Hey Floris,
congratulations to your Parker 21.
My 21 is lying in Lemmer, Bruce told me there shall be about 15 in the netherlands and 15 in germany.
On mine I use the stb. sheet winch on deck for keel hauling up. I use the handle from the winch - no problem. Do you have an extra winch for hauling up the keel?
The mast is about 8m long and it stands about 1m above waterline on deck so hight is about 9m.
I have a 4hp Mercury too and I think you are right it‘s not too strong to use it in Waddensea.
But the board for the motor is only screwed to the deck with four thin screwes on its downside - not perfect for powerhandling. I think the Sikka will do most of the powerhandling so 8hp is a lot for it.
And there ist not much place to stern. And the hole in the hull is just wide enough to get the prop form the 4hp through. Wirh a bigger diameter....
But you can get the same Mercury with 6 hp. I don‘t know if it‘s possible to modify the 4hp to 6hp.
I use the longshaft too. It is just long enough. The short shaft will not work well I think.
The draft of 21 is about 25cm with keel and rudder hauled up. The deepest point of the motor will not be deeper (on mine).
Best regards,
Matthias