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Ok guys I have found something on youtube that is exactly what happens to me when I
try to use jackplates on my boat.
I wanted to try my engine with the centre of the prop about 1 inch below the bottom of the boat.

So I put it in gear pull in centre of river , trim right under and hit the throttle.
the engine bursts into life and so does a terrible banging at the transom.
Now I know 100% that my transom is good, there is no way that is flexing its a
new transom that is ultra strong and no flexing.
anyway what I am getting is the same as what happens in the video below at around 50 seconds
into the video.
the bottom leg seems to be smashing back and forth making a terrible banging sound.
you can see the lower engine mounts moving terrible.

please see this video (not my boat and engine) as its exactly whats happening to mine and I want to know
why and how to stop this, unless its just something a jacked up engine does.
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https://www.youtube.com/watch?v=Pn8BP-nWQFI
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pull the whole mid apart and rebuild, looks like mounts and bushings shot.

What bushings ? or do you mean top mounts ?
that is not my engine in the video, But my mounts on my engine are good but its doing the same thing.
Does an engine need solid mounts to combat this ? or is this just a phenomenon that happens when
jacked up high just before planning out ?
I have seen this on a few others too, but this vid shows it best.

maybe under load the mounts are giving way, ill check that out on mine again.

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what are the pitfalls of solid mounts or polyurethane mounts, I know that Austin mini poly bushes fit in the top mounts on the 56cu in motor as seen them before, a bit harder but don't wear out like rubber.

I've had this happen if prop has too much ventilation, try a different prop, like a thru hub or something with a ton of cup and see if it goes away.

When i had this i was using a 13 3/4x 23 omc silver streak( Srx).
I will try a ssr 13x19 next time and see what happens.
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When running over hub props is it best to have a drilled exhaust if on a fixed jack plate to let exhaust escape and not just smother the blades.
My jack plate was raising engine up about 6 inches and back around 4.5 inches

Surface running is new to me