My guess would be that water resistant foam was not around back then so putting them on water was not the best thing to try. I would take a guess that if these plans were used using the motor placement changes and the in hull battery that it could do well on water if sealed and properly painted / covered..
I am getting the bug to finish the one I started last winter late and may get it up for this winters project.
Psyborg: if you do re-visit this project I's like to share some ideas if you're interested. I'm going to make my best effort to design an air boat that will hopefully look and behave at least similarly to a real one. I'm planning to design it around an Emax MT1806 (Power pack A) and a 6x3 prop to start, though I may move it to a 5x4x3 depending on it's behavior. Safe to say if everything is designed to accomodate a 6x3, the 5x4x3 or similar will also fit.
Currently I've got it in my head that I can use standard white foam board for the build and just minwax the heck out of it to ensure it is waterproof. Put on a coat, let it set, sand it, add another coat etc. I have seen a few forum threads here and there where this has been done with good results so that's my plan currently. It does seem to be relatively impossible to find a good set of dimensions for an actual airboat hull, or an RC airboat hull that.. Looks right? There's a certain look the real ones have and it's somewhat in contrast to the scratch built RC airboats or powered sleds people have built to run on snow. I probably won't build a prop shroud for my initial attempts, but I might make a sort of arched duct around the propeller. Will see how that goes when I actually start putting something together.
Is yours modeled after this community design or are you going for something a little different like I am? I'm going to do my best to rapid prototype something and do what I can to keep it looking as "Traditional" as possible while remembering that physics is still a thing and at the end of the day it not only has to float, but it has to handle well and come up on plane properly or I will kill it and start over. I just wish I could find some blueprints or any kind of graphical representations that I could scale down to make use of. There seems to be a ratio but I haven't gathered enough information to determine what it is. Seems to be that the width of the transom is just slightly wider than the propeller and the distance from the transom to the tip of the bow at least appears to be about twice that length. Dept, if I had to guess is about 5% the width of the transom. Lastly, what about the angles? The sides, I would guess slope outward around 15 to 20 degrees and the bow around 20 to 30 degrees? These are all guesses I've made from looking at a couple hundred images, designs that annoyingly lack dimensions, sketches etc that I have found on the internet in the last 72 hours. So with that, more research is still required on my end.
I'll be starting assembly of an HK Swamp Dawg in about a week once all of the parts arrive and I'm planning to take a serious look at the dimensions on that boat and look at how it actually handles on water.
If you would like to share some ideas, I can PM you my email, or we can just continue to do so publicly on the air boat thread I created yesterday. That might be helpful to others.