Bilge Pump Basics: What is it, and How Should You Use It?

Bilge Pump Basics: What is it, and How Should You Use It?

A bilge pump is an essential piece of equipment on almost any boat. For many new boaters, understanding its function can be a bit perplexing. What exactly is a bilge pump?

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The primary function of the bilge pump is to remove water that collects in the bilge, which is the lowest inner part of the hull. This water accumulation is often incidental:

• It can be rainwater or spray that enters the boat;
• Water from cleaning the boat that collects in the bilge;
• Or drainage from built-in coolers.

On larger boats, built-in coolers, hatch lips, and even drink holders may have direct overboard drainage. However, smaller boats typically route this water to the bilge. Minor leaks, such as water dripping from the shaft gland on an inboard boat, might also add to bilge water.

Most boats aren't equipped with a bilge pump powerful enough to prevent sinking in catastrophic situations, like a large hole in the hull. In such scenarios, the bilge pump may buy you time to repair the leak, call for help, or prepare for an orderly evacuation.

Where are Bilge Pumps Located on a Boat?

Bilge pumps should be installed at the lowest point in the bilge. Larger boats might have one in each enclosed area that can retain water. Discharge outlets must be at least eight inches above the waterline.

Accessibility is crucial for all bilge pumps on the boat to allow for inspections, debris removal, and maintenance. If a pump is difficult to reach, such as under the engine in a sterndrive boat, consider relocating it.

How and When Should You Turn on a Bilge Pump?

Some bilge pumps have a float or switch to automatically activate when water levels rise. This feature is particularly useful if the boat is docked, as it ensures the pump activates after heavy rain. There might also be a manual switch at the helm, often with 'auto' and 'manual' settings.

Regularly inspect the area around the pump for debris, which can clog the pickup and impede the pump. This is especially important for new boats, which might have construction debris in the bilge.

Additionally, periodically check the float switch's functionality, as it can corrode or become crusty over time.

• Route wiring for the bilge pump upward to keep it out of the damp environment, and ensure all connections are water-tight.
• If the pump isn't frequently active, test its functionality by pouring some water into the bilge.
• This practice not only checks the switch but also confirms the pump's ability to move water.

Selecting a Bilge Pump for Your Boat

Bilge pumps are rated by their capacity to move water, measured in gallons per hour (GPH). Although these ratings are nominal, it’s reasonable to expect the pump to move about 60% of its rated capacity due to real-world factors such as discharge distance and hose type.

Upgrading from a corrugated discharge hose to a smooth-wall hose can enhance the flow rate by up to 30%. Contrary to intuition, smaller boats may need high-capacity bilge pumps as they’ll fill with water faster in emergencies compared to larger boats.

Experienced boaters often use a two-pump strategy. They install a smaller (400-500 GPH) automatic pump low in the bilge and a larger (3,500 GPH) pump higher up for more serious situations.

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Foundations of Bilge Pump Systems

A bilge pump should not be regarded simply as a standalone unit but as part of a system. Each component, from the pump itself to the hose and thru-hull fitting, plays a vital role in the overall performance.

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Performance tests have shown that bilge pumps with hoses operate below their rated capacities. Smooth-bore hoses perform better than corrugated ones, and thru-hull fittings can significantly affect output due to their narrower diameters and additional friction loss.

Ensure the system head is minimized by considering both 'static head' (the vertical distance water must be lifted) and 'friction head' (resistance within the hose and fittings). The combined system head is what the pump must overcome.

Choosing the right hose and fitting, and maintaining a streamlined system, will help maximize your bilge pump’s effectiveness.