So You Want To Build A Warship...
So You Want To Build A Warship...Just to recap, we've got a hull frame that's 56.25" long, just under 6" wide, and about 3.5" deep, deck-to-keel. 1/8" ribs are spaced every 1" (some are a little farther apart, to compensate for hull contour lines that don't line up at even inches). A solid oak keel, 1/2" high and 1/4" thick, extends from the very first rib to the 38th, where a pair of parallel skegs angles upward toward the waterline at the stern. The skegs overlap the keel for a span of 3 ribs, making what should be a very strong point to eventually attach a recovery line. The cap rail is in place, and the whole assembly feels feather-light, but incredibly strong. No stringers are installed or planned.
Next on the agenda is to install propellors, shafts, and rudder while it's still relatively easy to reach in between the unskinned ribs. The expectation is that this job will only need to be done once - after the hull is skinned, it's got to be a lot more difficult than it would be now. Indeed, with the close rib spacing, it's still a challenge for somebody with fairly large hands.
General layout of equipment is partly dictated by the plans, and also highly dependent on the need to balance and ballast the ship. Locations of cannons, propellors, and rudder are specified in the plans. To a large degree, these locations also dictate where other equipment, such as the rudder servo, can go. Motors and propellor shafts are a little more flexible, however. Not quite recognizing and appreciating this flexibility was the first major error in this ship's construction.
I wanted to clear out as much space amidships as possible, for battery placement. To accomplish this, as well as to be able to drive all four shafts from two motors, I chose a chain drive system, using parts from Serv-O-Link Corp. Serv-O-Link has a good selection of 1/8" pitch Delrin chain and sprockets, as well as small gears. The use of sprockets and chain would allow the final drive ratio to be "tweaked" by changing sprockets, plus it should be a simple matter to drive all four shafts, or just a pair, from a pair of counter-rotating motors, in a very flexible arrangement. Also, the motors could be mounted parallel to the shafts in a sort of "folded back" configuration, instead of inline with them, making a larger space ahead of the motors for holding batteries and other equipment.
Ribs aft of Rib G were deliberately left with extra material at their bottoms to provide support for the propellor shafts. Locations where each shaft is to pass through each rib was marked on the rib, but no holes were cut prior to assembly. This presented a minor problem of having to drill what amounted to a hole over 12" deep, through multiple ribs. This problem was overcome by jamming a spade-style drill bit into a section of brass tubing to create an extension. A short piece of rod, the largest diameter that could fit inside the tubing, was inserted into the drill end, to keep the drill's chuck from crushing the tubing when it was tightened down.
Photo 1 shows the general layout, with propellor shafts set temporarily in place, as well as motors, batteries, and a stern cannon from another ship. A problem which is visible in this photo, but wasn't apparent to a rookie builder at the time, was the height of the #3 turret. In the photo, it can be clearly seen that the top of the magazine is a good inch above the cap rail, and the barrels themselves are probably 1-1/2" to 2" above where they should eventually be. Not only is this going to set the barrels to high, the weight of the cannon set up so high are going to hurt stability. However, as can also be seen clearly in this photo, there is ample space for the accumulator to sit between the inboard propellor shafts if the ribs are cut down to allow it to sit deeper. Right now would have been an excellent time to do this, but it wasn't - when it was finally done, it was much more difficult, due to the work that was done subsequently.
Photo 1.
Test Fitting.
The ship's plans show a stand-off just forward of each propellor. Each stand-off is apparently there to set the propellors' locations, and provide a bearing for the shaft. Stand-offs were build by soldering brass airfoil onto very short versions of the stuffing tube, as shown in Figure 1. A small wood block was set between each airfoil, to set the angle - this block is fastened to a rib to create a strong support for the shafts. Sections of 1/16" brass tubing are inserted through the airfoil, through a small hole in the short stuffing tubes, to allow greasing of the bearings. Note that the main stuffing tubes end where the shafts exit the hull. This leaves the shafts exposed to water, where it is possible to pick up weeds. This has proven to be much less of a problem than it might appear to be.
Figure 1.
Propellor Shaft Stand-off.
As for construction, the rudder was cut out of brass sheet and soldered to the post. This particular ship used a two-piece rudder - a fixed forward piece, and a rotating section that is attached to the post. The fixed section was soldered to the outer rudder post stuffing tube, making a complete rudder assembly that could be mounted into the hull as a unit. Photo 2 shows the completed rudder assembly and the propellors with their hafts and standoffs, after the ship's completion.
Photo 2.
Propellor and Rudder Details.
Control of the rudder provided an interesting challenge for a builder who had never seen a model warship before (or, for that matter, had even examined a R/C model boat!). Therefore, the rudder control mechanics are a bit unusual. Having lots of Delrin chain and sprockets available for the drive system, I decided to use the same method to control the rudder! The servo is mounted on a wooden "carriage", that really is only a pair of wood blocks that each end of the servo is screwed into. Each block has a pair of 1/8" holes drilled in it on either side, and a pair of 1/8" brass rods are passed through the holes. The rods are passed through a pair of expansion springs and mounted into brackets. The springs force the servo forward on the rods, to keep tension on the chain. One needs only to push the servo towards the stern to slip the chain on or off the sprockets. Sprockets on either the rudder or the servo can be changed to get the best steering performance. While this is an odd setup, dictated mostly by a combination of ignorance and materials on hand, nonetheless it has worked flawlessly. The rudder and its control servo can be seen immediately after installation, in Photo 3.
Photo 3.
Test Fitting (another angle), Showing Rudder Installation.
Figure 2.
Motor Mounting Bracket.
Attaching the sprockets to the motors was simple - the chosen motors have 1/8" splined shafts, which grip the Delrin sprockets quite nicely. The motors seem to generate pretty good torque, which is multiplied by the difference between the smaller sprocket on the motor and the larger sprockets on the propellor shafts. Slippage of the driven (propellor shaft) sprockets was a problem for quite awhile, however, and various solutions were attempted. What ended up working the best, and was quite simple as well, was to file a flat area on the propellor shaft, and place a dab of thick CA glue on it before sliding the sprocket over the shaft. Obviously, it can't be driven immediately - the glue needs to set up. Once it has, though, there has been no more shaft slipping.
Figure 3.
Motor Control Circuit, As Installed.
Note that the motors are wired in series, so that they turn in opposite directions. The switches are operated by a cut-away wheel on the servo - moving the servo one way closes one switch, moving the servo the other way closes the other switch. The motor(s) are connected through the "C" (common) pole of each switch, through the "NC" (normally closed) pole, to the same terminal of the battery. When the servo activates one of the switches, it breaks the C-to-NC connection, and closes the C-to-NO (normally open) connection. This connects the motor to the opposite terminal of the battery, completing a circuit and driving the motor(s). Operating the servo the opposite direction closes the other switch, completing a similar circuit, but with opposite polarity, driving the motor(s) the opposite direction. The circuit is not polarity-sensitive; it works the same regardless of where the battery and motor(s) are hooked up. If the direction of rotation is not what you want, simply reverse the battery connections or motor connections. Also, most radios have a switch to reverse the servo direction. A schematic diagram of the motor control circuit is shown in Figure 4.
Figure 4.
Motor Control Circuit Schematic.
However clever this arrangement was, there is no denying the fact that it would have been easier if done first! Everything else is secondary, but the guns have to fit or you don't have a warship!
With the cannons, propellors, shafts, drive motors, rudder, rudder control servo, and motor control servo and wiring installed, USS Pittsburgh is ready to be skinned. The CO2 delivery system was assembled per Safe, Effective CO2 Delivery. Items such as the adjustable regulator rack and various fittings were left unattached, so that they could be moved around as needed to balance the ship.
Skinning the hull, decks, and superstructure will be covered in the next article of this series.
Part 6 - Skinning the Hull; Decks, and SuperstructureNTXBG Home
Back to On The Ways