Pumps..
As far as bilge pumps go I think we need to be very careful taking any centrifugal pump at face value. Franklin has two large pumps a wise idea!
Flow rates, as rated by bilge pump makers, can be quite misleading. They should serve only as a rough guide of pump best case capacity compared to others of similar design.
Bilge pump capacity is usually rated at the “open flow” or what's called “open bucket” rate. This means the figures account for no, nada, zero vertical lift and also no discharge hose friction, radiused bends or discharge outlet restriction.
Actual flow rates, under real operating conditions, can be drastically lower. Water must be lifted up and out of the bilge and then be pushed through lengths of hose to the discharge point. This resistance is called head. Head pressure is basically the wieght of the water and the frictional resistance of the hose, bends and outlet. Because most centrifugal pumps, like the Rule or Johnson pumps, have large internal tolerances to allow passage of small debris, their flow rate decreases dramatically with increased head pressure. Unlike a vane pump, which are less affected by head loss, the pumps rotor or impeller does not come in direct contact with the pumps walls and this can create cavitation if the head pressure is extreme enough. I'm sure many of us have seen the bilge pump kick on and then not actually suck any water but instead just create noise and bubbles in teh bilge. This can usually be eliminated by removing those ridiculous head boosting flow checks..
The output of many centrifugal bilge pumps can diminish by as much as half with only a few feet of head and can cease entirely at between 13 and 20 feet, depending on the size. Remeber head is not just the peak height of the hose it is the friction, height, bends and fitting restrictions.
Impeller or vane pumps and diaphragm pumps are less affected by head pressure than a centrifugal pump is and they may actually be more effective despite a lower GPH rating. Some of the vane/impeller pumps can sustain up to 70 feet of head pressure.
Another consideration is voltage. What is your pump rated at? Is it 14.2 volts or 12 volts. This will and can make a differece as the pumps motor increases and decreases output based on voltage.
There have been a few tests like the ones conducted by West Marine and Power Boat Reports. They rated pumps based on voltage and head pressure. Most manufacturers ratings (open bucket / no head ratings) were off between 10-50%. the output on average was reduce by about
5% for every foot of head pressure. With voltage the drops from 13.6 volts to 12.2 volts were another 15-30% off on top of the head pressure loss.
I guess what I'm getting at is this; unless you have a "monster truck" grade bilge system, like Franklin, do not count on anywhere near the face value rated capacity from your bilge pump.
Some other thing sto consider:
- Use smooth walled dischage hose as it has significatly less head resistance than does corrugated.
- If the discharge outlet is close to the waterline you must use a high loop in teh discharge hose to prevent siphoning. It is advised to add a siphon break at the highest point to ensure it won't siphon. Even with a igh loop a bilge pump can still siphon. I have been on more than one boat that self siphoned when under sail and power..
- Do not succumb to the temptation of using a "check valve" with a centrifugal pump. If you have 5' of height, in a 1" hose, the pump will more likely than not cavitate before it can throw open that check valve..
Disclaimer: Many years ago I used to sell a line of submersible pumps to the plumbing and well supply wholesale distribution channels..
Franklin makes a very good point! I use three WatchDog Water Alarms (LINK) alarms on my boat and they are very, very loud similar to a smoke detector and they can be heard over the noise of the engine! I have one three inches off the bottom of the bilge, one in the engine compartment and one in the head.
As far as bilge pumps go I think we need to be very careful taking any centrifugal pump at face value. Franklin has two large pumps a wise idea!
Flow rates, as rated by bilge pump makers, can be quite misleading. They should serve only as a rough guide of pump best case capacity compared to others of similar design.
Bilge pump capacity is usually rated at the “open flow” or what's called “open bucket” rate. This means the figures account for no, nada, zero vertical lift and also no discharge hose friction, radiused bends or discharge outlet restriction.
Actual flow rates, under real operating conditions, can be drastically lower. Water must be lifted up and out of the bilge and then be pushed through lengths of hose to the discharge point. This resistance is called head. Head pressure is basically the wieght of the water and the frictional resistance of the hose, bends and outlet. Because most centrifugal pumps, like the Rule or Johnson pumps, have large internal tolerances to allow passage of small debris, their flow rate decreases dramatically with increased head pressure. Unlike a vane pump, which are less affected by head loss, the pumps rotor or impeller does not come in direct contact with the pumps walls and this can create cavitation if the head pressure is extreme enough. I'm sure many of us have seen the bilge pump kick on and then not actually suck any water but instead just create noise and bubbles in teh bilge. This can usually be eliminated by removing those ridiculous head boosting flow checks..
The output of many centrifugal bilge pumps can diminish by as much as half with only a few feet of head and can cease entirely at between 13 and 20 feet, depending on the size. Remeber head is not just the peak height of the hose it is the friction, height, bends and fitting restrictions.
Impeller or vane pumps and diaphragm pumps are less affected by head pressure than a centrifugal pump is and they may actually be more effective despite a lower GPH rating. Some of the vane/impeller pumps can sustain up to 70 feet of head pressure.
Another consideration is voltage. What is your pump rated at? Is it 14.2 volts or 12 volts. This will and can make a differece as the pumps motor increases and decreases output based on voltage.
There have been a few tests like the ones conducted by West Marine and Power Boat Reports. They rated pumps based on voltage and head pressure. Most manufacturers ratings (open bucket / no head ratings) were off between 10-50%. the output on average was reduce by about
5% for every foot of head pressure. With voltage the drops from 13.6 volts to 12.2 volts were another 15-30% off on top of the head pressure loss.
I guess what I'm getting at is this; unless you have a "monster truck" grade bilge system, like Franklin, do not count on anywhere near the face value rated capacity from your bilge pump.
Some other thing sto consider:
- Use smooth walled dischage hose as it has significatly less head resistance than does corrugated.
- If the discharge outlet is close to the waterline you must use a high loop in teh discharge hose to prevent siphoning. It is advised to add a siphon break at the highest point to ensure it won't siphon. Even with a igh loop a bilge pump can still siphon. I have been on more than one boat that self siphoned when under sail and power..
- Do not succumb to the temptation of using a "check valve" with a centrifugal pump. If you have 5' of height, in a 1" hose, the pump will more likely than not cavitate before it can throw open that check valve..
Disclaimer: Many years ago I used to sell a line of submersible pumps to the plumbing and well supply wholesale distribution channels..
