Will a fatter sink drain run faster through the same seacock?

[nigelmercier]

My galley sink drains are 3/4" from the barb on the sink outlet, the pipe, the seacock and through-hull.

Will putting in a fatter sink outlet and pipe (but keeping the seacock and through-hull) make it drain faster?

I think physics says not, but sometimes the sink takes ages to drain, as if there is an air-lock.

 

[prv]

I *think* physics says it will still drain a little faster with the wider pipe - but also, it will be harder to sustain an airlock and then it will drain much faster.

Pete

 

[sailorman]

the smallest dia will control the flow rate

 

[Hadenough]

The reduction in bore creating a debris trap would be my worry.

 

[PuffTheMagicDragon]

The widest part of the 'drain system' complex is the diameter of the sink itself because that is where it starts. Unfortunately this has no bearing on the rate at which the system drains because this is a function of the head of water above sea level and the cross-section of the smallest restriction in the system, usually the seacock.

 

[sparkie]

You'll get less loss of head due to flow (v2/2g) with the larger dia pipe, so in theory the available head at the seacock will be greater and the flow rate greater, but I doubt in practice if you'd notice much difference. S.

 

[Hydrozoan]

I *think* physics says it will still drain a little faster with the wider pipe - but also, it will be harder to sustain an airlock and then it will drain much faster.

Pete

I agree with the last part about airlocks, but am doubtful about the first part (in their absence) as there will be a greater pressure head for longer with the narrower pipe. Unless of course you were there reflecting Nigel's usage ('sink ... make it drain faster') and meaning by the first 'it' that the sink alone would drain faster simply because the bigger pipe will hold more volume pending ultimate discharge, that would otherwise sit in the bottom of the sink. I hope that makes sense.

 

[prv]

I agree with the last part about airlocks, but am doubtful about the first part (in their absence) as there will be a greater pressure head for longer with the narrower pipe. Unless of course you were there reflecting Nigel's usage ('sink ... make it drain faster') and meaning by the first 'it' that the sink alone would drain faster simply because the bigger pipe will hold more volume pending ultimate discharge, that would otherwise sit in the bottom of the sink. I hope that makes sense.

I meant something akin to what sparkie posted in #6 - a vague feeling that a bigger pipe will flow more freely even if it then goes through a smaller hole at the end. The smallest diameter is very important, of course, but if it was the only thing that mattered then we wouldn't have the advice to make bilge pump hoses as short and easy as possible, with gentle curves rather than right-angle elbows.

But as I implied by saying "I think", I am far from an expert on the topic and I might be wrong. The airlock reduction means it's probably worth doing either way.

Pete

 

[PuffTheMagicDragon]

While bends do slow down the flow through friction, the main advantage of short and easy bilge hoses is their being easier to clear in case of blockage.

 

[[3889]]

Yes, it will drain faster. Frictional loss is an inversely exponential function of cross sectional area. Pressure will remain constant, correlating to unchanging head, but flow will increase, determined by the mean frictional loss of the system reducing.

 

[tillergirl]

My MAWB has a pumped outlet. Very effective. One of those footpumps (at floor level obviously). Ever so easy. But then since the sink outlet shares one of the bilge pump outlets the other side of the boat it would only work if pumped.

 

[Pye_End]

If the boat is moving and there is some venturi 'suction' on the system, then a larger sink outlet may well drain slightly quicker. For a static system with pretty small head, and therefore low flow rates, can't see it making any difference.

 

[NormanS]

The chances are that the pipe and fittings are perfectly adequate, but partly blocked by grease. How do I know?

 

[JumbleDuck]

It's basically the same question as "will using heavy duty cables make a light bulb brighter?" to which the answer is "yes, but nly if the supply wires are very long". As with electricity, the resistance to flow of a pipe depends on both length and cross sectional area. It also depends on whether the flow is laminar or turbulent, the size of the boundary layer and so on.

In general, though, a narrow seacock will be by far the most restricting factor.

 

[Hydrozoan]

... The smallest diameter is very important, of course, but if it was the only thing that mattered then we wouldn't have the advice to make bilge pump hoses as short and easy as possible, with gentle curves rather than right-angle elbows. ...

Fair point, and I'll happily concede on the physics of flow (though in terms of time to seeing an empty sink I do wonder if just the extra volume of the larger pipe might have a greater effect than the reduced loss of head - but I do have a pretty small sink ). I find that occasional slow emptying of the heads basin, with a narrower pipe than the galley sink, is overcome by sticking the plug in again and pulling it out sharply, so I agree that airlocking is likely the key thing.

 

[mtb]

reading this reminds me of the thoughts I have watching the Jehovah's Witnesses WHY BOTHER

aside from that degrease the pipe it will flow faster

 

[ghostlymoron]

From my days of studying hydraulics, I can say that fluid flow is affected by three factors; length and bore of pipe, smoothness of pipe and the number of fittings (bends, elbows, valves, contractions and expansions). However for a short run of pipe, its likely to be controlled by the diameter of the smallest section/fitting. In your case the skin fitting. Its unlikely that increasing the bore elsewhere would make much difference and the sudden contraction would cause a significant increase in hydraulic resistance and potentially be a source of blockages. Replacing corrugated pipe with smooth bore might work.

 

[Anchorite]

An air lock??? Have you dual sinks on the circuit? Can you physically see a loop in the pipe run where there could be an air lock. I have 2 sinks, one covered by a cutting board - it has a serious effect on the evacuation rate!