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Bilge Pump

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Mick Favager24/09/2008 21:37:00
8 forum posts

I would like to fit a bilge pump to a Model Slipway AZIZ. The rear deck allows a lot of water to gain access to teh inside of the hull. Does anyone a have a design for a float switch to turn a pump?



Penny Lee25/09/2008 10:21:00
94 forum posts
12 photos



If you cover the postage you are welcome to my old model submarine bilge/compressor switch.It is 0.9 x 2.5 inches and will handle very big pump.

Alternatively, I got the above circuit from an old article.

For your application I see no reason why R2 can't be replaced by 1M and the probes be at Q2 input and the negative supply and Q1 be omitted.

The positive probe will become pitted due to electrolysis on d.c. probes.

I'll bet problems occur if the bilge water comes into contact with the negative supply (unless the probe sensitivity is increased by reducing R1to 10k or the more positive bilge probe shroulds the lower probe.)

If fitting a float switch is no problem,it could replace R4 while ensuring Q3 is turned "off" by a pull down resistor when the float switch is open.

The simplest solution of all could comprise a "TOPFET" in place of Q3, float switch and one resistor.


Lee Penny

Mick Favager25/09/2008 17:39:00
8 forum posts


I am more than willing to cover postage, thanks for all the information too.

Please let me know, how much and how I should pay.



Penny Lee25/09/2008 17:55:00
94 forum posts
12 photos

Hi Mick,

Just P.M.your address and I'll check it before sending it off with the connections and my own address.

I haven't posted a packet since the new rates but I doubt it will even come to a book of four 2nds.


Mark Leigh26/09/2008 06:29:00
34 forum posts
17 photos

 Hi Mick,

 Reference the above you can also get a 6 volt version with water sensor from Hunter Systems. The link is below though the cost is definately not "a book of four seconds!!!!" - 28k


Manxman02/10/2008 19:21:00
440 forum posts
133 photos

OK - I admit it - I'm thick !

water comes into the hull and operates the float switch or circuit probes and switches on the pump which pumps the water out, when the water is gone the float switch drops back down and stops the pump.

This much I understand, now comes the thicky part of me asking WHY, do you need a float switch or a fancy circuit board ?

if you connect +12v to the pump and run -12v to one of 2 small nails/screws set in a small block and fixed near to the hull floor and a 2nd wire from the other nail/screw to the pump.  surely water coming into the hull, which is a damned good conductor of 12v would, when reaching the nails make a connection and switch the thing on - when the water is gone the connection would be lost and it would stop !

Either there is something seriously wrong with my way of thinking or people just like to make life harder for themselves.

Ken - the stupid

60watt02/10/2008 19:58:00
253 forum posts
14 photos

You may not be stupid but just you have not crunched the figures.

Lee says "The positive probe will become pitted due to electrolysis" and that circuit has the probes leaking less than a milliamp !

It's also called sacrificial corrosion.

If you get your system to work the nail should disappear before your eyes and the boat will be full of an explosive mixture of gases. LOL

It won't actually work as the pump needs a rotating torque to overcome friction.The motor torque is proportional to the current through it.

You will get the full voltage bubbles at your probes because the pump is a tiny resistance compared to a few mm of water.


Mike Davidson20/10/2008 11:40:00
157 forum posts
9 photos

MIck   Regarding your bilge pump, I reckon the best way to go about this problem is to gey a smallD.C motor driven pump that works, and get an electronics chum to build a little sensing circuit with a 731 integrated circuitthat looks at the resistance between two electrodes right down in the bilges, and as soon as water is present, the circuit will enable a driver transistor which switches on a power transistorthat applies power to your little pump motor. there are lots of published application circuits available, so you don't have to worry about infringing copyright asthere are plenty in the public domain. the one I have in mind is very economical on power, as it only switches on when needed, and off again when the water's gone. just remember the word application when you are searching for the right one.

Cheers....................Mike     D

Mike Davidson20/10/2008 19:48:00
157 forum posts
9 photos

  Tom     the RNLI combat electrolytic or dissimilar metal corrosion by introducing a sacrificial anode placed strategiclyin a position where it will present an easier path for the electron flowthat erodes essential parts of the boat's anatomy, so it is that which is eaten away by the electron flow. you should wander down to a slipway when a lifeboat is up on the blocks, I have, and it opened my eyes I tell you. The only time I have seen worse corrosion is when I went to Cape Kennedy NASA space museum and I saw a Saturn Moon launcherassembled on its side in cradles. You know that rockets are manufactured from very light weightmagnesiun alloy, and that cape Kennedy is situated right next to the sea, Well, the onshore breeze carried with it a hefty amount of salt water spraywhich reacted with the magnesium alloy quite violentlyand you could see quite solid lumps of metal almost eaten through by the corrosion. The Saturn launch rocket was gradually turning itself into a pile of grey dust. NASA Put out a tender for an inhibitor which would stop this and an enterprising company came up with an oily based aerosol which could be sprayed on to stop the corrosion, and it didn't take a genius to work out the name of this inhibitor, they called it Rocket WD 40....Clever boys, so take note, WD40 is NOT a lubricant, it just stops things corroding. I use silicone oil in my propshafts, and WD40 on the rudders and propellors

Bob Abell20/10/2008 20:30:00
9317 forum posts
2976 photos

Simply get a fuel pump and motor...connect it to the an on/off switch...and use that when necessary


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