Hi all, my first post on here and just wanted to check something I'm working on for a project. The project is to use a pump to create a jet of water to manouvre small boat. I have found a formula to work out the jet reaction force in a fire fighting text book which gives: R = 0.157 * P * d^2 where R = Reaction force in Newtons P = Pressure in bar d = nozzle diameter in mm To avoid buying and testing different pumps and nozzles empirically to find the best reaction force I wanted to link a given pumps flow and pressure to find nozzle diameter and then using this and the pressure and the above formula to get the reaction force. I have been using L = 2/3 * d^2 * sqrt P where L = flow l/min d = nozzle diameter in mm P = pressure in bar re-arranged to give d = sqrt (L/ (2/3 * sqrt P)) this gives d in mm then putting this and the same pressure back into the above reaction formula to get reaction in Newtons. as an example: a pump giving P = 1.52 bar Q = 450 l/min d = sqrt (450/ (2/3 * sqrt 1.52)) = 23.4 mm then R = 0.157 * P * d^2 = 0.157 *1.52 *23.4^2 =130 N Questions: Are the formulas i'm using valid? If so could someone show me how to get to them from first principles or just tell me they're ok :) If not then why? Other losses: I have thought about losses due to pipe work friction but if the nozzle is underwater what sort of losses could I expect due to reduced flow because of higher pressure at/just after outlet Thanks in advance for any help.