I now have a working conversion of one of Herbie's previously fluorescent tube lights now replaced with LEDs.
It took a couple of prototypes to get the result I wanted, so here's what I learned.
You need a little 12v voltage regulator to protect the LEDs against over voltage - the boat's batteries can be anything up to 14.4 volts when the engine is running. I tested it with a 14v input. The first regulator I tried worked, but I soon realised that it wouldn't carry enough current to drive all the LEDs needed to make the light bright enough. No problem, I replaced it with an L7812 chip costing all of £0.46 from Bitsbox who are great for small quantities or electronic bits and pieces. A couple of cheap capacitors, one on the input and one on the output to protect against spikes and Bob's yer uncle.
The LED's come in a sticky backed strip
so they're easy to install on the plastic plate where the fluorescent tubes were. The LED strip costs about a tenner for 15 metres and I used less than a metre so call it about 80p. Other bits and bobs -a bit of stripboard, tiny bits of wire and a few blobs of solder. Lets say a couple of quid for the whole conversion - plus postage to buy the bits. £2.20 postage will be enough to buy all the bits to convert several strip lights. Herbie has 7 such units.
I measured the current it draws and its about 180 milliamps which at 12v makes about 2 watts by my reckoning. Compare with the two fluroescent tubes which are 8 watts per tube and you can see the power saving.
Next I need to install it in the boat and compare the brightness against the fluorescents on the other side of the saloon. I'm not sure whether they will be quite as bright. Then the only other thing is to see how long the LEDs last. I'm told they can be a bit dodgy if the electrics aren't right. Time will tell. I won't convert the rest of the strips until this one is proven over the winter.