Re: Bilge Pump Question
I’m assuming this is not a huge boat. If you need that much bilge pumping capacity I have to wonder why. But the fact that you are willing to add an additional bilge pump just sounds like good “pre-planning”.
Bilge pumps are not pressure pumps they are volume pumps… high volume @ low pressure. Since the pumping pressure of a bilge pump is not adjustable by the user there are three primary concerns when installing this type of pump:
1. DIAMETER of discharge hose and all fittings. It’s simple. The smaller the hose the less volume capacity you have. Take a look at the size of the discharge fitting on your pump. For maximum discharge capacity make sure there are no choke points from the pump discharge all the way through to the end of the thru hull connection. A choke point is anything that is smaller than the size of the pump discharge fitting.
2. LENGTH of discharge hose. This is not “head pressure” as was suggested. Check your installation instructions. It may tell you what the maximum length discharge hose is x number of feet or inches. If you exceed the maximum length discharge hose (within reason) it will still pump water. Exceeding the maximum length only means you cannot get the rated capacity of the pump in GPH out at the other end.
3. ELEVATION of the thru hull connection. Water in any application that is being pushed upward has to overcome gravity. Likewise water falling utilizes gravity. This is known as “head pressure”. It’s how we get water pressure into municipal water systems. A one inch square column of water will exert a pressure of .434 at its base. A 6 inch rise (as you suggested) in a 1 1/8” hose, from the center of the pump discharge to the center of the thru hull connection would create a pressure loss of about .217psi. A loss of pressure will have some affect your GPH. Even though you have a low pressure pump .217 is really negligible. Think about it. What does a bilge pump actually do. It drafts water from a low point and lifts it to a higher point for discharge. Again check your instructions for maximum “lift”.
Each of these three are factors of “friction loss” and will affect your ability to pump at your pumps rated discharge capacity… 1000GPH or 1250GPH. So, to fully understand how these three things affect your pump discharge capacity you must understand friction loss. Without giving a long lesson on friction loss, just suffice it to say this. In a newly installed application, if you have a significant reduction in pressure or GPH, it was likely due to friction loss and not the malfunction of a brand new pump. Remember the pressure in a bilge pump is low anyway. Depending on how you overcome the aforementioned three things will determine not if, but rather how much decrease you may have in your GPH discharge.
Considering what information you included about the application I too see no problem with what you’re trying to do. But, if you’re that concerned, before you mount it hook it up in your backyard and play with. Observe any changes and then make your final decision.
