Hi Ray,
if you want to go real technical there are several data logger type units that will give you any info you want, some even live, I've just got an Eagletree system, the base unit does amps and volts and with a cheap add on does RPM as well, another add on allows GPS data to be recorded. I wouldn't call it easy to set up as I HAD SOME WIN8 ISSUES but once its configured its pretty much plug it in, get the data then download the data. Most of the cheaper units don't have any recording, just live data, some will record peak data only
20 Kgs seems rather heavy ?
What performance do you need, you probably need to put the biggest prop you can to the outboard then use a lower RPM motor, the motor will depend on the size of prop and how fast you want it to go
AS an example though a 40 mm scale brass prop with a 1000 Kv 28 mm diameter motor will on 12 volts push a light 2kg hull to about 15-20 KPH
Ive got another hull that is about 7Kg called Axebow,by Damen Shipyards that does about 12 Kph on two 750 Kv motors also on 12 volts.
A 20Kg hull woild be heavy tug boat or similar and would use 60-100 mm props at from about 1500-5000 rpm max (smaller prop higher RPM)
http://www.rcgroups.com/forums/showatt.php?attachmentid=4261513&d=1315289844The above link might be of use to help with the numbers
Nick