the water in the kitchen drain (brand new 400) does not leave the sink in an acceptable time. Though the sink pipe is straight to the hull the only possible reasons could be the (a) link between pipe and sink or (b) construction error due to waterline/sink level. Has anyone experienced a similar problem?

 

Thanks for your help,

Alex

 

I have a similar problem on my 370. I believe it is due to the horizontal link hose between the drain pipe and the sink outlet. The way it is laid out, it allows air to be trapped and form a plug in the drain. I have cut the hose by 4-5 centimeters in order to get some slope and drainage was improved a bit. However, it is still not good enough.

 

You can manually test this by forcing the hose to slope more while you are emtying the sink.

 

Steve

 

If you let the water out say 0,5 m below the wl you have a pressure of 0,05 bar against and a watercolumn of about 1,5 m as a "driving force, equivalent to 0,15 bar, that gives you a net driving pressure og 0,1 bar in the beginning and 0 bar when the waterlevel in the tube is level with the waterline, i.e you will always have water in the hose at the level of the wl.

 

The only way to "fix" this problem is to let the water out above the wl. Then it will run out freely driven by the weight of the watercolumn.

 

This is by the way a good reason for why you should allways close your seavalves when leaving the boat.

 

Tingeling

 

Totally agree with you. Had the same problem  with my previous boat with the sink and I always closed the valves when I left the boat.

 

I always closed all the hull valves anyway leaving the boat. Why take the risk ??.

 

It's only a small job and it saves a lot of hassle at the end.

 

Willem

 

 

Anyway we still have the same problem.

greetings Marcel

 

 

Yes...bigger hole does not change the pressure balance in the hose....unless you drain to open air, then a bigger hole allows higher volume to escape.

 

Tingeling

 

The balance will not change unless you change the position, and not the size of your through hull valve.

 

Regards, Willem

 

I think both your observations confirm my theory that there is often air trapped in the hose since there is a horizontal leg on the drain.

 

On my previous Catalina 34 the sinks drained very fast, and when under way they absolutely whooshed out. I went and looked at the drains on a Catalina. They go straight down, without the trap the Hanse sinks have. I am sure that the hanse sink drains could be replaced with some that run straight down and don't trap air. 

 

I'll check the diameter next time I'm at the boat and see if I can find a suitable fitting.

I don't agree with the "physics" explanation, sorry tinqeling. The speed of the water in the drain depends of course on the water level difference (called water-head) but mostly on the diameter ,length and surface smoothness of the pipe used, where the smallest diameter in the entire length of the pipe matters the most. IMHO Hanse uses smaller through-hull fittings (usual Hanse penny savings) than it should. The smaller pipe gets also clogged easier by antifouling and sea growth and the result is slow drainage.

If the sink was below or at sea level then the water would flow in the inverse direction and the sink would fill or even overflow. Finally the boat would sink!!! Remember that the free surface of all connected water is at the same level. In Hanse boats the sea level is about 5-6 cm above the floor.

 

 

Hi again. So sorry, don't take me wrong, but I'm right. Maybe an easier explanation is this: When letting the water out directly to atmosphere the pressure against is net zero, letting the water out below waterline the pressure against is the depth in meter (x) dived by 10 = xbar. So if the the outlet is 0,5 m below waterline then the pressure against is 0,05 bar.

 

If, in addition, there are airpockets in the outlet hose you get into more difficulties. Therefor:

 

 

 

your humble friend

 

Tingeling

 

 

 

Another way to think of it is that the bottom X feet of the drain will normally be filled with "standing water" so you are really always draining to the waterline height, regardless of how low the drain goes.

 

The only difference between an outlet near the waterline and one further down should be a little bit more resistance due to a longer drain but there is no backpressure effect.

 

Sorry but I can't help entering discussions about physics and I have to vote with Panos on this one....

 

I do expect that the air in the drain hose is the problem and this is the problem that a "free fall" drain would fix.

 

 

 

 

Well, wrong, but both you and Panos actually are quite near in your reasoning, you are quite right that you are draining against the" stand still" water in the hose....wich in turn has to be pushed out against a final pressure of 0,05 bar (if 0,5m below wl), so there is a slight resistance there,,,which has to be overcome.

 

Now I am heading for my boat in a great spring weather.High pressure and 24 deg C, ...have a nice weekend and keep thinking about hydrostatics and flow of water. I will check this with a friend who is a hydro powerplant engineer....I'll admit it to you if I'm wrong. 

 

Tingeling

 

Picture two boats with the same drain, hose and through-hull diameter, but one with a "deep" drain opening near the bottom of the hull and one with a "shallow" drain opening just below the waterline.  

 

Pour a cup of water down both drains and the added cup of water creates the same "draining" pressure in both cases since it adds the same pressure head to each pipe.

 

The difference is not in the pressure but in the fact that the "deep" drain has to push a longer "plug" of water out of the pipe.  Since the force is the same but the mass that has to be moved is greater, it takes longer for a cup of water to come out of the deep drain than the shallow drain.  

 

The other difference is that there is more frictional resistance in the longer pipe.

 

If the owner of the "deep" drain boat moved his through-hull to the same location as the "shallow" drain boat but neglected to shorten his hose and left the same length of hose below the waterline as before the modification he would find that his problem was not solved and it would take just as long to drain his cup of water because it has to accelerate the same mass of standing water in the hose.

 

So Tingeling I think you are right about the yacht design, but only because a shallower drain reduces the standing water in the drain and the friction force, not that it results in a greater draining force. 

 

 

I had a beautiful long weekend, starting with fog with clear sight of 15-20 meters, ended with a night in an unprotected harbour and 40 knots wind.

 

The two days in beetween was just beautiful...thanks to plotter and radar...how did they do it in the past?

 

I will not trow in more arguments for the against pressure og 0,05 bar...I know it is there, but the arguments for bigger tubing is of course correct, but no solution. The solution is, as many manufacturers do....leading water out above wl and against athmosperic pressure.

 

Looking forward to the next problem to discuss.

 

Tingeling

 

It sometimes helpes to pour a kettle of boiling water down the drain to disolve all the greasy bits ?

 

The end of my involvement in this matter.

 

Happy draining.

Cheers Willem

Evtl also exchange the t-connection to a new one. Mine was too small - not the 25 mm it must be minimum.

This small operation totally solved this problem on my boat, and the drain takes water as quickly as I can fill it with the pump.

Azul/Peter

Thank you for posting , I almost instald a second pipe to solve the problem but now I am going to try this first!

greetings Marcel

 

My advice to everyone , do the same because it is just the way it should have been installed in the first place!

BDW all clams were very loose so check it before you sink!

Thanks anyway Jens

Greetings Marcel

 

A minor improvement might be to use Y-connection (instead of the T-conncection). Any nice theory or plumber experience to share on that idea?

 

 

stefan

Just like it shows on the picture in this topic .

Cost were only 7 Euro s fore some new flexible tube.

Just do it and you will see fore yourself , at my boat it runs empty like at my home.

succes

Marcel

 

 

I finally was back out to see my boat and confirmed the same plumbing error.  I re-plumbed my drains using the existing hoses, minus several inches and now have a safer boat, more space below the sinks, and sinks that empty quickly. 

 

I took pictures to add to the forum, but mostly wanted to pass on my thanks to SpilroC.   This website is the best feature that comes standard with every Hanse!

 

"BEFORE"  (original installation)

 

"AFTER"

Wenn the t-connection is below the watherline it gets to much friction .

greetings Marcel

on my boat the new situation is also perfect.

 

 

Johan you are an evil, evil man for raising the physics question again.  I was resisting the temptation but now that you've raised it I will probably be a Vice-Admiral before this topic is closed again (or maybe chief bottle-washer).

 

Others mentioned the "air-lock" problem earlier in this post and I think this is evidence that they were right. 

 

The first thing that has to happen to form the air-lock is that an air/water interface is formed across a section of the drain.   This happens easily because the cross-section of the drain is so small, especially at the fitting at the bottom of the sink, so it isn't hard to bridge the opening of the drain (forming a bubble) by splashing some water into the sink.

 

Now for the water to drain it either has to:

1)  break the surface (pop the bubble) by overcoming the surface tension

2)  push the air/water interface down the drain tube (move the bubble) or

3)  flow down along the the air/water interface to the water/pipe interface and seep down the side of the tube (go around the bubble)

 

In the case of the original installation neither (1) nor (2) can happen so only (3) is left, which is a slow process because the effective tube diameter is reduced so much by the air bubble. 

 

The reason why (1) doesn't happen is because the cross-section of the drain tube is so small, especially at the sink drain fitting.  The sum of the surface tension across the surface of the bubble results in a force that is proportional to the circumference of the bubble (proportional to the tube inner diameter), so if the diameter is reduced by a factor of 2, then so is the force supporting the bubble surface.  The downward force of the water on the bubble is proportional to the water pressure ("head" or height of water) and the cross-sectional area of the tube at the point where the bubble interface exists, so if the diameter is reduced by a factor of 2, then the force of the water is reduced by a factor of 4.  Therefore it takes more pressure to break an air-lock in a small tube than a wide tube, which is why bubbles get stuck in drinking straws but not in 2-inch household drain tubes.  Conversely, for a given amount of standing water in the sink, the downward force of the water on the bubble increases with the square of the drain diameter while the surface tension supporting the bubble increases linearly with the drain diameter.  As the diameter is increased, the water eventually overcomes the surface tension and "breaks" the bubble surface off the wall of the pipe.  Once this happens the water flows downward and smaller air bubbles rise upward or are washed away or just dance around in the water flow.  It may be possible to break the surface of the bubble with the original plumbing if the water level in the sink gets high enough but if the air doesn't escape the surface will reform with the water level lower and the drain will slow again.

 

The reason why (2) doesn't happen is that the only way to move the bubble downward is to push it out the bottom of the boat, and this requires enough force to displace the standing water in the bottom part of the drain (from the waterline to the through-hull opening).  That water is supported by a pressure head of about 0.5m because that is about the depth of the through-hull opening.  An equal pressure would be needed above the bubble to push it down but that would require 0.5m of water above the top of the bubble, so the sink would have to be filled to a depth of 0.5m which obviously doesn't happen very easily.  The bubble will be compressed a bit and will also move down the pipe as you fill the sink but I doubt that it ever sees the 0.5m pressure head needed to clear the through-hole opening.

 

The case with the new plumbing is very different.  First of all, the "T" provides an escape route for part of the bubble, so (2) happens very easily, at least for the top part of the bubble.  The bubble surface moves down the drain as air escapes through the other sink drain.  Once the bubble surface passes through the "T" the vent disappears and the rest of the bubble is trapped between the standing water below and the drain water above.  At this point, however, there is enough water in the drain above the airlock to overcome the surface tension and collapse the bubble, at which point it breaks into smaller bubbles, some of which will rise and vent out through the "T" to the open sink (with a gurgling noise).

 

I don't think it is strictly the difference in the friction between the two arrangements because you will notice that if you put the plug in the second drain (closing the vent) the old slow-drain problem returns.

 

I noticed that our aft head sink drains well, probably because it also has a "Y" going to the shower sump, which provides some venting but our forward head sink, which is just a single tube, is slow like the galley was before.

 

So presumably what we need to do for the forward head is to put a "T" in the sink drain (above the waterline!) and run a small vent hose upward to somewhere behind the sink and above the top of the sink -- this should vent that drain and as long as it is small and high enough it is no more dangerous than the sink drain.  Then I won't have to wait for several minutes to rinse away my whiskers after shaving, yes?

 

 

 

I guess that's why you're a Vice Admiral and I'm just a captain.  I think this comment of yours from long ago was bang on.   A bigger hose would prevent the airlock as well, as long as the top of the "bubble" was in the hose and not up in that narrow drain fixture.

 

I noticed that too.  While I had the hoses disconnected I opened the seacock and lowered the hose that was connected to it I could see the waterline.  I marked the height inside the cabinet for future reference.  I think it was about 12 inches above the floorboard. Does that agree with yours?

Peter, Denmark

 

I replumbed my 370 this weekend and the sinks now empty very quickly.

I am going to see what I can do to achieve the same with the basin in the heads.