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 |
[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.
