I am just finishing a fluid mechanics course this semester where we talked about modelling things that you are talking about. The first thing you want to do is to determine what parameters are important to you and how they depend on eachother.
Take thrust for example.
You want to find the thrust generated by a firehose. Now you must determine what parameters affect the thrust generated by the hose. Pressure would be one, velocity, diameter, legnth, density, and viscosity. Now you have to determine what parameters are going to be the same for the model and the real life situation and what parameters you can control by changing. Well I would assume you are going to using the same type of fluid in your model as would a real firehose so you can get rid of the density and viscosity parameters.
However, if you wanted to you can test the thrust using a different fluid then what would be used you would simple have to set up a relation between the two.
By understanding the relations of the parameters you can set up a much more meaingful experiment.