Accurate marking of a boot-top line is simple with a couple of blocks of wood and a small spirit level, as Richard Toyne explains

It is very common on boats of all types to paint a boot-top or stripe around the waterline. At first glance this line may seem like a simple bit of decoration, but it does have a couple of useful purposes.
Firstly, a coloured stripe can help to prevent fouling around the waterline by allowing antifouling painting higher up the topsides, and secondly, a boot-top will seem to reduce the freeboard, making the boat appear longer and lower in the water.
For both of these reasons I decided to paint a boot-top around Sigfrid, my 10.5m (34ft 6in) steel ketch, when she was ashore in Sheppards Marina, Gibraltar, a few years ago. In my case the existing waterline looked too high, so I was actually marking the boot-top below it.
It could be easy to assume that marking out a stripe would be a simple process of measuring the required distance, in my case 12.5cm (5in) from the existing waterline. If you do this, however, due to the changes in flare and tumblehome, the finished boot-top will look as if it varies in thickness – in most cases it will appear to taper at the ends.
My initial plan to avoid this problem was to use the same method I’d employed several years earlier to mark out the original waterline, by setting up a system of scaffold boards and tightly stretched strings.
As it turned out, on the day I tried to mark our boot-top, a strong wind was blowing, which made the strings vibrate to such an extent that this approach became impractical.
Noticing the trouble I was having, a passer-by in the boatyard described a technique he’d seen a local boatbuilder use.
This method, which can be used to either raise or lower the waterline, produces an accurate result, can be carried out by one person and requires minimal equipment: a couple of blocks of wood, a small spirit level and a pencil. The combined thickness of the two blocks of wood should equal the desired width of the boot top, and a chamfer needs to be planed on the end of one of them. The chamfer is important: its tip will enable your block of wood to touch the hull despite the changing angles involved.
n Hold the two pieces of wood one on top of the other, with the chamfered one underneath. In my case I held the upper edge of the top block against the existing waterline and used the spirit level to ensure it was horizontal. If you’re raising the line you’ll need to hold the lower edge of the bottom block against the waterline.
n Slide the lower block towards the hull until it touches, and mark the position of the chamfered edge with a pencil.
n Repeat this process at regular intervals all around the boat.
When the marks are joined together, as long as the blocks have been kept horizontal, the vertical distance between the new line and the old one will be constant, even though the actual thickness of the stripe varies.

Richard Toyne

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