Self Bailers. How do I choose a size ?

[Dave_S]

I'm fabricating bombay doors at the bottom of my outboard hole that goes straight down through the centreline of the hull, I want to include a self bailing system to evacuate the water when the doors are closed. I have looked at the small sail boat venturi effect self drainers and thought I would buy or fabricate something like it on one of the doors.

It has about 70lt of water to drain per hull.

2 questions.

1/. If I want it to drain at 15kn what size do I want
2/. If I want it to work at 2kn what size do I want

[psk125]

Are your "bomb-bay" doors actually watertight? If they're not, putting in self-bailers is not going to do much. Is there room with the outboards in place to reach down to open (or close) the bailers? 70L is a good bit of water. The largest bailer might be the best one to install if there is room for it. None of them are likely to work well at 2 knots. https://www.westmarine.com/andersen-...02_074_001_507

[Dave_S]

Originally Posted by psk125

Are your "bomb-bay" doors actually watertight? If they're not, putting in self-bailers is not going to do much. Is there room with the outboards in place to reach down to open (or close) the bailers? 70L is a good bit of water. The largest bailer might be the best one to install if there is room for it. None of them are likely to work well at 2 knots. https://www.westmarine.com/andersen-...02_074_001_507

I doubt I can make them water tight, the best I'm hoping for is a delay in refilling. I haven't seen the self Bailers work so I'm not sure how efficient they are to know if they can bail faster than my doors will leak and I don't really know how well I can slow the leak. I plan to fit seals but they will be below the waterline and will end up with some growth, they might work pretty well on day one but get worse from there on.

Lots of room.

Maybe I should look harder at how to seal it.

[psk125]

Sealing the doors to watertightness is going to be a major chore. Unless they open "in", opening them underwater may call for a good bit of effort. If water keeps entering the space, the bailer will always need to be working and creating drag while it does. The question becomes whether it is more worthwhile to carry 140L of water (or perhaps less if the water level in the windward hull is lower?) around with you, or to have the bailers creating drag the whole time. As the depth to the bottom of the space gets deeper and the water pressure increases it may take longer, or require faster speeds, to have the water go out through the bailers.

[Dave_S]

Originally Posted by psk125

Sealing the doors to watertightness is going to be a major chore. Unless they open "in", opening them underwater may call for a good bit of effort. If water keeps entering the space, the bailer will always need to be working and creating drag while it does. The question becomes whether it is more worthwhile to carry 140L of water (or perhaps less if the water level in the windward hull is lower?) around with you, or to have the bailers creating drag the whole time. As the depth to the bottom of the space gets deeper and the water pressure increases it may take longer, or require faster speeds, to have the water go out through the bailers.

Thanks for your help 👍

I still haven't decided which way to have them open, I'm 99% sure I'll have them open up. There are advantages and disadvantages to each. I imagine if they open down I can either flood the space first to open or just drop the motor through the door opening. The doors won't be flat, they will match the shape of the hull, so underwater they will create turbulence and the water flow may rattle the doors and prematurely wear the hinges and debris may deform the doors. The worst case is if they may hit the prop.

Opening upwards solves the above problems but sealing is much harder. I have found what I think will be a reasonable solution to sealing the doors provided I can hold them down in a controlled manner. The seal is a soft silicone closed cell foam that comes in round (thinking of 12 or 14mm with 3mm crush) it is very compliant and can be supplied as an "O" ring of any length and for the meeting of the doors a simple rectangular seal between the appossing faces of each leaf. The leaves will need to close together at the same time.

If I can seal the hatch then I might just pump the water out, that will solve the low boat speed functioning of the bailers.

I'm confident that 140kgs of buoyancy (less the weight of doors and other hardware) will be noticeable and worth the effort.

I'll draw it up and see how it works.

[Sparx]

These venturi bailers, left in the open position will slowly leak water into the hull when stopped or moving slowly. In my experience they want about four knots to function. Increasing speed increases the water removal rate slightly. At higher speeds the little flap door rattles with a distinct sound. I can detect no impairment on boat speed from having the bailer in the open position. I can detect the benefit of loosing water weight from the hull. From my experience the bailers work best with smooth flow; do not mount them directly behind a rudder or dagger board.

The choice of small or large is totally your choice, both will clear 70 l in a matter of 5 minutes or less.