The Joys of Trimming out a Wet-Hull type R/C Submarine

Transcription

1 The Joys of Trimming out a Wet-Hull type R/C Submarine By David Merriman A report to the Cabal: No, this is not directly connected to the series of Moebius 1/72 SKIPJACK r/c conversion articles I've sent out (more to follow). This is just a quick and dirty look at what it takes to trim out a new model. I'm letting your watch the sausage being made here. Later, when I get back into finishing the series, I'll present a more cogent discussion of the process as it applies to the finalized, commercially available lead weights and foam. Trimming is the process of installing fixed ballast weight low, and buoyant foam high. The objective is to fix the submarines center of gravity low, and its center of buoyancy high. The vertical distance between these two points of force is called the 'metacentric height' and the taller it is, the more statically stable the boat is about the roll and pitch axis. Trimming is done once; you should not need to alter the trim, from outing to outing, unless you change the configuration of the SubDriver, install/remove/replace devices or operate your boat in waters of varying density.

2 I've already installed a trial amount of lead weight in the bottom of the hull -- so placed as to put the assembled r/c model submarines longitudinal CG at the center of the ballast tank, just aft of the sails trailing edge. The trial pieces of foam will be rubber-banded to the outside of the hull. In the water, with the ballast tank full I'll work out how much and where, longitudinally, to put the foam to get the model to assume submerged trim with just the tip of the sail sticking out of the water and the boat stable on an even keel.

3 What I hoped would take only the morning turned into an all day affair as I made countless changes to the amount and location of weight and foam to get the trim right. The challenge was to use only two pieces of fixed ballast -- this in the interest of simplifying the process for our customers. I could have had the job done in an hour if I distributed the lead-ballast over the entire length of the boat as many pieces, but that would not lend itself to cheap weight production or easy installation by less than expert sub-drivers. Gotta keep it simple and 'user-friendly' -- so, all day fighting this beast! Sometimes simple ain't easy. Once the amount and shape (to get it to fit in the tight annular space between Sub-driver and hull) of these lead weights is worked out, I send that dope to Mike who will then produce tools and cast the lead weights up for SKIPJACK customers. You guys will get two lead weights that will conform to the assigned spaces within the lower hull -- we're making this a no-brainer for you.

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5 The boat in submerged trim. The ballast tank full of water, only a 1/4" of sail projecting into the air, and the boat at rest on an even keel. Perfect! The task here is to find the amount and longitudinal location of the foam bits required to establish this condition. Later, when working out the surface trim, the foam pieces will be moved vertically... only vertically!... to get the boat to float at the designed waterline. After getting the foam longitudinally positioned for submerged trim, I blow the tank dry and see where the boat sits in surface trim. I then move the hunks of foam vertically -- some above the designed waterline (causing that portion of the hull to loose some displacement, so it will sit lower), some below (retaining the displacement first established in submerged trim) -- till the boat sits with the actual waterline matching that of the designed waterline. Easy to do, as the kit has the designed waterline engraved onto the surface of the upper hull; just keep playing with the foam until the engraved line meets the waters edge. Simple! The boat is then dove again (several submerge-surface cycles actually) to affirm that submerged trim is as before, and the surface waterline is achieved when the tank is blown dry. The boat is then taken out of the water and a pencil outline of the foam pieces marked onto the hull. Time to transfer the foam pieces into the upper hull. (I had the SD working almost the entire day and surfaced-submerged the boat without a bit of trouble. I'm starting to fall in love with this SAS ballast sub-system. No propellant to dick with; no trim changes as the liquefied gas is used up, no frozen hands, ect. Thanks, Manfred and the rest of you who kicked in with ideas as I worked out the details).

6 I plot and layout in pencil the inside locations of the foam blocks. The foam pieces are moved from the temporary rubber-band held positions on the outer hull, to their permanent locations within the hull. Note how I get the foam blocks to conform to the curvature of the hull: I slit a block longitudinally so, with a little prying, it will splay out and adopt the face of the block to the curvature of the hull. I then use RTV adhesive to bond the foam block to the inside of the upper hull.

7 Foam blocks RTV'ed in place. Damn! It's dark now. December already! Tomorrow I finish the trimming chores in the kiddie-pool. Then, off to a local body of water and the Moebius SKIPJACK's first war-patrol: the Canadian Geese are loading up for they're trip south there. I anticipate a target-rich environment.

8 Mike Caswell produces resin encapsulated lead bird-shot ballast weights to specification. He did just that for this preliminary set of weights intended for the 1/72 SKIPJACK. Here the two weights have been temporarily RTV'ed in place within the lower hull, either end of the Sub-driver shockabsorber.

9 After an all day effort to find the ideal combination of weight and foam, I settled on this arrangement: two resin-lead weights low in the lower hull; and pieces of foam glued within the upper hull, some above and some below the waterline. Note all the blue RTV glue smears on the hull where I tried different weight amounts and locations. Not much science here -- it's all goshand-by-golly. Any of you guys see Iron Sky yet? I did, last night. What a hoot. W.A. Harbinson: call your office!