I cannot see a revolving pramhood being a good thing, either, and I cannot see how it could sensibly be done.

I think David's right: this small boat just needs a rectangular hood, it could be either soft or hard, over the offset hatch.

David wrote:

My "green box" was only 25mm wide - 500mm inside, 550mm outside diameters. That was wide enough.

Why as large as 660mm opening? It should be only your shoulder width, unless it's to be the only way in and out of the cabin.

Thank you for the response. I very much appreciate your help.

I attempted to measure my shoulders with some help from my family. I think they are about 520mm at the widest. Some of this may be due to me being overly fond of a  particular cheese cracker but I think most of the width is from bone, not extra insulation. I will plan on a 520mm opening.

Some how I got a 50mm thickness from a 550mm OD and 500mm ID. I guess I forgot that the radius is only half of the diameter! Thank you for the correction. I updated my sketch.

My boat is very small, has a relatively large cockpit and also has a keel trunk in the middle of the cabin. I am having a hard time imagining how I can add a second cabin entrance after adding a circular hatch. Even getting the hatch installed somewhere near the centerline is going to be tricky. Do you have any suggestions?

I need to find a way to wash my boat in storage. So much dirt.

I made a list of 21 boat-related items to do this winter. I tried to put them in a priority order. The first on the list is to address the checking on the mast partners. I really hope these are just cracks in the first layer of the plywood that can be repaired with some sanding and a layer of fiberglass.

But for some reason the only thing I can actually think about is item number 20 on my list. I don't urgently need a pram-hood but I very much want one. This is also a tempting project because I think I can build it mostly in my garage and then take the entire assembly over to the boat and glue it in place.

I came up with dimensions for the rings after studying 'Tystie's Pram-hood' by David Tyler, the photos on Annie Hill's FanShi build and, of course, PJR.

I think I want the actual opening to be about 660mm in diameter and the rings that make up the device approximately the dimensions in the attached drawing.

Red box: The bottom ring that will be glued to the deck of the boat.
Blue box: The two-part rotating carrier to which the hinges for the actual pram-hood will be attached.

Opinions on the proposed dimensions would be very much appreciated.