Using FreeShip Ship Design Software
The table of offsets development for the 18' Super Pelican from
the 16' Great Pelican.
Having an interest in building a scale model of the Super Pelican
as a study tool in preparation for building a full-sized version,
I decided to develop a table of offsets to describe the changes
in the stern needed when the hull is stretched. The plans for
the Super Pelican describe the process for stretching the hull.
Basically the process is to lay out the jig just like you would
for building the Great Pelican. At the transom, first the transom
is set vertically as opposed to the angle called for in the Great
Pelican plan. Then the location of the transom is moved 2 feet
aft preserving the vertical orientation and the same height from
the baseline. The dimensions of the transom remain the same as
those listed in the table of offsets for the Great Pelican. I
have worked with the software, FreeShip, in the past, and so I
decided to use that computer program to do the work. The FreeShip
software is designed to enter information for design points using
a table of offsets. It then displays the shape of the boat in
the usual design views. The greatest asset of the software is
that it displays computer generated "control lines"
and "curvature plots" that augment the ability to detect
sections of the curve of the hull that are not truly fair.
Setup From Plan
Based on the table of offsets for the Great Pelican provided
by Muriel Short as "Page A" of the Great Pelican plans,
I created a spreadsheet so that I could combine the measurements
for the station locations with the measurements for the heights
and half-breadths in one table. Then using "Page C"
from the Great Pelican plans which provides information for building
the strongback and "Sheet 1 of 2" of the Super Pelican
plan, I determined the measurements from the transom for each
of the station lines. I find the easiest form of data for entry
into the FreeShip software is the decimal form using the 4 decimal
places accepted by FreeShip. A table of the measurements for the
station lines is below.
The data points of the stretched hull without any fairing were
entered into FreeShip and the following lines were the result.
It is easy to see that fairing is needed at the stern of the
hull. It also appears that the data point for Station 1 at the
chine may not be at the right location. This is where FreeShip
really comes in handy. By turning on the fairing assist functions
with the control curves in the software, we can really see where
there are fairing problem. The following pictures show these control
curves. The pink shaded bulges along the lines show where points
need to be adjusted to optimize the fairing of the hull.
These images show the lack of fairness that would be expected
at the points where the hull has been extended without any attempt
at fairing. They also confirm a lack of fairness at the chine
at Station 1. There also seems to be a lack of fairness between
Station 10 and the Bow at the chine level. A closer look suggests
that the height of the chine at Station 10 may be a little high.
Fairing the Hull
Chine at Station 1 and Station 10
First let's look at the point at the chine at Station 1. Correcting
this does two things. First, it might provide useful information
to make some changes to the offsets for the Great Pelican. Second,
if that point is not correctly placed, the fairing efforts to
adjust for the stretching of the hull will not go as easily since
we will be chasing curvature around an improper point. After making
changes to the Station 1 chine point the control curves look like
As you can see, the pink bulge at the Station 1 chine has been
eliminated. The bulge at the bow remains as do those caused by
the stretching of the hull. Next we will deal with the lack of
fairness at the bow. The following images show the lines after
the fairness at the bow has been corrected.
Fairing the Stretched Stern
The next step in fairing the hull will be to clean up the lines
around the stern that resulted from stretching the hull. The plans
suggest that all of the offsets and mold forms between the newly
located and positioned transom and Station 3 should be ignored.
The bottom should be straight from Station 3 to the bottom of
the transom. Also, the mold form at Station 1 will be bypassed
during construction, allowing the plywood to define its own fair
curve. When we put FreeShip to the task of fairing the lines of
the stern using this guidance we come up with the following lines.
Overall this is starting to look pretty nice. There is one small
issue when we look closely at the Profile view. There seems to
be a hump at the location of Station 3. If we switch to the Perspective
view and turn the hull so that we are looking right down the chine
at Station 3 we can see that there is a small problem here. Remember,
the plans state to fair the stern to Station 3 so this point represents
a point from the original plan. Here is what it looks like with
the pink curvature indicator turned off.
This shows the same view with the curvature indicator turned
on. You can clearly see a lack of fairness at Station 3.
When we address this lack of fairness in the FreeShip software
we come up with the following new set of lines.
The boat is originally designed by Captain William Short.
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