Two pumps with one thru-hull fitting?

[Beefer]

Would you connect two bilge pumps together through one thru-hull? Rule says it's ok, and gives instructions on how to do it;

To install two pumps with one thru-hull fitting installation is critical. Both discharge hoses need to rise upwards as explained in Hose Installation. The two discharge hoses must rise up to the highest point possible then bend to slope down towards the thru-hull fitting. The two hoses need to merge together into a Y-Valve, a Tee fitting is not recommended. Then the single discharge hose coming off Y-Valve can be routed downwards to the thru-hull fitting. Installation in this way allows water to drain downwards to the thru-hull and not to have the ability to flow back through the other pump.

- ITT/Rule website

My only question is how large would the thru-hull have to be? If each is to use a 3/4" discharge, would you double the single (1-1/2"), or a size equivalent to the discharge hose of a single pump with the rated discharge of the combined 2 (1-1/8")?

 

[Home Cookin']

Re: Two pumps with one thru-hull fitting?

generally not a good idea.
If you plan to use both pumps at once, you need a bigger outlet, but I can't say whetehr it has to be double.
If you are rigging it so one pump is a back-up for the other, regular size.

 

[Gary H NC]

Re: Two pumps with one thru-hull fitting?

I would think you would double the discharge size.
I ran a second pump on mine and decided to just add another thru hull fitting.
Better than the hassle of the Y and adapting things from the original pump.
Plus now i can tell if each pump is working just by looking at the discharge fitting.

 

[Beefer]

Re: Two pumps with one thru-hull fitting?

I have one bilge pump, and I'm adding 2 more. Not too excited about more holes in the hull. I have an 800gph, and I'm adding two 500gph autos, but I would like to get a 1500gph or so eventually. When I get the big one, (which uses the 1-1/8"), I will eliminate the (2) 500 pumps. Cost of the 1500+ is prohibitive at this point to do it now. The 2 500 rule-mates cost me $32 for both.

 

[howlnmad]

Re: Two pumps with one thru-hull fitting?

I have one bilge pump, and I'm adding 2 more. Not too excited about more holes in the hull. I have an 800gph, and I'm adding two 500gph autos, but I would like to get a 1500gph or so eventually. When I get the big one, (which uses the 1-1/8"), I will eliminate the (2) 500 pumps. Cost of the 1500+ is prohibitive at this point to do it now. The 2 500 rule-mates cost me $32 for both.

You may want to fix that leak . Just kidding.

 

[DianneB]

Re: Two pumps with one thru-hull fitting?

I added a second bilge pump for redundancy (safety) so it doesn't make sense to have ANYTHING in common between them so there is no single point of failure.

It may be okay, by the rules, to have a single discharge but what are you hoping to accomplish with the second pump - maybe it doesn't make sense.

 

[109jb]

Re: Two pumps with one thru-hull fitting?

I added a second bilge pump for redundancy (safety) so it doesn't make sense to have ANYTHING in common between them so there is no single point of failure.

It may be okay, by the rules, to have a single discharge but what are you hoping to accomplish with the second pump - maybe it doesn't make sense.

I agree that total redundancy is best, but in the case of bilge pumps, or anything else, examining the likely failure modes and probabilities even in a subjective manner and weighing that against the desire to not put more holes in the boat can lead to a different decision. In the case of putting a y at the single outlet for these bilge pumps, what is the biggest danger of failure. In my opinion, it is still the pump failing. The y or the single outlet being the problem is minimal and if done carefully, I would say a problem there is very remote. I only have one bilge pump, but if I had 2 I would not have a problem putting them both to the same through hull outlet. I would also not increase the outlet size if the second pump was simply for a backup.

My 2cents

 

[dingbat]

Re: Two pumps with one thru-hull fitting?

I Cost of the 1500+ is prohibitive at this point to do it now. The 2 500 rule-mates cost me $32 for both.

A Rule 1500 is $60. Can't see doing all that work.....twice, to save $30
Yes, 1.5 TH will work. I would check on the availability of fittings before proceeding. Finding fittings for such a mismatch in connection sizes can be difficult.

 

[rwidman]

Re: Two pumps with one thru-hull fitting?

I would not connect two pumps to one discharge port. If it clogs with debris, neither pump will empty the bilge.

Additional hull penetrations are of little concern for a bilge pump discharge because it will be (we hope) above the waterline.

Best practice is to provide each bilge pump with a dedicated thru hull.

 

[joed]

Re: Two pumps with one thru-hull fitting?

Doubleing the diameter will quadruple the volume that can flow. 1 " would be equivalent area to two .75" hoses.

I think you need check valves in each line also to prevent one pump's discharge from retuning to the bilge via the other pumps discharge hose.

 

[rwidman]

Re: Two pumps with one thru-hull fitting?

Doubleing the diameter will quadruple the volume that can flow. 1 " would be equivalent area to two .75" hoses.

I think you need check valves in each line also to prevent one pump's discharge from retuning to the bilge via the other pumps discharge hose.

A check valve in a bilge pump discharge is a bad idea. You will be pumping debris (dirt, leaves, twigs, wire clippings, etc.) from the bilge. This debris may catch in the check valve and either hold it partially open, rendering it useless, or hold it partially closed and prevent the flow of water.

 

[109jb]

Re: Two pumps with one thru-hull fitting?

Personally, in the many years I have been boating I have never had a plugged bilge pump discharge hose. I also don't see that as being a likely event since anything that can make it past the pump screen will flow through the discharge. I would consider clogging of the pump inlet screen more likely to occur and have had that happen.

If you are having multiple pumps for increased flow capacity then I agree that you either need a larger through hull, or a biigger pump. Personally, I would just go with a bugger pump in that case.

 

[smclear]

Re: Two pumps with one thru-hull fitting?

Doubleing the diameter will quadruple the volume that can flow. 1 " would be equivalent area to two .75" hoses.

I think you need check valves in each line also to prevent one pump's discharge from retuning to the bilge via the other pumps discharge hose.

Correct. A 3/4" hose will handle 2-3 gpm (gallons per minute) while a 1" will handle 4-6 gpm. Don't double it. a 1 1/2" hose will handle 13-18 gpm. A check valve would not be required IF installed according to Rule's recommendations. That is if both pump discharge hoses rise above the thru-hull fitting and then drop down to the fitting.

 

[Frank Acampora]

Re: Two pumps with one thru-hull fitting?

Don't forget that bilge pumps are rated at free flow--zero head. Rule doesn't mention that. As you raise the height of the discharge hoses, you are increasing the head and can significantly reduce the pump capacity. YET, you must raise the hoses significantly above the discharge fitting with this arrangement.

Do-able, but all in all not good engineering design whether the second pump is backup or adding volume to the first. True, I am ignoring the siphon effect which would be there, but only if both pumps were running at the same time. Still poor engineering.

 

[Beefer]

Re: Two pumps with one thru-hull fitting?

Ok, I'll explain the set up again, I wasn't clear.

Currently, one 800gph manually switched bilge pump installed. I want to eventually add a 1500+ auto pump (Rule-Mate 1500 or 2000, $140 & $175 respectively). I picked up (2) 500gph Rule-Mate autos for $32 (brand new).

I am not touching the 800gph or its discharge. It will remain in place.

I'm adding the (2) 500's. I was going to install 2 new thru-hulls (3/4" each), but then figured that's 2 holes I won't be using when I upgrade. When I came across the Rule info on doubling 'em up, I thought it would work, and the thru-hull I use for the doubled will then become the t-h for the larger one.

To address some of the concerns mentioned above;

rwidman - the hoses from the pump to the Y is 3/4", and after the Y it's 1-1/8". A clog in one wouldn't affect the other, cause it would have to be in it's respective dedicated 3/4" hose, not the larger hose.

dingbat - we're working with very standard and common sizes

DianneB - the 2 500's will work as one system, and the 800 as another separate system, so it's 2 systems, 3 pumps.

 

[Mark42]

Re: Two pumps with one thru-hull fitting?

I would not share discharge between two pumps. I didn't find anything in the "back yard boat builders handbook" from the CG, but it is still not a good practice.

JMO

 

[NYBo]

Re: Two pumps with one thru-hull fitting?

How will the dual 500s be powered? If they are connected to the same switch(es), then the Y setup should work fine with no backflow problems since they will both be running. However, I would be leery of independent automatic operation.

But why not just give the setup a dry, or should I say, "wet", run and see how it works?

 

[Beefer]

Re: Two pumps with one thru-hull fitting?

Update

So I was going to go ahead with it, but ran into a logistics problem. There really isn't enough room to install the Y adapter where I want to mount the discharge, so I'm ditching the whole thing. Picked up a RuleMate 1100 gph, and just gonna do that. Just if anyone cared to know.

 

[Fireman431]

Re: Two pumps with one thru-hull fitting?

Just an added comment. There is nothing wrong with hooking two bilge pumps to a single discharge hose and a single discharge port. It's simply a matter of hydraulics:

Earlier it was mentioned that a 3/4" port would flow 2-3 GPM. Actually, at a nomimal pressure (5-6 psi), a 3/4" port will flow closer to 15 gpm through 10' of hose, and under the right plumbing and pressure, will handle up to 23 GPM. The amount of water flow thru the fitting depends greatly on the pressure applied and the friction loss that accompanies it. If you disregard the friction loss (which is high with 3/4" hose), the 3/4" fitting will handle 37 GPM (or 2220 GPH). Of course, with attached hose and friction loss, the flow is reduced. At too much pressure, the water flow actually decreases. The bilge pumps are made to move volume, not pressure.

GPM = 29.7 x diameter of the port squared x square root of the pressure
GPM = 29.7 x 3/4" [squared] x [square root of] 5
GPM = 29.7 x .5625 x 2.23
GPM = 37.25 GPM (x 60 min) = 2250 GPH
This is the optimum theoretical water flow.

Obviously this is ideal circumstances. What I'm simply saying is that a single 3/4" fitting can take 2- 800 GPH pumps, which would be a total of 26 GPM. This doesn't take into account kinks, leaves, ice bags, crappy pumps, etc.

Putting two in series is not a problem. Both pumps will actually flow just a hair under their rated capacity, but nonetheless, the 3/4" port will be able to handle 1600 GPH at the rated pressures of the pumps.

 

[DuckHunterJon]

Re: Two pumps with one thru-hull fitting?

Thank you fireman, I was about to type up the exact same explanation. Just one tiny nit - I wouldn't bring it up, other than, well, I can. Your last paragraph, I think you meant putting two in parallel. Putting them in series would allow them to have a small boost in pressure, but little boost in flow. Putting them in parrallel, assuming correct size discharge hose, will allow them to flow just under their combined rated capacity as you said.

Othwise, great explanation. Thanks!