I'm going to stick my neck out and hope it doesn't get chopped off.I think you are going to find that you are very much over propped.The 20" is what's causing it to clip the tab.
The min -max rpm's on your Merc 80HP are between 4800 and 5200 wide open.Me thinks,unless the Shakey is very light,you will be hard pushed to get up to 4800rpm's,and that will put strain on the engine.
I have a two blade aluminium 13.25x19" which I run on "NO-KIT" when I'm on my own and it pushes up to 5300rpm's,a wee bit over.When I have the girls on,I run a three blade aluminium 13.25x17" which just makes it to 5100rpm's.The three blade 19" pitch will only push up to 4900rpm's with four on board.
Another consideration is the s/s is stiffer than aluminium and needs less pitch to reach the recommended rpm's.
So as it was a Christmas present, get one of those Yamaha offset trim tabs.
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