Vented loop necessary?

[Foswick]

I think there might be some confusion about what the intended installation will look like. My understanding is that there is an existing drain tube from the sink through the hull below. The intention is to add an intersection to that drain tube and pump from the bilge into that tube.

If that is the case, there is absolutely no benefit to the vent. The only way it could possibly have any benefit is if you closed the sink drain with an airtight plug. Otherwise, the sink drain IS the vent.

The second part of this is the issue of how high the bilge hose needs to run. This is simple. In a worst case heeling attitude, draw an imaginary line the shortest path from the through-hull to the bilge pump, following the hose(s). If this line does not go above the waterline, there is a risk of taking on water in that attitude.

But, there is a third part to this that DJ addressed. If that intersection is below the waterline, you can still establish a siphon, even though the sink is a vent. That is because if there is no point along that imaginary line you drew which comes above the waterline, where air can enter the system, you can establish a siphon.

So, if your intersection is at a point that is safely above the waterline, then the sink does act as a vent, as there is a point along that imaginary line which comes above the waterline, where air can enter the system. If not, if you do not add a vent, you are at risk.

 

[thinwater]

This is boat specific. For example, I have had sinks on three multihulls, and in each case not only was the sink above the waterline, so was such an intersection, and even the thru-hull was >8 inches above any heeled waterline. There were no seacocks to close. As long as I routed the pump hose high under the countertop, the sink would not drain back down that hose into the bilge. As long as I routed the T so that the pump was the through leg, the pump did not back up into the sink. In no event was a siphon from the sea possible. There may be some senario (a full hose if the pump finished the moment the sink began to drain) where some water would return to the bilge, but I don't think it ever happened, and if it did, the pump would remove the water a few minutes later. Not worth installing a check valve or a drain plug (I do have a plug for when I want a full basin).

In a mono with a deep galley location, the math is different. One size does not fit all.

 

[capta]

You can't use a poppet (spring) check. They also tend to clog. A swing check will work, since they require practically no opening force. Their are types that are very clog resistant and are commonly used for bilges, sumps, and even wastewater.

I'd always figured it was the weight of the water behind the check valve that a centrifugal pump couldn't displace. OH well, live and learn.

 

[thinwater]

I'd always figured it was the weight of the water behind the check valve that a centrifugal pump couldn't displace. OH well, live and learn.

In fact, many centrifugal self-priming trash pumps have the check valve built in.

 

[Project_Mayhem]

Just a reminder - The question at hand isn't regarding water in the bilge or lake water coming up through the sink drain on occasion (check valve has been installed to prevent this) but is a vent needed on a loop connected to a sink drain line. The sink itself is an extension of this line and is above the waterline.

The pump used is a diaphragm pump, not a centrifugal so it should have check valves built in

Here is the proposed design

Incidental water that collects in the bilge as a result of ongoing spills and leaks is usually clear, but stagnant water being pushed into the bilge is a different issue. The average sailboat will have multiple gallons of water trapped in the hull stringers bellow the sole. That trapped water gets stagnant and with the growth of bacteria and decay matter it will stink. This trapped water will usually escape to the bilge as the boat heels or moves around when underway. It could also be displaced by new water entering the stringers area. Determine if the fouled water coming out is because the boat is infrequently used or if it may be excesive water leaking in. Installing a pump may only extract the fouled water being released but will not take care of the major infestation. Some sailors dump a chlorine solution or cleaners into the high sides of the inside hull and then go out for a brisk sail, hoping to flush out the trapped stagnant water to the bilge. A marine safe product should be used to discharge into the waterways. Lastly a bilge is designed to have a little standing water that the bilge pumps are unable to suck up as it will block the smells from invading the cabin. In my book, dry bilges in other than brand new boats are a no-no. A fun-fact is that if you are taking a long trip usually by the 2nd day at sea you will no longer smell the foul odor. Curing and drying up the stagnant water sipping into the bilge may take a week or more depending on the size of the boat and the effort applied, just to have to periodically repeat the procedure. Just keeping the animal caged and away from the nose is at a lot easier.

Standing water in a bilge leads to lots of growth on the surfaces. Those surfaces will likely be exposed when healing or bashing through waves, generating a smell which is likely moved throughout the boat by air that has been moved by sloshing bilge water

 

[dLj]

I've already answered this question. Let me ask you - why would you put that loop in? Just run straight to the thru-valve. That would be the exact same thing as putting that line in without a vent. You sink does not function as a vent. The connection is below the water line.

dj

p.s. you don't need the check valve in the sink.

 

[thinwater]

One tricky bit, which depends on the relative heights, pipe sizes, and the rate of the pump, is which way the Tee should be oriented. The way you show works well for the sink, but if the pump is fast and the Tee is close below the sink (mine was right on the sink) the sump pump could back up into the sink. The solution is to have the pump straight-through on the tee and the sink come in from the side. Of course, this restricts the sink flow a small amount, which probably does not matter. A sanitary tee or a Y would be perfect (they are designed for gravity flow), but you probably won't find one that small.

Not a big deal.

 

[Foswick]

In the proposed design, as you show, if there is no vent in that loop where you indicated, you run the risk of establishing a siphon. If you run the pump in fill the line from the pump to the through-hull, then when the pump turns off, water will try to return along that line. That is because pump is below the waterline and water will want to flow to the lower point, and since you established a siphon, your system will happily oblige and water will continue to flow backward until it is stopped or unless air is introduced (via a vent).

I think DJ summed it up correctly here. The fact that the T-fitting and pump are below the waterline are the key factors.

It feels a little counterintuitive that a siphon can be established with the sink drain open, but it really doesn't matter, as you've laid it out.

However, if you attach the T-fitting well above the waterline, then yes, your sink will act as a vent. This is because for water to flow backward, it must travel up the drain pipe ... leap several inches ... and enter the T-fitting to travel back down the bilge tube.

With the fitting below the waterline, the water has no such gap, once the line is filled with water.

 

[thinwater]

In the proposed design, as you show, if there is no vent in that loop where you indicated, you run the risk of establishing a siphon. If you run the pump in fill the line from the pump to the through-hull, then when the pump turns off, water will try to return along that line. That is because pump is below the waterline and water will want to flow to the lower point, and since you established a siphon, your system will happily oblige and water will continue to flow backward until it is stopped or unless air is introduced (via a vent).

I think DJ summed it up correctly here. The fact that the T-fitting and pump are below the waterline are the key factors.

It feels a little counterintuitive that a siphon can be established with the sink drain open, but it really doesn't matter, as you've laid it out.

However, if you attach the T-fitting well above the waterline, then yes, your sink will act as a vent. This is because for water to flow backward, it must travel up the drain pipe ... leap several inches ... and enter the T-fitting to travel back down the bilge tube.

With the fitting below the waterline, the water has no such gap, once the line is filled with water.

Correct. The tee must be above the waterline (where there is a suction condition during siphon). Below the water line it won't draw air in. On my boats it was always >1.5 feet above the WL.

 

[Project_Mayhem]

It appears that everyone is pretty unanimous that this application needs a vent. Does anyone know of a vented loop with threaded ends or 3/8" barbs? Forespar used to make a 3/8" barbed unit but I'm not seeing it in stock anywhere. They also have some units with threaded ends. Does anyone know the size of the threads?

@dLj The check valve prevents water from coming into the sink in heavy seas. Only mildly related to the topic in this discussion. The loop isn't needed but I'll sleep a lot better at night

 

[Ward H]

Forespar used to make a 3/8" barbed unit but I'm not seeing it in stock anywhere. They also have some units with threaded ends. Does anyone know the size of the threads?

Ask Forespar. I've found Forespar's customer support to be very good. I've emailed them a few times in the past with questions about thru hull and valves and other products. I always received quick responses.

 

[dLj]

@dLj The check valve prevents water from coming into the sink in heavy seas. Only mildly related to the topic in this discussion. The loop isn't needed but I'll sleep a lot better at night

I wouldn't use a check valve, I'd use a stopper in the sink drain. In my boat I simply close the thru hull for the sink but putting that drain there complicates things...

The reason I asked you to think about why you would have that loop is to try and make you think that having a non-vented loop is virtually the same thing as just running a straight line between the tee and the pump. It would be false peaceful sleep....

dj

 

[Hayden Watson]

I'm installing a pump that dumps into the sink drain. Since the drain is open, I assume no vent is needed?

I do not leave the boat with any open below waterline thru hulls. I installed a new 1/2"ø above waterline thru hull in the transom for this purpose.

In your drawing, you have a closed hose end below the waterline and an open thru hull below the waterline. The hose in question is the one from the "T" to the pump and it is below the waterline so the sink cannot allow air into that hose. Check valves and diaphragm pumps increase the safety, but nothing trumps physics. If you have a closed pipe with both ends below the waterline, you have a syphon. In your case, it is a closed pipe because the "T" is below waterline. and cannot pull air down below the waterline from the sink.

You have a pipe that is fully below waterline that COULD have a leak at some point and if you have a leak at any point between the thru hull and the intake of the pump, you will flow water into the boat without the need for a syphon.

I prefer to reduce the potential leak points from the full system to just the thru hull valve and let the rest of the system back up that single point of stoppage. Checking that ALL thru hulls are closed is the final check on the "leaving boat checklist".

 

[Project_Mayhem]

I do not leave the boat with any open below waterline thru hulls. I installed a new 1/2"ø above waterline thru hull in the transom for this purpose.

My transom through hulls dunk under water on calm days. My feet get wet on medium to high waves. Is there a loop on the bilge pump line?

I wouldn't use a check valve, I'd use a stopper in the sink drain. In my boat I simply close the thru hull for the sink but putting that drain there complicates things...

The reason I asked you to think about why you would have that loop is to try and make you think that having a non-vented loop is virtually the same thing as just running a straight line between the tee and the pump. It would be false peaceful sleep....

dj

I think it'll help if there is a leak in the pump but it wont protect against siphoning

 

[Hayden Watson]

My transom through hulls dunk under water on calm days. My feet get wet on medium to high waves. Is there a loop on the bilge pump line?

I think it'll help if there is a leak in the pump but it wont protect against siphoning

I went back and looked at your drawing again and was not thinking correct. [No first cup of coffee.] You have a syphon from the pump to the "T" so if there is any leak in that length of pipe you will get water intrusion. I have the same hull as you and while you are correct in that the transom will occasionally dip below the waterline, that it not the issue. When you are sailing, you are on the boat and can monitor the situation below. I have a very loud bilge electronic alarm that goes off if the pump comes on more frequently that every 5-minutes and have had it go off when boisterously sailing when forgetting to close the galley sink thru hull. As you know, we can ship water into the galley sink and to the drain on fun days. The issue is can you have water back flowing from the thru hull to the pump when you are not on the boat to monitor the situation. That is why I always close all thru hulls before putting Papillon to bed.

The thru hull through the transom is above the waterline most of the time and the line goes up to a point higher than waterline as it crosses into the port locker. This section of line will drain down through the thru hull breaking the potential syphon to the pump.

 

[Project_Mayhem]

I went back and looked at your drawing again and was not thinking correct. [No first cup of coffee.] You have a syphon from the pump to the "T" so if there is any leak in that length of pipe you will get water intrusion. I have the same hull as you and while you are correct in that the transom will occasionally dip below the waterline, that it not the issue. When you are sailing, you are on the boat and can monitor the situation below. I have a very loud bilge electronic alarm that goes off if the pump comes on more frequently that every 5-minutes and have had it go off when boisterously sailing when forgetting to close the galley sink thru hull. As you know, we can ship water into the galley sink and to the drain on fun days. The issue is can you have water back flowing from the thru hull to the pump when you are not on the boat to monitor the situation. That is why I always close all thru hulls before putting Papillon to bed.

The thru hull through the transom is above the waterline most of the time and the line goes up to a point higher than waterline as it crosses into the port locker. This section of line will drain down through the thru hull breaking the potential syphon to the pump.

This is some excellent info and the bilge alert system is a great idea! I've used cheap water sensor alarms in the past. It has alerted me when something went wrong a couple of times. Did you buy or build the 5 minute alert system?

 

[Hayden Watson]

This is some excellent info and the bilge alert system is a great idea! I've used cheap water sensor alarms in the past. It has alerted me when something went wrong a couple of times. Did you buy or build the 5 minute alert system?

It came with my boat when I bought it 28-years ago. I cannot find the exact model currently but this one has the same features and might be the updated version. It will alarm if the pump runs for 2-minutes. Mine is a bit different. It will alarm if it runs continuously but will also alarm if the pump come on too frequently.