Calculating venturi effect when not in a pipe ? 4

[TMcRally]

I'm considering using a self bailer on a boat to draw out water from a well in the hull (Self bailer image linked). It swings down into the waterflow. Waterflow can be anywhere from 3 to 20kn. When the well is empty the bailer will be 350mm below the waterline. For it to be successful it will need to work reasonably efficiently at that depth. Reasonable efficiency is somewhere close to an average of 1/2 litre per second ideal would be 1 lps.

I can't find manufacturer specifications.I can find the online information for pipes but these are very different (I assume different) and I can't find any leads. I'm not sure where to start, any leads would be much appreciated,

Thanks,
Dave

https://www.ronstan.com.au/marine5/product.asp?ProdNo=RA554133

 

[1503-44]

I can't see how that works. Do you have more info?

 

[TMcRally]

They typically use them for small one or two persons racing sail boats that are much shallower.i.e. they might be 100mm below the waterline.

As it is displayed in the image, it is open. The rectangular plate drilled for 6 screws, screws to the inside of the boat. The "mouth" is the triangular shaped component you can see protruding at the mid-right hand side, it is the same piece as above the rectangular plate that has the wire through it. When open, as shown this triangular section penetrates through the hull and drags in the passing waterflow. The section that runs deeper in the waterflow is facing aft. The aft section is open facing backwards and the U section forms a channel that opens to forwards to inside the boat. The Venturi effect of the passing water forms à low pressure area behind the bailer and draws air/water via the U channel from inside the boat.

When it is closed the triangular section clicks up via an over centred spring and is then flush under the boat and sealed.

 

[1503-44]

So the water's entry velocity is the speed of the boat.
Entry flow is the boat's speed x area of the water's entrance.
Need the diameter of the draw tube.

 

[hydtools]

Here is a MIT publication.

Ted

 

[LittleInch]

I think these things just "are", i.e. no one really "designed" it, but just modified it over time for a particular use.

They all say they don't work at "low" speed without ever qualifying that.

At 350mm below water level it might work but I think your min speed will increase as you go deeper. The drop in pressure at the venturi will need to be more than the pressure due to depth at that point to be able to "suck" water out of the boat.

why not just buy the biggest one you can find and fit it?

Can't you just fit a 12V sump pump? They are very small and will give you the flow you need.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TMcRally]

Thanks all

1503-44, yes the boats speed through the water creates the waterflow relative velocity. The back of the mouth is appropriately 40 X 30mm but if I make my own it can be any size.

Hydtools, who would have thought. Exactly what's needed, thanks.

LittleInch, I agree, development most likely came from trial and error, There's a lot of work in makeing the cover and hatch and I wouldn't like to make fitting changes after finding it couldn't draw the last of the water.

I haven't given up on the electric pump, it has the not insignificant advantage that it doesn't create another point of possible leaks and is not speed dependant but we run on solar power exclusively so I like to exhaust working on non powered options first until they don't work or aren't practical. If I was confident I could seal the inward opening door from any leaks then the electric pump would only need to drain once and would be my preferred option but I think it will leak slowly and need to switch on and off continually.

 

[TMcRally]

I've read the MIT paper linked by Hydtools (thanks again) and it doesn't look like self bailers will work acceptably at 350mm depth.

I was hoping the NACA foil bailer they trialed would work. That would have been a neat solution.

Electric pump it is, I'll just have to do more work on hatch seals.

Thanks all
Dave