Well, I checked off pretty much the last major item on the ever-growing TODO list for WiTHiN today. I was supposed to be out on my bike all day today, but it snowed yesterday and rained all day today, so I postponed the ride for tomorrow and made further progress on WiTHiN today.
The only major item still left is the lake test – and this is the big one. I know WiTHiN floats and is fairly stable from the pool test. I also know that the drive, prop and steering works. What I do not yet know, and this is HUGELY important, is how fast WiTHiN is. If for some reason, our calculations are wrong about the hull shape and prop specs using my known rpm and power, then there is a possibility WiTHiN won’t be efficient enough for a 24 hour distance record attempt. So far, everything has worked like it should, but I know from experience that you just can’t assume things until they have been tested.
Again, the calculations that I really hope to verify this weekend at Glenmore Reservoir, are as follows:
RPM = 78 rpm (verified)
Power input = 149 watts (verified)
Weight = 122 kg (verified)
Drive efficiency = 95% (verified)
Prop efficiency = 84.7%
Speed = 10.2 kph
If I spend 24 hours at 150 watts, then my ending average including everything should be about 110 watts. 110 watts converts to 9 km/hr average speed, so I should be good for 216 km or so. The current HPB record as recognized by the human powered vehicle association is 168 km. The rules are here.
Now, if I really messed my prop up, and the hull drag is WAY higher than we estimated, then I would need to be no more than 12% slower (7 km/hr) to still travel 168 km.
I have a separate update coming detailing all of the progress on finishing WiTHiN that has been completed over the last week.
Anonymous on May 7, 2007
found polymorph in the UK – great stuff
i suppose you could wax the hull?
stephen
Anonymous on May 7, 2007
Hi Greg,
It looks like you are making great progess. It will also be great to see how you do once you hit the lake. I'm sure you're wishing for warmer weather.
I looked at drag figures in "Fluid-Dynamic Drag" by S. F. Hoerner and estimated that the 1/2" thick drive leg strut would have a coefficient of drag of around 0.3. Based on that I calculated that the strut will cost you about 1 watt per inch of length at 10 kph. I was guessing that your strut is around 10" long, so that would be 10 watts just to push the strut through the water.
This is not going to stop you from setting the 24 hr record, but it's one area for potential improvement for the ocean crossing design. With the right design you can probably cut the power consumption by a fact of at least 4. Every little bit helps!
Hoerner also has some good information on things like the fairing of the strut to the hull. Joints can significantly increase the drag.
Peter Raymond