Excellent question, Matti! Here is the evidence we have: 1. The attachment points for the ends of both the upper and lower railings survive, which provide the location and cross section of the railings, as well as the joinery. This shows, for example, that there is a horizontal knee between the after face of the upper railing and the fiferail. 2. The large horizontal knee abaft the lower railing survives on the starboard side, which indicates the angle at which the end of the lower railing meets the side. 3. One of the balusters survives, which shows how the balusters join the railings and provides the distance between the upper and lower railings at some point. The angles at the ends of the baluster show that it was not located at the centre but towards one side or the other. 4. The lead of the mainstay collar has to run over the lower railing, since there is no other place to attach it than the bowsprit (this is another long logic train we can discuss elsewhere - for now just accept that this is so). This means that there cannot be a centre baluster or shield/sculpture. 5. Related to the above, the baby head sculpture which Landström reconstructed as part of a large frieze on the face of the balustrade actually comes from the beakhead end (I can provide details for those who are interested). 6. Stairs on the face of the beakhead bulkhead. 7. The surviving starboard cathead has a mortise and bolts for the attachment of the lower railing, thus showing the height of the railing above the deck at this point.
So, from all of this we can determine directly: 1. The dimensions and joinery of the railings. 2. The angle at which the lower railing meets the side. 3. The space between the railings and the height of the lower railing above the deck at the catheads. 4. There must be an even number of balusters. 5. The space between the balusters must be large enough for a man to pass through relatively easily (or it would not be possible to use the stairs on the face of the beakhead bulkhead).
This leaves a few things to determine: 1. What is the curvature of the upper and lower railings? 2. How many balusters? 3. What decorative details and other fittings might have been attached to the balustrade?
The asnwers we can provide are: 1. The angles at the ends of the lower railing leave very little scope for the arc of the railing, so connecting the ends with a fair arc is relatively simple. The height of the arc in the middle might vary, but only within a range of about 50 mm. This is how the arc on the reconstructed railing on the ship was determined. Knowing the spacing between railings and the location of the attachment points at the ends shows that the upper railing is approximately parallel to the lower. 2. We cannot be sure of exactly how many balusters, but it cannot be more than eight and still leave room to pass between them. Six balusters, as are reconstructed on the ship, give plenty of room. The spacing also has to leave the area directly over the cathead clear, since we believe there is a large cleat here for the cattackle fall and cat stopper (these lines cannot go anytwhere else, and there are a pair of bolts at the right angle for a cleat). This space is clear with six evenly spaced balusters, but it is also possible to have more balusters and leave extra space between one pair where access is wanted. 3. There is almost certainly a large cleat on top of the lower railing where it crosses the cathead each side (see above). There may have been a bolster on the lower railing where the mainstay collar crossed it. The lower railing is also the best place to belay the fore tacks. The size of the lodging knees at the ends of the lower railing suggest that the railing, which is more of a beam, was subjected to some stress against which it had to be reinforced, and the cat tackles and tacks would be a good explanation. As for decorative mouldings, there is no direct evidence but the connection of the upper railing with the fiferail suggests that it might have carried a similar profile to the fiferail.
I hope this answers your question, and shows how we work through the process of reconstruction. The reconstructed balustrade on the ship is based on similar reasoning, and so it the best approximation we can come up with. The 1:10 model was built in the 1980s, based on an earlier reconstruction, as are the 1980 plans and Landström. As with everything else about the ship, our ideas are constantly in the process of refinement as we do a better job of recording or locate more information. This is why we decided in 2007 to start over from scratch and record the whole ship in one go, at a consistent level of detail.
Thanks for taking the time to make a thorough explanation Fred, I find stuff like this really interesting. How you manage to decide the original details, and the changes due to new methods or approaches.