Thanks for the explanation and believe me I appreciate the simplification of the science! If time and money allow I plan to give packrafting a go this year. I see so much scope for it over here. The larger fjords scare me a little as I’m not the strongest swimmer so I want to make sure that if (when…) anything goes wrong that I give myself a solid chance of living to tell the tale. Although the MoPacka concept seems great I guess it does mean you have to carry the requisite amount of Platypi (I usually only carry 1.5 litres worth). But then they’re dual purpose and still lighter than carrying a PFD, even if it can replace your sleep mat…. see? I guess I need to do some maths. I could use the kids at school to brainstorm this…

So by your calculations if I managed to eat 7.5 litres of pies I should be good to go?!

I’m going to use some pretty nasty approximations here, for starters, that 1Kg corresponds to 10 Newtons (it’s more like 9.80665…..), and that 1 litre of air displaces 1Kg of water (sorry Archimedes). That’s INCREDIBLY rough, but for simplicity it’ll have to do. A normal foam BA is rated at 50 Newtons, whereas an inland/coastal lifejacket is rated at 150N, offshore 275N etc etc. The way my back-of-an-Esbit-packet calculations work – I need 5 litres of Playpus distributed about the MoPacka to have the same effect as a 50N BA (my physics teacher is crying right now), so as I managed to cram in 7.5 litres that’s almost the same as a 75N buoyancy aid (sob). I also thought about WHERE the ‘buoyancy’ is – if it’s mostly in the back of the vest it’ll assist me to float face down ( I reckon that’s bad), vice versa if it’s mostly on the front pocket (this is good). Body fat is also more buoyant than muscle, so I’ve been eating pies to aid my personal safety.

Another recognised technique is to ram a Thermarest up your jacket – this works, but it’s not quite as elegant or comfortable as the MoPacka. It’s possible to cut arm holes and reseal the mat too, but my little torso-length Thermarest wouldn’t be alot of use. Plus it’ll possibly probably get wet, or pop out in all the excitement of capsizing.

Remember that kayakers use a Buoyancy AID rather than a lifejacket – a BA on its own only assists you to keep afloat, whereas a lifejacket has a much greater chance of overcoming gravity and keeping you above the water with its clever negative-Newtons. Incidentally a deployed lifejacket (like one that uses a little CO2 canister) isn’t that big – it’s surprising how little gas is involved really.

I work in engineering and the gross misuse of principles above makes me slightly bilious – please don’t judge me or risk your life based on ANYTHING I’ve said above or in the past.