One issue that crops up from time to time with Hollow Earth Expedition is the vehicular rules. For small craft like cars, or even smaller boats up to a tramp steamer, they work well, with the size and mass of the vehicle roughly doubling for each size doubling. The issue is once the scales start to outstrip the clean doubling rules at about size eight.
In Secrets of the Surface World, Exile Games, gave us a few very large ships — all size 16. Most of these bigger vehicles had a length of 600-800′ and often in the 15-30,000 ton range, or much more that the doubling that Hollow Earth Expedition was initially settling into. This gives you a couple of issues when it comes to combat vs. massive ships and aircraft: either they are so weak that a good roll with a machine gun might incapacitate them (something as unlikely for the airship Graf Zeppelin as for the mighty Arizona…), or you have to find some fudge factor like adding defense/structure points to model armor or size.
So I see two options to fixing vehicles for Ubiquity:
The First: Keep the scales consistent. A Size 16 would be roughly 101-200′ in length and 100-200 tons or so in size. Size 32 200-400′, Size 64 400-600, and 128 600-1200.
Under this option, USS Arizona would be Size 128, with a Defense of six for metal and and extra, say, 2 for armor giving her an 8. But her structure would be 136. Even with some great bombing runs, it would take dozens of bomb runs to kill the ship.
This is more accurately modeled, when you consider that a similar sized ship, the IJN Yamato, took 11-13 torpedo and 6-8 bomb hits — somewhere around 120 points of damage for just the weapons, if you took the average.
Option 2: Create steps between Size 8 and 16 to keep the scaling from Secrets of the Surface World consistent. Here, you would add a Size 10, 12, and 14 — with a size of 100-200 for Size 10, a 200-400 for Size 12, a 400-600 for Size 14, and-600-800 for size 16. You would add the size to the material to get the Defense and Structure as usual, but ignore the negative modifier given for the lack of Dexterity. In other words, normally, the Size modifier for defense cancels itself out — Body bonuses lost to Dexterity — so that you are trying to beat the material. A wood vehicle of size 8 would have a Defense and Structure of 12. this would, in practice, make it nearly impossible for someone with a .45 Colt to damage a 50-100′ yacht, say, but a Tommy gun on full auto might make a bit of a mess. That’s mostly accurate. A decent pilot with a medium-sized torpedo could do a few points to Arizona, but it would take a remarkably good hit to sink her with one shot.
Personally, I rather like the first option, but your mileage may vary…