Photo 40. Hull planking taper gauge
Copyright © 2006, James Allen
The hull planking taper gauge

Constructing the graph:

This the hull planking taper gauge graph that Robine has drawn for her model. It is constructed by drawing a centreline along the length of a sheet of paper or white card. A "maximum plank width line" is then drawn at one end at a normal (90 degrees) to the centreline.

One side of the "maximum plank width line" is labelled as the starboard side and the other as the port side. Tick marks are then stepped out on both sides of the centreline along this "maximum plank width line" exactly one full plank width apart. There must be enough tick marks to represent the number of planks to completely cover the maximum distance between the keel and the gunwale. Hopefully the number of planks will be the same for both sides, otherwise there may be a serious error in the symmetry of the hull shape.

A focus point for plank tapering is then arbitrarily marked near the opposite end of the centreline. Lines are drawn from each tick mark on the "maximum plank width line" to this focus point.

A second line is drawn at a normal to the centreline at the point where distance between tapering lines is exactly half a plank width. This marks off the no-go zone where a plank width must not be tapered to less the half their full width.

The finished graph is shown in the photo above.

How it works:

Plank width reference lines are drawn around the hull running directly from keel to gunwale at convenient intervals along the length of the hull. On larger models the hull is divided into two or three sections between the keel and the gunwale, and each section is measured and planked separately.

Robine chose to plank the hull without tapering from the gunwale down to the rubbing strake, and then taper the planking between the rubbing strake and the keel. To do this she glued three full width planks from the gunwale to the approximate position of the rubbing strake, which on her model was to be attached on the outside of the second layer planking. She used contact gel to attach the second layer planking.

Robine then laid out the first tapered plank along the hull beside and firmly against the lowest of the untapered planks. She then filed the end of the plank to butt accurately against the stem post and cut the aft end of the plank to overlap the transom slightly. This would be sanded flush with transom after the plank had been glued in place. She then marked each of the plank width reference points from the hull onto the new plank.

A strip of paper was laid around the curve of the hull from the edge of the last installed plank to the keel at the reference line with greatest distance. This measurement strip was then laid on the graph at 90 degrees to the centreline on the relevant side of the graph (port or starboard) and slid along the graph as close as possible to the maximum plank width line to the point where the calculated number of planks for full coverage of the remaining space would fit into the distance marked on our strip of paper. The distance between the tapered lines at this point represented the required width of the plank at that reference point. This width measurement was transferred to the new plank at the corresponding reference point. Planks were always tapered on one edge only and the untapered edge was always butted against the previously installed plank for consistency and to minimise irregularities in the gaps between planks. Note also that hull planks often have to be bevelled on their edges to avoid excessive gaps appearing between them where they need to roll around sharp curves on the supporting bulkheads or frames.

Each of the other width reference points were measured in the same way as the widest point bearing in mind the number of planks required to cover the remaining distance at the widest point. To avoid confusion each plank taper line used should be removed from the graph as each plank is completed and installed on the hull. Also the main trick with successful planking is to measure, taper and install one plank at a time. That way the next measurement will automatically compensate for any previous measurement and tapering errors.

Robine also installed a number of short stealer planks at the stern of her model to avoid tapering her planks excessively at the bow. She consulted various model planking publications to determine how best to do this.