You have one blade too many on that prop . No way you can turn a 13x11- 3 blade with 13 hp unless it was in a 400lb skiff . At 2100 rpms your are not even getting 13hp. That is a huge prop for a small boat .

My figures were for a 2 blade 13x11 , that extra blade needs more hp to spin it.

It is probably a motor boat prop too , and has huge surface area .

Get a 12x6 sailor prop (2 blade ) and you will be happy for the rest of your life .
You will sail faster too (less drag )
1216 shaft rpms should give you 6+or- knots with a 12x6 ( 6" pitch ) prop , so if you throttle back a little 5.5 knots is easy .
You know you can have props re-pitched , so get a 2 blade that is close and try it and if it is not exact either pitch it up or down until you can get 5.5knots at 2800 rpms .
There are a lot of other factors involved , but this gets you in the ball park .
I have re-proped dozens of boats , half were right on the 1st try and the others took one more pitch change to get close enough . planing motorboats are a lot harder to get right .