, manifolds, large filters or main connections. | Connection with larger thread or higher flow. | 15 mm or 22 mm x 3/4" BSP adaptor depending on the point. |
| NPT / NPS / NPTF | Pumps, heaters, taps and imported accessories, especially from the US market. | Non-BSP threaded connection. | Specific adapter after identifying the exact standard. |
Do not use in just any circuit on the boat
This guide focuses on domestic freshwater and applications compatible with push-fit fittings. It must not be applied directly to fuel, gas, hydraulics, steering, engine, engine cooling circuit, bilge, black water, grey water, seacocks, fire-fighting systems or critical circuits unless the component manufacturer explicitly states compatibility and the applicable regulations are met.
Why 15 mm pipe is so common on board
15 mm outside diameter pipe is very common because it offers a good balance between flow rate, ease of installation and fitting availability. It can supply kitchen taps, bathrooms, showers and heaters on recreational boats without taking up too much space or requiring complex tools.
- Very widespread size in European and UK marine plumbing.
- Compatible with many JG Speedfit ranges and equivalent systems.
- Good balance between flow rate and space.
- Works on cold and hot water if the correct range is used.
- Allows quick repairs with elbow, tee, straight connector or valve.
- Actual outside diameter of the pipe.
- Whether the tube is plastic, copper or a mixed installation.
- Pump and accumulator pressure.
- Maximum hot water temperature.
- If it requires an insert or clip depending on pipe and fitting.
- If the fitting will be accessible for inspection.
When 22 mm pipe appears on boats and yachts
22 mm tube is not usually necessary at every point of use, but it appears when several zones must be supplied, pressure losses reduced or higher flow is required. On larger yachts, it can make sense as the main line from tank/pump/manifold, then reducing to 15 mm towards each outlet.
| Use of 22 mm | Why it is used | Common parts |
|---|---|---|
| Main line | Allows you to feed several branches with lower pressure loss. | Straight connector 22 mm, elbow 22 mm, tee 22 mm, reducer 22–15 mm. |
| Manifold | Centralise distribution to galley, heads, shower and boiler. | 22 x 15 manifold, outlet valves and fixing clips. |
| Water heater / boiler | May require higher flow or 3/4" thread depending on the model. | 22 x 3/4" BSP adaptor, 22–15 mm reducer. |
| Yachts with several heads | Several simultaneous outlets may require a larger cross-section. | Manifold, shut-off valves, reducers and non-return valves. |
Imperial sizes and imported systems: 1/2", 3/8", NPT and NPS
On imported boats, American yachts, US pumps, imported heaters or water treatment equipment, imperial sizes may appear. This is a common source of errors because 1/2" can refer to outside pipe diameter, BSP thread, NPT thread or NPS thread.
| Size or thread | What it means | Where it appears | Warning |
|---|---|---|---|
| 1/2" OD | Tube with an outside diameter of approximately 12.7 mm. | SeaTech/AquaLock systems, imported RV/marine, US equipment. | Does not fit properly into a 15 mm fitting. |
| 3/8" OD | Pipe with an outside diameter of approximately 9.52 mm. | Filters, reverse osmosis, auxiliary drinking water, fridges and dispensing. | Not equivalent to 10 mm. |
| 1/2" BSP | British/European plumbing thread. | Taps, pumps, filters, flexible hoses and heaters. | It is not 1/2" OD tube. |
| 3/4" BSP | British/European thread with a coarser pitch. | Boilers, tanks, large filters and outlets. | Confirm male/female and flat gasket or sealing thread. |
| NPT / NPTF | American tapered or sealing thread. | Pumps, heaters and imported accessories. | Do not mix with BSP without the correct adapter. |
| NPS | Straight American thread. | Some SeaTech/AquaLock connectors and marine accessories. | Requires precise identification of thread and gasket. |
To expand equivalences, you can consult our inch-to-millimetre conversion table.
Which John Guest range to use in marine applications
Not all John Guest ranges are used for the same purpose. In boating you must distinguish between hot/cold sanitary water, auxiliary potable water, dispensing equipment, purification, air/inert gases or installations with specific requirements.
| Range or family | Usual use | When it fits on a boat | Caution |
|---|---|---|---|
| JG Speedfit / Twist & Lock | Cold and hot sanitary water plumbing. | Fresh water circuits on board: galley, bathroom, shower and boiler. | Respect temperature, pressure, inserts, clips and tube compatibility. |
| PM metric fittings | Liquids, inert gases, water treatment and dispensing depending on the range. | Auxiliary equipment, filtration, drinking, technical potable water or custom setups. | Do not assume they are suitable for domestic hot water or critical circuits. |
| PI imperial fittings | Inch sizes for beverage, water, filters and imported systems. | 3/8", 1/2" OD, filters, dispensers or US equipment. | Do not confuse with metric 10, 12 or 15 mm. |
| DZR brass fittings | Robust threaded connections, fixed points or high-stress areas. | Threaded adaptors, fixed outlets or connections with greater mechanical stress. | Check material, corrosion, tightening torque and galvanic compatibility. |
| Valves and non-return valves | Shut-off, zoning, backflow prevention and maintenance. | Pump, boiler, filters, tanks and branches by zone. | Check flow direction, pressure and access for maintenance. |
In marine applications, the environment matters a lot
Humidity, salinity, vibration and confined spaces make it advisable to keep fittings accessible, secure the tube properly, avoid lateral stress, inspect periodically, and not conceal joints without the possibility of inspection. In professional marine projects or vessels subject to classification, the technical specifications and regulations required by the project must be followed.
List of the most common John Guest connectors for boats and yachts
The following table lists the most common parts for repairing or assembling fresh water circuits in marine applications. Exact references may vary depending on range, material, thread, colour, certification and availability; the important thing is to identify the function, size and thread standard before buying.
| Connector or accessory | Common size | Indicative references | Typical use on board | Priority |
|---|---|---|---|---|
| Straight equal union | 15 mm x 15 mm | PM0415E / PEM0415W | Repair a cut, join two sections or extend a water line. | Very high |
| Equal elbow | 15 mm x 15 mm | PM0315E / PEM0315W | 90° turns in furniture, technical bay, bathroom or kitchen. | Very high |
| Equal T | 15 mm x 15 mm x 15 mm | PM0215E / PEM0215W | Branch off to tap, shower, basin, boiler or filter. | Very high |
| Male BSP adaptor | 15 mm x 1/2" BSP | PM011514E | Connect 15 mm tube to a part with 1/2" female thread. | Very high |
| Male BSP adaptor | 15 mm x 3/4" BSP | PM011516E | Connection to equipment or tanks with 3/4" thread. | High |
| Plain barb to thread adaptor | 15 mm x 1/2" BSP | PM051514E | Convert a push-fit connection into a male thread. | High |
| Straight tap connector | 15 mm x 1/2" BSP | PEMSTC1514 / PEMSTC1514-FS | Connect a tap, hose, filter or accessory with 1/2" male thread. | Very high |
| Angled tap connector | 15 mm x 1/2" BSP | PEMBTC1514 | Connection to tap or appliance in shallow spaces. | High |
| Shut-off valve | 15 mm | Depending on the range available | Isolate pump, boiler, tap, filter or area of the boat. | Very high |
| Non-return valve | 15 mm | Depending on the range available | Prevent water from flowing back towards the pump, tank or boiler. | High |
| Pipe plug | 15 mm | PSE4615W or equivalent option | Temporarily blank off a line during maintenance or refurbishment. | High |
| Reducer | 22 mm x 15 mm | PM062215E / Speedfit alternative | Reduce from 22 mm main line to 15 mm branch. | High |
| Reducer | 15 mm x 10 mm | PM061510E / Speedfit alternative | Connect auxiliary equipment or small branches. | Medium |
| Stem elbow | 15 mm x 15 mm | PM221515E / PEM221515W | Create an elbowed outlet that can be inserted into another fitting. | Medium‑high |
| 22 mm straight coupling | 22 mm x 22 mm | PM0422E / PEM0422W | Repair or extend main lines. | Medium‑high |
| 22 mm elbow | 22 mm x 22 mm | PM0322E / PEM0322W | Turn of main line or manifold. | Medium |
| 22 mm tee | 22 mm x 22 mm x 22 mm | PM0222E / PEM0222W | Branch off from a higher-flow main line. | Medium |
| 22 mm to 3/4" BSP adapter | 22 mm x 3/4" BSP | PM012216E / PM052216E | Boiler, tank, large filter or main connection. | Medium‑high |
| 3/8" imperial fitting | 3/8" OD | PI0412S, PI0312S, PI0212S | Filters, water treatment, dispensing or auxiliary systems. | Medium |
| 1/2" imperial fitting | 1/2" OD | PI0416S, PI0316S, PI0216S | Imported systems, SeaTech/AquaLock or US equipment. | Medium‑high |
| Safety clip | 15 mm / 22 mm | CM1815, CM1822 by colour/range | Lock or identify connections in areas with vibration. | High |
| Tube insert | 15 mm / 22 mm | Depending on pipe type | Internal reinforcement when the tube or connection requires it. | High |
| Tube cutter | Up to 15 mm / 22 mm depending on model | John Guest tool | Square cut to avoid leaks at the O-ring. | Very high |
Looking for a specific John Guest size for a boat?
At Install Beer we are official John Guest distributors in Spain and can help you identify the correct fitting if you have a photo, the outside diameter of the tube and the type of thread or equipment you want to connect.
See John Guest connectors Check compatibilityWhich fitting to use depending on the area of the boat
A fresh water system on a boat can have many connection points. The key is to divide it into sections, leave maintenance access and not mix standards without confirming compatibility.
| Area of the boat | Common connectors | Common sizes | Practical tip |
|---|---|---|---|
| Freshwater tank | Through-bulkhead fittings, threaded adapters, shut-off valve, tube-to-hose. | 15 mm, 22 mm, 1/2" BSP, 3/4" BSP. | Plan access for cleaning, draining and repairs. |
| Pressure pump | Threaded adapters, shut-off valve, check valve, filter and elbows. | 15 mm, 1/2" BSP, NPT/NPS on imported pumps. | Do not overtighten plastic threads and check seals. |
| Accumulator | Elbows, straight connectors, adaptors and shut-off valves. | 15 mm and 1/2" BSP. | Check pressure and access for maintenance. |
| Galley / sink | T, elbow, tap connector, shut-off valve and reducer. | 15 mm, 10 mm, 3/8" BSP, 1/2" BSP. | Check whether the tap uses a flexible hose, thread, or hosepipe. |
| Bathrooms and indoor showers | T, elbow, shut-off valve, adapters and reducers. | 15 mm and 10 mm. | Separate hot/cold water and identify lines. |
| External shower / bath | Through-bulkhead fitting, elbow, shut-off valve, fixed outlet and plug. | 15 mm x 1/2" BSP or the manufacturer’s configuration. | Check watertightness and exposure to salinity. |
| Boiler / water heater | BSP adaptors, safety valve, check valve, elbows and drain. | 15 mm, 22 mm, 1/2" BSP, 3/4" BSP. | Do not bypass safety valves or install direct without checking the manual. |
| Water treatment / filters | 3/8", 1/4" fittings, adaptors and small valves. | 3/8" OD, 1/4" OD, 15 mm to thread. | Differentiate the domestic water circuit from the purification circuit. |
| Leak repair | Straight connector, elbow, tee, plug, valve and tube cutter. | Mainly 15 mm; 22 mm on main lines. | Carrying spare parts on board prevents leaving the boat without water. |
Connection to pressure pumps and freshwater systems
Freshwater pumps on boats may use BSP, NPT, NPS threads, hose barbs or specific connectors. Before buying a John Guest for a pump, it is advisable to identify whether the pump has a male or female thread, whether it is 1/2" BSP or NPT, and whether the installation is in 15 mm, 1/2" OD or flexible hose.
- 15 mm x 1/2" BSP adaptor, male or female.
- 15 mm elbow for outlet in confined spaces.
- Shut-off valve before and after the pump.
- Pump filter or pre-filter.
- Non-return valve if the layout requires it.
- Straight union for quick repair.
- Safety clips and tube supports.
- Check the pump brand/model.
- Identify BSP, NPT, NPS or hose tail.
- Measure the outside diameter of the tube.
- Do not overtighten plastic threads.
- Depressurise before disassembly.
- Leak-test before closing panels.
Connection to taps, showers, sink and heads
In marine taps you may find flexible hoses, BSP nuts, compact threads, flexible hose, imported connections or fittings already installed from other manufacturers. That’s why asking for “a John Guest fitting for a boat tap” is not enough: you need to know whether the tap ends in a male thread, female thread, spigot, hose or pipe.
| Situation | Probable part | Common size | What to confirm |
|---|---|---|---|
| Tap with 1/2" male thread | Straight female tap connector. | 15 mm x 1/2" BSP. | Type of seal, thread length and available space. |
| Tap with little space behind it | Angled tap connector. | 15 mm x 1/2" BSP. | Elbow orientation and tube bend. |
| Imported tap | Specific BSP/NPT/NPS adapter or hose. | Variable. | Measure actual thread before buying. |
| Branch to kitchen and bathroom | 15 mm T + shut-off valves. | 15 mm. | Identify cold and hot water. |
| Outdoor shower | Bulkhead fitting, elbow, fixed outlet and valve. | 15 mm x 1/2" BSP. | Protect against splashes and salinity. |
Useful photo to identify the fitting
To choose correctly, prepare a clear photo of the old fitting, a photo of the equipment it connects to, the outside measurement of the pipe with a caliper and, if there is a thread, a photo of the thread with a diameter reference. On boats it is very common to find mixed installations with metric, BSP, NPT, NPS or flexible hose parts.
Heaters, boilers and hot water on boats
For hot water you must check temperature, pressure and distance from the unit. Heaters, boilers or marine calorifiers may have safety valves, mixing valves, drains, check valves and specific threads. A safety component must not be replaced with a generic fitting.
| Point to check | Why it matters | Recommendation |
|---|---|---|
| Maximum temperature | Hot water requires compatible pipe and fittings. | Use a range suitable for potable hot water and check the technical data sheet. |
| Pump/accumulator pressure | Pressure spikes can cause leaks. | Check working pressure and condition of seals. |
| Safety valve | Protects the boiler against overpressure. | Do not disable or replace it without technical criteria. |
| Mixing valve | Control outlet temperature for safety and comfort. | Respect connection direction and maintenance requirements. |
| Non‑return valve | Prevents unwanted backflow between cold and hot water. | Install only if the schematic requires it. |
| Technical access | Boilers are usually installed in lockers or tight compartments. | Allow for inspection and disassembly. |
How to correctly install a John Guest push-fit fitting on board
Most leaks in push-fit fittings are not due to the fitting itself, but to a bad cut, oval pipe, pipe not fully inserted, burrs, side load, overtightened thread, wrong seal, vibration or incorrect size.
1. Depressurise the circuit
Switch off the pump, open a tap and release pressure before undoing any connection.
2. Measure the pipe on the outside
Use a caliper if possible. Don’t buy on intuition: 15 mm, 1/2" OD and 1/2" BSP are different things.
3. Cut square
Use a tube cutter. A crooked or chewed cut can damage the internal seal.
4. Remove burrs
The outside of the tube must go in clean, without deep scratches or deformation.
5. Use insert if required
Some tubes or product ranges require an insert to keep the tube round and reinforce the connection.
6. Push fully home
Push the pipe straight in until you feel the internal stop of the fitting.
7. Pull gently to check
Check that the collet has gripped the pipe and that it does not come out.
8. Lock if applicable
Fit a clip, collet cover or use Twist & Lock depending on the range and installation area.
9. Secure the pipe
Prevents boat vibration from putting stress on the fitting.
10. Pressure test
Switch on the pump and check for leaks before closing panels, lockers or access hatches.
Vibration, salinity and access: three critical points
On a boat, the system is subject to vibration, movement, humidity, salty atmosphere and changing loads. That’s why it’s important to secure the pipe properly, avoid side load on the fittings, keep joints accessible, inspect after sailing and replace parts that show signs of damage, discolouration, cracking or leakage.
Recommended John Guest spare parts kit for boats
A small selection of fittings can solve many issues during a trip: leak from a cut pipe, broken elbow, section taken out of service, pump change, tap repair or the need to shut off a line.
| Item | Recommended quantity | Function |
|---|---|---|
| 15 mm straight connector | 2 units | Repair a cut or extend a line. |
| 15 mm elbow | 2 units | Solve changes of direction in tight spaces. |
| 15 mm tee | 1 unit | Create or repair a branch. |
| 15 mm shut-off valve | 1–2 units | Isolate a section or appliance. |
| 15 mm non-return valve | 1 unit | Prevent water backflow if the system requires it. |
| 15 mm plug | 1–2 units | Temporarily blank off a tube or branch. |
| Adaptor 15 mm x 1/2" BSP | 1 male and/or 1 female | Connect pump, tap, filter or threaded accessory. |
| 22–15 mm reducer | 1 unit if your boat has a 22 mm main line | Branching off from the main line. |
| Clips / collet covers | Various | Locking and identification of connections. |
| Pipe inserts | Various | Pipe reinforcement where applicable. |
| 15 mm tube | 1–2 metres or short coil | Replace a damaged section. |
| Tube cutter | 1 unit | Make straight, safe cuts. |
Common mistakes when buying John Guest connectors for boating
| Error | Consequence | How to avoid it |
|---|---|---|
| Confusing 15 mm with 1/2" | The pipe will not go in or it leaks. | Measure outside diameter and distinguish metric from imperial. |
| Mixing up tube with thread | The wrong adapter is purchased. | Confirm whether we’re talking about OD tube, BSP, NPT or NPS. |
| Not knowing whether the thread is male or female | The adapter does not fit the pump or tap. | Look at the equipment and the part it must thread into. |
| Mixing BSP with NPT/NPS | Leak, damaged thread or forced assembly. | Identify the standard before tightening. |
| Reusing marked or deformed pipe | The internal seal may not seat properly. | Cut a new, straight section before connecting. |
| Do not use a tube cutter | Crooked cut, burrs and leaks. | Use a John Guest pipe cutter or suitable tool. |
| Forcing plastic threads | Cracks, leaks or breakage of the fitting. | Tighten carefully and use a compatible gasket. |
| Concealing fittings without access | A future leak will be difficult to detect and repair. | Leave access panels and inspection points. |
| Using water fittings for fuel, gas or bilge | Serious risk of incompatibility and safety issues. | Use approved components for each system. |
| Not securing the tube | Vibration puts stress on the connection. | Use clips, supports and stress-free routing. |
What to buy at Install Beer to repair or install fresh water on a boat
Install Beer works with John Guest every day in real-world beverage, water, filtration, gas and technical systems. For boat and yacht customers, the most useful selection is usually concentrated in 15 mm fittings, threaded adapters, valves, check valves, pipe, reducers and shut-off accessories.
| Need | Recommended product | Internal link |
|---|---|---|
| View the full John Guest range | Push-fit fittings, valves, adapters, pipes and accessories. | John Guest collection |
| Search fittings by type | Elbows, tees, unions, reducers, check valves and adapters. | Connectors and fittings |
| Connect or replace lines | Food-grade tubes and hoses compatible with water or beverages. | Food-grade tubing |
| Cut pipe correctly | John Guest pipe cutter for a clean, square cut. | John Guest tube cutter |
| Isolate a section | John Guest shut-off valve for maintenance or repair. | John Guest shut-off valve |
| Adapt to BSP/BSPT thread | Male, female or swivel elbow BSP/BSPT adapters. | Male BSPT thread adaptor |
| Connect to hose | John Guest pipe-to-hose adapter. | Pipe-to-hose adapter |
Not sure which connector you need for your boat?
Send us a photo of the fitting, the outside diameter of the tube and the equipment it connects to. We’ll help you tell apart 15 mm, 22 mm, 3/8", 1/2" OD, 1/2" BSP, 3/4" BSP, NPT, NPS, male, female, elbow, T, valve or adapter.
Buy John Guest Check the sizeFrequently asked questions about John Guest connectors for boats and marine use
What pipe size is most commonly used on boats and yachts?
The most common size in domestic fresh water systems is 15 mm outside diameter. On main lines, large yachts or manifolds you may also find 22 mm.
Are 15 mm and 1/2" the same?
No. 1/2" outside is approximately 12.7 mm, whereas 15 mm is a different metric size. In addition, 1/2" may refer to a BSP or NPT thread, not necessarily to a tube.
Which threads are most common in marine applications?
In Europe and the UK, 1/2" BSP and 3/4" BSP are common. On imported equipment you may find NPT, NPS or NPTF, especially on pumps, heaters or accessories of American origin.
Which John Guest connectors are most useful for a boat?
The most common are 15 mm straight coupler, 15 mm elbow, 15 mm T, 15 mm x 1/2" BSP adapters, tap connectors, shut-off valves, check valves, plugs, 22–15 mm reducers, clips, inserts and tube cutters.
Can I use John Guest on a yacht’s hot water line?
Yes, as long as the chosen range, pipe and fitting are suitable for domestic hot water and operate within the pressure and temperature limits specified by the manufacturer.
Are these fittings suitable for salt water?
This guide focuses on potable freshwater. For salt water, deck wash, livewells or other systems, you must check product, material, pressure, temperature and specific application compatibility.
Are they suitable for fuel, gas or bilge?
They must not be used for fuel, gas, hydraulics, bilge, black water, engine systems or critical circuits unless the product is specifically approved for that application.
Do I need safety clips?
On boats they are highly recommended in areas with vibration, strain or difficult access. They help prevent accidental disconnections and can also be used to identify lines.
Can I mix John Guest with Whale, SeaTech or AquaLock?
There may be installations where brands coexist, but you must not assume automatic compatibility. You need to confirm outside diameter, tolerances, tube type, need for insert, working pressure and temperature.
Can Install Beer help me identify the fitting?
Yes. As official John Guest distributors in Spain, we can help you identify sizes and connectors if you send us photos, the tube’s outside diameter and details of the equipment it connects to.
Technical note: this guide focuses on freshwater systems and applications compatible with push-fit fittings. It must not be used as instructions for fuel, gas, engine, hydraulic, bilge, black water, critical circuits or systems subject to specific approval. On commercial vessels, classed craft or regulated naval installations, always confirm compatibility, pressure, temperature, certification and applicable regulations.