Bilge pump pipes

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nathanlee

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I seem to be coming up with some remarkably boring questions lately, but I'm making progress, so I'll keep going :)

I don't have the space in my bilges to run a standard size bilge inlet pipe, so I'm wondering if it is acceptable to use two smaller pipes and a Y adapter to run them into the normal size further up stream where there's enough room? The two pipes would be routed together so there would be no worry of one sucking air.
 
nathanlee said:
I seem to be coming up with some remarkably boring questions lately, but I'm making progress, so I'll keep going :)

I don't have the space in my bilges to run a standard size bilge inlet pipe, so I'm wondering if it is acceptable to use two smaller pipes and a Y adapter to run them into the normal size further up stream where there's enough room? The two pipes would be routed together so there would be no worry of one sucking air.
Click to expand...

more susceptible to blockage
1 x 1.25 inch would be better
 
MrCramp said:
I am puzzled to know what you mean by a "bilge inlet pipe"!! Maybe that is why you have not yet had a reply.
Click to expand...

I'm missing a 'pump'. The bilge pump inlet pipe, or pickup pipe. The bilge pump will be in a shallow locker, but there is no room to run a normal sized pipe under the bilges (cabin sole boards) on my bilge keel boat. The previous owner achieved this by cutting a hole on the cabin sole, with the pipe exiting a locker, and diving down in to the bilges. It was visible and looked a bit naff, so I'm trying to avoid this if possible.

Cheers,

Nathan
 
Resistance to flow through a pipe is a function of the Hagen–Poiseuille equation

Essentially flow is limited by the length of a pipe...longer = less flow for a given pressure, the viscosity of the fluid (obviously) and a function of the radius of the pipe to the power of 4. Now I’m no mathematician but as I understand it, if the cross sectional area of the two smaller pipes is about the same as the one larger pipe then your max flow rate will be about half. It’s rough and ready maths...can you accept a higher blockage risk and half flow? If so, fine!
 
If you can fit two smaller pipes can you instead fit one elliptical pipe. What I mean is can you flatten a pipe from a circle to an ellipse. I did this with the six inch exhaust on my boat. The area is the same just the shape that is different!
 
I cut my pump inlet hose long, (7 metres), and fitted a non return strum box on the end. The idea is that I can move it anywhere it's needed. The coiled 38mm hose does need a bit of room to store though.
 
PCUK said:
If you can fit two smaller pipes can you instead fit one elliptical pipe. What I mean is can you flatten a pipe from a circle to an ellipse. I did this with the six inch exhaust on my boat. The area is the same just the shape that is different!
Click to expand...

That would work, although I'm not sure how to effectively flatten the pipe?
 
PCUK said:
What I mean is can you flatten a pipe from a circle to an ellipse. I did this with the six inch exhaust on my boat. The area is the same just the shape that is different!
Click to expand...


Oh no it isn't. Once you start distorting your circle into an ellipse (or any other shape for that matter) the same circumference encloses a smaller area. If you were right, squashing or kinking my garden hose would have no effect on the outflow.
 
No, don't flatten the pipe, you'll seriously reduce the cross sectional area and limit the flow.

I'm also a bit worried about the 7m long pick up hose mentioned earlier - I think it would take a seriously powerful pump to make it work. Even with no kinks, bends or loops that's about 12' of equivalent vertical pressure head at 50 gpm, many pumps can't handle that. But if it works, it must be OK; you live and learn.

If Nathan's bilge pipe is on the main bilge pump I'd try to maintain the pipe diameter throughout and keep it as short as possible. If it isn't that critical a pump I'd Y to two hoses and then Y back to a single hose again after the obstruction. For example a 1.25" hose to two .75" hoses and then back to the 1.25" hose as soon as possible. But I'd also want to mount the strum box a little bit proud of the bilge surface to avoid picking up debris which could clog the smaller diameter pipes.

Entirely my own opinion, of course, and worth about what you paid for it!
 
Salty John,
The long hose does not seem to be a problem, the pump is a Whale mk5 and although I have yet to try the pump in anger, I can report that it will very quickly empty a bucket of water from the bow locker. I think the joker valve fitted to the strum box helps by preventing any back flow from the hose.

regards,
 
Nigel,

That's a good pump, same as the Henderson MkV which is fitted on many quality boats. It's rated at about 18 gallons per minute (when operating at 80 strokes per minutes) so the hose losses will be lower than those at the 50gpm I used. The pump is rated at up to 15' of head but the pump maximum capacity is at 1.5' of head and 3' vertical lift. I would think you'd lose about half the pump capacity with the long hose run and a few kinks and bends.

The scarey thing about bilge pump systems is that none of them can keep up with even a small hole below the waterline. A 1.5" hole, such as that resulting from a broken seacock, 3' below the surface will let in over 70 gpm! So the trick is to stem the flow as quickly as possible because just pumping isn't going to keep the boat afloat. Even the biggest engine driven pumps struggle to handle 70 gpm. The biggest manual pump I know of is the Edson which pumps one gallon per stroke, but doing more than 60 or 70 strokes a minute is hard work!
 
John,
I take your point about reduced capacity, hopefully I'll never need to pump huge volumes of water, most of the boat is divided into separate deep lockers with no limber holes, hence the need for a "roving pump".

regards,
 
pumps

nigel,
i`m going through the same prob at present. i bought a whale bilge pump made to fit into the smallest of bilges.
basically its long and thin as opposed to the usual "tall" pumps.
dunno if this helps but it def looks the part for the sort of probs u just mentioned.
good luck.
james.
 
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