In my previous seagoing life, having done the trip from Chatham to Ramsgate numerous times, not always by choice as our destination was Oostende with Ramsgate being the Bolthole.
We used to average 12 hours from Chatham to Oostende. Thats in a boat with an engine that provided a cruising speed of 7 knots over the ground with no wide or tide.
Times from Chatham to Ramsgate varied between 6 and 8 hours, again depending on tide and if we could use the inshore passage (Gore Channel) when approaching Margate.
The advise to choose forecasted winds of force four and below is often stated but you will also need to plan the direction of the wind and tide as I have done the section from Sheppy to North Foreland on an outgoing tide with easterlies blowing force 4 and had waves breaking over our bow and drenching the wheelhouse.
Generally, the most lumpy bit is around the North Foreland and once clear of that things calm down as you head up the Thames Estuary.
Its more than doable in a Broads style cruiser provided you choose the right conditions and your happy with the servicability of the engine and fuel supply.
One trip I did in a 90' 100 yr old Dutch Barge was quite interesting. We left Neuiport in Belgium in a dead calm with the sea like a mill pond.
Halfway across the wind and sea picked up and we were bouncing around all over the place for several hours.
Finally got round North Foreland and all was calm and peacefull again all the way into Chatham. That particular crossing took just under 24 hours.
Approaching Broadstairs