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Hanno,

re: Bygrave etc

As you know from my Fuller 2, I am no mechanical engineer! However, I found the Bygrave replica relatively easy to build, using ready available precision plastic pipes, ordinary domestic ink jet printer/gloss photopaper/+felt friction brakes/water proof paint spray. Easier than the Fuller (no clumsy brass cursers to adjust).No engineering skills required.Only real skill was the scales,(in my case, courtesy Wayne Harrison).

I've used mine satisfactorily at sea for 5 years. Totally seawothy, waterproof and consistently accurate to 1-2'. Never had a "slippage" which is the main design problem. Only maintenance is annual service of the felt pads to prevent slippage. I think my replica could easily be built commercially at very litte cost, but again, I'm  no expert on that.

The basic Brown-Nassau even easier. My polycarbonate replica is good at sea too.

Obviously, not  necessary pieces of kit. I just use them because it is fun (for me anyway).

The calculator is  better, but boring, and anyway, if calculator, why not smart phone/tablet with easy celnav /LOP app for pennies? In rough weather, any "calculation" is likely to be error prone just because of the rocking and rolling! Also, how many of us consistently get sextant readings to 1' in the back yard? Now how about in a 35ft boat in a seaway? Umh! Not easy! So total error likely to be mostly sextant readings in my experience.

Hav Doniol best manual back-up for me, but I just like to use slide rules too.

I'm hoping to test and compare different methods at sea in the next few weeks, health/doctor/children/weather and wife allowing!

I still owe you some emails on the various  Hav-Doniol and BN adaptations.(which, thanks again for your  help, and Greg)

Many thanks and keep up the great work.

Francis