stainless-steel = running engine higher on transom

Hi all. This is my boat 16.5ft, with a Force 70hp (4500-5500 wot). The prop is a standard aluminum 10x15p. Note the transom is high on this boat, I draw lines on one of the pics attached so to have an idea of how the hight and trim is set.

A week ago I made some boat trail-runs(with no anti-fauling). I was getting 40.6mph @ 5600rpm. Was quite happy with this

Yesterday I tried the same setup but now I had the bottom of the boat painted with anti-fouling.
I was expecting some drag and loosing a couple of mph obviously, but I was very disepointed with the new reaction of the boat.
The boat ran fine till reaching 4000rpm, then at this point the prop seems to loose grip of the water and the motor over revs to 6000rpm. Top speed 26mph.
I cannot lower the engine more than that. So:
Do you think it is neccesary to use a stainless-steel prop at that hieght I am running? should I go with 15P as was before?
What do you think the problem may be ?

Re: stainless-steel = running engine higher on transom

If everything is really still set up the same, it sounds to me like you spun the hub on the prop, i.e., the hub is turning but the prop is slipping. I can't think of anything else that would explain such a radical change.

Do a search- there's a way to test this.

Re: stainless-steel = running engine higher on transom

Put a indelable line across the hub and prop run the boat and check if the mark is still lined up.

Re: stainless-steel = running engine higher on transom

I will check for it, I will let you know soon.