Bilge Pumps

[pmagowan]

Sticking to the subject of sinking and the prevention thereof I was looking into bilge pumps. Now it seems that the standard 12 or 24V jobs really are more for the removal of small amounts of water from the bilge and even if they met rated capacity (which they never will in our application since they are rated for pumping water on the flat) they would be less effective than a bucket. Assuming that the average hole in a hull is likely to be a through-hull fitting of about 1 or 2" and that this would quickly swamp a standard bilge pump and perhaps even damage the electrics I was looking for alternatives of higher capacity.

I found engine driven pumps which are clutched such that they can be engaged at need. They pump 10-20 times the amount of water of a standard pump and will do so against a head of about 3m. I was wondering if anyone else had used or fitted this type and what your thoughts are in general. It might buy some time to find a leak which is, IMO, the whole point.

 

[jon711]

Engine driven clutch pumps are great - assuming the water level has not got too high to stop the engine!! They can be either electric or manual clutch, but the problems with the electric clutch is that if the water level is too high you may have lost your electrics, the problem with the manual clutch can be an access issue.

If the outlet pipework is arranged so that there is a column of water on the outlet (vertical outlet pipe) then you should also get some dry running out of the pump, as the remaining column of water in the outlet pipework, will lubricate the impeller (But it will not dry run for long!)

By the way not all manufacturers quote pump outputs at zero head!

Jon

 

[pmagowan]

I had thought about the engine being swamped but it would not be hard to design an engine system such that the air intake was high up and the engine had a watertight bulkhead of its own. That way the engine would potentially keep running right up until the point the boat sank. This, of course, would be very useful as it not only drives the pump but many other bits of equipment that could be necessary in a fix, lights being one. Perhaps the whole electrical system should be treated in the same way.

 

[prv]

Agree that most small 12v pumps are really only suitable for removing accumulated water, not dealing with an actual leak. Centrifugal pumps are rated at zero head, but diaphragm types are generally described more realistically.

Engine power is the only way to go for an actual "crash pump". Either clutched (I would go for a manual clutch with a bowden cable to a convenient position) or in the form of a separate motor. I know of at least one oceangoing yacht which carries a 4" builder's trash pump, and I don't think it's that unusual.

Something like this:

can be had for less than the Jabsco clutched pumps. The pump materials may not be long-term corrosion resistant in seawater, but it's not something you'd use every day.

Pete

 

[Salty John]

An engine driven pump is a great asset but is physically large and difficult to accommodate on a smaller boat.

Here are some thoughts on bilge pump system design:http://www.saltyjohn.co.uk/resources/Bilge_pump_system_design.pdf

No, we don't sell bilge pumps.

 

[prv]

An engine driven pump is a great asset but is physically large and difficult to accommodate on a smaller boat.

Very true, but he's planning to build a big one

Pete

 

[rogerthebodger]

There used to be an emergency bilge pump available that fitted around the prop shaft that was designed to shift large volumes of water. It must have been a centrifugal type as it would be running dry for most of the time.

The name Edson the steering component comes to mind but it not on their web site currently.

 

[abraxus]

Ok, this may sound really stupid as I'm not a boat builder and so don't know how practical it would be, but if the primary concern is the failure of a through hull fitting, then would it possible to design something that stops the water getting in, rather than dealing with getting it out?

Maybe some kind of sealed enclosure surrounding the fitting, with a sealed access hatch?

 

[sarabande]

in extreme power-less conditions, for example, coming on board and finding the batteries submerged, there is very little to beat a Patay DD120. Double action, and simple to unblock.


Easy to mount on a board so you can stand on it and put some effort into the handle.



Also very useful for de-watering a flooded dinghy or tender.

Relatively cheap (£60 -£90 on ebay.

 

[prv]

Ok, this may sound really stupid as I'm not a boat builder and so don't know how practical it would be, but if the primary concern is the failure of a through hull fitting, then would it possible to design something that stops the water getting in, rather than dealing with getting it out?

Maybe some kind of sealed enclosure surrounding the fitting, with a sealed access hatch?

I don't think an enclosure would work, as you still need the hose to pass out through it so you just move the problem along the line.

The recognised solution in steel and aluminium boats is to build them with welded-in standpipes that reach above the waterline, and fit the valves to the top of these. However, these really need to be considered and designed in right from the beginning, so that they can be supported by nearby bulkheads etc, and they are less flexible than just bunging a valve wherever you need one. If the boat has a deep body (ie, the cabin sole is a long way below the waterline) then it can be difficult to arrange gravity drainage from sinks etc.

Pete

 

[abraxus]

I don't think an enclosure would work, as you still need the hose to pass out through it so you just move the problem along the line.

The recognised solution in steel and aluminium boats is to build them with welded-in standpipes that reach above the waterline, and fit the valves to the top of these. However, these really need to be considered and designed in right from the beginning, so that they can be supported by nearby bulkheads etc, and they are less flexible than just bunging a valve wherever you need one. If the boat has a deep body (ie, the cabin sole is a long way below the waterline) then it can be difficult to arrange gravity drainage from sinks etc.

Pete

I was assuming that the where the hose exited would be sealed too. However, a standpipe above the waterline achieves the same thing, but as you say depends on the feasibility of being able to build one in.

 

[prv]

I was assuming that the where the hose exited would be sealed too.

But why do you trust that seal and not the original seal to the seacock?

It's turtles all the way down

Pete

 

[pmagowan]

Thanks everyone. I had already read your document Salty John and have had good service from you in the past so would be happy if you sold them anyway . The boat I am thinking about all this for is going to be in the region of 40-45' but am happy to discuss the concepts for any boat as it is all helpful and not just for me.

I like the idea of the stand alone builders pump also but it would be a lot slower to get into play than a plumbed in bilge pump system. If I was doing serious offshore stuff I may vacuum pack one of these and put it in storage onboard for if additional pumpage was required. I don't like the idea of extended through hulls as I suspect the leverage on these would be so large as to increase the risk of failure. I have thought about isolating areas of risk with full or partial watertight bulkheads but that is on another thread.

Bottom line is that you need to stop the water getting in. It would only be in minor situations where a bilge pump would keep up with the influx but a good one might buy you time. Perhaps the most important thing is to detect the leak early enough to give you a good chance of identifying where it is from. From the stories I hear most people only know there is a leak when the floorboards are awash and by this stage it is very difficult to find out if it is even from the bow or stern. Perhaps some sort of sensors placed at various levels of the bilge would help or even in or at the throughhulls.

 

[abraxus]

But why do you trust that seal and not the original seal to the seacock?

It's turtles all the way down

Pete

Well I guess you wouldn't trust it any more than the seacock Pete, nor any more than a bilge pump for that matter, but you'd be working on the basis that it would be unlikely for both to fail at the same time.

It's just a matter of cumulative odds, which increase in your favour with each turtle

 

[rogerthebodger]

When I built my steel boat I did just that. I did not have standpipes to above the water line st they would intrude too far into the living space but if I wish I can retrofit them.

The cable have sealed cable glands to prevent the ingress of water if the skin fitting of the transducer is breached and the main reason I did it was that the transducer fitting is plastic so it would be possible through unlikely for the plastic to be hit by underwater debris.

 

[prv]

When I built my steel boat I did just that. I did not have standpipes to above the water line st they would intrude too far into the living space but if I wish I can retrofit them.

The cable have sealed cable glands to prevent the ingress of water if the skin fitting of the transducer is breached and the main reason I did it was that the transducer fitting is plastic so it would be possible through unlikely for the plastic to be hit by underwater debris.

So, to be clear, are these only around the transducers, or around the seacocks as Abraxus suggests?

Metal pots around plastic transducers are common in metal boats, as I'm sure you know, but I've never heard of them around seacocks.

Pete

 

[prv]

Well I guess you wouldn't trust it any more than the seacock Pete, nor any more than a bilge pump for that matter, but you'd be working on the basis that it would be unlikely for both to fail at the same time.

It's just a matter of cumulative odds, which increase in your favour with each turtle

You still only have one turtle on the hose itself (outside the pot) and whatever is on the other end of the hose, though.

Pete

 

[pmagowan]

You still only have one turtle on the hose itself (outside the pot) and whatever is on the other end of the hose, though.

Pete

Yes, I don't quite get it as it seems that it is still a single point of failure but now it is raised a bit and perhaps more vulnerable. The only way I can see of reducing the risk is to have the flexible pipe inside a steel tube or similar which went above the waterline but this would be very intrusive and make any inspection or working of the seacock almost impossible. If you simply move the seacock to the top of this tube then it is still a single point of failure at the bottom. I am not sure I am a fan of the idea in any of its variations. I would prefer a real bulkhead in the area in question.

 

[prv]

Yes, I don't quite get it as it seems that it is still a single point of failure but now it is raised a bit and perhaps more vulnerable. The only way I can see of reducing the risk is to have the flexible pipe inside a steel tube or similar which went above the waterline but this would be very intrusive and make any inspection or working of the seacock almost impossible. If you simply move the seacock to the top of this tube then it is still a single point of failure at the bottom. I am not sure I am a fan of the idea in any of its variations. I would prefer a real bulkhead in the area in question.

You seem to be mixing up the separate proposals of a) standpipes and b) seacocks in pots.

Transducers in pots are relatively common in metal boats, because the fittings are generally plastic and this seems like an unacceptable weak spot in an otherwise very strong hull.

Seacocks in pots (Abraxus's suggestion) I have never heard of before, and don't really like for the reasons stated, but it would prevent a few classes of failure and maybe someone out there is doing it.

Standpipes are a known technique in metal boats, though I think less common than transducer pots. The seacock does indeed go on the top, above the waterline - in calm water you can completely remove the seacock for maintenance and look down the pipe! You can also rod out a blockage the same way. It's true that the pipe itself is a single point of failure, but for this reason they are heavily constructed. Thick-walled pipe is used, generally significantly thicker than the hull plating, so the chance of breaking the pipe itself is remote. To prevent this strong pipe lever ripping the hull plating instead, the plating is thickened with a welded flange around the pipe. Triangular plate braces between the flange and the pipe take the load off the welded joint itself. And wherever possible, the top end of the pipe is additionally braced to the framing of the hull. The designer I first heard of this from said he always put the standpipes next to an internal bulkhead (either full or half-height) so that the steel bracing to the hull side could run along the (usually wooden) bulkhead without intruding on the available space. Of course if there was a steel bulkhead, the standpipe would be welded to that.

Anyone worrying about a properly-constructed standpipe failing ought to be worrying about their hull plating instead

Pete

 

[pmagowan]

How do you enclose a seacock in a pot? Surely the pipe from the seacock has to go through the pot and therefore it simply raises the problem from the hull to the top of the pot. the only way I can understand it working is to raise it such that it is the same as the standpipe idea and I am not a big fan of that one.