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Foam Sandwich construction - Part 4
Drawing the molds:
This file is an excerpt from the foam sandwich boat building instructions supplied with our plans.
The instructions supplied with the plans are more detailed and contain information specific to the boat.
For faster loading, we divided the smaller online version
in several sections.
Boat building books describe different methods to transfer the station lines to the molds material.
All these books require the builder to draw the outline of the molds
full size on the shop floor or on a large plywood platform and transfer
that outline to the molds material.
Note that this is not lofting. Lofting is a much more elaborate
task during which the lofter draws the all the boat lines, WL's,
buttocks and stations full size and corrects drafting errors due to
scaling problems.
Boats designed in 3D CAD do not require lofting. The dimensions
extracted from the 3D models are much more accurate than what can be
achieved through lofting.
The picture above shows a traditional stations drawing.
There may be more stations and the diagonals are not always shown.
The designer can show the station dimensions in different ways. The
traditional method use a table of offsets instead of dimensions.
How to read the offsets:
Reminder: no drawing is necessary if you use Mylar templates or have your molds cut by a CNC shop from our DXF files. In that case, skip this paragraph and go to the molds set up.
A table of offsets is simple to use. All the points used to draw the curves are located either on a waterline or on a buttock. The table gives either a width or a height for each point.
The picture above shows a traditional table of offsets. Each column shows points on a station. For example, column 5, row WL1 is 2-11-7. This means that on station 5, the intersection of waterline 2 is at 2' 11-7/8 from the centerline.
We use another type of offsets drawings. We supply one drawing per station and show each point on that drawing.
The drawing above does not show a real hull station.
If the boat has 20 stations, we provide 20 drawings as above.
In this case, the Designed Water Line is the also the baseline. The
drawing is in millimeters (metric) but units do not matter here.
We like to name the grid lines by their distance to the baseline or center line.
WL750 is a waterline 750 units above the baseline (DWL) and V150 is a buttock or Vertical 150 units away from the centerline.
Our offset system is much easier to read and reduces the risk of errors.
Often, less points are required. In the case of hard chine hulls, the offsets can be very simple. Here is an example of a station drawing taken from our LB26.
In this case, at that station, the bottom section is straight and requires only two points but the side is cambered and we show more points.
Stations outline :
To draw the molds from a table of offsets or from our stations drawing, the first step is to draw the grid on which we will mark the points.
Start by setting up a "drawing board. This is usually sheets of plywood, often painted white.
Our grid will correspond to the WL (waterlines, horizontal lines) and Buttocks (Vertical lines) show on the plans
On that grid, we will mark all the points that define one station and join them with a fair curve.
Note that you will probably use this drawing to cut not
only molds but bulkheads and frames. It is much easier to draw the
lines once and take all outlines from the drawing instead of taking the
dimensions of bulkheads and frames dimensions from the inside of the
hull after it is built.
Taking dimensions from the drawings is also more accurate. While it may
take you a day to draw all the curves, later you will save you several
days of work when building the inside.
If the designer takes it in consideration ( as we do), the stations
will be located as much as possible at the main bulkheads and frames.
Reading the dimensions:
While the drawing shows two sides for clarity, we will only draw one side and use it to draw the other side.
Start by identifying the "corners of your station: keel (we call it fairbody), chine and sheer.
The keel point is on the centerline at 343 below the baseline.
The chine is 64 below the baseline and 1039 away from the centerline.
The sheer is 779 above the baseline and offset 1208 from the centerline.
We can draw the bottom by joining keel and chine.
For the cambered side (this boat has tumblehome), we need more points.
On waterline 600, we mark a point at 1211, on WL 450, another point at
1205. We proceed the same way for all the waterline points.
Then will draw a fair curve through all those points, from the sheer to the chine.
To join those points, use a batten and weights or
nails. We show pictures of the method with weights in our other
tutorials but here is one using nails and a thin batten for a small
boat station.
The batten can be wood or plastic. Some have used a PVC pipe with success.
Once we have the station, we can draw the mold outline.
Most of the time, the station shows the outside of the hull but the
molds must fit inside the hull material. The mold is smaller than the
station.
Almost all designers draw the stations as sections through the outside surface of the hull.
There are good reasons for that: the designer doesn't know what exact
size of mold battens will be used if any and there may be material
thickness options.
In the case of foam sandwich, the molds will be on the inside face of
the foam and we must offset the outline of the mold by the thickness of
the foam plus the average thickness of the outside skin.
As an example, for our LB26, from outside: fiberglass skin average 8 mm, foam 15 mm, total offset 23 mm.
There are many ways to offset that line, we like to use
a reducing wheel: a simple round piece of plywood with a hole for a
pencil in the middle, the radius is equal to the offset.
Run the reducing wheel along the batten we used previously and it automatically draws the correct mold outline.
Now that we have the mold outline, we must transfer it to the mold material.
Transfer outline to molds material:
The Gougeon book shows 4 methods of transferring the
stations outlines to the molds. We like the tracing paper method. Copy
the line to tracing paper (translucent bond) and transfer from the
paper to the mold with a pounce wheel.
You can either make one copy per mold or better, make a copy of several
molds on a large piece of pattern paper and transfer from there to the
molds with the pounce wheel.
Another valid method is the nails system:
This works well with particle board molds. Line up a row of nails with their heads along the outline of the mold then press the mold on the nails. The nails will mark the material and you can draw a line through those points.
Last but not least, instead of drawing on a separate table and then transferring to the molds, it is valid to draw each mold directly on the mold material. The drawing lines are visible on the molds in the picture below:
Whatever method you used to draw and cut your molds,
each mold should have crucial lines clearly marked: centerline and
baseline, sheer line and DWL if shown on the plans.
If you plan to use a laser beam to check alignment, mark the location
for the holes now. The alignment method is described later in the set
up section.
Molds can be made from solid sheets of particle boards, plywood or framing wood. For large boats, it is often necessary to have access to the inside of the jig, this may require to cut large holes in molds made from full sheets of particle board
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The information above is based on Jacques Mertens experience with foam sandwich construction since 1977 and on technical literature from:
- CoreCell
- Airex
- DIAB products: Divinycell, Klegecell, Renicell
- Dupont Nomex
- Nidacore products
- Raptor
Thanks to Evan Gatehouse for the technical proof reading and suggestions.
Thanks to our builders who volunteered to proof read for comprehension and grammar, in particular Glover Housman.
Copyright 2007 Jacques Mertens
