Hi, Thanks for the info. The measurements at bow and stern are paticulaly useful to know. I was wondering just what line to follow at the bow. (I am sure I have seen the waterline sweeping upthe bow following the profile on some models.) its clear now so thanks very much

Excellent pic of an acutual setup 👍 . So much easier to see the thing in action. Please let us know how you progress. Additional info is always welcome and I would never object or keel haul anyone 😀

Lesfac - the information Dave has given is spot on but this was one area that had me confused for some time and therefore thought it appropriate to add a little more info which may make life easier.

Hopefully the Fleet Admiral won't keel haul me as a result 😰

The measurements (for the 34" Fire Boat) were worked out from the original plans and are all measured from the deck with the Stern being 6.4cm and the Bow being 9cm.
Then all that is needed (as Dave points out) is to make a suitable pencil holder and arrange the markings to achieve the level needed to scribe the line evenly all the way round.
You will notice from the photo that I used a load of books to achieve the appropriate level and an area where I could work all the way around without any interference (kitchen table)
Once the line was on I then found that the boat was slightly twisted/not level and so a few adjustments were made to ensure equal spacing.
Good luck 👍

Any paint masking tape would provide the straight edge you require. I have found the Frog tape very good but its not cheap. Tamiya provide masking tape and this is also good.
I am assuming you have already fully cleaned and prepped the hull before you started the restoration. As it has been stored for many years any splits in the wood or glue joints will also need to be fixed.
To achieve a level line you should place the boat in a cradle (use a spirit level) on a flat table, then attach a soft pencil with rubber bands to a piece of wood, Adjust the height of the pencil to where you require the waterline and mark all round the hull, being careful to keep the wood and pencil perpendicular.
Your chosen masking tape can then be applied to the marked line, making sure there are no gaps.
Paint the base and put some paint on the masking tape. Check from time to time and when a test on the masking tape paint indicates the paint is finger safe, carefully remove the masking tape, ensuring you do not touch the new paint until it is completely hardened.
If you leave the tape on until the paint hardens you run the risk of leaving a ridge between the new and old paints.
Good luck 😀

I have some information in an email and a couple of photos on how I did this will post tomorrow night as currently using my tablet.