CAD Model for an I-Boat Hull

With Strike Models no longer selling hulls, there isn't an HMS Invincible class (aka I-Boat) kit available for purchase anymore. It seems that it would be a good candidate for a crowd sourced beginner's ship. I've made a little progress on drawing the hull.
I've got the section lines sketched out using some scans from the British National Maritime Museum as a reference.

 

Using those section lines, I was able to make what I think is a pretty nice looking hull form, in FreeCAD.
I still need to sketch in the stem and the stern and to add in the flat bottom.
Here's what the resulting STL file looks like in Slic3r :

 

I was able to get the stem and stern profiles blended in and closed up the flat bottom and the deck with ruled surfaces.
It's not perfect but, I'm pretty pleased with the shape of the ram bow and how smooth the mesh is.
Slic3r was able to process the exported STL file, so I'm encouraged that it will probably print OK in 4 or 5 sections.

 

As a reality check, I scaled this model to roughly 1:350 and printed it in 2 halves on my FDM printer. It looks pretty good in plastic : )
I'll glue it together and post up some pics with a Dreadnought model, in the same scale, for comparison, later.

 

Here's my I-Boat hull printed in 1:350 scale next to the hull from Trumpeter's 1:350 HMS Dreadnought injection molded model kit.
Unfortunately, perspective makes the I-Boat look longer much longer relative to Dreadnought in this quickly composed image but, they are both the proper LOA and Beam.
I think the curves compare pretty favorably.

 

With a little more tweaking of the model, I refined the shape of the stern and got the hull to be a solid, so that it can be booleaned.
The decks are now flat and the step is in the right place.
Now to see if I can work up a rib layout that would be legal for both fast gun and big gun builds?

 

I played around with the spacing for a total of 16 ribs that are each 1/4" thick.
If I put one at the deck step, use a 68 mm spacing for the forward ribs, and a 64 mm spacing for the aft ribs then, I believe it meets the requirements for both the IRCWCC and the WWCC rules.
Here's what that looks like :

 

That works for Big Gun all right.

 

Here's a better view of what the ribs and the solid sections would look like :

 

I found some time to refine the model. It's now a thin walled solid.

 

This thing is looking better by the day. How thin are the walls? And how far do you plan to develop it?

 

Thanks : ) Right now, the hull is 3.2 mm or 1/8" thick. It's parametric, so that can change based on materials. I guess how far this progresses depends on the community response? I figure I'll get it to at least the laser/cnc hull kit stage. If there's more interest, I'll make a mold for fiberglass hulls. A fully 3D printed hull could be a further option. I'm trying to keep the design modular and flexible.

 

It looks like I can break this up into 5 pieces to print it on my FDM printer.
The joints will be split bulkheads where ribs would be located.
Here's the midship chunk in Slic3r :

 

Here are the updated ribs :

 

Looks like I'm going to have to get creative.
The aft turret needs to go almost exactly where the easy location for the drive motors is ; (
I'm thinking I'll try a V-drive.

 

Push the motors further forward. One frame and you can easily fit some Fast Gun cannons. Two frames and I can drop a Big Gun twin turret back there. This is a spacious ship, there's no need to do anything complicated like a V-drive.

 

Hi!

so a couple of things, you need to have the motors much father foreword than you have. A pair of 550 motors will be over 3” wide, and you will need to have them well amidship to sit low and not butt up against the side of the hull.

also, you also need to bring your props forward as well. If they are too far back the blades will extend past the profile of the deck and be clipped by other ships and the narrow hull will make it hard to install rudders.

You will want to run 1.5” props, so your profile of 2 props will be around 3 wide, and your rudder posts will need to be 1.5” apart and ideally be 1/2” from the sides of the hull (so farthest optimal placement of rudders will be where you stern is at a minimum 2.5” wide.

Below are some shots of my I-boat for reference.

cheers,

john

 

It's tight but a 2:1 gear reduction fits in there.
One would probably have to notch the two frames to accommodate the solenoid and the accumulator but, getting @Kotori87 's 7/32" twin in there looks do-able. Maybe even with a rotation drive?
Moving on to the steering arms : )

 

Listen to Warspite, he has what a perfectly functional Invincible and combat experience with it. His advice also mostly lines up with the other Invincibles I've seen and the one I half-built a few years ago. That said, I personally don't think it needs 1.5" props, especially with nice 3d-printable kort props able to push some serious water. 1.25" props will give you more rudder-redirectable thrust, which has a very noticeable effect on turning. There's also the consideration of weight distribution - you want all your heavy things in the middle of the boat, to reduce the moment of inertia when turning. Yes, that includes motors. To give you an idea, the Invincible I half-built had its motors around about the deck step, which was nice and flat and roomy and easy to access/work on.

As far as the Big Gun cannon goes - it's not just the accumulator you have to worry about, but the MPA-7 actuator on the other end. Right now it looks like the motors will be running into that. For the Mikasa installation, I ended up mounting the cannon firmly to the bottom of the boat with a #6-32 screw. I wouldn't worry too much about Big Gun compatibility, since both I-boats and VDTs are matched or beaten in every category by the pocket battleships of WWII. There's a very good reason why I've only ever seen photos of one, down in Australia.

 

Hi,

So prop size is one of the single most important decisions you can make for your ship, because that defines where you put your motor mounts, install your stuffing tubes, mount your rudders etc. Once your ship is completed, you can always go to a smaller prop, but you cannot go to a larger one (it would require ripping out your entire drive train and re-centering your rudders).

In fast gun larger props leads to better performance (larger drag disks etc) not sure about other formats. You absolutely could go with 1.25” props on this ship. I used 1.5” and had to go to nearly a 4:1 year ratio to get her on speed. 1.25 would be plenty to get her to speed without issue.

so more pictures attached for building examples.

Cheers,

John