Part 4 – Framing The Hull
As noted in prior installments, I believe in preparation. I also like cutting, bending, sanding, filing, and in general making the dust fly! All the preliminaries up until now have been about getting ready to actually build something, but even the best planning and preparation won’t settle everything – there are going to have to be decisions made while the actual work is in progress. Things that you think will work won’t, or you’ll figure out a better way of doing something, while you’re in the process of doing it the way you originally planned. This is normal, and is part of the process. Knowing this, you can take things in fairly small, logical steps, so as not to over commit yourself to a particular direction and allowing yourself the luxury of being able to change your mind later.
One topic that isn’t getting it’s own chapter in this series, although some people might think it deserves one, is materials. I may change my mind later, and insert a materials selection chapter into this series if I think it warrants it. For now, though, we’ll tackle that issue as a subset of this chapter. Balsa is a given for the skin, of course, since that’s what the Big Gun rules says to use. Plywood for the ribs – thickness, again, dictated by the rules of the hobby. The more plies (layers), the better, so go with aircraft or modeling plywood from a hobby shop, not the stuff you’ll find at the lumber yard. Since I’m building with 1″ spacing, I have to use 1/8″ plywood for ribs. Cap rails also get plywood – I chose 1/4″, but 1/2″ is allowed. I’ve also got 1/16″ aircraft plywood for decks, and balsa blocks ready for superstructure and the solid bow and stern ends of the hull.
One of the first decisions that needs to be made is whether to build the hull using a baseboard or a keel. Randy Kehr’s Big Gun Page has some great information on baseboard construction. For the Baltimore class, however, the plans indicated that the bottom was fairly rounded. I figured that a baseboard could only be a little more than an inch wide at the widest part, so I chose to go with a keel-and-rib method. At my mentor’s suggestion, I chose oak for the keel – I found a nice straight, clear 1/2″x2″. The plan was to use a friend’s table saw to cut a 1/4″ strip off the short side, to make the keel. This proved to be a bit more challenging than it looked – a case where the standard-sized tool just won’t work at the small scales and tight tolerances needed for modeling. Getting a small, straight cut off of such a small piece wasn’t easy – they kept tapering on us – we finally got one, but burned through the entire piece in the process (with a lot of nice oak shims left over!). A band saw might have worked better than a table saw. Subsequent hulls have been built using plywood keels, and worked out just fine. Also, having a straight keel to start with may not be that important – any curve can be straightend as the frame is assembled. In fact, you want to make sure you don’t introduce a curve into a straight keel during assembly, anyway.
The keel was notched at the appropriate locations, to accept the ribs. Again, the table saw was used, with the blade height set to the appropriate depth of the notches. Again, the standard-sized tool turned out to be too much for the task – the saw just sort of dances around – you’d never notice it if you were cutting 2″x4″s or something – but for small precise work like this, it was just about impossible to consistently hit the marks close enough. Several notches came out too narrow, which could be corrected, but others were too wide! With no more raw material to make another keel, we had to go with it as is. This created some problems later on, when it came time to assemble parts – fortunately, there was an ample supply of oak shims!
From a point about 3/4 of the way back from the bow, the bottom of the hull needs to angle upwards about 15 degrees to about 1/4″ below the water line at the very stern – the props and rudder go under here. To accommodate this, I decide to use a “dual skeg” setup – the main keel would end where the hull bottom starts to slope upward. Twin parallel partial keels, or skegs, would be attached to the main keel and angle up to the stern. When put together, it looks a little like a tuning fork. An added bonus of doing this is that there’s a natural gap right on the center line of the hull where the rudder sticks through, so there’s no need to monkey around trying to get a rudder through a keel – just place it between the skegs! For a ship with an odd number of shafts (Baltimore class had 4, so it doesn’t apply here), this would also make it very easy to run the center shaft. Also, the portion of the keel where the main keel and skegs overlap will eventually be a very strong point – perhaps a good choice for attaching a recovery line?
In Part 2, we designed the individual ribs. I printed each rib’s outline, cut off the excess paper (leaving about 1/2″ outside the lines), and used a spray adhesive to stick the patterns onto the 1/8″ plywood. I used a scroll saw to cut the ribs, on or just outside the pattern lines. I didn’t worry too much about being precise with the saw, except to be sure not to cut INSIDE the lines! Excess material can be sanded off easily. I mounted my Dremel tool into its drill press, with a sanding drum in the chuck. Set the height so that the bottom of the drum us just below the drill press table. Then, with the Dremel running, you can set the rib on the table and push it up against the spinning sanding drum where needed – zip! zip! zip! Nice, square edges, too! The best technique I found was to move the piece back and forth across, past the spinning drum, just easing it into the drum a little at a time. This made nice, smooth, even tapers and made it easy not to cut too much.
Note here that the rib patterns have center lines and water lines printed on them. These lines are very important, to make it possible to align pieces when the frame is assembled. Depending on the quality of the spray adhesive you used, the paper patterns may easily peel off or they may be there to stay for awhile. It won’t hurt anything to leave them on, but you may want them off for aesthetic reasons (then again, the model snipes may appreciate the wallpaper!). It’s not a bad idea, and now would be a good, easy time, to transfer the center lines and water lines directly onto the ribs themselves. Ribs should be notched to mate with the keel, just like the keel is notched to mate with the ribs. Note that those ribs aft of the main keel will need two notches to mate with the twin skegs, and the ribs right at the keel/skeg joint get extra wide notches.
I had pre-determined that there would be cross-over braces at three points along the length of the hull. The ribs at those points had a top cross member left on them, so that they were “O”-shaped, rather than “U”- or “V”-shaped. One of these cross members was later removed to allow the guns to fit properly. In addition, some ribs were designated for possible supporting roles for various hardware, and had a little more material left on them – that could be cut away later, if necessary. In particular, those ribs forward of where the prop. shafts would enter the hull, up to the point where I intended to mount the motors, were made extra tall, with holes cut that the shafts would be passed through. The points on each of these ribs where the shafts would pass through should be marked and cut before the ribs are assembled to the keel. I marked the points on each rib, but didn’t drill the holes though, and was forced to invent a drill extension to let me drill the shaft holes after the frame was assembled. The drill bit was stuck into a piece of tubing with an inside diameter such that the bit would fit snugly (a spade, rather than a twist drill was used). A rod the same size as the inside diameter of the tubing was placed inside, so that the drill chuck wouldn’t crush the tubing, and the whole works chucked into the drill. It worked, but I know it would have been easier to drill the holes first.
Finally, a pair of rib-like pieces are cut, to form the curve of the transom. These will be mounted horizontally, attached to a short “mini-keel” that extends upward from the end of the twin skegs to the cap rail. A solid stem is also cut, that will be attached to the front of the keel and defines the shape of the ship’s bow.
Out of the 56 ribs in this hull, I doubt if more than 4 or 5 of them actually fit correctly on the first try. There was a lot of “tweak” cutting, to get notches deep enough and/or wide enough. As noted previously, some of them had a sloppy fit that had to be taken care of – shims to the rescue!
Cap rail was plotted by measuring from center line to gunwale, at each rib, then connect-the-dots. Notches were cut on the outer edge of the cap rail to mate with the tops of the ribs. The cap rail was not a single piece, contrary to some advice I had received. There is a “U”-shaped segment at the stern, up to the first cross member, a “V”-shaped segment at the bow, back to the first cross member. Between each pair of cross members are separate segments of cap rail.
To assemble the frame, I didn’t have the rig described in the previous section – the process of doing it the way I did led me to the idea of the rig. Instead, the keel was clamped down onto the work table, and a pair of tight lines were strung on either side of it at the water line. Note that the depth of the hull is not uniform from stem to stern – from the water line to the bottom-most point of the hull, it’s deeper at the bow than at the stern. The lines, therefore, would not be level but rather slope downward slightly from bow to stern. Water line height at the bow was measured, as was it at the stern. These two points are where the lines were attached. As each rib was placed into the keel, its drawn-on water line was aligned to the two lines on either side. Ribs were tacked down with CA, then epoxied for the final strong bond. Once the rib-to-keel bond had set, the cap rail was attached using the same method.
Although it looked rather flimsy at times, the resulting hull frame ended up surprisingly strong, but light-weight. It just had a very strong feel to it – so strong, in fact, that I was tempted to see if I could stand on it! Only the still-vivid memory of all the work that I had put into it kept me from trying that stunt, but I seriously thought it might support me!
Cutting all those ribs and the keel took the better part of a weekend. Finishing them took a full Saturday, and the initial keel assembly took most of the following Sunday. Rib-to-keel assembly occupied the following Saturday, with Sunday used for attaching the cap rail. So, working weekends only, I was able to complete the frame in three weeks. Here are some photographs of this process.
Keel With Ribs.
Here, the only two ribs that have been cut out are set without glue into the completed keel, to get a feel for how the ship’s going to look in 3D. The twin skegs can be seen at the top of the picture. The work surface is a 6-foot folding table, with a copy of the plans taped to the top. The assembly is placed on top of the plan (overhead) view, to give it a sense of scale. Also on the table are some deck sections – note the center line and rib locations drawn on the decks. Also, on either side of the keel, are what were originally intended to be cap rail sections, but it was decided they were too thin – one of many changes to the plan that evolved as construction progressed. Notice how one of the ribs has extra material remaining in its bottom portion – this has been left to support the prop. shafts, which will eventually pass through and be epoxied to it.
Assembled Keel With Stem and Stern.
Here we have a good look at the stern framing, to shape the transom. Note, on the plan below the stern, where I had drawn in the correction to the original plan, based on actual photographs of Baltimore-class cruisers. Stacked up on the right side are the roughed-out ribs.
All Ribs Temporarily Placed.
Here, all of the ribs have been set dry into the keel/stem/stern assembly, just to get another look at the overall dimensions. Note the bulb shape of the bow. This would prove a little difficult to skin later, but is authentic. We’ll see how it affects ship handling. Also, stacked to the left of the bow, are the superstructure decks. These were cut out of 1/16″ plywood, by gluing copies of the plan to the plywood and cutting them with a scroll saw.
Keel, Ribs, Decks, Mock Superstructure.
Sometimes, you just can’t resist seeing what the thing is going to look like. Here, in addition to the ribs set dry into the keel assembly – you can see the forward ribs have a decided lean to them caused by some of the keel’s notches being a little too wide – the main deck sections have been laid on top. Also, the superstructure decks have been stacked on top, using little chunks of styrofoam to give them an approximate height. This was a pretty delicate operation – a lot like building a house of cards – and took several tries to get it to stay put for the photo. Beer can added to provide scale ;-).
Ribs Attached To Keel.
Here, the keel has been clamped down to the table and the ribs glued in – note the brown epoxy stains at the bottom middle of the ribs. Also, the alignment strings on either side can be seen. It’s pretty obvious that there’s going to be some sanding needed, to even out the lines of the hull. Note the wax paper underneath, to keep epoxy from dripping onto the working plans. Also note that the prop. shaft holes are visible in the thick-bottomed ribs aft.
Fully Assembled Frame With Cap Rail.
Still clamped to the table, the cap rail is installed and glued down. Note in the background that the main superstructure has been carved out of balsa blocks, while the epoxy was drying on the frame. The superstructure was built by stacking balsa blocks on top of each other, with plywood decks in between.
Assembled Frame With Decks, Superstructure.
Another “couldn’t resist” opportunity. The forecastle deck has been epoxied to the frame, and is clamped in place with clothespins. It’s hard to tell from this photo, but the forecastle deck is curved, with a crown along the center line. Main deck segments are laid on top, with the finished but unpainted superstructures setting on top of that. Starting to look like a real ship, now!
Let’s see how things are going to fit! Here, a cannon from the Adm. Scheer is set approximately into the position where Pittsburgh’s #3 turret will eventually be. Also, a pair of gel cell batteries and a pair of motors are set where they are expected to go. Just visible between the motors are the inboard prop. shafts, which have already been installed. Also visible is part of the chain drive setup, details of which will be covered in a future article in this series.
In the next installment, we’ll start fitting out. Things like prop. shafts, rudders, motors, and other equipment are easier to install before the skin goes on, when you can reach between ribs – even if they’re only 1″ apart!