If you can use a calculator
you can design a boat…

by Morten Olesen

Part 1

Part 2 - Part 3

Who 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
Hull with interior
Engine and tanks

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.