Venturii effect help

[TMcRally]

I am trying to drain 25lt of water from a boat to below sea level using the water passing the underside of the hull at speeds between 6 and 20kn.

The boat is a sailing catamaran. It has 2 outboard motors that slide up and down through a hole in each hull. When sailing the outboards are pulled up, clear of the water flow but this leaves an exposed hole under the boat which creates drag. I intend to manufacture a cover for the hole to minimize the drag. This will leave 25lt of water in the hull.

If I can remove that water I will have added buoyancy and that is worth having.

I'm outside of my comfort zone here but it occurred to me that maybe for minimal expense in drag I may be able to draw the water down by creating a form on the cover that empties the water without moving parts.

Ideally the water would drain in minutes at the lowest speed possible.

The hole in the boat can be up to 30cm in diameter or can be another shape. The hole is about 270mm below the waterline at rest at its Centre point, perhaps 50mm less at speed.

The hole is towards the back of the boat where the hull is rising and the hull is roughly semicircular in x section.


P. S. I am not concerned with drag when the hatch is open and the outboard are down.

 

[TMcRally]

This is a pretty rubbish video but hopefully you can see what I'm talking about.

This view is one hull cut lengthways down the middle.

The grey is the hull as it is today.
The purple is the insert I will fit that will fill the 117lt hole. So imagine the hull as the grey and purple. Because I am fitting the large insert I can make the 25lt hole any shape needed.
The gold is the cover plate that will swing around and cover the outboard leg hole which holds 25lt of water

At the rear of the gold plate is my interpretation of what the form to draw the water out might look like. (now you have an understanding of why I need help )

Thanks
Dave

 

[berkshire]

Why don't you take a look at an Anderson / Elvstrom self bailer, these things will work at boat speeds down to 3kts .

If you design your cover right, I think you would have room for a small one. Also if room above inside the cover is a problem I have seen these things installed in the permanent " Down" position. Or you can incorporate a similar looking unit of your own.
The secret to this device is the non return flap which whilst not preventing all water ingress when you are stopped considerably slows it down.
B.E.

You are judged not by what you know, but by what you can do.

 

[TMcRally]

Thanks berkshire

I Googled it and I would be able to fit it, or something similar. I imagine fabricating it as part of the cover, fashioned to represent one like the one you mentioned but as a fixed, permanently down version (i.e. with no moving parts (KISS principal)). I am not concerned about it re-filling when stopped so long as it empties in a few minutes once underway. It will only fill up to 25lt, not flood the boat. (this is really because I don't think I will manage a good seal on the cover so I wont try and just let it fill)

My only real concern is wether or not it has the drawing power to pull 25lt of water 270mm below the outside surrounding sea level ? I searched for ratings etc. but couldn't find figures and the design is for applications where the draw down is <50mm. There were forums that discussed a "Super Sucker", same idea with a more traditional tubular venturii design. Would this draw better than an open venturi ? Or do I just go bigger.

Bigger = more drag so I'd like to make it just big enough + a margin, is there a rule of thumb type calculation for this ?

 

[rb1957]

self-bailers are (in my experience) pretty efficient at get water out, and not letting too much in.

I think your big problem is the cover panel ... keeping it flush with the hull.

What happens to the outboards once lifted ? how stowed ? (damn things must get in the way unless this is a big cat ... I'm used to Hobies)

Could you have a hinged cover ? attach/hinge on the fwd edge (or side), motor pushes it out of the way, a light spring to retract it ... ??

another day in paradise, or is paradise one day closer ?

 

[TMcRally]

It is a 48' Schionning Waterline, the outboards are fully contained in the hulls and simply slide up and down on rails.

This is a bigger challenge than I first thought.

The original design had a hinged coverplate attached to the bottom of the outboard leg. The hinge point forward of centre which keeps it parallel to the waterflow going forward but terrible in reverse, When you pulled the motor up the oversized plate cups the bottom of the hull. Sounds great, except the leg then sits permanently in the salt water.

I want the new design to be automatically operated by the action of raising and lowering of the outboards to reduce the complexity if you need the motors urgently...... MOB for example.

I'm still considering a few options which all have the self bailer idea, I like the simplicity of it: the 3 front runners are:
[ul]
[li]A coverplate that slides forward, captive in guides, attached to the lower leg by a pair of arms. As the leg lowers the coverplate is pushed forward and the arms hold the plate there to prevent it coming backwards when the engine is in forwards. When the outboard is up, the leg is clear of the water by means of the length of the arms while still holding the cover in position.[/li]
[li]A cover plate that pivots sideways around the radius of the hull, pushed and pulled by the outboards movement[/li][li]A two door "bomb bay" type arrangement[/li]
[/ul].
I am leaning towards option 1

Hinged covers run greater risk of picking up rubbish and maybe breaking or jamming.

 

[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.