John Guest guide for boats and yachts: sizes, threads and fittings
Choosing a John Guest connector for a boat is not just about buying “a similar fitting”. In marine systems you find metric tubes, inch sizes, imported pumps, BSP, NPT or NPS threads, cold‑ and hot‑water circuits, filters, boilers, showers, taps, watermakers, pontoon pedestals and dock bollards. This guide explains how to identify the correct size and avoid mistakes before replacing or installing a connection.
Quick summary
On boats and yachts, the most common John Guest tube for potable fresh water is usually 15 mm OD. On main lines, manifolds or higher‑flow installations you may find 22 mm OD. On imported vessels or auxiliary equipment, inch sizes such as 3/8" OD and 1/2" OD are also common, as well as BSP, NPT or NPS threads.
To buy specific parts, see the John Guest Marine collection. To view all available families, you can go to the general John Guest collection or the dedicated John Guest page.
This guide complements the John Guest Marine collection
This page is designed to help you identify sizes, threads and compatibilities. To buy specific parts, check available products or search for fittings by application, visit the dedicated John Guest connectors for marine, boats and yachts collection.
If you need to see the full range of push‑fit fittings, tubes, valves, reducers, adapters, clips and accessories, also check the John Guest collection. To learn more about the brand, its applications and technical advantages, see the dedicated John Guest page on Install Beer.
What is used in boating, yachts, sailboats and ships
On a boat, the fresh water system is usually distributed between tank, pressure pump, accumulator, filter, taps, showers, galley, head, water heater, transom shower and auxiliary outlets. Unlike a domestic installation, many connections are in hard-to-reach areas, with humidity, vibration, movement, ambient salinity and little room to work.
For that reason, John Guest push-fit connectors are common in onboard fresh water systems: they allow quick, removable, clean connections, with no soldering, no glue and no traditional metal hose clamps at many points in the installation. At Install Beer you can find both a selection specifically for boats in the John Guest Nautical collection and the full range in the general John Guest collection.
Recreational boats
On sailboats, motorboats and family boats, 15 mm tube is usually predominant for fresh water, with branches to taps, showers, pump, tank and water heater.
Yachts and mega yachts
On larger vessels you may find 22 mm in main lines, manifolds, feeds to several areas, multiple bathrooms and higher-flow technical systems.
Harbours and pontoons
On service pedestals, bollards and water supply points in the harbour, quick‑connect fittings make it easier to repair, replace and maintain internal connections.
Most common sizes in John Guest marine plumbing
The key is to distinguish between outside tube diameter, thread and flexible hose. A push‑fit fitting is chosen by the actual outside diameter of the tube. A threaded adapter is chosen by the equipment’s thread. A hose tail is chosen by the hose’s inside diameter.
Once you have identified the size, you can go directly to the John Guest Marine collection to find 15 mm, 22 mm, 3/8", 1/2" OD fittings, threaded adapters, valves and accessories.
| Size | Where it appears | Usual use | What to order |
|---|---|---|---|
| 15 mm outside diameter | Hot and cold potable water circuits on boats, sailboats, and European yachts. | Taps, showers, pump, tank, head, galley, filter and water heater. | John Guest / JG Speedfit fittings for 15 mm tubing. See nautical collection. |
| 22 mm outside diameter | Main lines, manifolds, larger yachts or distribution to several zones. | Higher flow rate, lower pressure drop and supply to several branches. | 22 mm fittings, 22–15 mm reducers, and 22 mm x 3/4” BSP adapters. |
| 10 mm outside | Small branches, compact equipment, or auxiliary connections. | Small taps, filters, specific equipment, or flow reducers. | 10 mm fittings or reducers from 15 mm if the installation requires it. |
| 12 mm outside | More common in caravanning, although it may appear on small boats or mixed installations. | Repairs, auxiliary branches or non‑standardized systems. | 12 mm fittings only if the pipe actually measures 12 mm outside diameter. |
| 3/8" OD | Filtration, reverse osmosis, dispensing, watermakers, chillers and imported equipment. | Auxiliary lines for drinking water, treated water or technical equipment. | John Guest fittings in inches for 3/8" OD tubing. See also the general John Guest collection. |
| 1/2" OD | American installations, imported boats, SeaTech/AquaLock or RV/marine-type spares. | Fresh water on imported boats and US‑origin equipment. | Fittings for 1/2" OD tube. Do not replace with 15 mm metric fittings. |
| 1/2” BSP | Taps, pumps, filters, heaters and European/UK plumbing connections. | Adapt push-fit tube to the equipment thread. | 15 mm x 1/2” BSP adapter, male or female, straight or angled. |
| 3/4" BSP | Boilers, tanks, manifolds, large filters or main connections. | Connections with larger threads or higher flow rates. | 15 mm or 22 mm x 3/4" BSP adapter depending on the connection point. |
| NPT / NPS / NPTF | Pumps, taps, heaters, and imported accessories, especially from the American market. | Equipment with non-European threads. | Specific adapter after identifying the exact standard. |
The key idea: 15 mm is not 1/2”
One of the most common mistakes in marine spares is confusing 15 mm with 1/2". Although they may look similar, they are not equivalent. A 15 mm OD tube needs a 15 mm metric connector. A 1/2" OD tube is approximately 12.7 mm OD and needs a 1/2" OD imperial fitting.
Do not mix sizes by approximation
If you insert a 1/2" OD tube into a 15 mm fitting, it may not seal properly. If you try to force a 15 mm tube into a 1/2" OD fitting, you can damage the tube, the collet, the seal or cause a leak. Before buying, check the John Guest marine collection by actual size.
In addition, the term “1/2” can refer to several things: 1/2" OD tube, 1/2" BSP thread, 1/2" NPT thread or even a hose with 1/2" inside diameter. That’s why, before buying, you must identify what is being measured.
Which John Guest fitting to use depending on the area of the boat
The correct choice depends on the function of the line, the connected equipment, working pressure, temperature, accessibility and the type of tube installed. For a complete overview of the brand and its applications, also see the dedicated John Guest page.
| Area of the boat | Common connectors | Technical tip |
|---|---|---|
| Freshwater tank | Threaded adapters, hose-barb fittings, shut-off valves, plugs and straight connectors. | Confirm whether the tank outlet is threaded, barbed or a specific connection from the manufacturer. |
| Pressure pump | BSP, NPT, NPS adapters, elbows, shut-off valves and check valves. | Imported pumps may not use BSP thread. Do not force plastic threads. |
| Kitchen tap | Elbows, angled connectors, male/female adapters and shut-off valves. | In narrow cabinets, an elbow can prevent twisting and forced bends in the tube. |
| Bathroom and shower | Tees, elbows, 15 mm x 1/2" BSP adapters and shut-off valves. | It is advisable to sectorize each area so you can carry out repairs without shutting off water to the entire boat. |
| Boiler or water heater | 15 mm or 22 mm adapters, check valves, shut-off valve and temperature-rated fittings. | Always respect the manufacturer’s safety, pressure and temperature valves. |
| Filters and reverse osmosis | 1/4", 3/8", 1/2" OD fittings, reducers, valves and threaded adapters. | In water treatment it is very common to find sizes in inches. |
| Manifolds | 22 x 15 mm manifolds, reducers and shut-off valves per branch. | Useful on yachts with several consumption zones or for orderly system refits. |
| Harbor pedestals and bollards | Straight connectors, elbows, shut-off valves, check valves, bulkhead fittings and threaded adapters. | Prioritize accessibility, quick replacement and confirmation of pressure/outdoor conditions. |
John Guest ranges useful in marine applications
John Guest is not a single family of fittings. There are metric, imperial, plumbing, beverage, water treatment, compressed air and technical application ranges. In marine applications more than one may appear, especially on imported boats or modified installations. You can see a filtered selection in the John Guest Nautical collection or explore all the families in the general John Guest collection.
JG Speedfit 15 mm and 22 mm
Common range for cold and domestic hot water, especially in European or UK-influenced installations. It is the most common base for boats, yachts, sailboats and onboard fresh water systems.
PI range in inches
Very useful in filters, reverse osmosis, watermakers, ice machines, dispensers, imported equipment and auxiliary lines in 1/4", 3/8" or 1/2" OD.
PM metric range
Metric fittings for liquids, inert gases and technical applications. They can be useful in auxiliary installations where small metric diameters are used.
Valves and accessories
Shut-off valves, check valves, safety clips, inserts, plugs, bulkhead fittings and tube cutters are just as important as the main fitting for a reliable installation.
Certifications and approvals
In shipyard projects, megayachts, OEM, professional refits, marinas or installations requiring specific approval, you should not assume that any John Guest reference is suitable for any use. You must check the specific technical data sheet for the range, material, seal, pressure, temperature, fluid and applicable certifications. For general questions about the brand and product families, you can review the dedicated John Guest page.
Connection to Shurflo, Jabsco, Whale, Johnson Pump and similar pumps
Many enquiries arise from a leak or a pressure‑pump replacement. The usual mistake is to buy a 1/2" adapter without checking whether the pump uses BSP, NPT, NPS, its own thread, a hose tail or a specific connector.
Once you have identified the thread or size, look for adapters and valves in the John Guest Nautical collection. For other non-nautical applications, see the general John Guest collection.
| Item to check | Why it matters | How to proceed |
|---|---|---|
| Pump make and model | It can determine whether the thread is European, American or specific. | Look for the manufacturer’s manual or technical datasheet. |
| Thread type | BSP, NPT and NPS may look similar, but they do not seal the same way. | Do not force adapters. Confirm the standard before assembling. |
| Outer tube diameter | The push‑fit fitting is chosen by the tube’s actual outside diameter. | Measure with a caliper: 15 mm, 22 mm, 3/8", 1/2" OD, etc. |
| Flow direction | For check valves and pumps it is essential to respect inlet and outlet. | Check flow arrows and pump orientation. |
| Accessibility | A future repair can be complicated in a bilge or technical locker. | Install shut‑off valves close to the pump whenever possible. |
Connection to boat taps, showers, and kitchen
On galley taps, bathroom taps and deck showers, tube-to-thread adapters, elbows and shut-off valves are usually used. The goal is not only for the fitting to connect, but also to ensure the tube is not under stress or kinked behind the cabinet.
Elbows
Recommended in narrow cabinets, taps with little depth or outlets where the tube cannot enter straight.
Tees and branches
Useful for supplying a shower, washbasin, galley or secondary branches from a main line.
Shut‑off valves
They allow you to repair a tap, filter or shower without draining or shutting down the entire water circuit.
Boilers, heaters and domestic hot water on board
In hot water systems you must pay special attention to pressure, temperature, safety valves, check valves, thermal expansion and compatibility of the range used. Not all fittings, tubes or seals are suitable for every temperature or circuit.
Do not disable safety systems.
If the boiler or water heater includes a safety valve, check valve, pressure limiter or specific manufacturer instructions, they must be followed. A fitting must not be used to correct an overpressure, expansion or undersized installation problem.
For connections to water heaters or boilers, always check the equipment data sheet and confirm whether you need 15 mm x 1/2" BSP, 15 mm x 3/4" BSP, 22 mm x 3/4" BSP adapters, shut-off valves, check valves or reducers. These formats can be grouped within the John Guest Marine collection.
Filters, reverse osmosis and watermakers on boats and yachts
In filtration, reverse osmosis and watermakers, the measurement system can change completely. Even if the boat’s main installation is 15 mm or 22 mm, filters and auxiliary equipment may use 1/4", 3/8" or 1/2" OD.
For this type of application, it is worth checking both the John Guest Marine collection and the general John Guest collection, since many water treatment and dispensing parts use inch ranges.
| Team | Common sizes | Common connectors |
|---|---|---|
| Drinking water filter | 1/4", 3/8", 1/2" OD or threaded adapters. | Straight couplers, elbows, valves, reducers and BSP/NPT adapters depending on the equipment. |
| Reverse osmosis | 1/4" and 3/8" OD in many auxiliary connections. | Inch‑size fittings, shut‑off valves, tees, elbows and locking clips. |
| Watermaker / desalination unit | It depends entirely on the manufacturer and the stage of the system. | Confirm material, pressure, treated water, salinity and technical data sheet. |
| Ice maker / chiller | Usually 1/4" or 3/8" OD. | PI/PP fittings in inches, valves and adapters to the water line. |
John Guest connectors in pedestals, pontoons and dock bollards
In marinas and yacht harbours, service pedestals and dock bollards may integrate potable‑water connections, shut‑off valves, branches and internal fittings. In these cases, it is important that the installation is accessible, easy to maintain and quick to replace in case of leak or breakage.
Which parts can be useful
- Straight couplers.
- Compact elbows.
- Shut‑off valves.
- Non-return valves.
- Threaded adapters.
- Skin fittings or bulkhead fittings.
- Safety clips.
What to check before installation
- Harbor mains pressure.
- Type of existing tube or hose.
- Sun exposure, humidity and salinity.
- Accessibility for maintenance.
- Marina regulations or requirements.
- Compatibility with drinking water.
For this type of installation, the John Guest Marine collection should work as the main entry point, while the dedicated John Guest page helps reinforce the brand’s technical information.
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 an incorrect size, a bad cut, a scratched tube, incomplete insertion or a forced thread.
- Depressurize the system. Turn off the pump, open a tap and release the pressure before disassembling.
- Measure the tube on the outside. Use calipers, not a visual estimate.
- Cut the tube square. Use a suitable tube cutter. Avoid scissors that crush the tube.
- Remove burrs. A damaged edge can mark the O-ring and cause a leak.
- Check the condition of the tube. It must not be oval, scratched, hardened or deformed.
- Push it in all the way. Push the tube fully into the fitting.
- Pull gently. Check that the collet has gripped the tube.
- Pressurize and check. Look for leaks before closing furniture, bulkheads or inspection hatches.
- Use clips where appropriate. In areas with vibration or frequent maintenance they can help secure and identify the connection.
To find fittings, clips, inserts, valves and accessories related to this process, you can check the John Guest Marine selection.
Common mistakes when choosing John Guest connectors for boats
| Error | Consequence | Solution |
|---|---|---|
| Confusing 15 mm with 1/2” OD. | Leak, poor retention or inability to insert the tube. | Measure outside diameter with a caliper and buy by the actual size in the nautical collection. |
| Confusing BSP with NPT. | Poorly sealed thread, cracks or persistent dripping. | Confirm the equipment standard or the manufacturer’s manual. |
| Cut the tube square. | The seal does not seat correctly and may drip. | Use a John Guest pipe cutter or suitable tool. |
| Reusing scratched or hardened tube. | The seal may not seat or may fail after pressurizing. | Cut back to a sound section or replace that run. |
| Forcing a plastic thread. | Crack, leak or break in the threaded body. | Tighten sensibly and use the recommended sealant. |
| Not providing a shut-off valve. | Slower repairs and the need to shut off water to the entire boat. | Divide the pump, filters, boiler and consumption zones into separate circuits. |
| Choosing only by visual appearance. | Similar but incompatible fitting. | Confirm size, thread, fluid, pressure and temperature. |
Recommended John Guest spare parts kit for marine use
For basic maintenance on boats, yachts or recreational craft, it can be useful to carry a small spare‑parts kit. It does not replace a technical diagnosis, but it allows you to solve many leaks or simple fresh‑water repairs.
For 15 mm installations
- 15 mm straight connectors.
- 15 mm elbows.
- Equal 15 mm tees.
- 15 mm x 1/2" BSP adapters.
- 15 mm shut-off valve.
- 15 mm non-return valve.
- 15 mm end plugs.
- 15 mm safety clips.
- Compatible insert if the tube requires it.
- Tube cutter.
For yachts or mixed installations
- 22–15 mm reducers.
- 22 mm couplings.
- 22 mm elbows.
- 22 mm x 3/4" BSP adapters.
- 3/8" OD fittings.
- 1/2" OD fittings.
- BSP/NPT adapters depending on the equipment installed.
- Additional shut-off valves.
- 22 mm clips.
- Spare tube sections.
The most practical way to assemble this kit is to start from the John Guest Marine collection and add extra references from the general John Guest collection if the boat combines metric sizes, inch sizes, water treatment or special connections.
Useful John Guest links on Install Beer
To continue from this guide, here are the three main links:
- John Guest Marine collection: specific selection for boats, yachts, fresh water and dock bollards.
- General John Guest collection: full range of connectors, tubes, valves and accessories available.
- Dedicated John Guest page: general information on the brand, applications, advantages and product families.
Buy John Guest connectors for marine use at Install Beer
This guide helps you identify the size and avoid mistakes. To buy fittings, tubes, adapters, valves, check valves, clips and accessories selected for boats, yachts, fresh water systems and harbor bollards, go to the specific collection.
What information to send if you don’t know which fitting you need
To reliably identify a marine connection, the ideal is to send an overall photo, a close-up photo and some basic data. This prevents buying a part that looks similar but is incompatible.
- Photo of the current fitting installed.
- Photo of the connected equipment: pump, filter, tap, boiler, tank, shower or service pedestal.
- Outside diameter of the tube measured with calipers.
- Brand and model of the pump, tap, filter or heater if shown.
- Indicate whether it is cold water, hot water, filtration, reverse osmosis, main line or branch.
- Indicate whether the connection is tube, male thread, female thread, barb or flexible hose.
You can also first check the John Guest Marine collection to locate similar parts, or the John Guest collection to look up references for other applications.
Frequently asked questions about John Guest for boats and yachts
Which John Guest size is most commonly used on boats?
In fresh domestic water circuits on European boats and yachts, one of the most common sizes is 15 mm outside diameter. On main lines, manifolds or larger vessels, 22 mm may also appear. You can check selected parts in the John Guest Marine collection.
Are 15 mm and 1/2” the same?
No. 15 mm is a metric outside diameter. 1/2" OD is approximately 12.7 mm. In addition, 1/2" can also refer to a BSP, NPT or NPS thread, so you must confirm what is actually being measured.
Can I use John Guest with a Shurflo, Jabsco or Whale pump?
It may be possible, but you must confirm the pump connection. Some use BSP thread, others NPT, NPS, hose barb or specific adapters. You shouldn’t buy based only on the apparent thread size.
Are John Guest fittings suitable for domestic hot water?
Some ranges are designed for hot and cold potable water, but you must always check the specific reference, the tube, pressure, temperature and system conditions, especially in boilers or heaters. To learn more about the available families, see the dedicated John Guest page.
Which fitting do I need to connect a boat tap?
It depends on the tap and the installed tube. In many cases 15 mm x 1/2" BSP adapters, angled connectors, elbows or shut-off valves are used. If the tap is imported, it may use a different thread.
Can John Guest connectors be disassembled and reused?
Many John Guest connectors are removable if the system is depressurized and the tube is in good condition. If the tube is scratched, oval, hardened or deformed, it is advisable to cut back to a sound section or replace that run.
Which parts are worth carrying as spares on a boat?
For a 15 mm installation, it is usually useful to carry straight couplers, elbows, tees, 15 mm x 1/2" BSP adapters, shut‑off valves, check valves, plugs, locking clips, inserts and a suitable tube cutter. You can start from the John Guest Marine collection.
Can I use John Guest in dock bollards or pontoon pedestals?
It can be a practical solution for water distribution, maintenance and replacement of internal connections, provided the chosen part number is suitable for the pressure, temperature, fluid, protection and real conditions of the installation.
Where to buy John Guest connectors for marine use?
You can check the specific John Guest connectors for boating, boats and yachts collection at Install Beer. For other uses, see the general John Guest collection or the dedicated John Guest page.
Final technical note
Final compatibility depends on the exact part number, tube type, pressure, temperature, fluid, thread, applicable standards and real installation conditions. In professional projects, shipyards, megayachts, marinas, OEM installations or systems requiring specific approval, request technical confirmation of the part number before placing the order.