Relay control of bilge pumps.

jhall767 · Nov 7, 2011

I'm thinking about changing my bilge pump wiring so that the power switching is done by a bosch type relay instead of the float switch or dashboard control.
Currently I have the traditional smaller boat setup where the power goes through a switch (MOM-Off-ON) on the flybridge and back down to the bilge pump. Either directly to the pump or through the float switch depending on the position of the flybridge switch. Probably 60-70 feet total run.

Under the new design only the relay circuit would carry power directly to the pump. The flybridge switch and float switch would only carry signal current (<.2 amp) for the relay. This would shorten the wire run by 40-50 feet.

I will also wire the high water alarm switches so they will turn on the bilge pump(s) in their area if they detect high water instead of just setting off the alarm. This would be similar to having redundant float switches.

Also the relays are a proven technology and should be more reliable then the current crop of float switches.

Are there any drawbacks to this system? How are the larger yachts designed where there might be 100' of wire between the flybridge and a bilge pump?

Capt J · Nov 7, 2011

You're going to want to still use float switches to keep the bilge pumps automatic.

I would leave it well enough alone. I wouldn't want to add another part (the relay) that could fail. Usually bilge pumps are wired with the hot wire going to the float switch coming from a power source in the engine room, and there isn't a long run on wiring going to the float switch. Then the manual on switch power goes to the Flybridge or lower helm or both, then back down to the bilge pump. If the wires are properly sized, there shouldn't be much of a voltage drop, and if the system currently works, I'd leave it well enough alone.

Capt J · Nov 7, 2011

You're going to want to still use float switches to keep the bilge pumps automatic.

I would leave it well enough alone. I wouldn't want to add another part (the relay) that could fail. Usually bilge pumps are wired with the hot wire going to the float switch coming from a power source in the engine room, and there isn't a long run on wiring going to the float switch. Then the manual on switch power goes to the Flybridge or lower helm or both, then back down to the bilge pump.

Most large yachts 100' + have one large pump (and a backup usually) that you have to manually turn on, hard piping that goes to each bilge with a shutoff valve on each, and you manually open the valve for the bilge that you want to drain and it's all manual operation. Which means someone has to physically check all of the bilges. I know of a brand new 160' MY that sank at the dock about a decade ago, because the a/c discharge broke and nobody was living on it, or on it to turn the bilge pump(s) on.

jhall767 · Nov 7, 2011

Capt J said: ↑

You're going to want to still use float switches to keep the bilge pumps automatic.

I would leave it well enough alone. I wouldn't want to add another part (the relay) that could fail. Usually bilge pumps are wired with the hot wire going to the float switch coming from a power source in the engine room, and there isn't a long run on wiring going to the float switch. Then the manual on switch power goes to the Flybridge or lower helm or both, then back down to the bilge pump.

Most large yachts 100' + have one large pump (and a backup usually) that you have to manually turn on, hard piping that goes to each bilge with a shutoff valve on each, and you manually open the valve for the bilge that you want to drain and it's all manual operation. Which means someone has to physically check all of the bilges. I know of a brand new 160' MY that sank at the dock about a decade ago, because the a/c discharge broke and nobody was living on it, or on it to turn the bilge pump(s) on.
Click to expand...

Maybe I need to make up a schematic?

1) Not sure why you think I'm talking about not making the pump automatic? I'm talking about using the high water alarm as a REDUNDANT automatic float switch. So doubly automatic.

2) I don' think there are many boats wired the way you describe it. It doesn't match my experience nor the wiring diagrams provided by the bilge pump/switch companies. As you describe it you cannot turn off the bilge pump with the man-off-auto switch on the control panel. Look at any manufactures wiring diagram. Power goes through the control panel switch and then to either the float or directly to the pump depending on the switch position.

3) The float switch itself should become more reliable when used at low current with a relay vs. at high current directly controlling the pump. The bosch relay MTBF in hours or cycles will far exceed the rest of the components in the system. These are the same relays that are used in your car to control everything from headlights and flashers to the ABS and airbags.

Does it work the way it is now? Yes. But I have seen more than one boat, including one of mine, get into trouble when a single float switch fails or is obstructed. They also seem to be less reliable since they stopped using mercury in them.

Capt J · Nov 7, 2011

2. A lot of boats have the float switches direct wired ( of course there is a circuit breaker involved in case of a short), and only have an off/manual on switch at the helm to bypass the float switch. It's safer, because the manual switch is not involved (could fail), or somebody cannot accidentally turn it to off and the auto switch does not work.

3. The Bosch relays may work well in a car. However, they don't like to get wet, or be in a moist environment and it's an additional part to fail

I agree, the float switches are not as reliable as they used to be since they went mercury free. However, now the government (US) is pushing CFL light bulbs which have mercury in them. It makes no sense.

Capt Ralph · Nov 10, 2011

Keep it simple and close to a standard. Somebody else may need to steep in and fix it one day for you.

We also use a second pump / switch setup as backup in each bilge area running from the second battery bank. This backup setup also sounds alarm bells around the boat.
When updating all the pump systems after we purchased our old Bertram, we did go thru a lot of the new (doa & short life duds) float switches at first. The last few years we are down to one a year failure out of fifteen (including shower and a/c sumps). I'd say the factory (Danforth/Rule) makes one good switch out of three that keeps working. You have to weed thru them like we did. The new clip in / out is nice also. Always keep spares.
And then, The original factory backup is a belt driven pump with a manifold selecting which bilge to de-water.

Anybody that has seen a pump / switch setup from a Bayliner would be able to service my stuff with minimum problems.

KISS

jhall767 · Nov 10, 2011

Interesting points. I agree with the KISS principle but not where its keeping it to a low reliability point also. The failure rate of a completely sealed bosh type relay (marine rated) is thousands if not millions of times better than a float switch. Its also documented and verified. Definitely not one out of 3! Also any control switches life span will be improved by only having to carry .2 amps vs 15 amps. Plus the high water alarm circuit for that area will function as a backup float switch.

Controlling bilge pumps with relays is not a new idea. All those auto sensing bilge pumps out now do it. Also some of the better float switches already incorporate a relay such as Wema. Their schematic is similar to what Capt J said about only being able to turn the bilge pump on from the helm except they control the relay from the helm switch. I do know that bombadier uses relays to control their bilge pumps. Not sure how many other manufactures have switched.

I don't really see an issue with someone else working on the system. Modern boats already have relays in them to control lighting, fuel pumps, blowers, etc. If they can't figure out how to pop out a relay from it's socket and replace it then they aren't too much help as it is.

Capt J · Nov 10, 2011

Rule super switches are rated for 20 or 30amps (I forgot without looking at one.) So a 15 amp draw should not effect them any differently than a 3amp draw. One can find a Rule superswitch at just about any marina on the water. You're going to have hell to play to try to find a bosch relay in most places. Not only that, they do indeed fail, and often. I've replaced several that have failed, and it was a pain to find them. If the bilge pump locks up, it fries the bosch relay. IMO, you're adding another part that could fail and has no need to be there. Direct wire the float switch from the batteries, with either an in-line fuse or circuit breaker (my preference) and you're good to go.

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Relay control of bilge pumps.

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