Corrugated hose or smooth for bilge pumps?

[Bigjim]

Originally Posted by Mithril Bham

I assume that when heeled the outlet is below the waterline.

The stern of the boat is an old wine glass design and the outlets are well above the waterline at all points of sail.

[Bigjim]

Originally Posted by Exile

The transom IS the highest point above the pump. I don't think you're understanding my setup, or maybe I didn't describe it well enough. But thanks anyway.

Perhaps this image will help you more to understand.



In the top image the bilge hose runs from the pump directly to the transom. There's nothing stopping the water from running backwards into the bilge once the pump stops pumping.

In the bottom image the hose has a hump in it to create a high spot. Water can run back to the bilge only from the high point of the hump. After the water is over the hump it will only run out of the boat.

Further, in my boat the outlet is about a foot above the water line so once the water flows over the hump it breaks the siphon action to the water so it cannot suck water backwards into the boat.

My transom is at least 4 feet in height. The outlets are at the bottom of the transom. So, although my transom is higher than the bilge, there's quite a bit of difference between the bottom of the transom and the top of it.

[Exile]

Originally Posted by Bigjim

Perhaps this image will help you more to understand.



In the top image the bilge hose runs from the pump directly to the transom. There's nothing stopping the water from running backwards into the bilge once the pump stops pumping.

In the bottom image the hose has a hump in it to create a high spot. Water can run back to the bilge only from the high point of the hump. After the water is over the hump it will only run out of the boat.

Further, in my boat the outlet is about a foot above the water line so once the water flows over the hump it breaks the siphon action to the water so it cannot suck water backwards into the boat.

My transom is at least 4 feet in height. The outlets are at the bottom of the transom. So, although my transom is higher than the bilge, there's quite a bit of difference between the bottom of the transom and the top of it.

Thank you Jim. The pictures are most helpful too. On my boat the outlet is also at the bottom of the transom, which I'd say is closer to about 3' in height. The hump or loop is right at the transom with a siphon break positioned at the very top. There is only one possibility (i.e. space) for creating another loop closer to the pump that may exceed the height of the outlet, and that would be under the fwd part of the aft cabin bunk. I'll take another look when I'm back onboard, but am doubtful there's room. That space is pretty tight with the h/w heater, emerg. manual bilge pump, and all of the batteries housed there. From that point on back the hull climbs up to meet the cabin sole and there's no room to create such a loop.

But I did not realize that positioning the loop anywhere other than the transom was what Terra Nova or others may have been referring to, so many thanks. It would certainly be preferable not to have any check valve for all the reasons stated, and with the loop positioned closer to the pump there would be that much less "nuisance" water the pump would have to repeatedly cycle to remove.

[Bigjim]

There is a lot of talk about siphon action and I'd just like to pass along a tale of warning: When we launched our new boat for the first time this spring, it had not been in the water for nearly a decade. I was concerned that many of the fittings may have dried out and would be faulty. I tried to be on hand on launch in case we had a stuffing box blow-out or a thru-hull failure but we missed the drop by about an hour.

It was raining hard the day of the launch and when we finally climbed aboard at the transient pier there was foot of water in the bilge and not a single working bilge pump on the boat. I brought our portable sump pump and threw it into the bilge. I handed the hose to my 19-year-old son and told him to toss the end over the side rail. We pumped out the water in a matter of seconds and the bilge was nearly dry. Great.

As we worked on the boat getting ready to start the motor for the first time I noticed that the water in the bilge was over a foot deep again. The portable sump did not have a float switch so I had to plug it in again. The water was gone. But now I was concerned that something was leaking. A boatyard employee showed up to check the stuffing box and he tightened it, but said he didn't see any excessive leakage. Frantically, we checked each of the thru-hulls but none of them seemed to be leaking. What the hell was going on! In the mean time, the water in the bilge was back to a foot deep!

Then, suddenly it dawned on me. I looked at the hose and at my son standing there totally unconcerned.

"Is the hose over the side? Or under the water?"

He climbed into the cockpit and looked over the side. "Under the water."

I sighed and dropped my head, "Have you ever heard of siphon action?"

He kind of nodded.

The end of the hose was in the water; the bilge pump was in the bilge and underwater. There was complete circuit with no siphon break. As we pumped the water over the side, the pump and hose were filled with water. Once the bilge was empty we unplugged the pump and the water was being siphoned back into the bilge since the pump was lower than the end of the hose and the pump was still in a few inches of water.

I picked up the pump and could see the water flowing from the bottom. I grabbed the other end of the hose and pulled it out of the harbor. The water stopped flowing.

Needless to say, I now have one bilge pumped installed with an automatic switch and a second one to be added soon. A third, big pump, will replace the old main pump which makes noise but doesn't seem to pump water. In all three cases, the outlets will be above the water line or be installed with a siphon break.

No matter how smart you are you can still be the victim of basic physics.

[Exile]

Useful example of siphon action!

[Andina Marie]

As a safety you should use a siphon breaker at the top of the loop. Commercial siphon breakers are notorious for valve sticking but see this article on a DIY Siphon Breakers - Vented loops valve that I used on 4 bilge, 1 grey water and 1 black water pump out for 14 years without a single failure.

[Kolchac]

The corrugations cannot reduce the flow (roverhi is right) according to hydrodynamics the flow depends only on cross-section – it is the science, period. The valve has same diameter as an output of the pump, so no problem here.
Also, when a boat is leaning, outlet is underwater, waves could create a hydraulic stroke, which can lift the water on several meters.
Also, if you have a narrow and deep bilge and long hoses, the pump could be turning on more often because of returning water without a valve.
And finally – keep the bilge clean.
Resume: use valve, loop and corrugate hoses

[darylat8750]

Originally Posted by Exile

When I purchased my now 32-year old boat 11 years ago, it had a Rule centrifugal primary bulge pump with the type of check valve that has the rubber flap. It was working fine back then but I replaced it with the same type, and recently replaced it again, still working fine. It's called a Bosworth (see pic below) but I believe Jabsco makes a similar one.
......

Those who are absolutist about not having check valves are in good company, however. In other threads Maine Sail has recommended a second diaphragm pump for the "nuisance" water that drains back into the bilge after the primary cycles. Other than fitment, etc. issues, my concern about such a set-up is that a centrifugal pump requires priming, so if it is sitting in a dry bilge it may cavitate rather than pump water out of the boat. I could go with a diaphragm pump as my primary I suppose, with an intake hose that sits down in the bilge while the pump stays high & dry. But after 11 years of flawless duty from both the check valve and a couple of Rule 2000's, I'm not sure I need to make any changes.

I remain open-minded for any better/safer solutions, however, and understand why the ABYC and most experts seem to frown on any sort of check valve.

My check valves that failed were name brand bronze. I did what MainSail recommended and the the lowest pickup point is emptied by a diaphragm pump. Sort of built in check valves. When cleaning the bilge a couple of times a year I turn off the diaphragm pump and check each of the others.

[Bigjim]

Originally Posted by roverhi

The corrugated hoses for bilge pumps that I've seen are smooth on the inside. The corrugations keep the hose from being easily kinked and/or crunched reducing the volume of the hose. Nothing will decrease flow more than having a heavy object flatten it or a kink.

There are two kinds of corrugated hose on the boat. The older ones are smooth on the inside. The newer ones look like they are corrugated inside and out.

[Exile]

Originally Posted by Bigjim

There are two kinds of corrugated hose on the boat. The older ones are smooth on the inside. The newer ones look like they are corrugated inside and out.

I'm betting mine might be the newer style and it does look corrugated inside & out, but is in fact smooth on the inside. It was sold as "bilge" hose and was cheap -- too cheap I think. Looks like thin, flimsy plastic that is prone to movement when the pump comes on. I'm going to replace, probably with some heavy white sanitary hose. Only problem is my Rule 2000 has a 1 1/8" outlet which appears to be an odd hose size. May need to go with 1 1/4" and use adapters.

[TreblePlink]

Has anybody had good luck with the flow-max ball scupper? I was going to add an additional cockpit drain (to the Lazarette which has an outboard) - but in testing it, the ball would not reliably seat into the hole ...

[thinwater]

Originally Posted by Kolchac

The corrugations cannot reduce the flow (roverhi is right) according to hydrodynamics the flow depends only on cross-section – it is the science, period. ...

This is pretty funny. EVERY engineering calculation handbook or formula for pipe flow includes a roughness factor and factors for bends. Even rust in a pipe makes a big difference. Roughness affects the speed of a boat.


(I'm a chemical engineer and used to do this for a living.)



You just flunked fluid dynamics 101.

[TreblePlink]

"The corrugations cannot reduce the flow (roverhi is right) according to hydrodynamics the flow depends only on cross-section – it is the science, period. ... "


By this logic, there would be no reason to streamline airplanes ...

If a fluid moves very slowly past rough surfaces, the impediment is mild - but as speed increases the impediment rises with either the square or cube function...

[fivecapes]

But, but, but the dimples on a golf ball!

[Bigjim]

I think the issue with the corrugation is that it produces cavitation and inefficiency. A smooth hose will waste no energy since all the water is moving in the same direction.