Replacing Your Bilge Pump

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That sinking feeling


Click on the intro video (above) to go to our youtube page and see 3 more videos on replacing a new bilge pump.

You are enjoying a dreamy and lethargic day on the water, with bright sunshine, a balmy eight-knot breeze and flat water with sparkling six inch wavelets-about as good as it gets. One of your passengers goes below for another drink from the icebox, and suddenly exclaims four words that immediately send your pulse rate into overdrive, “What´s all this water?!?”

We experienced this unfortunate situation recently, jumping up and peering down the companionway to see water sloshing over the floorboards. After a few choice expletives combined with “we´re sinking!” we were fortunately able to keep the flow of saltwater under control with a vigorous aerobic workout on the manual bilge pump.

Returning to the marina, we quickly located and repaired the source of the leak, with the major damage adding up a few hours of lost time on the Bay, along with a new bilge pump. While we were well equipped with a manual backup pump, this was NOT the way we wanted to learn that our electric centrifugal pump had failed. As a result, we recommend that you briefly flip the pump switch onto MANUAL to check the status of your pump each time you use your boat, a simple routine that we neglected to follow in this instance.

Skill Level Description
Replacing your current pump with an identical model:
2 out of 10
Upgrading your pump to automatic operation:
3 out of 10
Tools Required
To replace your pump with an identical replacement:

  • Flat screwdriver or 5/16″ or 7mm nutdriver for hose clamp.
  • Wire cutters
  • Wire strippers
  • Wire terminal crimpers (ratcheting type if possible)
  • Heat source for heat-shrink terminals (heat gun, small butane torch, disposable lighter)
To upgrade your pump to one with a larger pump diameter:

  • Electric drill, drill bit set (1/8″ to 3/16″ especially)
  • Sealant suitable for underwater use
  • c) Screwdriver for mounting fasteners
To upgrade your pump to one with a larger discharge hose:

  • Drill motor and hole saw for larger through hull fitting
  • Hole saw that is slightly larger than the hose diameter
  • Sealant for through hull
  • Wrench for nut on through hull
Shopping List

Overview

A do-it-yourself project [back to overview]

Installing a new bilge pump is a do-it-yourself projectReplacing your electric bilge pump is an easy do-it-yourself project that you can accomplish using normal mechanic´s tools. We´re going to focus on the most common style, the submersible centrifugal pump, although there are three popular kinds of bilge pumps (manual, self-priming diaphragm and submersible). If you´re happy with the performance of your current pump, we´ll talk about how to replace it with an identical model. For those of you who would like to either increase your pump´s capacity or add a float switch to your current pump, we´ll also address how to make these upgrades.

What bilge pumps should and should not be expected to do: [back to overview]

The American Boat and Yacht Council (ABYC) reminds us that bilge pumps are “intended for control of spray, rain water, and normal accumulation of water due to seepage and spillage” but not for emergency removal of water from a serious leak or an underwater hole in the hull. They´re not damage control pumps.

Therefore, never leave a boat with a known leak alone with an automatic bilge pump, depending on the pump to keep your boat afloat. If your boat has a leak, get it fixed or haul the boat out. It’s good practice to make sure your pump is also be capable of being reached easily for inspection, removal or maintenance, without pulling apart any permanent part of the boat´s structure.

Should you upgrade to a higher capacity pump? [back to overview]

Few boaters ever complain about having too large a bilge pump. In conditions where you have to remove a lot of water in a hurry, a large pump can be a lifesaver. Our advice is to install the largest pump you can fit in your bilge. This is especially true if, due to your boat´s configuration, the discharge hose is either lengthy or has a lot of vertical lift.

Why do we advise you to buy a big pump (other than for our own self-serving reasons)? [back to overview]

Centrifugal pumps are rated in gallons per hour (gph) or gallons per minute (gpm) using an extremely unrealistic standard: they´re rated at open flow (with no hose connected) where they are not lifting water (at zero vertical lift, or zero head pressure, which is a combination of the vertical lift distance and the resistance the water encounters from the hose and fittings). They are also rated at 13.6V, rather than the more conservative 12V. Pump performance decreases with lower voltage. As a general rule to help you estimate the water-lifting performance of a centrifugal pump, don´t expect more than half of the pump´s rated gph performance at 5´ of vertical lift.
There are, however, three problems to consider with a large pump:

  • Big pumps draw more power, draining the battery bank more quickly
  • Big discharge hoses (1 1/2″) hold more water, which drains back into the bilge when the pump turns off, so the float switch may then re-start the pump (known as short-cycling)
  • Big pumps (with bigger strainers) do not drain the bilge as completely as smaller pumps

Two-pump system [back to overview]

The two-pump bilge pump system The best solution we´ve found (suggested by boat maintenance guru Don Casey) is to install two pumps: 1) 500gph (or smaller) pump with an integral float switch, such as a Rule-Mate 500 automatic pump, located in the deepest part of the bilge and connected directly to the battery. Rule-Mate pumps have a small 3/4″ hose size, and a “spongeability” feature that keeps them running for several seconds after the switch turns off to suck out every bit of water. 2) 3500gph (or larger) pump connected to a manual switch, and located high enough in the bilge to only be submerged in an emergency situation, with a 1 1/2″ discharge hose.

Maximizing pump output [back to overview]

Since everyone wants a pump with as much capacity as possible, what can you do to ensure that your pump has the highest output? Here are several points to remember:

  • Use the correct wire size. Larger wire will minimize the drop in voltage (see below)
  • Use a smooth-walled hose for the least internal resistance. Corrugated Series 120 Bilgeflex hose reduces flow capacity and pump performance with its ribbed interior surface. Series 148 hose costs more, but its smooth bore creates less friction.
  • Keep hose runs as straight and short as possible. Be sure the hose runs continuously downward to the bilge (at all angles of heel) with no hills and valleys than can create air pockets that cause a vapor-lock and stall the pump.
  • Discharge at the lowest practical height, but above the vessel´s waterline or above the maximum heeled waterline for sailboats. If you must install the bilge pump opening where it may submerge when the boat heels, you´ll also need to install a vented loop (located as close to the boat´s centerline as possible). Otherwise, the thru-hull may back-siphon into your bilge when you´re heeled over, and flood the boat.

Replacement with the same model of pump [back to overview]

Let´s assume that you have a pump like the 1100gph Rule 27D (Model 316430) that has finally failed after years of service. What are the steps to follow for installing a new one? If you are satisfied with the performance of your old pump, replacement is a straightforward task. Here´s what to do:

  1. Disconnect the discharge hose from the pump.
  2. Remove the pump from its base.
  3. Snip the wire leads.
  4. Strip the supply wire and inspect for corrosion; strip back to clean wire or replace if the tinned copper has turned green.
  5. Strip ends to correct length, verify the polarity, then double crimp a waterproof splice and heat to shrink the insulating cover.
  6. Snap pump back in its base.
  7. Reconnect the discharge hose.
  8. Check operation.

Replacement with a larger sized model [back to overview]

If you want to upgrade to a larger-capacity pump, the process might be only slightly more complicated. Three modifications may be required, so you need answers to these questions:

Can I use the same strainer base? [back to overview]

Many pumps share the same strainer base, so you can upgrade the size of the pump and still keep the original mounting arrangement. For example, several round Rule pumps (the 360, 500, 800 and 1100gph models) all use the same blue Model 275 base, which simplifies your installation.

If you do have to swap out the base, the problem becomes how to drill new mounting holes in the bilge of your boat without drilling through the hull and creating a leak. We recommend that you carefully check the surface to which you´re mounting the pump to ensure that you won´t drill through. Alternately, you can bond the pump base to the bilge with a sealant like 3M 5200, although adhesives like 5200 are extremely tenacious, making them almost impossible to remove when cured.

Can I use the same size hose? [back to overview]

There are three common sizes of discharge hose: 3/4″, 1 1/8″, and 1 1/2″. You´ll find that many times you can increase your pump size and retain the same size hose, which greatly simplifies the project. With the popular Rule pumps mentioned above, the 360, 500 and 800gph models all use 3/4″ discharge hoses. The 1100gph model uses 1 1/8″ hose.

Upgrading your discharge hose diameter requires that you replace the discharge thru-hull to accept the larger hose diameter, and if the hose passes through bulkheads or other barriers, those holes will also need to be enlarged. This can be quite a big job, and those locations may be very difficult to access. But at some point, if you´re upgrading your pump, the hose will have to be upgraded too.

Can I use the same wiring? [back to overview]

Wiring standards for your bilge pump installationABYC and ISO standards allow a 10% voltage drop for bilge pumps, but you can increase performance by using the 3% voltage drop tables found in the Marine Wire Advisor in our Annual Catalog and on this site. A 10% voltage drop at 12V reduces pump voltage to 10.8V, with a corresponding decrease in pump capacity, compared to 11.6V with a 3% voltage drop.

Most small bilge pumps will be adequately supplied by 16-gauge wire since you can power a 4A load at distances over 30´ with this size wire. Medium-sized pumps, in the 1500gph range, can use a 14-gauge wire up to about 30´. The largest pumps, which draw about 15A, can use 12-gauge wire up to 30´ from the panel. Shorter wire runs will have lower voltage drop, and your pump will operate closer to its optimum performance.

Making sure the connections are waterproof [back to overview]

Make sure your electrical connections are waterproofSince we´re dealing with an electric circuit in a damp or possibly submerged location, it´s vital to make any wiring connections impervious to water. The wire´s insulation will protect it, but any wire termination is a potential source of water intrusion. Use waterproof butt connectors, since most pumps don´t come with enough pigtail length to ensure that the connections are out of the bilge.

Both Ancor and 3M make solderless terminals with adhesive-lined heat-shrink tubing for exactly this application. The way these work is to crimp them using a quality ratcheting crimper, like Model 5764568 from Ancor, then apply gentle heat to cause the shrink tubing to contract. This will also melt some adhesive inside the tubing, which will bond to the wire insulation for a 100% waterproof connection. While you can use a disposable lighter as a heat source, we find that an electric heat gun does a great job without causing the plastic to burn.

Adding a float switch [back to overview]

Adding a float switch to your bilge pump installationA lot of boats came from the factory with a manually-activated bilge pump. You need to flip a switch to turn the pump on to evacuate the bilge, and then turn it off again. This isn´t much help if the boat is left overnight and there´s a downpour. That´s when having an automatic float switch is a good idea. While an automatic float switch requires a modest amount of wiring and a new three-way panel switch, it´s a great idea for boats that are left unattended.

It used to be that float switches relied on a small mercury capsule to turn the pump motor on and off. Modern switches are mercury-free, so they are better for the environment. Plus, we offer some switches that are entirely electronic, and sense the water level without any moving parts.

Many bilge pumps are designed so that the matching float switch attaches directly to the base of the pump. In other cases, you simply mount the switch in the bilge in the vicinity of the bilge pump. Mount the switch at the same height as the pump, and in the same fore-and-aft plane, so they will work together properly. Otherwise your pump may run dry, not remove all the water, or perform differently depending on which way the boat heels. Finally, you can buy a pump like the Rule-Mate we discussed earlier with an integral float switch. Regardless of the style you choose, you´ll find that having a pump with automatic operation is a big convenience.

An Automatic Bilge Pump Caution [back to overview]

We should mention one caution, however. An automatic bilge pump can often mask a more serious problem like a split hose or a crack in a thru-hull, since it dutifully pumps the water out as quickly as it comes in. A bilge pump indicator light that shows when the pump is operating is a good safety feature so that you can monitor how frequently the pump turns on. A cycle counter is even more precise.

Good luck, and don´t be afraid to tackle these hands-on projects. With the cost of everything spiraling upward, doing it yourself is the best way to keep boating affordable. You´ll also improve your ability to respond when something goes wrong far from shore, and perhaps prevent an early termination to a day enjoying perfect boating weather.

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