Hi ,
I want to calculate radial thrust vs voltage or current graph and create a plot or table for a mini water pump motor . I have 4, 12 v hobby DC motors with flow rate given as 700 L/H without any datasheet. What method should i use to measure the radial thrust ? .I am using this motor to create a water surface vehicle and need the thrust values in small error range, enough to get accurate actuation. Here is a photo of the motor

Link from where i bought the motor

 

You should be able to connect a hose to the outlet and raise the hose until your pump cannot overcome the gravity pulling on the weight of the water in the hose. Assuming minimal wall friction in the hose (don't use a tiny hose diameter), you can assume a column of water h meters above the pump outlet times the cross sectional area of the hose to get volume of water, then use gravitational constant to calculate force at the nozzle.

 

The motor current will vary dependent on Head/lift etc, the current will be highest with outlet unconnected, and reduce accordingly as the head back-pressure increases.

 

Is "Radial-Thrust" an actual measurement ?,
I've never heard of it.

You will be pushing XXX pounds of water out of the back of the Boat per minute or second,
at a particular velocity.
You may have to fabricate some reducing-nozzles to get the velocity up,
if You expect this Boat to run fast.
They need to be long, with a taper of no more than ~7-degrees.

But the biggest challenge You will have is hand-fabricating a
properly designed "Inlet-Scoop" that can efficiently "turn" the water running under the Boat
from a horizontal-direction to a vertical-direction,
with the least amount of turbulence disrupting the smooth-flow.
This is absolutely critical.
.
.
.

 

This is a very low power Pump.
4 of them will move roughly ~3-Pounds of Water per Second.

There are better ways of Powering your Boat,
unless it's supposed to be slow.
.
.
.

 

small error range, enough to get accurate actuation.

and if it bumps out of water ( don't misread "pumps out the water" ) . . . or passes over water with air bubbles in it

many pumps have the efficiency relative to the flow rate . . . which will vary from the still to settled flow rate