Duckworks - Development of a Table of Offsets

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Development of a Table of Offsets - Super Pelican

By Ken St.Andre - Auburn, Maine - USA

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 this.

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.