Sorry Derek far to technical for me. If you are referring to the bit sticking out in the centre that is part of the hub. Obvious not shown before it is on the inside. It will rest on the centre of the ball bearing, fixed to the shaft by a grub screw this will stop lateral movement, as the other wheel will be the same obviously. May need a washer or two for adjustment.
I have added wood to the outer edge of the deck, (the scuppers) it projects out from the hull, forming the knuckle. That's what I call the half round strip around the hull, another needed below the sponsons, this one just stuck on. There is a raised section over the rudder arm, with a grating. Small gratings can be a problem, here is a solution, not my idea but a good one. A plastic hit & miss air vent, D.I.Y. store. I cut it with a band saw, as the blade cools down, were a gig saw melts it to back again.
Milled the frames from square bar for the engine. Drilled the pivot hole & hole to take the bearing. Second photo shows the layout. The gearing is lower than me previous models at 5.7 to 1. I have used 3.5 to 1 before. We will see.