I was making a general observation about the relative capacities of pipe sizes (or tubular tankage) - wherein a small increase in pipe diameter (ie: from Ĺ to 3/4" = 50% increase) results in a much larger (ratio squared) delivery
(or storage) capacity. This is a basic principle of geometry, which is always valid, and has nothing to do with pump type.
A pipe or hose has a circular cross-sectional area.
The area of a circle can be found by multiplying pi (3.1416) by the square of the radius (Ĺ diameter).
Area (thus capacity) of Ĺ" diameter (1/4" radius) Pipe:
3.1416 x (0.25 x 0.25) = 0.19635 square inches
Area (thus capacity) of 3/4" diameter (3/8" radius) Pipe:
3.1416 x (0.375 x 0.375) = 0.4418 square inches
0.4418 ų 0.19635 = 2.25 (1.5 squared)
Doubling a pipe size quadruples itís cross-sectional area.
Given the fairly low flow rates of engine cooling water
systems, I have little doubt that a Ĺ" supply (thru-hull & hose) will be adequate for the purpose
(as Richhh implied).
On the other hand:
- A 3/4" Thru-hull will deliver 125% more water than a Ĺ"; providing reserve capacity to overcome possible obstructions (fouling), and permitting manifolding for other purposes.
- A 3/4" thru-hull is more physically substantial, reducing the likelihood of accidental mechanical damage.
- A 3/4" thru-hull inlet will produce a more laminar flow of water than a 1/2".
- There is little cost premium associated with increasing to 3/4"
There are two important factors that limit pump efficiency: frictional losses & fluid flow characteristics.
Frictional losses are directly proportional to: Length of pipe, Flow rate, Pipe Diameter, & Fluid Viscosity.
Each PD pump will have a specified NPSHr (net positive suction head
requirement), requiring that restrictions and pressure drops in the suction lines be avoided.
Fluid flow losses in pipes occur in two principle modes: Laminar &Turbulent. Smaller inlet orifices (thru-hull) tend to cause more turbulent fluid flow, which can lead to pump cavitation.
Some basic Pumping system design criteria:
1.Restricting the inlet port size and the inlet pipe ID will cause cavitation and damage the pump.
2. It is best to have a straight run of pipe leading into the pump inlet.
3. The NPSHa must be greater than NPSHr of the system.
NPSHr (Net Positive Suction Head
Required) determines the required suction head (maximum suction lift). It is inherent to the design of the pump and is measured in feet of water. NPSHa (Net Positive Suction Head Available) is determined by the pipe system on the suction (inlet) side of the pump.
You should configure your system so NPSHa > NPSHr (the head available from the system is greater than the head the pump requires). Failure to meet this requirement will cause reduced flow rate, cavitation, and vibration of the pump.
I donít disagree with Richhh, inasmuch as thru-hull & pipe size isnít really critical in this application - but a larger thru-hull and pipe certainly canít hurt.