Les,
You are totally right in doing things your way, in fact a lot of the actual 'water line' markings on the lifeboats are applied using an adhesive tape - I've seen them pulling them off at RNLI HQ Poole when de-commissioning a boat in the past. I've also seen them removing the tape at Ramsgate, on their Trent, and then masking and taping a new line on after a small 'accident' during a storm and the rescuing of a yacht. At least you should get parallel lines. The hardest part will be the wrapping it around the bow [where it sometimes has a mind of its own, due to the bend in two directions], but on a warm day you might just be able to coax it with a few rubs of the them. The tapes from BECC cure on over a 24 hour period, but will require gentle warming if you need to remove them. The only problem here is the lengths you get on each length, they are enough, but there will be joins, but so are there on the real lifeboats. Again here and on the overlaps required to get the tape across the spray rails and hull moulded lines it is best to overlap and then out of sight, underneath, cut through with a sharp knife [scalpel] so as not to damage the finished paintwork, peel back and remove unwanted sections and re-position - it will hardly notice, it at all. It is sometimes easier to apply the line with the hull upside down, once you have marked where it is to go and are satisfied with the position.
Lastly, as a non-rivet counter there are two blessings. Firstly you should live longer than the rest of us and secondly, remember what Phil Warren the famous surface warship fleet builder said, 'it's a hobby and you should build to your own standard and at your speed, anything else is a chore!'. He is so right. Although we all want to get it right, the main thing is to enjoy it. All boats in real life are slightly different and you will sometimes hear even a 'rivet counter' say 'that's good enough' or 'that will have to do!', so don't beat yourself up with such a big beast as the 1/12th Severn. Remember what I have said before too, just keep asking away, we've all done the same and still do [sometimes off the record though!!], then you get umpteen suggested ways of doing it and you can choose the best way for yourself.
Keep up the good work and keep smiling and taking the pills, you'll get there long before I have finished one of mine!!!!!
Kim