Pump Out Holding Tank

[Mitchellmoxo]

So did we decide if "medium density polyethylene" was a good enough material or not?

 

Thanks

[dmr]

2 hours ago, Mitchellmoxo said:

So did we decide if "medium density polyethylene" was a good enough material or not?

 

Thanks

lots of useful and interesting information about engineering plastics here

https://www.directplastics.co.uk/#

..............Dave

[WotEver]

Cak Tanks know all about plastic tanks..,

http://www.caktanks.co.uk/

[John Hartley]

The plastic tank on Hawthorn is the original fitted in 2000. It has a plastic hatch on top, [no I have never opened it] and is still fine.

Whats the betting now I have said that something goes wrong with it. Why did I open my big mouth 

 

[Robini]

Hello,

My boat "Owlet" is 37 years old (going on 38). I was worried about the toilet tank and corrosion so I contacted the David Pipers owners club and was assured that there are many Piper boats much older than mine that are not having problems. The tank is mild steel and to my knowledge has not been replaced. Fingers crossed!

[dmr]

11 minutes ago, Robini said:

Hello,

My boat "Owlet" is 37 years old (going on 38). I was worried about the toilet tank and corrosion so I contacted the David Pipers owners club and was assured that there are many Piper boats much older than mine that are not having problems. The tank is mild steel and to my knowledge has not been replaced. Fingers crossed!

So there can only be two conclusions here,  either David Piper uses a type of mild steel that is not subject corrosion, or Piper owners have a special type of wee and poo that prevents corrosion. This obviously needs further investigation as there is potential to make a lot of money.

.............Dave

[Robini]

Okay, facetious comments or not I'm just reporting the actual situation. There is a third conclusion but maybe we  go into that here. Perhaps this is not the forum for me!

[Phoenix_V]

On 25/09/2017 at 16:10, cuthound said:

 

 

Of more concern to me would be having a waste tank that utilised the bottom and side of the hull for two of its sides. Failure then ensures costly hull repairs as well as a leaky tank. 

 

But then you would never need to pump it out

[mrsmelly]

On ‎26‎/‎09‎/‎2017 at 08:07, junior said:

If i were you I'd save your money and get a cassette toilet. Much better option.

Stop talking sense you are on the wrong thread.

[Dr Bob]

On 26/09/2017 at 08:12, Mitchellmoxo said:

So did we decide if "medium density polyethylene" was a good enough material or not?

 

Thanks

I know I am a month too late but the answer is yes, a medium density PE tank will be fine. You should get 20 years+ out of it. Make sure it is 9-10mm thick.

Some pointers if anyone is interested. The key properties you should be looking for are stress crack resistance (SCR) and permeability. SCR is a failure mode in all plastics and starts as micro cracks caused by stress in the plastic which then grow with time. All plastics are designed for their use so you need a plastic designed for good SCR.

The choices for plastic tanks are:

Medium density polyethylene (MDPE)

High density polyethylene (HDPE)

Injection moulded PE

Polypropylene (PP)

Firstly polypropylene, this is good for permeability (it is still permeable to gases) BUT it has really bad low temperature properties and below 5deg C becomes quite brittle, has very poor impact resistance and therefore its SCR is very poor at the low temps. I personally would not use PP for a boat tank in the UK. PP tanks are designed to be used at higher temps and can go up to 100degC which PE cannot do.

MDPE is the most widely used material. In the UK, it is formed into tanks using the rotomoulding technique which gives thick section tanks (which is good as that reduces the smell getting out – the thicker the section the better). I would look for a 9-10mm thickness. MDPE is also quite flexible which is good and its SCR is good enough for the application. All the green Oil tanks you see in gardens are rotomoulded MDPE and they last 20 years in the sun. All these polymers will contain good levels of UV stabiliser but likely the tanks will never see the sun.

Injection moulding can be used for smaller PE tanks but these inevitably will be thinner in section and likely a lower molecular weight so will be more brittle and have worse SCR – so I wouldnt think these will be nearly as good as the rotomoulded MDPE tanks.

HDPE may be the best choice as they use a type of PE that is stronger, better SCR and probably better permeability (i.e., higher density and molecular weight) BUT their thickness will be much lower as they are Blow moulded rather than rotomoulded. I am not aware of any tank manufactures in the UK that blow mould PE tanks but it is far more common on the continent. The better SCR will give this type of tank a better resistance to falling apart so may last longer BUT the thinner section (likely 4-6mm) will mean more smell getting through. On balance the thickness of the rotomoulded tank would swing it for me.

I would avoid any welded PE or PP tank as the weld will be a weak point and that is where tank failure is likely to happen.

Make sure the tank manufacture is making to the relevant BS standard – ie they have had their tanks tested by an appropriate test lab. I cant really comment on PE tanks vs Stainless tanks as I have no experience of stainless vs lifetime. I certainly wouldnt go for mild steel tank. There are 10,000s rotomoulded MDPE tanks made in the UK per year for water, diesel and gas oil and they have been around 30 years and more. Yes there are many instances of failure but these are almost certainly down to problems of manufacturing or the odd bad batch of polymer.

[Sea Dog]

16 hours ago, dmr said:

So there can only be two conclusions here,  either David Piper uses a type of mild steel that is not subject corrosion, or Piper owners have a special type of wee and poo that prevents corrosion. This obviously needs further investigation as there is potential to make a lot of money.

.............Dave

I have a Piper boat, albeit a later one made by his son, Simon. I'm pretty sure (or do I mean hopeful?) that both of the things you point out are absolutely correct. The third thing, which is missing from your post, is that the only odour associated with the sanitary fittings in these boats is the delicate smell of violets.

[dmr]

28 minutes ago, Dr Bob said:

I know I am a month too late but the answer is yes, a medium density PE tank will be fine. You should get 20 years+ out of it. Make sure it is 9-10mm thick.

 

We have a plastic tank and I do worry about it. You say 20+ years? Are you aware of any long term ageing mechanisms that might become an issue after 20 years?

Ours is currently 16 and it would not be easy to remove and replace.

.........Dave

[Dr Bob]

1 hour ago, dmr said:

We have a plastic tank and I do worry about it. You say 20+ years? Are you aware of any long term ageing mechanisms that might become an issue after 20 years?

Ours is currently 16 and it would not be easy to remove and replace.

.........Dave

Ours is also 15 years old and not easy to remove or replace! Built into the boat at birth and likely lots of wood needing to be removed to get the old one out.

No, the only long term ageing is likely to be Stress cracking in the pump out tank application. The major degradation path on PE is sunlight (UV radiation) causing free radicals which then split the polymer chains weakening the polymer and thus cracking. The MDPE will be loaded with UV stabiliser to enable green oil tanks to last 20 years - so your natural colour tank (I assume) will be well away from any light so this weakening will not happen. Stress cracking will be happening but on a good resin and a well made tank this should be low. Poorly made tanks will have a lot of stress built in and therefore stess cracking will show up earlier but then that should be in the first 10 years and you should see early failures. As i said in the earlier post, there are a lot of green garden tanks going after 30 years and that is a more demanding application with the weathering. There will be more stress in the pump out tank as the filling and emptying will apply some stress (so that cycle is more than green oil tanks) but I cant see it really decreasing lifetime much.

I dont think the water/effluent will have much of an effect. It doesnt tend to weaken the structure as PE is very hydrophobic.

Will it last more than 20 years? I dont think it will last more than a GRP tank would but we should be able to bet 20-40years life. I am not aware of any lifetime predictions on these PE tanks as most manufactures are seeking 10 years performance and offering 10 year warranties. No one manufacturing thinks about 20 year lifetimes. I know these materials well as I am part owner of a test lab who tests the majority of these rotomoulded tanks in the UK -but I cant be specific on 20 year+. I will ask our technical expert!

Usually PE articles break well before 10 years - washing up bowls, buckets etc, but that is all down to UV. These tanks are different but as an example of my scepticism over performance 20+ years, we have a vacu-flush system fitted to ours. This has a vacuum generator between the toilet and the tank at the back of the boat. This vacuum generator has a pump and a load of valves and is now also 15 years old. To get it serviced is circa £500 and a new unit is about £1.2K. I am worried if I service it then I may only get another 1-5 years out of it as the body is made of PE (looks like MDPE) and that is subjected to far more stress when the pump operates. I am therefore going to buy a new unit and have 15 years rather than find the PE body fails. It could well go on for another 15 years but I dont want to risk it. Unfortunately, we cant do the same for the pump out tank!

[Dr Bob]

Dave, just checked with my technical expert, who has spent 30 years with BP Chemicals working on structure and performance of MDPE/HDPE and sits on the BS standards committe for MDPE oil storage tanks. He thinks 30 years is a good bet - based on industry experience of green oil tanks - although these do adsorb a bit of gas oil and so soften a bit to aid in stress crack resistance. The pump out tank will be a slightly acid environment which will accelerate stress cracking but unlikely to reduce life significantly. He confirmed there are no other long term mechanisms that might come into play (other than UV). When I asked him "40 years" he said to leave it at 30 for now! We are at the sharp end of the business and if tanks were failing at 20years, we would be hearing about them.....but they dont seem to be.

No one can be certain over life expectancy but from my discussions today, it sounds like 30 years maybe more relastic than 20 years. I would still worry....but not as much!

I'd be interested to hear of anyone who has had a split in their pump out tank! It may help us gain a bit more info for the forum members.

[dmr]

2 minutes ago, Dr Bob said:

Dave, just checked with my technical expert, who has spent 30 years with BP Chemicals working on structure and performance of MDPE/HDPE and sits on the BS standards committe for MDPE oil storage tanks. He thinks 30 years is a good bet - based on industry experience of green oil tanks - although these do adsorb a bit of gas oil and so soften a bit to aid in stress crack resistance. The pump out tank will be a slightly acid environment which will accelerate stress cracking but unlikely to reduce life significantly. He confirmed there are no other long term mechanisms that might come into play (other than UV). When I asked him "40 years" he said to leave it at 30 for now! We are at the sharp end of the business and if tanks were failing at 20years, we would be hearing about them.....but they dont seem to be.

No one can be certain over life expectancy but from my discussions today, it sounds like 30 years maybe more relastic than 20 years. I would still worry....but not as much!

I'd be interested to hear of anyone who has had a split in their pump out tank! It may help us gain a bit more info for the forum members.

The tank is actually made from a brown plastic, somewhat textured on one side, and of welded construction (I assume). I don't know what sort of plastic it is, though I should because I recently had a battery box made recently which looks to be of the same stuff.

The tank is fully enclosed in wood so does not get any UV. It is also internally baffled to reduce liquid movement stress. The top does have a just little "give" if I move the bog from side to side. I have never heard of a major plastic tank failure, but think there was a report on this forum of a weld failure where a pipe goes into the top of the tank.  A split tank would be a bad thing to happen!

Like most boats, ours was fitted out without too much thought as to how it might come apart for future maintenance.

..........Dave

[Dr Bob]

7 minutes ago, dmr said:

The tank is actually made from a brown plastic, somewhat textured on one side, and of welded construction (I assume). I don't know what sort of plastic it is, though I should because I recently had a battery box made recently which looks to be of the same stuff.

The tank is fully enclosed in wood so does not get any UV. It is also internally baffled to reduce liquid movement stress. The top does have a just little "give" if I move the bog from side to side. I have never heard of a major plastic tank failure, but think there was a report on this forum of a weld failure where a pipe goes into the top of the tank.  A split tank would be a bad thing to happen!

Like most boats, ours was fitted out without too much thought as to how it might come apart for future maintenance.

..........Dave

Dave, all my comments above are for rotomoulded MDPE tanks. I wouldnt like to say about lifetime on tanks that have been welded as welds are not simple to do in PE. If sitting on the toilet puts stress on the tank then that will not be as good as a rear mounted tank with no weight on it. Are you sure it is welded?

[dmr]

1 hour ago, Dr Bob said:

Dave, all my comments above are for rotomoulded MDPE tanks. I wouldnt like to say about lifetime on tanks that have been welded as welds are not simple to do in PE. If sitting on the toilet puts stress on the tank then that will not be as good as a rear mounted tank with no weight on it. Are you sure it is welded?

I have never actually seen an edge of the tank but assume its welded, the connections look welded. I have assumed that most "dump through" tanks are custom built and so welded. Is there any other way to make a plastic tank with internal baffles?

Hendersons and Goodwin Plastics are both well known custom tank fabricators and both appear to supply welded tanks.

Every time the dog farts I run to the back of the boat and check the bilge just in case the tank has split

.................Dave

[Dr Bob]

No,  its common for custom built tanks to be made by rotomoulding. It is pretty cheap to knock up a mould with various ribs etc. Difficult to be sure until you look at the edge of the tank. If its lasted this long there cant be much wrong with it!