Still working on them but I won't release Probes with Speed until I develop and test a design that is reliable and consistent.
Having said that...there are plenty of "Dive Curve" charts available. Typically those charts are used to determine depth. If you know the line-out and measure the down-speed you can look on the chart to determine the depth. Those same charts can be used in reverse. In other words, if you know the line-out and Smart Troll tells you the depth then you can look on the same chart and determine down-speed.
If your setup is 50 feet deep and you are traveling with a SOG of 2.5 MPG in the same direction as a 0.5 MPH current then your down speed would be 2.0 MPH. If you turn around 180 degrees so you are now going against the 0.5 MPH current with the same 2.5 MPH SOG then your down-speed would be 3 MPH (1 MPH faster just by changing direction). Even without measuring down-speed you would know it changed because with the same setup your lure would be 20-30 feet less deep and there could be a depth change of even greater than 30 feet in many cases.
In this scenario if you slow the boat down until you achieved the same 50 feet of depth then you would also be at the same down-speed. So, you would have to slow the boat to 1.5 MPH SOG to achieve the same 50 foot depth...which would also be the same 2.0 MPH down-speed (1.5 MPH SOG + 0.5 MPH for the current you are traveling against).
Pretty dramatic...in one direction your SOG is 2.5 MPH and in the opposite direction your SOG would be 1.5 MPH just to get your lure to the same depth.
For any particular setup if you duplicate the depth then you are also duplicating the down-speed. Documented results and feedback from Captains using Smart Troll have shown that as little as 0.5 MPH change in speed will affect lure depth up to 30 feet and maybe even more for some setups.
Attached is a chart you can use to help determine the depth of your downrigger ball when trolling. The depth of the ball is determined by the angle of the cable and line-out...this chart helps you to indirectly measure the angle. The chart assumes the DR boom is 3 feet above the surface of the water. If yours is different or if you want numbers for different amounts of line-out let me know and I will add to the chart for ya. Please note that I rounded the numbers to avoid decimals and small fractions. The first column represents the difference (in feet) between the point where the cable would enter the water if the boat was not moving (cable straight down) and the point where the cable enters the water when trolling.