Mariner 4hp 1985 thumping.

[MustardKeen]

Hi guys,
I have a 1985 4hp mariner outboard and when I'm travelling along, I get an intermittent thump from the outboard.
It occurs every 20 seconds or so.
I have tried it on another boat (Aluminium pontoon) and it does the same.
Here is a video.

If you listen you can hear the thump over the engine and you will see the transom thump every 20 seconds or so.
It happens about 4 times in this video.
I have it on a 10' portabote.
I should also mention that the propeller shaft is slightly bent.
Any ideas?

Thanks