Boat Plumbing

Boat Plumbing

Boat Plumbing can be a bit of a nightmare. To avoid mismatches between equipment and pipes or hoses, and untoward plumbing failures, it is necessary to understand how pipes and hoses are specified, the suitability of different pipes and hoses for different applications, and proper installation practices.

The nominal sizes of most pipes, hoses, and fittings do not correspond to their actual sizes, either the outside diameter (OD) or the inside diameter (ID). In addition, there is no universal standard governing sizes. Given two items that are nominally the same size (e.g., 3/4 inch or 20 mm) it is quite likely that neither will actually be this size, and, what is more, that they will not be the same size as each other!

Pipes and fittings are commonly copper, metal (black or galvanized), or plastic (PVC or CPVC). Copper pipe in the USA is subdivided into water pipe or tubing and refrigeration pipe or tubing (pipe comes in straight lengths; tubing in coils).

Copper Pipe or Tubing

Water pipe in the USA has a purely nominal size. In other words, its nominal size does not correspond to either its ID or its OD. Refrigeration pipe or tubing, on the other hand, is measured by its outside diameter (OD)-1/inch (13-mm) refrigeration tubing really has a 1/-inch (13-mm) outside diameter. Copper fittings (connectors, elbows, tees, etc.) are made mostly to water pipe sizes. Since water pipe of any given nominal size is actually one size larger than refrigeration pipe of the same nominal size, when buying fittings for use with refrigeration pipe or tubing, generally you must buy the next smaller size of fitting down (e.g., 3/8-inch fittings for 1/2-inch refrigeration pipe or tubing).

Copper pipe and tubing is further defined by its wall thickness. In the USA it is given a K, L, or M rating, with the K having the thickest wall. Type L is generally used for household and refrigeration plumbing. In the UK copper pipe and fittings are all standardized according to the outside diameter of the pipe or tubing (8-, 10-, 15-, 22-, 28-mm, etc.).

Specific refrigeration tubing or pipe should always be bought for refrigeration purposes – it is specially cleaned, dehydrated, and capped to keep out moisture. For water systems, either refrigeration or water tubing or pipe can be bought from any hardware store. Tubing has the advantage that it can be worked around the awkward shapes of a boat with very few joints. It generally comes in 50-foot rolls. When making tubing runs, the copper should be bent as little as possible; with constant flexing it work-hardens and then is prone to kink or crack. If tubing does become hard, it can be resoftened by heating to a cherry red color with a propane torch and then dousing with cold water (annealing – copper is annealed the opposite way to other metals, which are heated and then cooled slowly).

Tight bends are liable to flatten out, kink, or both, unless made with proper bending tools or springs. Bending springs, which simply slip over the outside of the tubing and are then removed when the bend is complete, are quite cheap, and available from plumbing suppliers.

If possible, tubing and pipe should always be cut with tubing cutters since this is the only way to ensure a smooth and square cut (especially important for making flare connections-see below). To make a cut, clamp the cutters lightly around the tube or pipe and make a full turn. Tighten the handle a half turn after each turn until the cut is complete. On the back of the cutters will be a hinged arrowhead fitting-use this to clean any burr on the inside of the cut.

In both the USA and the UK, copper pipe and tubing can be joined with compression fittings, flare fittings, and solder fittings.

Boat plumbing fittings

Compression fittings. Boat plumbing compression fittings are in many ways the easiest to make but the least reliable in service.

Flare fittings.

Flares are made with a special tool. There are a number of relatively inexpensive flaring kits on the market. The cheaper kits consist of a clamp that fits around the tube or pipe and has a machined bevel in its face. A horseshoe-shaped bracket with a threaded bolt in its centre fits over the clamp. On the base of this bolt is a spinner-a block of metal cut to the same taper as the bevel in the clamp. The spinner is screwed into the mouth of the tube or pipe and forces its sides out against the bevel. The flare is complete.

Here are some tips:

1. The nut must be put on the line before making the flare! If possible use long-nosed flare nuts (as opposed to the more common short ones), since they provide more support to the joint and reduce the chances of cracking due to vibration.

2. The end of the tube or pipe must be cut square and cleaned of all burrs inside and out.

3. Warming the tube or pipe before screwing down the spinner will help prevent cracking (ideally, the tube should be annealed).

4. Depending on clamp configuration, the tube or pipe may need to protrude above the face of the clamp by one-third to one-half the depth of the bevel to permit an adequate flare. If it is set flush with the face of the clamp, the resulting flare will be skimpy and prone to leak. When made, the flare should just fit into, but not hang up on, the sides of the flare nut.

5. The spinner should be oiled when making the flare.

6. The spinner should not be screwed down too tightly-it will weaken the flare. When the joint is done up, the flare nut will pull the flare snugly onto the flared fitting.

7. If the flare looks uneven or in any other way unsatisfactory, it must be cut off and remade. Doing so right away will be a lot easier than doing so later.


Solder fittings (sweat fittings) can be used on both copper tubing and pipe. A wide range of fittings is readily available. Some fittings are presoldered, but most are not.

1. Clean both surfaces to be soldered with fine (400- to 600-grit) wet-or-dry sandpaper until the entire surface to be soldered is shiny. Do not use emery cloth-oils in the cloth backing will spoil the soldering.

2. Apply soldering flux immediately but sparingly to both surfaces. Push the fitting onto the tube or pipe and twist to spread the flux evenly. The flux is there to keep out oxygen and contaminants once the joint is clean. Flux is no substitute for cleaning.

3. Heat the joint evenly with a propane torch.

4. Touch solid solder to the joint periodically until the tube or pipe and fittings are hot enough to melt the solder. The solder itself is not heated by the torch-if the metal in the joint is not hot enough to melt the solder, it will not flow properly.

5. Let solder flow into the joint. Apply only enough heat to keep the solder melting; more will overheat the joint. Generally the flame can be held at some distance or turned away from the joint and passed over it quickly a couple of times.

6. When solder shows all around the fitting, the joint is complete. Remove the heat. Any more solder added to the joint will merely flow out the other side and into the tube or pipe, causing a partial blockage.

7. When the joint is cool, thoroughly clean away the excess flux.

If you experience problems in getting leakproof joints, the most likely culprit is contamination of the soldering surface on the pipe or in the fitting. To be certain of making successful joints, the surfaces to be soldered can be tinned (i.e., given a thin film of solder) prior to being mated. If the solder will not spread out evenly, the copper is not clean enough. Once tinned, the pipe may not push into the fitting, in which case you need a little heat to melt the solder while seating the pipe.