Training aid idea

This not intended as something to be a commercial product just a bit of fun. It stared with thinking about speed through tacks and wouldn’t it be good if to knew that you were slowing down, by how much and then use this information to improve your technique. I figure that a system that would work would be an audio system, initially thinking that beeps would get closer together as you sped up and further apart as you slow down. This would only really work if it was comparing relative speed over the past den seconds say. This would ensure it works at two knots and twenty knots. 

I have started messing about with Arduino programming and it seams relatively easy. I’m thinking supper simple. You have a piece of rope tie it to the rudder stock and throw it overboard, near the boat end is a small container with the electronics and the speaker, the outboard end has a spinner, think Walker log. 

I appreciate there are all sorts of fancy pucks with logging and readout but this is just about in the moment understanding what is happening to your speed without looking at a display 

Any thoughts would be appreciated, again nothing commercial just a bit of fun.