Coxmate has made an alternative to the speed measurement impeller for its top of the range SCT unit.

The GPS attachment mounts behind the cox and plugs into the base of the SCT magnetic mount.
The Coxmate manufacturer has this to say comparing the impeller and GPS measurement systems.

GPS vs Impeller Speed sensing
Now we offer both speed sensing options, we can offer some advice which will not be seen as having a commercial bias.  Like many technological choices, there is not a simple answer. Each has its merits. We can summarise the argument:

• GPS is simpler to install, produces no drag, is not prone to interference from weed or damage from impact – on and off the water, and requires NO calibration. A GPS gives land speed. It can easily be moved between boats.
• Impellers give a speed reading every few milliseconds, compared to around a second for GPS, they use less power – GPS uses comparable power to the audio amplifier. This high frequency speed reading enables stroke velocity profiles to be analysed – not possible with GPS. Impellers give speed through the water which is a better indication of rowing performance. Impellers are not affected by overhead line of sight obstructions – bridges and city high rise can affect performance of GPS.

The main questions in making your determination are:

• Are you rowing on moving water?
• Is water free of weed or similar?
• Are you interested in (and will you do it) analysing the stroke velocity profile?
• Do your crews look after equipment to minimise damage to fin?
• Do you have time and a measured course for calibrating speed sensor?
• Do you use speed sensor on same boat – GPS can easily be transferred between boats?
• Is there restricted line of sight to sky?

If all the answers are all YES then an impeller is the way to go. If they are all NO then a GPS is. For most people the answers will not be all YES or NO.

My experience is that despite good intentions, most impeller sensors are not calibrated by users, and few coaches use the stroke profile analysis capability of the SC, and so for most users the convenience of the GPS is hard to beat.

Some people will argue there is a slight lag with the GPS – due to the delay in getting speed signal. Whilst this is true the Coxmate SC/SCT firmware has compensation for this. Finally to avoid giving you an easy economic choice – they cost about the same for the SC/SCT! I would also like to add that we are continuing our developments with impellers to make them smaller – less drag and less chance of damage. We do not see any end to the use of impellers – and GPS is unlikely to take over for small boats due to their power requirement.

GPS unit GPS and SCT on magnetic mount