Back on the Oxford Canal, immediately the noise returned. I am convinced it is turbulence , cavitation noise. As suggested I wrapped rope around the bottom plate as shown but that made no difference. Also have released the vacuum by nicking the seal but to little effect. There is no noise at 1000 revs but put it higher and back it comes. Go into deeper water and it greatly reduces. I’ve been asked for some measurements,
top plate to bottom plate is 30 cm
tip of prop is 18 cm below water line
Prop is 8cm from bottom plate
any more suggestions please