How can you calculate water pump capacity for a jet boat?

I need to know how many litres per minute a jet boat pump is outputting. I know that the outlet diameter is 28mm and the thrust is 5kg. Power source is 2x7.2V 3300mAh batteries.How can you calculate water pump capacity for a jet boat?It depends on how you are figuring out the thrust, how much current your motor draws, or its efficiency. If you knew the wattage your motor uses and it's efficiency you could just calculate using the formula for pump head.