Plumbing on a sailboat delivers fresh water from the tanks, expels sewage from the heads, and moves seawater both through the engine and out of the bilges. Important parts of the plumbing are the thru-hull fittings and their seacocks, the hoses and the pumps. There might also be filters, check valves or other valves, and vents.
Thru-hulls
Every opening or thru-hull below the waterline has a seacock, which is a quarter-turn valve used to open or close it. The handle lines up with the thru-hull when open and is perpendicular to the direction of flow when closed. Ordinary gate valves are sometimes found and must be promptly replaced.Proper seacocks have ball valves or tapered plugs. They are made of bronze, sometimes stainless steel on metal hulls, or a strong plastic called Marlon. There should be a backing plate or adapter between the seacock and the thru-hull and the hose connected to the seacock should have two clamps.
Tapered
plug seacock
The traditional tapered plug seacock is simple and effective but requires periodic maintenance. An adjusting nut, a lock nut, and a washer secure the tapered plug in the seacock body. The shaft that these fit onto has a flat that aligns with a flat in the washer. The adjusting nut is tightened down only enough to prevent leaks, but not too tight, and the lock nut is then tightened while holding the adjusting nut in position.One or two drain plugs are the only other parts of this simple device. A layer of waterproof grease on the tapered plug seals out water. It should be removed with solvent and replaced with fresh grease annually. Sometimes a zerk fitting in one of the drain plug holes allows refreshing the grease with a grease gun.
If fresh grease does not provide a waterproof seal, the mating surface between the tapered plug and the seacock body can be cleaned and reconditioned with lapping compound. If the plug is difficult to remove, try tapping the shaft with a hammer, using the nuts to protect the end of the shaft from damage. If this fails, try a careful application of heat to the body. When the boat is left in the water during winter, and if ice will form down to the thru-hull, each seacock must be closed and drained to prevent it from freezing and cracking.
Marine
hoses
Hoses do a surprisingly wide range of jobs. What matters most is what the hose carries—fresh water, seawater, fuel, waste, air, or exhaust—and whether it must withstand heat, pressure, chemicals, or movement. Common types of hose are:Fresh water hose: Used for potable water from the tank to faucets, showers and water heaters. Usually reinforced, flexible tubing designed for pressurized systems and rated for drinking water.
Sanitation hose: Used for marine toilet, holding tanks, and waste plumbing. Especially important because ordinary vinyl or rubber hose can allow sewage odors to pass through the hose wall. The most expensive kinds are not necessarily the best.
Bilge hose: Carries water from the bilge pump to an overboard discharge. Smooth-bore reinforced bilge hose gives better flow. Corrugated bilge hose is easier to route around corners.
Engine cooling water hose: Carries raw seawater through the engine's heat exchanger, water pump, and exhaust system. Must withstand continuous exposure to seawater and, depending on location, high temperatures.
Exhaust hose: Connects the engine's exhaust outlet to the water-injected exhaust system and transom outlet. This high-temperature, wire reinforced marine exhaust hose is thicker and much more robust than other boat hoses.
Fuel hose: Used for gasoline or diesel fuel between the tank, filters, engine, and diesel fuel return system. Designed to meet applicable fire-resistance and permeability requirements. Do not use ordinary automotive fuel hose, nor any hose not rated for fuel, especially if the fuel is gasoline.
Hydraulic hose: Found on boats with hydraulic steering or autopilots. It operates under substantially higher pressures than most plumbing hoses and must have the correct pressure rating and fittings.
Head intake/discharge hose: Marine toilets commonly have separate hoses for seawater intake and waste discharge. Intake hose can be plain reinforced water hose. Discharge requires special sanitation hose.
Deck and cockpit drain hose: Carries rain and wash water from cockpit drains, scuppers, and deck drains to through-hulls. Because these are often gravity fed, large diameter and unobstructed routing with no sharp bends is what matters most. If there are any tight angles, debris will surely block them.
Propane or LPG hose: On boats with propane cooking systems, LPG-rated hose is used between the regulator and appliances. These deserve special attention because propane is heavier than air. It can settle in the bilge and explode from the slightest spark, such as a bilge pump arcing on.
Air or vent hose: Used for tank vents, engine-room ventilation, battery-box ventilation, and anti-siphon loops. Although often not pressure hoses, it must tolerate the environment and, in some cases, fuel vapors.
Marine hose is not simply interchangeable based on diameter. Two hoses may both fit a particular size of barbed fitting while having totally different pressure, temperature, chemical, permeation, or fire-resistance characteristics. For example, using ordinary clear vinyl hose on a holding-tank discharge can result in persistent sewage odor, while using ordinary water hose for fuel can create a much more serious problem.
Hose
clamps
Hose clamps are just as important as the hose itself. A bad clamp can turn a small hose problem into a major leak, especially below the waterline. Main types of hose clamp are:316 stainless-steel worm-drive clamps: The standard choice for much marine plumbing. 316 stainless is preferable to 304 because it has much better resistance to saltwater corrosion. Look for clamps where the band, screw, and housing are all corrosion resistant. Inexpensive clamps from a hardware or home supply store are probably not what you want. Although the worm gear mechanism will be slotted for a screwdriver, it is far easier to use a nut driver or a small ratchet wrench. For this reason, avoid mixing sizes, especially regular and metric.
Non-perforated (solid-band) clamps: Generally, better for marine use than inexpensive perforated worm-drive clamps. The smooth inner surface is less likely to cut or damage soft hose.
T-bolt clamps: Stronger and more uniform than ordinary worm-drive clamps. Used for larger hoses, high-vibration applications, and some engine and exhaust plumbing.
Constant-tension clamps: Spring-type clamps that maintain clamping force as temperature changes. Commonly found on engine cooling systems where the hose repeatedly heats and cools.
Ear clamps or Oetiker-style clamps: Crimped clamps that provide a highly consistent seal. Common in factory installed and compact applications. Replacement requires the correct tool and usually a new clamp.
Below the waterline, best practice is to use two clamps, side by side but not overlapping. Both should be appropriate for the hose and fitting, fully engaged on the fitting's barb, and tightened just enough to seal without cutting into the hose. Some fittings and hose systems are specifically designed around a single clamp.
Do not put a clamp over the very end of a hose. It should sit behind the barb or ridge of the fitting with enough hose extending past the barb to provide a proper sealing area.
Do not overtighten the clamp until it cannot move. Its job is to create a seal around the hose and barb. Over-tightening can deform or cut into the hose, damage the barb, create a leak and actually weaken the connection. Tighten ordinary worm gear clamps firmly while watching the hose. If the hose is being crushed or the band is digging deeply into it, you've gone too far.
A clamp can look perfectly fine from above while being badly corroded underneath. Inspect by loosening the clamp and rotating it on the hose.
If, after loosening a clamp, the hose cannot be removed from the barb, as often occurs, use gentle application of heat from a heat gun or hair dryer.