What's a float switch?

jezjez

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What\'s a float switch?

I only had an electric pump on board when I bought my boat in the spring. I'm told this needs a float switch. Is it complicated to fit - do I need an elextrician or a boatyard or is it a simple task I can do myself?
(I've temporarily added a manual pump which needs properly fixing into the bilge for emergency use.)
 

Casey

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Re: What\'s a float switch?

A float switch is a very simple device that is basically a flap, hinged at one end with a lump of mercury inside. As the flap goes vertical in the rising bilge water the mercury moves down the flap triggering the switch. When the water is low enough the flap falls, the mercury moves and the switch is triggered off. If you can wire in a switch you can fit a float switch. When fitted it must be checked frequently to ensure that no debris is fouling it.
 

Porthandbuoy

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Re: What\'s a float switch?

Or you could fit an automatic bilge pump. Rule do a range of them. They switch on every few minutes and they have clever electronics which can tell if there's a load on the pump or not, If there is, it continues pumping until all the water is gone.

Edit
PS. Don't get complacent though. You need to be able to tell if a leak is getting worse or one day the water might come in faster than the pump can get it out!
 

Cliveshep

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Re: What\'s a float switch?

Try A.S.A.P. supplies, they seem to sell the cheapest bilge pumps with auto float switches. Mine is on all the time, but only works if I get bilge water. Keep hold of your old pump as if you fit a shower sometime they are ideal to pump out the water or to attach to a lanyard and hose and drop over the side to use as a deck wash pump as we do!
 

Peterduck

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Re: What\'s a float switch?

The idea of a float switch is that it can turn the pump on and off when you are absent. If your boat doesn't let any water in then you may be able to get away without one for a while. However, for peace of mind,it is better to have one. It should go in the wiring onthe positive side of the pump leads between the battery and the pump. If you can crimp cable joiners then you can fit one. It does need to be fastened in position in the lowest place which you can reach; ideally the top side of the keel as far aft as you can get to. Secure all wire joints as high as possible to keep them out of any bilge water. At one stage I became fed up with these switches being unreliable, and I asked a chandler if there was any brand which was very reliable, and his answer was simple; "No!"
Peter
 

fisherman

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Re: What\'s a float switch?

Mine is a magnet in a buoyant tube, which floats up close to another tube containing a reed switch. It has been very good for 15 years. I would still prefer the tube in the bilge with a washing machine pressure switch on the top end: keeps all the lectrics up in the dry.
 

fishermantwo

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Re: What\'s a float switch?

[ QUOTE ]
Mine is a magnet in a buoyant tube, which floats up close to another tube containing a reed switch. It has been very good for 15 years. I would still prefer the tube in the bilge with a washing machine pressure switch on the top end: keeps all the lectrics up in the dry.

[/ QUOTE ]

The washing machine pressure switch is by far the best method. Pick up a secondhand one from a repair shop for about $5. I use one with a 240 volt bilge pump that looks after the boat when I'm ashore and at sea. 12 volt one as well driving a small submersible. The last one I bought even has multiple adjustments for water level. Tricky bit is guessing the right size tube that goes into the water. I have a 3 inch bit of water pipe that steps down to a 4 mm rubber tube to the switch. All the wiring can be kept well clear of the water. They are a very common fit here with commercial fishing and charter boats. Float switches are about $50 for a quality one. If you go this route ensure you wire in an auto relay to ensure a longer life for this very unreliable unit.
 

fishermantwo

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Re: What\'s a float switch?

[ QUOTE ]
This may help visualise it

Fitting a float switch

[/ QUOTE ]

If you use this method ensure the float switch can handle the current demands of the pump. Small pumps are ok but the switch wont last long unless a relay is used to actually run the pump. If you decide to go the expensive route and use this sort of switch, consider mounting the switch well clear of the water and use a rod and float to activate the switch. The ideal system is to have the minimum wiring subjected to bilge water.
There is a commercial version of the washing machine switch available, I think its Jabsco that sell it. Its easy to see why ships chandlers like the old float switches.
 

cliffordpope

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Re: What\'s a float switch?

Apart from considerations of practicality and simplicity, you also need to consider what depth range of bilge water you want it to operate over. The standard kind of plastic flap with a mercury switch trigger after a rise of about 3".
I imagine a floating magnet and a detector switch could have a narrower range with a greater sensitivity - like an intruder sensor, which triggers when a door is opened by even a fraction of an inch. On the other hand a washing machine switch has I think a wider range of operation. For example, I use one in my well to tell the motor to stop pumping if the level has fallen below a certain point. But it then has to rise by about 6" before it will cut in again. You may not want that much water in the bilge before the pump cuts in. Equally you don't want it responding too quickly if it can't fall sufficiently to trigger the off position.
There is an advantage in sensitivity, but you don't want it cutting in and out repeatedly. Remember because of obstructions in the bilge it may take a while for water to flow along, and you don't want it switching on and off just because it has drained one small section but the water is still flowing slowly from the next. Nor do you want it triggering from a passing wash slopping the water about. It would be better if it let the water rise a bit before doing a longer pump.
Also a by-pass switch is useful.
 

fishermantwo

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Re: What\'s a float switch?

[ QUOTE ]
On the other hand a washing machine switch has I think a wider range of operation. For example, I use one in my well to tell the motor to stop pumping if the level has fallen below a certain point. But it then has to rise by about 6" before it will cut in again.

[/ QUOTE ]

Yes this is the tricky bit. Some experimenting is required with the size of the pipe that goes into the bilge water. The pipe size for my 240 volt pump is 4 inches. I can adjust the depth of water by just sliding the pipe up or down. Because air compresses under pressure this gives a much "softer" switching than an ordinary float switch, the level has to rise about 3 inches before it switches on. It pumps all the water out before switching off. This is ok with a 40 foot fishing vessel. I'm fiddling about in my workshop at present with another washing machine switch, this time to run a 12 volt pump over a more limited range.
 

cliffordpope

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Re: What\'s a float switch?

"Yes this is the tricky bit. Some experimenting is required"

Exactly. There are several variables and lots of possibilities.
Washing machine pressure switches vary between makes, trigger at a wide range of different pressures. Usually there are screws to adjust the operating pressure within the pre-set range. Sometimes there are two sets of contacts operating at different pressures.
The volume of the "bulb" bit that goes in the water varies, and can be altered by cutting down, or adapted from something else, and has a big influence on the trigger point.
For example, the thin connecting pipe alone would probably need a range of about 20 feet before it caused the switch to click. But put a fat bowl on the end and the pressure effect can be magnified.
One weakness in the principle is that being under constant pressure the hose and connections have to be perfectly air-tight. Any slight leakage reduces the pressure so delays the point when the switch cuts in. Ultimately, if the rubber perishes, the switch doesn't work at all.
My well installation is designed to prevent pumping unless there is sufficient depth of water, so is fail-safe. (I have another reverse-sense set-up in the water tank, to turn the pump off after the tank has filled) A bilge pump situation would be the opposite of fail-safe..
Also there has to be a residual depth of water. Once it has lowered the level to below the end of the pipe, no further pressure reduction is possible so it will never switch off and will run dry.

It's all great fun when it works, but does need constant checking and monitoring.
 

jezjez

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Re: What\'s a float switch?

Thanks for all the advice on this one.
I'm not sure when I'll get to this job but I know I have to.
I'm having someone from the boatyard look at a couple of other things so we'll see.
 
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