Bilge blower warning light.

sunquest

sunquest

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On my flybridge I have fitted a switch to turn on the bilge blowers. If I forget the engines are starved of air so I want to fit a light or buzzer to indicate the blowers are off but goes out when the switch is thrown and the blowers are active. Can anyone illustrate the wiring? Thanks in advance.
 
sunquest said:
On my flybridge I have fitted a switch to turn on the bilge blowers. If I forget the engines are starved of air so I want to fit a light or buzzer to indicate the blowers are off but goes out when the switch is thrown and the blowers are active. Can anyone illustrate the wiring? Thanks in advance.
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Depends how intelligent you want it to be...
Simplest is a DPDT switch (dual pole{circuit} dual throw {each circuit has a set of on and off contacts for both switch positions)
You wire one circuit so that in its on position +12V goes to the blower feed. You wire the other set so that when the switch is in the blower off position then engine run(ignition) goes to the alarm. So, if you turn the blower on the switch isolates the alarm, if you dont then the feed from the engine on switch goes to the alarm.
More intelligent would monitor the air flow, but thats maybe a bit ott for now.

edit add:
blower.png

shown with switch off
 
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Thanks for your reply, I should have worked that one out, but the switch is a simple two contact on/off, another senior moment.:sleeping: Will source a DPDT switch.
 
Instead off manually switching on the blowers do you not have any spare relay contacts on your engine controls that could be used to switch on the blowers when you start the engine
 
Maybe try a differential pressure switch with one leg to fan intake and one leg to fan exhaust. The 'off position would be equal pressures and the on position when intake drops below exhaust pressure (e.g. Fan running).
 
Piers said:
Keeping it simple, why not just have a relay that energises to power the blowers when the engine ignition switch is ON?
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Because that does not tell you that the blower is actually running just that there is power available to it.
 
Ok - even simpler fit a manometer made from clear plastic tube and using food colouring dyed water, one leg to atmosphere and one leg tapped to engine compartment. Get the fan running and mark the tube with an indelible black pen, turn the fan off and mark the 'off' position with a red one.��
 
The initial request was 'something' to remind sunquest to check the blowers were on. There was no requirement to check that with power to the fans that they were actually running. If this is an additional requirement, it can be achieved in a number of ways, such as having an ammeter in series which shows the current draw. A reduction in the reading would indicate the fans were not operating.

On Play d'eau, part of my pre-departure checks are to check the blowers operate. Then I run them as required. So, I'd go for a relay activated by the ignition switch to complete the circuit to the fans, with a pre-departute check that they were working.
 
sunquest said:
On my flybridge I have fitted a switch to turn on the bilge blowers. If I forget the engines are starved of air so I want to fit a light or buzzer to indicate the blowers are off but goes out when the switch is thrown and the blowers are active.
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Have I understood you correctly, in particular is there a possibility the warning light could be left on when there is no source of battery charging?
I think NormanB is on the case with positive and negative air pressure, although my reply is not directly associated with your original question, it may be of interest.
Blowers were originally fitted for bilge air extraction when domestic gas installations arrived. Are yours for this scenario? In either event when the engine is running you run the risk of either the fan motor subjected to over speed or overload!
May be a good idea if you wish to run the blower with the engine running to compare combustion air with the blower fan flow volume. As combustion air volume increases with revs there comes a point at which the fan motor may be beyond its design limits, depending upon the the actual fan design.
Forums can be unintentionally misleading and in this case because your engine combustion air may be directly ducted from the intakes, in which case to remove the radiated/convection engine heat you will need fans far bigger than those generally available in the chandlers!
If you have already covered the above design criteria then I take this opportunity to apologize.
Just posted and then read nigelmerciers reply above, that maybe complements my attempt.
 
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nigelmercier said:
I find it difficult to believe that an air pump of several (probably hundred) horse-power can be in any way assisted by an electric fan. A 2 litre engine at 2000 RPM pumps air at 2000 litres per minute, 30 litres a second: that's 6 buckets of air.
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Hi Nigelmercier. A major issue in engine room design on motor cruisers is ensuring sufficient air can be drawn into the engine room without creating a drop in pressure within the room itself. Many motor cruiser, such a some of the Marlow trawler yachts, have to have blowers on all the time the engines are running. A simple check is to try opening/closing the engine room door whilst running at full chat to see if a pressure difference is felt.
 
Piers said:
Hi Nigelmercier. A major issue in engine room design on motor cruisers is ensuring sufficient air can be drawn into the engine room without creating a drop in pressure within the room itself. Many motor cruiser, such a some of the Marlow trawler yachts, have to have blowers on all the time the engines are running. A simple check is to try opening/closing the engine room door whilst running at full chat to see if a pressure difference is felt.
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You've hit it on the nail Piers, the reason for ensuring the fans are on is when opening the engine room door after a run there is a resistance caused by a vacuum caused by insufficient air flow. The solution to me is to have a lamp or buzzer on when the engines are started and switched off when the bilge fans are turned on. Thanks to all for your input. Geoff
 
Dougal Tolan said:
It would be really interesting (for me anyway) to know just what the air pressure would be in the engine room with no blowers and WOT.
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Apologies for the subject drift, but many years ago using a simple water gauge I measured the differential engine room pressure. As I recall you need a calm sea, or alternatively and easier/accurate in harbour when no one is listening.
By good fortune or design the result was well within the manufacturers turbocharger tolerance, and therefore a reassuring exercise.
As for the fans and boat at rest there was little or no visible reading although my memory has dimmed somewhat.
 
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Perhaps im reading things wrong but an engine that is starved of air would only become more starved of air with the 'Blowers' running because blowers usually suck air from the engine room and dump it overboard via the vents on the side of the boat. As someone has already said, the engine is far more powerful that the electric fan anyway. if it were to help we'd all fit one as an electric remote supercharger...
 
Antony Collis said:
Perhaps im reading things wrong but an engine that is starved of air would only become more starved of air with the 'Blowers' running because blowers usually suck air from the engine room and dump it overboard via the vents on the side of the boat. As someone has already said, the engine is far more powerful that the electric fan anyway. if it were to help we'd all fit one as an electric remote supercharger...
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Blowers on boats that need air in the engine room blow air in, not out. When we went to see the Marlow range when I was thinking about changing our Fleming, the thought of having to run a generator full time whilst cruising so provide sufficient AC power for the blowers to keep the engine room from a reduced air pressure relative to ambient, was an absolute 'no. no'. Crazy.
 
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