Saving Old Seagulls

continuing saga of my tc102 ( posts..tc102 cooling problem)
After a delay of a couple of weeks, I finally got to test the engine out in the river. I have now found that when the engine is in neutral the cooling water flow is good and just warm . But when in gear the flow reduces and the engine starts to overheat. Against an estimated 0.7 into current, I cannot get enough revs to raise the water flow. Its almost like the prop is interfering with the water inlet. Surely this cannot be right? Has anyone experienced this?
Red.

We need to see a picture of the prop (plan view) & the lower unit to ensure you have the right combination.

Thanks Charles, I will endeavour to post a picture, though I'm not sure how to do that yet. Bear with me.

Yes got that just fine, photo of engine will be on the way in a couple of days. I left it on the boat.
These swans are on the river Arun in West sussex by the way.