Pressure Hull
WARNING: The building of an inner pressure hull for this 1:72 U-Boat model may incite cries of “crazy modeller” from the modelling community at large and/or lead to temporary insanity.
I can hear the questions already:
“Why would you make your own inner pressure hull if you aren’t going to make a cutaway model?”
And also,
“Why not simply buy a commercially-available resin casting?”
To which I reply:
“While you won’t see most of the inner pressure hull after the model is completed, at least you will see (approximately) the proper curvature of the inner pressure hull. And it’s cheaper and easier than you think (now that someone has done the legwork and is willing to share it).”
If you choose to undertake this mission, consider doing some calming exercises each time you sit at your workbench. Am I being overly dramatic? Probably... though at some times I did think I had gone off the deep end in the über-detailing pool... all in the name of having some “proper-looking curves” that most people will never see nor care about. But I’LL know that they’re there... along with anyone that I can point them out to.
When/Why to Build an Inner Pressure Hull?
If you are putting any sort of detailing effort into this fantastic U-Boat model, it is very likely that you will open up all of the flooding holes in the plastic outer hull, including the long flooding channels atop the saddletanks. See the previous chapter on Hull Detailing for more information.
So after you open up all of the flood holes in the kit hull, what would you see when you look through them? Nothing! Well, since a U-boat was not just a big empty outer shell, it makes a certain amount of sense to have SOMETHING inside when you look through the holes. And what better to have inside than the properly-shaped inner pressure hull that was in the real thing?
Scratchbuilt vs. Commercially-available Pressure Hulls
In my previous U-boat project (U-625), I installed the resin upper pressure hull set from Yankee Modelworks. It certainly is nicely detailed, but I wanted to try my hand at making a simpler one myself. It killed me that most of the great detail on the YMW pressure hull was hidden in my U-625 boat, so by making my own pressure hull I would only need to put in larger structures that are more readily apparent through the vent holes.
By building my own inner pressure hull mockup, I could make the upper AND lower forms at the extreme bow and stern ends; the lower parts of the pressure hull are visible through the lower flooding vent holes in the hull. Unfortunately, the YKM pressure hull does not include lower parts of the pressure hull... only the top.
Designing & Scratchbuilding an Inner Pressure Hull
After careful deliberation, I decided to make my own inner pressure hull mock-up. I was inspired by several very good sources:
- Daniel Jayne’s FSM article “Inside the Wolfpack”
- Wink Grisé’s site about his U-557 build
- Juri Melichar’s U-201 build
- Anatomy of the Ship: The Type VII U-boat by Dr. David Westwood, which provided a wealth of information about the cross-sectional shapes of the inner pressure hull, as well as all the components inside & outside of it
I won’t bore you with all the details about how long I spent researching, calculating, drawing,
prototyping, etc.
Suffice to say that I did end up spending a fair bit of time on it as others have done before me.
What I ended up with was a skeletal inner pressure hull made out of styrene sheet and rod which will
install very nicely into the kit hull. The top of the pressure hull is skinned along the entire top
from bow to stern, while only the most extreme bow and stern sections are skinned along the bottom
as well.
Plans
Here are the links to download the plans for making the pressure hull.
Note that the PDF plans are full-size (39" x 27") and were made originally in CorelDRAW.
List of Materials
Here is the list of materials that I used to construct the inner pressure hull:
- Spine:
Evergreen Scale Models StripStyrene #230 hollow tube, 5/16" (.312" dia.)
with 1/4" (0.25") brass tubing joiner - Ribs:
Sheet styrene 0.080" - Skin:
Sheet styrene 0.010"
For drilling the spine hole in each rib, I first drilled pilot holes, and then finished with a Dremel #115 High-speed Cutter (cylindrical) which gives an 8mm hole and a tight fit for the plastic spine.
Making the Ribs
Following the dimensions in the plans, lay out and draw each rib onto a sheet of 0.080" styrene, marking the outer curves, flats, and the location of the spine hole. (For circular ribs you can use a compass device.)
TIP: Use a black marker to mark the centerlines of each rib. This will help you to align all the ribs later on when they are installed on the spine.
Cut out each rib using your tools and methods of choice. I opted to rough cut each rib outside the lines, and then sand to the proper shape.
Using pilot holes of increasing size, I carefully drilled out the spine hole, and to finish I used the Dremel #115 High-speed Cutter (cylindrical) to make the final 8mm spine hole.
Making the Spine
I joined 2 full-length pieces of the styrene spine tube using a 3" length of 1/4" brass tubing (which fits inside perfectly), using CA glue to cement the pieces together.
Starting at one end of the spine tube, I measured back approximately 144mm, and marked that as the “0” reference mark (the origin point). Using the “0” mark as reference, measure and mark the location for each rib along the spine according to the plans.
The “0” mark on the spine aligns with the location of the kit’s forward bulkhead. You do not have to use the kit bulkheads when you install the pressure hull.
Installing the Ribs on the Spine
With all of the ribs cut and the spine assembled and marked, press each rib onto the end of the spine, and then slide to the proper location. The spine hole in each rib should give a nice tight fit on the spine. When all of the ribs are installed, look down the spine and twist each rib into the proper orientation... this is where the centerlines you marked on each rib will make it very easy to align the ribs.
Use CA glue to secure each rib to the spine tube. Cut off any extra protruding spine tube, and sand flush the end ribs.
Applying Skin to the Pressure Hull Skeleton
I used thin 0.010" sheet styrene to apply the “skin” to the skeletal framework; it was easy to cut and glue with CA glue. As you can see from the photos, you can apply skin across a few ribs at a time in some places. This will help ensure the least amount of seams.
I used plain paper to create templates for skinning some of the more “cone-shaped” sections that spanned between ribs of different sizes. Wrap the paper around the ribs, and hold it tight. Hold the skeleton up to the light, and use a fine pencil/pen to mark where the paper touches the MIDDLE of the rib edge; this is fairly easy to do since the rib is made of 0.080" styrene. Repeat for the other end of the skin section. Cut the paper to shape, lay the paper template on a piece of thin 0.010" styrene sheet, and cut the styrene sheet to match the template.
Pre-curve the skin sections by rolling into a tight cylinder; a curved piece of skin will be much easier to attach to the ribs than a flat piece. Hold the skin section in place and use thin CA glue to cement it to the ribs. Repeat for the remaining sections of styrene skin.
Use putty to fill any gaps between the skin sections.
Apply skin to the top of all ribbed sections from bow to stern. Only the last 2 sections (extreme bow and stern ends) will require skin on the bottom.
Installing End Caps
The last step is to apply end caps to the bow and stern of the pressure hull. For the bow I made a 3-layer “sandwich” of 0.080" styrene (CA’d together), cut to the rough shape of the bow end section; for the stern I made a 2-layer “sandwich” of 0.080" styrene. After cementing the rough-cut “sandwich” sections to the bow and stern ribs, I used a sanding drum to shape them, puttied them, then performed final sanding with sandpaper.
Test Fitting in the Hull
I had to do a lot of test fitting of the inner pressure hull inside the outer plastic kit hull to get the proper size & shapes of the ribs. Even after the pressure hull was completed and skinned, I had to go back and change a few of the ribs to ensure proper fitment.
If the inner pressure hull is too wide in spots, it will cause the two hull halves to push apart; this will dramatically impact the proper fitting of the deck.
The pictures below give an idea of how the finished pressure hull will look like when it is installed in the hull (without the saddletanks and other fixtures completed).
Centerline Baffle Plates
I used thin 0.010" styrene sheet to create two vertical baffle plates that would go behind the opened lower flood holes in the bow and stern. These vertical baffle plates would be placed along the hull centerline, between the kit hull and the lower edge of the inner pressure hull.
Using a 1:1 photo of the kit hull with inner pressure hull in place, along with detailed plans of the sub interior from my reference books, I used CorelDRAW to create the draft shape of the bow & stern baffle plates. I deviated slightly from the scaled plans, in order to align the baffle holes with the rows of holes in the outer hull vent plates.
These baffle plates were designed for and worked with own kit; they will differ slightly for everyone else’s kit. I recommend making paper patterns first, cutting them out, test fitting them, and making modifications as required BEFORE making the final styrene versions. Expect to make 2 or 3 templates before the final versions, and even then minor modifications will probably be required before final fitting.
