“As you know, I've been playing with stove and kettle design and have now got some new test figures to share. You can help me with the maths if you like but I think it's around 60% efficient :o) My stove also weighs 6g including the primer cup and the kettle is 2oz without insulation, which weighs another 1/4oz.
425ml water at 10C to boiling at 99.6C
9.4g (12ml) methhylated spirit plus 0.5ml for priming. ( roughly 95% ethyl alcohol 5% methyl alcohol)”
With your figures I calculated 61.8% thermal efficiency and for the efficiency ratio that I use I calculated 9.85g/80ºC/0.5l, not to bad at all. A 2 oz (57 gram) pot or kettle is very light, I look forward to finding out a bit more about your design but I suspect greater surface area of the kettle design and slowing down the hot gas flow helps with efficiency and boil times.
You are right about it is nice to get a cuppa sooner rather than later, when my walking partner came back from the US with his JetBoil I could not believe how fast it was for a cuppa, in testing I have had times of 2m 30s to boil using only a little more than 4 grams of fuel. My tests are done in my garage with no wind and I try and do them with an ambient of around 20ºC to give consistency, there is nothing like testing a stove in the field especially in windy cold condition, a few if my earlier remote canister stoves have failed in the field, I always took a backup.
”Now here's the reason for quoting your comment about boiling times, my kettle has the boil time down to 5mins 45secs for the 15 UK floz (around 14.4 US fluid ounces. Fast boiling isn't everything, but it's nice to get that cuppa sooner rather than later on a windy cold day. ;-)
Gives the pot less time to lose heat to the atmosphere and a stronger stove flame is less affected by cross winds in adverse field conditions too. I guess there are different optimum compromises to be made depending on the type of country you like travelling through and the seasonal weather you encounter.”
You have brought up a very good point about fast boiling times and I do not fully understand what goes on with efficiency and boiling times but I will go back to some earlier testing that I have done, this might help explain my statement about time and efficiency. Below is a graph that I have previously posted the results of boiling 1 liter of water from 20ºC to boiling 98.5ºC using a Trangia (Alcohol) red line, MSR Whisperlite (White gas) blue line and a MSR pocket Rocket (canister gas) yellow line, in all tests the flame was set to as fast as the stove could go then graduated to the slowest (or smallest) I could set it. In these tests I used a 1l Snow Peak Ti pot.
I will start with the Pocket Rocket (Yellow line) I was trying to find the optimum valve setting for fuel used vs time and in the graph it is clearly shown that with boiling 1 l of water boiling faster than 12 minutes uses more fuel and the faster the boiling the more fuel used, but after 12 minutes this there is no improvement in fuel used, I have repeated this test many times with 1l and 0.5l (which produced a time of 6 minutes for the optimum boil time vs fuel usage) and it was always the same results. On this information I base my statement “slower heating is more efficient that fast boiling (fast boiling and efficiency with stoves is a myth that manufactures have perpetuated to sell stoves)” for it is mainly the canister gas stove manufactures that use fast boiling times as a claim of high efficiency.
With the Trangia (red line) I was able to slow the flame down by the simmering ring that comes with the Trangia. The Trangia did not use more fuel the no matter how much longer it took to boil the 1l (this is the same for 0.5l). I do not know if it has any significance but please note that the fastest boil time for the Trangia of 12 minutes seemed to be the same as the optimum boil time for the Pocket Rocket. I have not tried these tests with a faster heating alcohol stove as I do not have one that can heat faster. The times of 6 minutes to boil 0.5l fits in nicely with your 5m 45s to boil 0.425l.
I have only started testing my Volcano stove, but on one of the faster test that I did, it took 8 minutes and used 10.7g/80ºC, slightly more than other tests, I have not done enough tests on the volcano to see what is the optimum efficiency time, I will do some more testing on the weekend.
With the Wisperlite (blue line) it is a very inefficient stove and blasts most of its heat out the sides of the pot when the flame was slowed down the efficiency improved and I suspect that a point could be reached that the efficiency would not improve with time, this stove was very hard to adjust.
I hope this helps