Vacuum Bilge Pump

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TomTom
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Vacuum Bilge Pump

Post by TomTom »

This intrigues me - particularly as a soln to no scuppers on a trot line…

Any thoughts?

https://www.osculati.com/en/11139-15.26 ... rettyPhoto

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Jaysen
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Re: Vacuum Bilge Pump

Post by Jaysen »

Pretty common on old old old boats. Uses Bernoulli principle to get a siphon started. Works very well when you are moving. Isn’t all the great when you aren’t!

I’ve created the same thing by just dragging a weighted hose over the side at idle. Bigger hose needs more speed so keep that in mind.
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OrangeQuest
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Re: Vacuum Bilge Pump

Post by OrangeQuest »

Using the same principle on the nozzles of my bait and live wells. Water pressure from the pump causes the nozzle to suck air from a hose that supplies fresh air/O2 to the wells instead of just making bubbles with the air in the wells. Very noisy when sucking just air and water flow is only as fast as the prop wash can produce negative pressure. Because the system has to pull the water up against gravity, always needs that vacuum or negative pressure. That is because the inside bottom of the boat would be lower than the waterline. And without prop wash the hose could fill up the boat when the motor is not running. It never uses the siphon effect from the boat to outside water, once the prop wash is removed the siphon effect will force surface water into the boat. But that will only happen if you kill the motor with water still in the boat and the hose is full of water or you get hit with a wave big enough for the highest point of the hose to go under water forcing water to go up to the highest point of the hose and it would start the siphon effect. Kick the motor up every time you kill it would stop the risk.

If the inside boat bottom (like a sole) was higher than the waterline, then scuppers would be more effective so any water coming into the boat just drains out.
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TomTom
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Re: Vacuum Bilge Pump

Post by TomTom »

My boat would be moored in a creek/ tidal area - wondering if the current in and out would be enough to drain rainwater at anchor?

OneWayTraffic
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Re: Vacuum Bilge Pump

Post by OneWayTraffic »

I wouldn't say so. You need forward motion. I'm fitting DIY elephant trunks to mine, same idea but a straight run with no siphon loop. The trunk stops backflow, at least most of it. On the other hand a couple of 50mm elephant trunks will drain faster than any pump that sensible money will buy. At a fastish non planing speed you are looking at 5000GPH each.

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OrangeQuest
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Re: Vacuum Bilge Pump

Post by OrangeQuest »

Here is how that pump works.
Image

The "fast air" is the prop wash hitting the cone. The hose is connected to the smallest diameter of the cone and the high-water flow then expands when exiting the cone causing lower pressure than atmospheric pressure and that is the vacuum that will pull the water up into the hose. It needs a continues flow of water at a fairly good speed like that that comes off a spinning prop.

Bernoulli's principle is also the same effect that causes the prop to cavitate and vent. One is small bubbles forming on the edges of a prop blade due to extremely large pressure differential from one side of the blade and the other. Water is expanding so quickly it vaporizes like when water boils in a pot. Venting is where the low pressure caused by the prop causes a negative pressure to close to the surface of the water and it sucks air.

Hope that makes it all a little clearer and why the pump will not work without the motor running in gear.
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