Propeller pairing for dual Flipsky 7070 outboard
I am tinkering with making an (eventually) autonomous electric outboard for my 12 ft semi v hull tin boat.
So far I have used two Flipsky 7070, 110kv motors, running submerged. Two Flipsky 75200v2 pro ESCs and a Dyness 38.5V 55ah lifepo4 battery.
At present it works as a life sized RC boat and I have tested it carrying between 1 and 3 adult passengers on 5 mile round trips.
I tried two different props, one was an APC marine 10x5 inch prop which worked fine but the top speed was about 2.5 knots so I jumped to the 10x8 inch version and that got me to about 4.5 knots at 60% throttle. I can cruise at 3.5 knots on 35% throttle.
My issue is that whilst it's all very efficient for slow speed cruising the 10x8 prop cavitates at 65% throttle. I want to be able to temporarily go to 8 knots if I have to move out of the way of another boat. Im moving around a sheltered bay which experienced up to 3 knot currents so I don't want to get in trouble.
I have tested the boat by going through a bridge choke point of a creek that experiences very fast flow of water, definitely above 3 knots and I go against that no problem with two adults on board but I cant get my over ground speed to be more than 4.5.
I have measured all speeds with a GPS speedometer so everything is over ground rather than through the water.
What would be a better prop setup? I am happy to sacrifice a little cruising efficiency in the name of temporary speed potential.