Part 1Who hasn’t worked with ideas or sketches of ones own dreamboat? Shouldn't it be better, faster, cheaper and prettier than all the other boats designed up 'till now? It is not difficult to design a boat if you follow some easy design steps. Step 1) be aware of what you want Always start by defining what you want. Most people start with some ill-defined ideas about their dreamboat. The first thing you must do is to define exactly what you want. In order to get the ideas well described, write down a list that defines the boat. This list can be called a design brief. It will help you through out the entire design process and it will help you keeping focus. Here is an example of some questions you can ask yourself when you make the list: What do you intend to use the boat for? Should it be racing, cruising or fishing, or is it a combination of different purposes? Is it a boat for sail or power? How many bunks do you want? Should there be a toilet and pantry? How big a boat do you what? What materials do you want to use for the boat? Step 2) look for inspiration All designers look for inspiration in other boats and there is nothing wrong with that. So see if there are other boats that meet the demands in your design brief or take fragments from different designs and combine them to form a concept you like. Be aware of the size of the boat that inspires you. It is not at all possible to just scale a boat beyond a certain limit of approximately 10-15%. If you, for instance, scale the length, beam and depth of a boat by 2 then the sail area would be scaled by a factor of 4 and the displacement of the hull will be scaled by a factor of 8. So you see the proportions of the boat no longer match. Step 3) make some sketches Now that you have made the design brief and found some inspiration, you should have a better idea what the boat should look like. The next step is to start sketching. (click image to enlarge) The sketching does not need to be fancy. Remember you are at the start of the design process and there are things that will have to be corrected later on. Simply make a sketch containing a waterline/chine plane, a deck plane, a profile and a midship section. Use a reasonable scale, perhaps 1:20 or 1:25, and find paper that is 11”x17” or A3. Don't make the boat too peculiar. For an amateur, the odds of designing a good boat are much better when you stick to something known. Also try not to make your boat too narrow. It is much easier to give a boat better stability or different trim if you have some displacement to work with, and the wider beam will give you that. At this time it is a good idea to make some preliminary sketches regarding interior. Make some copies of your hull sketch and draw in your interior to see if the hull fits the requirements in the design brief. You may have to correct either your design brief or hull. If, for instance, you have stated that you want to make a 14’ boat with 4 bunks, pantry and toile, you will need to make some serious considerations about what is possible. Step 4) estimating the weight To determine the stability of your hull, you need to know the weight of the different elements you place in the boat. There is no easy way to do this but to do it right, you have to make a complete list of all elements' weights and centres of gravity (cog). At this stage you can start with some estimations and in part 3 of this article series there will be a deeper discussion of this issue. In order to estimate the weight you can start by comparing your boat with other designs. From step 2 you should have some good ideas about where to start. Compare the weights of the different boats and make some simple calculations regarding your own boat. Be aware of what weight you compare with what. Different designers have different ways to state their weight; e.g. some state it only for the bare hull and some for the hull with all gear, crew etc. The simple calculation may look like this: Element Weight [ kg] Hull with interior 1900 Engine and tanks 350 Gear 450 2700 At this stage it is not necessary to take the distribution of the weight into account. That will be considered in more detail in part 3 of this series. Now the question is: How does the preliminary weight match the hull sketched previously? Again it is necessary to make estimations. When designing a hull there are certain coefficients that can be useful. One of them is the prismatic coefficient. This is a number that reflects the relationship between the displacement and a prism with equal area as the midship section and with a length equal the waterline length of the hull. E.g. if a hull has a waterline length of 6,0 m and the midship section has an area of 0,658 m2 the prismatic volume will be: 6,0 x 0,658 = 3,95 m3 If the same hull has a prismatic coefficient of 0,7 it means that the hull has a displacement of: 3,95 x 0,72 = 2,84 m3 If the hull has to float, at this waterline, in fresh water it means that the boat will have a weight of 2840 kg. From your hull sketch you have the midship section. So the only thing to do is find the area of the midship section. Remember that the area has to be the underwater area of the section. Multiply the area with your waterline length and you have the prismatic volume. To determine the displacement use a prismatic coefficient between 0,54-0,75. This means that for fine ended hulls use the number 0,54. Now see if the hull you have sketched and the weight you have estimated can be made to fit together. You are not at all done yet, but you have managed to get started and make some preliminary sketches and calculations. In the example you can see that the weight estimated is smaller than the displacement of the hull. This is not a problem since it is normal that the weight will be higher when you get in to details with your weight calculation. The whole process of going through the different steps is called a design spiral and you have to go through this spiral several times before your design is finished. Every time you go through the spiral you should come closer to the final result making your dreamboat better and better every time. Not every round in the spiral needs to cover all 4 steps, but it may be necessary for you to correct all of them. In the next 2 articles there will be an more about line drawing and hydrostatics and also an elaboration of the weight issue. Part 2 The Line Drawing (click image above to enlarge) The most useful tool when designing boats is the line drawing. The line drawing clearly defines the hull geometry. Furthermore it is used when calculating hydrostatic properties for the hull you are designing. (click image above to enlarge) If you have followed the steps in part 1 of this article series you will already have the start of the line drawing. But there is more drawing to do before it is finished. Start with drawing your design water line (DWL). The design water line is the waterline you expect your hull will float at. Later on it may show up that the hull does not float at exactly this waterline but don’t worry about that at the moment. Next you divide your waterline in 10 equal lengthen pieces making 11 stations. In the profile plane the stations 0 and 10 are made where the profile of the hull is cut by the DWL. Now you have a grid to start with. The next thing to do is to begin constructing the stations. The stations cut the hull in the same way as the frames will be placed in the boat later on. But the stations have in principle nothing to do with the frames in the boat. (click image above to enlarge) Now with the grid started it is time to draw some more stations in the body plane. As an example you can see above how it is done. In the profile plane the intersections between the hull and the stations are continued into the body plane. Then the ‘Dist 1’ and ‘Dist 2’ from the half breadth plane are set out from the centreline in the body plane. In the body plane you now have 3 points that define the section. In the example there is used a chine hull so the 3 points are sufficient for defining the section. Continue with section 1, 7 and 9. You can fill in the rest later. If you are designing a traditional hull the 3 points will not be enough to define the entire section so you will need some waterlines and buttocks as well. (click image above to enlarge) You can add as many waterlines (WL) and buttocks (B) as you want. They have the purpose of helping you define the hull shape more accurately. When you add the waterlines and buttocks you make more intersection points so you will have more points helping you to define your curves. Normally waterlines and buttocks are made with an equivalent distance between them, but you can place them at any distance you choose. (click image above to enlarge) The rest of the line drawing is now made. Normally curves should be smooth but since the example is a chine hull you can see some bends where the waterline cuts the chine. Making a line drawing is hard work. It is not unusual for a designer to spend days working on one, so don’t expect to make yours in an hour or so. You must also be aware that the lines may have to be changed many times during the work. Even lines you thought were right may later need to be altered. This is normal and there is no shortcut to that. Drawing tools Beside a ruler, pencil and eraser some tools for making curves are necessary when making the line drawing. If your drawing is large you can use a thin batten held with weights, but if your drawing is smaller or there is large curvature some curved rulers are necessary. It is possible to get a large variety of curved rulers but they are not all well suited for the purpose of drawing hull curves. The most complete set of ship curves is the Copenhagen ship curves. The ship curves are produced by Linex (www.linex.dk). The set of curves are outstanding but unfortunately it will also be quite expensive to buy them all. I can recommend that if this is the solution you want you can buy curves A1, A2, A11/15 and A11/29. They will fit most needs. If you want to spend less on curves you can use 3 curves from the ‘103DT Burmester Set’. The curves can be used for making hull curves but they are not outstanding. I have often seen the curve set in bookstores for $15-20. CAD In this article series cad for hull design is not described but there are many cad applications available for hull design. They range in price from free to many thousand dollars. So all the sketching and drawing on paper can be avoided and instead you can do all the work on your computer. It shall be said at once that using cad for hull design is great, it saves a lot of the hard work for you get results quickly. But there are problems when designing with cad programs. First of all, are you sure that the hull you designed with the cad program is the hull you wanted in the first place? Many times, if you are not careful the hull you get is the hull the cad program or the limits in the cad program designs for you. So you sort of loose control over your hull design process. Anyway it can never give you any problems knowing the old fashion way of doing the design process. It might even give you an advantage when using a design software. Part 3 Hydrostatic There are many hydrostatic properties that can be calculated for a hull. Within the limits of this article series only the displacement and the longitudinal centre of buoyancy (LCB) is calculated. From the line drawing it is possible to calculate every station's underwater area. In the example a chine hull is used, so calculating the areas should not be a problem. If you are designing a traditional hull it can be more difficult calculating the area under a curve. One possibility could be to draw the stations on graph paper and then count the squares under the curve. This will give a reasonably accurate result. Remember that your line drawing is only made with a half station, so the area has to be multiplied by two in order to get the entire area of a station. Station Area [m 2 ] 0 0 1 0,1584 2 0,3474 3 0,4936 4 0,5952 5 0,6578 6 0,6880 7 0,6920 8 0,6750 9 0,6418 10 0,5972 When you are through calculating the areas you should have a table like the one above. Next step is to use Simpson’s rule for approximating definite integrals. It sounds complicated but it is quite easy to use. Every station area is first multiplied with Simpson’s factor (SF). Then the sum of all the products are divided by 3 and multiplied with the longitudinal distance between the stations. Station Area [m 2 ] SF Res. 0 0 1 0 1 0,1584 4 0,6336 2 0,3474 2 0,6948 3 0,4936 4 1,9744 4 0,5952 2 1,1904 5 0,6578 4 2,6312 6 0,6880 2 1,3760 7 0,6920 4 2,7680 8 0,6750 2 1,3500 9 0,6418 4 2,5672 10 0,5972 1 0,5972 15,7828 In the example the longitudinal station distance is: s = 0,648 m. The displacement for the example hull becomes: 0,648 x 15,7828 / 3 = 3,409 m 3 If the boat has to float in freshwater the weight at the construction waterline will be 3409 kg. But since saltwater has a density of approximately 1025 kg/m3 the boat will have a weight of 3,409 x 1,025 = 3495 kg when floating at the construction waterline in saltwater. Thus saltwater has a greater buoyancy than freshwater due to the higher density. To determine the longitudinal centre of buoyancy (LCB) it is necessary to make some further calculations with the station areas. The next technique introduced is the moment calculation. This technique can be used in various situations. First it is necessary to define a fixed station and in this example the fixed station will be station 10. Then every station contributes to the moment calculation by their area multiplied with the longitudinal distance from station 10. Station Area [m 2 ] Arm [m] Moment 0 0 6,480 0 1 0,1584 5,832 0,924 2 0,3474 5,184 1,801 3 0,4936 4,536 2,239 4 0,5952 3,888 2,314 5 0,6578 3,240 2,131 6 0,6880 2,592 1,783 7 0,6920 1,944 1,345 8 0,6750 1,296 0,875 9 0,6418 0,648 0,416 10 0,5972 0 0 5,5464 13,828 The longitudinal distance becomes: 13,828 / 5,5464 = 2,493 m This means that LCB is laying 2,493 m forward of station 10 i.e. between station 5 and 6. The moment calculation is made from station 10 but the moment calculation can be made from any station desired. Just remember from witch station the calculation is made when evaluating the result. Weight calculation The weight calculation is carried out in order to calculate the longitudinal centre of gravity (LCG) for the boat. In principles all elements, even every screw or nail, should be counted in the calculation. But of course it would be a large job to do that, so in practice all major parts should be counted in and then a certain amount added for the small things left out or forgotten. It is normal to divide the weight into different main areas. You must determine which areas are suitable for your hull, but a minimum must be; hull structure, superstructure, interior, installations, ballast, rig and sail (if a sailboat) and payload. In order to make the rest of the weight calculation you must make some serious considerations regarding the construction, appearance and layout of your boat. You have to decide how you would construct the hull, what the cockpit and roof should look like, how the interior should be and what engine installation you will have. When these things are decided and drawn you can make a trustworthy weight calculation. For every area considered, not only must the weight be known, but also the centre of gravity must be known. With many things the centre of gravity is not known precisely so an estimated centre must be used. The centre of gravity for every component is used in a moment calculation similar to the one made in the hydrostatic part. Later on, when the calculation is finished the resulting centre of gravity for the boat is found. Below you can se an example of the weight calculation for the hull structure. Hull Structure Item Weight [kg] LCG [m] Moment Scantlings plywood 431 2,910 1254,21 Scantlings epoxy and glass 312 2,910 907,92 Transom plywood 38 0 0 Transom epoxy and glass 23 0 0 Frames 172 2,490 428,28 976 2590,41 LCG hull = 2590,41 / 976 = 2,654 m forward of station 10 Now it is only a question of finishing the rest of the main areas in the weight calculation. When that is done you can make a new weight and moment calculation that gives the total weight and LCG. Item Weight [kg] LCG [m] Moment Hull structure 976 2,654 2590,30 Superstructure 586 2,937 1721,08 Interior 637 3,236 2061,33 Installations 779 1,345 1047,76 Payload 590 2,493 1470,87 3568 8891,34 LCG = 8891,34 / 3568 = 2,492 m forward of station 10 As you can see, the weight is a bit less than the displacement calculated. The difference is thus so small that in practice it is nothing compared to the uncertainty in the calculations. Furthermore it can be seen that the LCG is laying a bit aft of the LCB. This means that the boat will have a small trim, but again it will have no influence in practice. It is not certain that your first set of hydrostatic and weight calculations will come out like the example above and therefore it may be necessary to go back through the design spiral, each time correcting and refining the drawings and calculations.
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