In the video clip I watched, the sub was not underway. It was sitting still and then submerged on ballast. That's a static submergence, not a dynamic dive. So the nose down effect was not induced by hydrodynamics, it is occuring under static conditions, and is therefore a factor of weight and balance.
Actually, the CG of most NAUTILUS replicas I've seen (mine included) is slightly forward of the salon windows; yours is the only one I've seen where it's aft. Probably due to variables in the systems, weight and buoyancy distribution, etc. Whatever works, eh?
And I tend to agree: shifting Bob's CG aft could compensate for the nose down dive, but then it would sit tail low on the surface when at rest.
I think it comes down to the variables between weight and buoyancy of the front half of the sub; and the difference in those variables between a pressure hull type submarine where the wheelhouse is dry, and a model where the wheelhouse is flooded. The latter lacks the buoyancy to offset the weight of the structure, and that's why models nose down.
With the NAUTILUS MINISUB, I can sit still at a level attitude on the surface, flood the tanks, and submerge on an even keel; the boat will descend on an even keel, straight down, and not pitch down during the descent. That's because my foredeck and wheelhouse structures are filled with air, and that buoyancy offsets the structural weight.
Models where the hull is primarily a freeflooding shell with a smaller cylindrical WTC inside don't share that same additional buoyancy offsetting the weight of the wheelhouse structures. Thus, while sitting on the surface the boat might be trimmed level. But when water fills that foredeck and wheelhouse area, she drops at the bow.
It seems to me this is pretty clear from the video clip Bob has on his website. She settles nicely; the tail starts to drop, even. But when the water covers the wheelhouse, down she goes nose first. Check it out. It looks like that's what's happening to me.
I would think the way to go would be for Bob to try to add some buoyancy to the area of the wheelhouse, that wouldn't come into play until the boat was submerged. Maybe if he sealed off the wheelhouse floor (and maybe also part of the foredeck) and somehow made it a dry chamber inside? That way he'd have the additional buoyancy where it seems he needs it, when he needs it: i.e., when that part of the boat is underwater.
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