Using venturi effect to empty a water column in a boats hull ?

[TMcRally]

I hope you are the guys to ask......

I have a performance cruising catamaran sail boat. The higher it sits in the water the better its potential. It has outboard motors that slide vertically on rails to lift the leg and propeller up out of the water flow to reduce drag.

As a consequence there is a hole on each side through the hulls that allows for raising and lowering them. When the outboard is up and we are sailing each "well" holds about 25kgs of water. If I can empty the water from the well I will have 25kgs of extra buoyancy on each side, and that's worth having.

I can try to seal it with a cover and pump the water out (least attractive from a design point of view) or I hope I can have the passing water drain it through venturi effect with a clever shaped hinged cover plate. I assume there is a penalty for this in pulling the boat down (negative lift?) or in drag from the plate. Is it equal to the 25kgs or only partially or until it draws air or will drawing air cause the effect to collapse.

Are the speeds required to work out of my range ?

Hole size - round 30cm diameter (that is the max size possible - actual is smaller) smaller the better from the boats point of view
Water column height at rest - 240mm
For good benefits I'd like it to fully drain by 12kn (24kphish)

Thanks
Dave

P.S. Max boat speed 50kph

 

[LittleInch]

Interesting. It would help to have a little sketch to define the issue, but as I understand it you have two cylinders of water which currently are open ended?

Some questions which form the design.

What time scale are you looking for to empty the 25 litres or water? 1 minute, 10 minutes?
What is at the top of the well?
Can air freely enter / exit?
Could you simply pump air into the well and out through a non return valve?
If the well is sealed you'll

Anything creating a venturi effect will add to the drag, but maybe not a lot if the flow rate out isn't high. There won't be any negative lift at those sorts of volumes / flows. It will continue to draw air, but that shouldn't be an issue.

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[itsmoked]

I'm not too clear on the specifics of your configuration. A sketch would probably help a lot with any discussion.

Have you seen the usual aluminum dingy drain plugs on the rear bulkhead? I remember that if you pulled the plug and were going more than about 5 knots it would suck all the water out in about 10 seconds. Can you include something like that? Perhaps electric solenoids that you energize with a timer that closes them in 60 seconds. That way in a complex operational environment there's no need to remember to close them again.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TMcRally]

The attached image is of the opening through the hull before the closed cell insert is glassed in. Once the insert is in a hole matching the plan view of the cross section of the outboard leg will be cut through, open at the top and with a hinged flap at the bottom that is hinged inline with the water flow. The back of this flap I imagine a lip to create the venturii to draw the water out. When the motor is lifted The flap is held closed by spring pressure and water pressure as it drains. It is held open by the outboard leg when the outboard is lowered through.

[ul]
[li]The top of the well is open and air can freely enter / exit the top[/li]
[li]I would not be able to pressurise the top of the well[/li]
[li]I remember the flappers on the drains on the Tinny's,[/li]
[li]I will make up some drawings of what I have in mind[/li]
[li]A small amount of drag is acceptable[/li]
[li]In my minds rough sketch I have a lip about 50mm x 30mm is that the sort of sizing[/li]
[/ul]

I would hope to draw the water out in a 2-3minutes but it is not critical, 10min would be ok-ish, typically we are on a tack for half an hour+ and it probably won't refill on a slow tack anyway.

 

[Compositepro]

The water in the well is not adding any weight to the boat. It is reducing the buoyancy of the boat. Isn't the flap going to create a lot of drag when on engine power?

 

[EdStainless]

What about a sealed door and a self draining bilge plug (either ball check or rubber duck bill)?
Would you have enough speed to have one of those to work?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

What's the water in that hole actually do when at speed? Pile up at the back? Swirl around furiously? Stay calm?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TMcRally]

It builds up then rushes out,
This cycles at approximately 10-12 times/minute.
It is turbulent.
I would estimate it gains 100lt then almost empties = 1 cycle.

 

[LittleInch]

I would just use a circular disc which rotates to seal the opening and terr just use a12v sump pump. Will clear 25 litres in a few minutes.

Kind of like a torpedo tube cover.



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

 

[IRstuff]

Given that you want to clear the water at low boat speeds, I don't think you'll have enough airflow to generate sufficient suction to pull up much water.

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[TMcRally]

I'm crap at this uploading video's but I think it works in the most Low definition eva, sorry

This is an image of the hull in X section (grey) the purple is the closed cell foam to permanently blank off the majority of the wasted space and the gold is the hatch that I hope to make rotate open and closed activated by the raising and lowering of the outboard. At the back of the hatch is a profile that I hope will draw the water out of the well. This is the part I am unsure of. I am not sure if I will create enough low pressure to pull the water down ?

The water at rest comes to about 120mm from the top of the closed cell foam and if it all works as planned it should raise the back end by a further 30mm. The height of the foam at the front (LHS in the image) is 495mm.

https://vimeo.com/278106667?utm_sou...vimeo-cliptranscode-201504&utm_campaign=28749

 

[chief]

A quick easy and flexible solution is to use a type of spray painting pistol. These have a suction line to fit in a barrel or boat well and the motive power can be air or a tap off from a seawater supply.

Offshore Engineering&Design

 

[TMcRally]

Thanks all.

I have decided to go with the forward sliding hatch with either a fixed or Open/closed type venturii drain. I may make it replaceable in case I need to go larger or smaller.