Measuring a Boat or Ship
How can a boat or a ship be measured when no plans or drawings exist.
The measuring has to result in a plan containing the necessary measurements and interrelationships, so that a boat builder can reproduce the measured boat.
We shall first look at the equipment, material and tools that the ship gauger will use. The successive steps from taking the individual measurements to producing the lines plan shall then be explained.
Material, measuring equipment and tools that the ship gauger will use.
The spirit level is the basic instrument for measuring.
If a correct plan is to be produced the spirit level will be used from the first up to and including the last measurement to check the basis from which measurements are taken for squareness.
The plumb line, used in conjunction with the spirit level, ensures that all measurements are taken from a square basis.
A tensioned rope will show the line through the keel and the posts.
This can be either a folding rule, a measuring tape, a tape measure or a laser measuring device.
The plumb lines end up on the batten and the distances are measured.
The batten is used both horizontally and vertically.
In the example a straight beam is used as the batten.
Hammer and nail(s)
The plumb line is secured to a nail which is lightly tapped into the hull.
Note book and rough sketch
The principal (numbered) lines are drawn as a rough sketch and the associated information is recorded in a note book.
The flexible ruler allows different points to be joined by a flowing line.
- First of all a tensioned rope is tied between the stem post and the stern post (image 1). The first important piece of data, namely the length between the posts, can be noted on the rope. The length of the keel line is easily measured on the keel beam.
- The height of the stem and stern posts is now recorded.
- To record the curvature of the stem post a plumb line is suspended from the highest and most forward point. From this plumb line a number of points are taken at 90 degrees. The distance (horizontals) between the plumb line and the post are recorded. The more points are taken the more flowing and accurate the drawing will become (image 2 & 3). In the example shown, six intermediate points were taken, in addition to the highest and the lowest points. After this we have the profile of the stem post. The same procedure is used to measure the sternpost (image 4).
- A second important step is to determine the outside lines. For this, the measurements are taken from underneath the wale. After all, this is a free, uninterrupted line. The drawing will be more flowing and accurate when more measuring points are taken towards the stern post and towards the stem post. The different distances are recorded using the batten positioned at the height of the keel beam. The widest beam is an important result here (image 5,6,7 & 8).
- The number of strakes is recorded (image 11). The distances to under the wale are recorded.
- Deadrise. The deadrise is the degree to which the plane runs to the edges. This is measured in exactly the same way as the skin. In the example the plane does not have any deadrise.
- Any bulkheads are recorded and measured in respect of both the stem post and the stern post.
- As a check, the distances are also recorded from the plane through the keel and posts to the skin (diagram 1). The plane through the keel and the posts is formed by the centre line of the keel and the rope that is tensioned between the stem post and the stern post.