Are you sure it's overreading ? I had a new impeller fitted on my ST50 and the log was underreading by a factor of about 0.75. You can recalibrate the ST50 to take this into account. Can you do this with the Stowe ?
The impellor causes electro-magnetic impulses [you will see the magnets in the impellor] and so it is the pulses that determine the speed rather than the resistance [length] of the cable. There is adjustment on the reverse of the box and also on the repeater.
The people at SIMRAD who [ultimately] took over the Stowe business have experts in the Stowe instruments; ring up and ask for them as they are very helpful.