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3.1. Abstract

Let’s begin with a basic concept: there’s no sharp, absolute border between planning and displacing hulls. It’s rather a smooth and gradual shape transition from one type to the other. The drawing of a displacing hull is trickier than the previous one: we don’t have any more sharp lines joining sheer, chine and hull, but curved forms. The designer needs a great sensibility for the hull shape and the capability of figuring out a three-dimensional body: all things which only time and experience shall enhance.

3.2. The hull lines

The design work is similar to the one already described for the planning hull. The reference network is the same and such are the fundamental lines of the hull. This time the common interval is 840 mm and for sure it doesn’t coincide with the actual framing of the vessel.

The vessel that we consider has a 25 m. LOA and 6.30 m. B. Figure # 12 shows some important differences with the drawing we saw before.

The more evident is the transverse sections drawing: it’s not drafted on the vessel’s lines but apart, on the right side of the hull. It’s only a stylish choice. Another remarkable difference is the quick works profile: it shows a kind of “nose”: it’s a bow bulb, and we shall examine it later on. Probably the most outstanding dissimilarity is the transverse section shape: once again we deal with the main section, # 9, which shape is totally unlike the planning hull form.

Fig. 12

Side and bottom aren’t anymore straight lines but fair polylines, joined by a roundish section, called “bilge”. In fact these kinds of vessels are known as “round bilge hull”. The lines show the vessel’s deck in plan view and elevation, and the topgallant bulwarks. The bulwark is a part of the hull which rises above the main deck. In figure # 13 it’s highlighted with a square box.

Fig. 13

3.3. A short preliminary calculation

We do a preliminary check like we already did in 2.7: in other words we verify whether the designed floating line (LWL in figure # 12) fits the forecasted displacement of the vessel. The average Cp for such a family of hulls ranges between 0.5 and 0.6. Let’s assume a 0.50 figure. We measure the half area of the immersed section, which equals 3.679 m². The whole area is therefore 7.358 m². The LWL length is 22.75 metres. The renown formula is: = Cp * Am * LWL : in other words = 1.50 * 7.358 * 22.75 = 83.697 m³.

It’s a correct volume that we take for good as half load displacement, being = 83.697 * 1.023 = 85.622 kilograms.

3.4. The weight modifications

Let’s briefly suspend the displacing hull design to introduce an important concept: the vessel’s weight range and change between the empty and full load conditions. On such a kind of yacht we shall expect to have tanks for fresh water (3,000 litres), black waters (250 litres), white waters (400 litres), oily waters (500 litres), used engines oil (400 litres), new oil for machineries (200 litres), fuel… About the fuel topic we need to do a digression inside the digression: let’s suppose that the vessel has two four-stroke diesel engines releasing 600 hp each. This is the maximum output power, but at cruising speed we would only use 80% of it, aka 2 * 600 * 0.80 = 960 hp. The specific fuel consumption of any engine is shown on a chart, supplied by the engines builder. As a matter of principle it’s placed around 190 grams x hp x hour. The fuel consumption on the vessel of our design would therefore be 190 * 960 = 182,400 grams per hour, equal to some 183 kilograms per hour. Such type of vessel is normally equipped with two diesel engine electric generators. Lets’ presume that one generator releases 30 Kw/h (aka 30 kilowatts per hour) and the second one 12 Kw/h, for a total of 42

Kw/h. Also in this calculation we presume that the generators work at 80% of their maximum load, therefore do roughly release 34 Kw/h. The ratio between Kw and Hp is 1.341: therefore 34 Kw/h equal 45.6 hp. The generators fuel consumption shall be 190 * 45.6 = 8,664 grams per hour, equal to some 8.7 kilograms per hour. The total fuel consumption on a hourly base is. 183 + 8.7 ≅ 192 kg.The maximum speed of the vessel comes from the renown formula , or knots. We deem a consistent speed reduction, as we plan to use only 80% of the power, thus assuming a constant speed of 10 knots. So, the vessel covers 10 nautical miles in an hour, burning 192 kilograms of fuel. Let’s figure out the vessel range as 450 nautical miles: it takes 45 hours to cover this distance at 10 knots speed. During this time lapse the fuel consumption shall be 45

* 192 = 8,640 kg. Unfortunately this is not the amount of fuel that we need to board: as it goes, part of the fuel cannot be drawn because it remains in the tanks bottom and part of it fills the pipes, the filters etcetera. The percentage of unusable fuel is roughly 10%: thereafter the total amount of fuel we need is 8,640 + 864 = 9,504 kg. I wish to highlight that, up to now, the unit of measure has always been kilograms: but at the fuel station you buy diesel by the litre, not by the kilogram. Diesel fuel is lighter than water, on equal volume: it actually weights 850 kilograms per cubic metre. To accommodate 9,504 kilogram of fuel we need a total tanks volume of 11.2 m³.

Coming back to the variable weights calculation we have 4,750 kilograms between miscellany tanks and 9,500 kilograms of fuel, for a total amount of 14,250 kilograms.

The huge difference between full load and empty vessel conditions affects many parameters: the centre of gravity position, the stability, the draft, the speed. Actually the ship is never completely empty, thus for the stability calculations a 10% load is assumed, and this condition is called “ship at arrival”. It’s not difficult to calculate how the draft changes with the weight modifications: an issue that will be later debated.

3.5. Back to the drawing

Similarly to what we already did with the planning hull, let’s add more elements to the drawing.

We decide what we want station # 0 and station # 18 look like. We also introduce a new element: the mark of waterline # 10. We have now four points through which the waterline must surely pass: three half breadths that we measure at the intersection of the transverse sections # 0, 7, 13, 18 and 23 with the LWL, plus the LWL beginning (see figure # 14). Let’s assume that the WL 10 mark is good for the design we’re determined to achieve: this trace gives us the half breadth on the waterline of all the stations. Now we add stations # 5 and # 14.

One might be puzzled by the ostensible arbitrariness of waterline # 10 shape. Actually, the trick is examining body plans of similar existing vessels and gathering how, why and which parts our design work assimilates, which ideas it’s worth blending into our project and which are the elements of innovation.

Going back to figure # 14: there’s a brand new section, 45° inclined, which starts from the transverse sections centreline. This section is called “diagonal”: its origin position and tilting angle are totally arbitrary. It’s a very useful section to verify the fairing of the round bilge area. The designer can trace as many diagonals as he wants: the more you have, the easier is the control of the hull shape trend. Figure # 14 shows the line generated by the intersection of the diagonal with the transverse sections. It goes without saying that the diagonal line, as well as all the remaining lines

representing the hull, must be fair: in case it isn’t we’ll have to go over an adjustment work, as described for the planning hull. Now we add more transverse sections, waterlines and buttocks, always checking the fairing of lines and the collation of the intersections in the three views, till we get to the complete hull lines drawing. I highlight a new element, in the aft section of the hull: it’s kind of a fin or a centreboard, called “skeg”. Its main purpose is to increase the vessel’s course keeping attitude in following sea, but also to protect from impacts the appendages, such as the propellers, the brackets, the rudders. And what about the bulb bow? Books have been written and designers have severely fought about this appendage, about its benefits and its shape. No doubt that the bulb bow improves the performances of very large ships, where it has remarkable dimensions and its effect on the waves generated by the hull is substantial. On the contrary its usefulness on minor vessels is controversial. As for the form, in our design example it has an egg shape. Yet the debate on the topic is open.

3.6. The decks

We design the deck and the deckhouse exactly with the same criteria we used to draft the hull, naturally overlooking any hydrostatic issue. We cut the deck and deckhouse by means of the same stations that we used for the hull, drafting this part of the vessel in three views.

Fig. 14

Chapter 4