This is a subject that can really confuse some people when it comes to purchasing a pump to run their highbanker. Often flow rating is referred to by the HP of an engine "beware of any seller of supplier who refers to Horse power" when talking about water flow it has zero rating of flow and any engine can run any pump to some degree.
HP or horse power is the driver not the mechanism that pumps water so you want to focus more on the fitted pumps output rating not HP.
You have two primary factors to consider, you would opt for low HP if you want something small and easy to carry but with small often comes lower efficiency so there is a trade-off and some exceptions to this rule.
Hi HP generally means big and bulky so from a 2.5hp engine to a 8hp engine we're looking at a difference in weight of 5 to 55kgs dry. Quite the variation when considering carrying this into the bush.
I personally have no issue carrying anything up to 6.5hp at a dry weight of 26kgs because i like the economy of a large engine running low rpm all day on a tank of fuel.
Pumps and flow ratings
Manufacturers will make claim to ratings to sell a product, its not uncommon to see a listing on ebay for a 1" pump that put out 18,000 litres per hour. In order to put out that level of water through a 1 inch pipe that pump would have to produce over 100 PSI so you have to ask yourself "How can they make and sell a pump for under $150 that puts out that much psi?"
The answer is usually they can't, not at 2.5HP or even 3HP for a 1" pump. 4HP and we are in the realm of possibility. Shop around and compare some more respected brands like Honda, Thornado, Rato to name a few. You will find most 1" pumps put out about 9000 litres at 2.5hp and up to 12,000 litres for 3hp engines. Again there are some exceptions to the rule but they are never cheap.
Here's a table to gauge capacity of flows
How much Flow do i need for my Sluice or Highbanker?
For your average 25cm wide sluice you will want to have available in flow around 14,000 litres per hour, for a larger sluice 300-400 wide i would recommend not less than 15,000 and as high as 34,000 litres per hour.
This accommodates common lift, pressure requirements for cleaning in your spray bars and assuming you will block some of your take with debris at some point.
There are a few different types of pumps and really they can be broken down three categories, High Flow, low flow high pressure and hybrid hi flow high pressure.
In the picture below we have at the top a Honda hi flow pump rated at 16,800 litres per hour, then to the left a small 12,000 litres per hour, then to the right of it 16,000 litres per hour and right at the bottom the hybrid turbo pump 24,000 litres per hour, commonly used in applications that require great pressure and volume like small dredges and highbankers that need lots of water.
These are common small engine pumps using engines under 3hp.
Here we have the more common 6.5 HP engines running either Hi Flow pumps or Hi pressure pumps. The Hi Flow pump runs a single large impeller and simply moves allot of water really fast without much head height and is susceptible to friction loss.
While the hi pressure pump runs twin impellers which create a mass of pressure inside the smaller housing which limits flows but can pump straight up a hill very well, this type of pump is more suited to fire fighting or small commercial dredges.
With a hi pressure twin impeller pump it is crucial to make sure you do ot suck any stones into the impeller or you will ahve to disassemble the whole assembly to get the pump flowing again, high flow pumps do not suffer form this as the impeller veins are wide open.
For a highbanker you will want the hi flow pump it is the most economical option and you can create your own pressure where you need it at the spray bars.