{{ summaryTitle }}
{{ summaryValue }}

{{ summaryLine }}

Lower net{{ resultsReady ? formatFlow(computation.values.lower_net_lph) : '—' }}
Use the exact pump model’s open-flow rating at the expected supply voltage.

{{ fieldIssues.rated }}

Manufacturer head at zero flow, at the same voltage as the open-flow rating. The example is not a real pump curve.

{{ fieldIssues.shutoff }}

From the bilge water surface to the highest discharge point; use the greatest lift over the clearing cycle.

{{ fieldIssues.lift }}

Total hose run, including the vertical section.

{{ fieldIssues.length }}

Use the actual internal bore. Narrow outlets and reducers need separate loss data.

{{ fieldIssues.diameter }}

Illustrative constant Darcy factors, not measured hose data. Corrugation and restrictions can cause much greater losses.
Short-radius elbows at K = 0.9 each. An outlet loss K = 1 is included.
count

{{ fieldIssues.bends }}

Water volume to remove; 0 checks ongoing inflow only.

{{ fieldIssues.volume }}

Assumes a steady rate throughout clearing; unknown or increasing flooding is outside this model.

{{ fieldIssues.inflow }}

{{ allowance }}%
Reduces modeled flow for the lower scenario. The 20% example is your assumption, not a guaranteed minimum.
%

{{ fieldIssues.allowance }}

Add manufacturer or measured K for restrictions beyond the counted bends and outlet. Default 0 adds nothing.
1

{{ fieldIssues.extra_k }}

{{ flow_loss }}%
User-supplied reduction in open-flow capacity; 0% preserves the entered rating. Do not infer this directly from voltage.
%

{{ fieldIssues.flow_loss }}

{{ head_loss }}%
User-supplied reduction in shutoff head; 0% preserves the entered head. Use matching manufacturer voltage data.
%

{{ fieldIssues.head_loss }}

Optional pump current while running. Enter 0 to omit the charge estimate; this does not predict battery runtime.
A

{{ fieldIssues.current }}

Installed-flow check

{{ comparisonLabel }}. Compare the exact manufacturer curve and test the installed discharge before choosing equipment.

{{ row.label }}: {{ row.display }}
Installation verification
  • Confirm both pump endpoints at the expected voltage; real curves may be nonlinear.
  • Check the complete hose route, outlet bore, airlocks, debris and actual discharge flow.
  • Follow pump and vessel instructions for wiring, circuit protection, siphon prevention, alarms and backup pumping. A check valve is not siphon protection.

This screening calculation does not establish safe capacity, compliance or emergency dewatering performance. A lower-flow allowance does not bound every failure.

{{ batteryNote }}

Basis: BoatUS system tests; EPA hydraulic loss equations; Rule installation manual.

Calculation method
{{ formulaEquation }}
  1. {{ row.label }} {{ row.value }}

    {{ row.note }}

The chart renderer is unavailable. The plan and ledger still contain the flow values.

Measurement Value Unit Copy
{{ row.label }} {{ row.display }} {{ row.unit }}
Sump Pump Size Calculator Size a sump pump by combining peak inflow with total dynamic head, then compare manufacturer-rated capacity and check cycling risk at that duty point.
Water Well Pump Size Calculator Size a well pump at the required flow and total dynamic head, then screen well yield plus pipe velocity, tank drawdown and pump-curve fit.
Tank Capacity Calculator Calculate ideal tank capacity, partial liquid volume, headspace, and reserve allowance for rectangular or cylindrical geometry.
Battery Charge Time Calculator Estimate battery charge time across a chosen state-of-charge window using charger limits, efficiency and taper plus realistic derating and overhead.

Water already in a bilge and water still entering it create two different demands on a pump. The pump must discharge faster than the incoming flow before it can reduce the standing volume. A stream at the outlet shows that water is moving, but it does not establish that the water level inside the boat is falling.

Routine bilge pumping deals with water collected from rain, spray and minor seepage in an intact boat. When comparing a replacement pump or a different discharge route, the useful quantity is the water delivered through the complete installation. Flow is a volume per unit of time, commonly expressed in litres per hour, litres per minute or US gallons per hour. A manufacturer's open-flow rating describes a particular test condition, not every installation.

Open-flow rating
The stated delivery at zero head and a specified supply voltage. Connecting a hose and lifting the water changes the conditions under which the pump operates.
Head
Pressure expressed as an equivalent height of water. The pump must provide enough head for the vertical rise and for resistance in the hose and fittings.
Shutoff head
The head at zero discharge. It is an endpoint of the pump's performance curve, not a recommended working lift.

A long hose, a small internal bore and restrictive fittings can reduce delivery even when the pump itself is unchanged. Hose diameter means the opening available to water, not the outside diameter. A fitting sold for a particular hose size may have a smaller opening, so fitting losses cannot be judged from the hose label alone. Supply voltage also matters, which is why open-flow and shutoff data need to describe the same pump under the same electrical conditions.

The vertical rise and the hose length are separate measurements. Lift begins at the bilge water surface, while friction acts along the entire hose route, including its vertical section. As the water level falls, the required lift can increase. A high discharge loop may also require more lift than the outlet's height suggests.

For clearing a standing volume, only the delivery left after subtracting continuing inflow is available to lower the water level. A small surplus can mean a long clearing time. Calculated delivery helps compare assumptions, but measured discharge from the installed system remains necessary. Neither a label rating nor a favorable estimate establishes the ability to cope with hull damage or unknown, increasing flooding.

How to Use This Tool:

Gather the pump's endpoint data and measurements of one discharge route. Results update as you change the inputs. The starting values illustrate a calculation and do not identify a real pump.

  1. Enter Rated open flow and Shutoff head for the exact pump model at the expected supply voltage. Obtain matching data rather than combining ratings from different voltages. Changing a unit converts an existing numeric value rather than merely changing its label.
  2. Enter Vertical lift, Hose length and Hose inside diameter from the installation. Use the greatest lift from the water surface to the highest required discharge point during clearing, the complete hose run, and the actual internal bore. No siphon assistance is credited.
  3. Select the Hose friction scenario and enter the number of 90° bends. Both hose choices use illustrative factors. Under Advanced, add supported Additional fitting loss K for restrictions beyond the counted bends and the outlet loss already included.
  4. Enter Standing water and Declared inflow. Use zero standing water to compare ongoing inflow alone, or zero inflow to assume no more water enters.
    Do not stop necessary pumping or alarms to measure Declared inflow. Use existing observations or a supervised test. An unknown or increasing rate cannot be represented reliably by a single steady value.
  5. Set a Lower-flow allowance to examine a reduced-delivery scenario. The initial 20% is an assumption, not a verified reserve. Advanced also offers independent Flow reduction and Shutoff reduction adjustments when you have a reason to change the entered endpoints. Do not infer these percentages directly from voltage.
  6. Review Estimated installed flow and the net-removal and clearing results in the Installation report. The Installation ledger records the assumptions and calculated head losses for comparison with pump data.
    Installation inputs need attention means a hydraulic entry must be corrected before results return. Follow the field message and check its unit. An invalid optional Operating current is different: it omits the charge estimate without hiding the hydraulic results.

Interpreting Results:

The displayed Screening range runs from the lower-flow scenario to the modeled installed flow. These are two estimates, not measured minimum and maximum outputs or a statistical confidence interval. Actual discharge can fall outside the range.

Modeled net removal and Lower-scenario net removal subtract declared inflow from the corresponding delivery. Positive net removal reduces the standing volume under the stated assumptions. Zero means delivery only matches incoming water; a negative value means water accumulates despite the pump's discharge.

For a positive standing volume, Cannot clear means no finite clearing time exists in that scenario. The ledger marks the corresponding unavailable value as Not available. With zero Standing water, the clearing times are zero because no initial volume was declared, even when the flow comparison says the pump cannot outpace inflow. That zero is not an assurance about future water accumulation.

Main flow results follow the Rated open flow unit. The flow comparison is a comparison of rates, not a measured pump curve. The report and ledger retain the calculated values if the chart cannot load.

Before choosing equipment, compare the result with the exact manufacturer's head-flow curve and test discharge through the actual installation at the expected supply voltage. A result that exceeds declared inflow confirms only the arithmetic of that scenario, not the boat's pumping safety.

Technical Details:

Centrifugal-pump delivery depends on the balance between the head the pump can provide and the head required by the discharge route. Increasing flow raises hose and fitting losses. This screening model represents pump head with a straight line between two endpoints and represents installation losses with constant coefficients. The resulting operating point is an approximation, not a fitted manufacturer curve.

Pump head and discharge resistance

At a positive operating flow, pump head equals vertical lift plus hose and fitting losses. All head terms in this equation are in metres.

H0 (1−qq0) =z+ (fLD+K) v22g

Here, q is modeled discharge in cubic metres per second, q0 is adjusted open flow in the same unit, and H0 is adjusted shutoff head. Vertical lift is z, full hose length is L, and internal diameter is D, all in metres. Water speed is v in metres per second. The dimensionless coefficients f and K represent hose friction and fitting losses. Gravity g is 9.80665 m/s².

Pump endpoints and water speed

Let R be Rated open flow converted to L/h and Hs be the entered Shutoff head converted to metres. Let rF and rH be the flow and shutoff reductions expressed as fractions, so 10% becomes 0.10. The two endpoint adjustments are independent.

q0= R(1−rF) 3600000 H0=Hs (1−rH) A=πD24 ,v=qA

A is the internal cross-sectional area in m². Convert the entered bore from millimetres to metres by dividing by 1,000 before calculating area. Dividing L/h by 3,600,000 converts it to m³/s. Zero endpoint reductions preserve the entered pump data; neither reduction is calculated from a voltage reading or from Operating current.

Hose and fitting losses

Hose friction uses the Darcy–Weisbach relation. Fitting losses use a total loss coefficient K, including one outlet, the counted short-radius elbows and any additional entered loss.

K=1+0.9n+Kextra hhose=fLD v22g hfittings=K v22g Hsystem=z+ hhose+hfittings

The bend count is n and Additional fitting loss K supplies Kextra. These losses use the water speed in the entered hose bore. A reducer or narrow outlet needs suitable loss data; changing the hose diameter alone does not describe a separate restricted opening.

Assumption Value Meaning
Smooth-bore hose f = 0.03 Illustrative constant Darcy friction factor.
Corrugated hose f = 0.06 Illustrative higher-friction scenario, not measured hose data.
Outlet K = 1 Always included, including with zero hose length and zero bends.
Each short-radius 90° bend K = 0.9 Added once for each counted bend.

The friction factors do not change with flow regime, water temperature or measured hose roughness. Corrugation and restrictions can create losses larger than these assumptions suggest. The hydraulic equations provide the loss framework; they do not validate the selected constants or the linear pump approximation.

A smaller bore raises water speed at a given flow and increases the length-to-diameter ratio in the hose-loss term. Increasing hose length or adding bends therefore changes the operating point. Delivery is solved again rather than reduced by a fixed percentage per metre of hose.

Solving the operating point

Combining the straight-line pump relation with the velocity-squared losses gives a quadratic equation. Define α as the combined resistance coefficient, β from the adjusted pump endpoints, and Δ as the head remaining after vertical lift.

α= fLD+K 2gA2 β=H0q0 ,Δ=H0−z αq2+βq−Δ=0

The nonnegative discharge uses the following solution. This form avoids subtracting nearly equal terms when the remaining head is small.

q={ 0 when Δ≤0 2Δ β+β2+4αΔ when Δ>0

Lift equal to or greater than adjusted shutoff head produces zero modeled discharge. Hose and fitting losses are then zero, while total system head remains the entered lift. Above shutoff, this is a no-discharge condition, not a positive-flow balance between pump head and system head. With a 4 m adjusted shutoff head, a 4 m lift already reaches the cutoff.

Even with zero lift, zero hose length and no bends, the included outlet loss remains. The modeled flow therefore need not equal the adjusted open-flow rating.

Lower flow, net removal and clearing

Convert the solved discharge back to L/h, then apply the lower-flow allowance a as a fraction. Let F be Declared inflow in L/h. Net removal is the delivery remaining after that inflow is subtracted.

Q=3600000q Qlower=Q(1−a) Nmodeled=Q−F Nlower=Qlower−F

The allowance reduces the solved flow; it does not solve a second pump curve. Reported hose loss, fitting loss and total system head belong to the modeled operating point. Standing volume and declared inflow do not change that point in this fixed-lift approximation.

For standing volume V in litres, the modeled clearing time t in minutes follows the conditions below. A positive volume needs strictly positive net removal.

t={ 0 when V=0 60VQ−F when V>0 and Q>F no finite clearing time when V>0 and Q≤F

Use Qlower in place of Q for lower-scenario clearing. Set F to zero for the comparison with inflow stopped. Doubling V doubles a finite clearing time without changing pump delivery. When delivery only slightly exceeds inflow, the small net removal makes the time highly sensitive to either estimate.

A valid positive Operating current, I in amperes, gives ideal clearing-charge demand C in ampere-hours using the modeled clearing time, not the lower-scenario time.

C=It60

Zero current omits this estimate. No finite charge estimate is available when a positive standing volume cannot clear. For zero standing volume, a supplied current gives zero clearing charge. Battery capacity, usable discharge limits and other electrical loads are not included, so ampere-hours here do not predict battery runtime.

Flow-comparison boundaries

The comparison uses unrounded flows and does not depend on standing volume. Q is modeled installed flow, Qlower is the lower-flow scenario and F is declared inflow, all in L/h. Equality with inflow does not count as outpacing it.

Message Condition
No modeled discharge Q = 0. This condition takes priority.
Lower scenario exceeds inflow Q > 0 and Qlower > F.
Lower scenario cannot outpace inflow Q > F and Qlower ≤ F.
Modeled flow cannot outpace inflow 0 < Q ≤ F.

Units, accepted bounds and precision

Flow inputs accept L/h, L/min or US GPH, meaning US gallons per hour. Lengths accept metres or feet, bore accepts millimetres or inches, and standing volume accepts litres or US gallons. The exact conversions are 1 US gal = 3.785411784 L, 1 ft = 0.3048 m, 1 in = 25.4 mm and 1 L/min = 60 L/h. Imperial gallons are not supported.

The bounds below are inclusive after conversion to the listed units. They define accepted calculation inputs, not recommended pump sizes or verified installation dimensions.

Input Minimum Maximum Unit or condition
Rated open flow1100,000L/h
Shutoff head0.1100m
Vertical lift and hose length, each0100m
Hose inside diameter5200mm
Standing water0100,000L
Declared inflow0100,000L/h
90° bends020Whole-number count
Lower-flow allowance, flow reduction and shutoff reduction, each090%
Additional fitting loss K0100Dimensionless
Operating current0100A; zero omits charge

Calculations use unrounded values throughout. Displayed results use at most two decimal places, without requiring trailing zeros. A displayed zero or two matching rounded flow values can conceal a small nonzero difference, so the comparison message and availability of clearing time follow the full-precision calculation. Unit changes do not deliberately round intermediate values.

Limitations:

This calculation compares one pump and one discharge route under stated assumptions. It does not establish safe capacity, regulatory compliance, emergency dewatering performance or a manufacturer-approved pump selection.

  • Lift, pump delivery and inflow remain fixed throughout clearing. Changing water levels, priming, switch cycling and residual water are not simulated. A zero-time result does not mean a real bilge can be pumped completely dry.
  • Airlocks, debris, blocked inlets and other obstructions are not detected. Passing the input limits also does not verify that the entered hose length, lift and bore describe a physically consistent installation.
  • Follow the pump and vessel instructions for wiring, circuit protection, alarms, backup pumping and siphon prevention. A check valve is not siphon protection. Do not remove protective discharge arrangements simply to obtain a lower lift or loss estimate.

The hydraulic arithmetic runs in the browser. Input values and units can be retained in the page address and restored on reload, so sharing that address can disclose the scenario. The optional chart also loads an external resource.

Worked Examples:

These cases use illustrative pump endpoints. Their numerical results describe the screening model, not measurements of a particular pump.

Clearing 100 litres while inflow continues

Use 4,000 L/h open flow, 4 m shutoff head, 1 m lift and a 3 m hose with a 25 mm inside diameter. Choose smooth bore, two bends, no additional K, no endpoint reductions and a 20% lower-flow allowance. The area is about 0.000490874 m² and total fitting K is 2.8. Solving the head balance gives 2,398.75 L/h, with about 0.338203 m hose loss and 0.263047 m fitting loss, for 1.601249 m total head. The lower scenario is 1,919 L/h. With 1,000 L/h declared inflow, net removal is 1,398.75 L/h modeled and 919 L/h lower. Clearing 100 L takes about 4.29 minutes modeled or 6.53 minutes lower, compared with 2.50 minutes if inflow stops. The message is Lower scenario exceeds inflow, subject to checking actual installed discharge.

More resistance and reduced pump endpoints

Keep the same pump entries and route dimensions, but choose corrugated hose, add K = 2, reduce open flow by 10% and shutoff head by 20%, and enter 500 L/h inflow. The adjusted endpoints are 3,600 L/h and 3.2 m, with total fitting K = 4.8. Retaining 100 L standing water and the 20% allowance gives about 1,778.12 L/h modeled delivery and 1,422.49 L/h lower delivery. Clearing takes 4.69 or 6.50 minutes respectively. At an entered operating current of 5 A, the modeled clearing-charge demand is about 0.39 Ah. That is charge for this estimated clearing event, not a battery endurance prediction.

The modeled estimate clears, but the lower scenario does not

Return to the first case's pump and hose assumptions and increase declared inflow to 2,000 L/h. Delivery estimates remain 2,398.75 and 1,919 L/h because the hydraulic calculation still uses the same fixed lift. Modeled net removal drops to about 398.75 L/h, extending the 100 L clearing time to 15.05 minutes. Lower-scenario net removal becomes about −81 L/h, so Lower-scenario clearing reads Cannot clear and the comparison says Lower scenario cannot outpace inflow. Check actual discharge rather than relying on the positive modeled surplus.

References: