Since the weight of the model will not be true to scale, and therefore will have a completely different underwater hull, I will not use / need any information regarding the real hull below the waterline. Besides, the underwater part of the hull is a closely guarded secret of competitive character, which I would never compromise. Build light, or build heavy? Well... Neither! Build right. Building a surface ship is quite different than building a submarine, so a bit of research is necessary. The sub had to be quite heavy for it to dive at all, but that’s not the case with the surface ship. Keeping the weight low in everything above the waterline is objective no. one throughout the construction. First, let’s look into a few basics concerning floating things.
Wind sensitivity is an expression that covers the effect of the wind pushing on whatever is above the waterline; hull, superstructure and deck details. Large, light ships do battle this in real life. Ever seen a ferry having a bit of trouble docking at high winds? The same will apply to a model ship. One workaround is to give the hull below the waterline a steep keel, rather than a flat bottom. Another countermeasure is to implement thrusters, normally found at the bow, sometimes both bow and aft. The real Emma has got two thrusters in both ends, each with a thrust of 25 tones. Please read the chapter about rudder, prop. and bow thruster for information about the bow thrusters used in my model.
A submarine can adjust it’s buoyancy using ballast- and trim tanks, going from positive, over neutral, to negative. Positive buoyancy causes it to float on the surface, neutral buoyancy enables it to hover at a given depth, and negative buoyancy causes it to dive deeper. (Being submerged with neutral buoyancy also enables it to change depth using the dive planes (horizontal rudders), as long as there’s forward motion. This is called "Dynamic diving") A surface ship floats on the surface, displacing an amount of water with the part of the hull that is below the waterline. The weight of the displaced water is 100% equal to the weight of the ship. We cannot change that, it’s the laws of physics. What we can change though, is the weight of the ship, using ballast- and trim tanks. Making the ship heavier pulls more of the hull down into the water, thus increasing the amount of displaced water. If we take it too far, the ship will sink, and here another law applies: All ships can sink, only submarines can resurface. With the above in mind, we need to form the hull so that it displaces a volume of water equal to it’s own weight, but how do we know the finished weight in advance? Well, we don’t. We have to estimate it.
(The number for the actual weight will be added upon completion.) The above idea will make it bottom-heavy, as the electronics, keel ballast, and to some extend the hull weight, should be added into one figure. (Electronics and batteries are at the very bottom of the hull.) An idea: Instead of putting in lead ballast, add batteries. Might as well utilize the weight.. If the submerged part of the hull ends up displacing too much water, weight can be added to correct the design error, but if the submerged part of the hull ends up displacing too little water, then we have a problem. In the last event some structure needs to be build on to the bottom. I plan to make little removable containers to put on the deck, and that might call for a ballast system to compensate for them, when they are not there. Rather than making real ballast tanks with automatic adjustment like I did with the sub, I’ll just make one or two of the deck cargo hatches removable, allowing me to insert weight(s) to the very bottom of the ship, when the container modules are removed. Important: A typical flaw on model boats and submarines is the lack of rudder response. Modelers often oversize the rudder by 10-15% to compensate for this. Another trick is to place the center of gravity a few inches in front of the center of the length axis. This improves the force x arm that the rudder uses to turn the stern.
If you have read the section about the submarine, you might have noticed that the center of gravity (G) is below the center of buoyancy (B). This is so for submarines and sailing yachts with keel weight only. On normal surface ships the center of gravity (G) usually lies above the centre of buoyancy (B). The concept that explains the stability is called a metacentre. Common for them both is that if the center of gravity (G) is not vertically aligned with the center of buoyancy (B), then the object will roll and / or pitch until this is true. First, a couple of rules apply: A) The centre of buoyancy (the centre of the submerged ship hull volume) will be always below the waterline, B) The centre of gravity (its vertical coordinate) can be at any point (below, on or above the waterline). To increase the stability (to increase the righting moment) the centre of gravity should move downward - toward the B or below the B, and away from the metacentre (M). The forces of gravity and buoyancy are illustrated here at zero list: They all line up.  Red ball = center of gravity / negative buoyancy. Green ball = center of positive buoyancy. Now, let's list the ship and have another look at the forces in play. The list used for illustration is at app. 35 degrees, JUST before the deck in the port side is submerged. Going further would change the hole picture, as the now submerging part of the ship does no longer add to the buoyancy in that side, because the deck is flooding. (No "more" hull is added to the water.) The result would most likely be a capsize. 
Red ball = center of gravity / negative buoyancy. Green ball = center of positive buoyancy. Blue ball = metacentre. What is the metacenter (M)? The point where the line of the buoyancy force action crosses the old line BG (upright ship) we call the metacenter. When the ship incline then the centre of buoyancy moves to the more submerged ship side. The part of the hull near the waterline submerges, other part emerges, so B moves sideways following a line parallel to ze - zi, which cuts the emerging- and the submerging hull parts in half. By now, the ship is inclined, so the direction of forces (buoyancy and the gravity changes) are perpendicular to the new waterline (as they were perpendicular to the old one, for upright ship). The lines of the action of the buoyancy force and the gravity force do not coincide now (for upright ship they were coinciding - G was above or below B, on perpendicular to the waterline). There are some distance (U) between the lines of action of B and G forces, and these equal forces create a so called couple, and this produces the hull righting moment. To increase the stability you can move the M up (causing B to move more sideways when you incline the ship) without touching G - this is done on old ferries and RoRo ships by using so called sponsons - you increase the ship breadth on the waterline and above it. For modelers who have a given hull form - to increase the stability you move G downwards as much as you can (putting the ballast and the batteries on the bottom of the model). This is also done on tall ships (sailing ships) - they have fixed ballast near the keel to increase the righting moment. A very helpful guest on this site directed my attention to the work of Professor J.M.J. Journée and W.W. Massie (Delft University of Technology) The theory of ships are very interesting, and I can highly recommend reading the following two PDF documents for more information.
....or visit
http://www.shipmotions.nl/LectureNotes.html
(NOTE: This drawing will change as the hull shape is not shown quite right) The upper image shows the bow. Remember that the bow has a guard against waves at the top, making it higher than the rest of the hull. The lower image shows the stern. The oval end in the center is where the prop shaft will exit the hull later on. Next is to calculate the amount of water displaced by this design, but it's in the ballpark of what is needed. The planned draught is about 2.75 in. / 7 cm (Est.), which displaces app.16 liters of water over the center 1 meter part of the hull. The stern will displace only very little as most of it is out of the water, so the bow will account for the majority of the rest. At the bow it'll narrow in and end up in the bulk, and at the stern it'll narrow in just before the propeller, thus not obstructing an easy flow to the prop. In large numbers, I'd say that the hull will have this profile for about half it's length.
|
