Hi Larry,
Something I didn't mention in my earlier post was that the drag tests I did were on a motor on a lifting carriage - so when the motor is not needed for generating power it was pulled up through the hull. So there was no drag from any of the paraphenalia of prop, motor and supports.
Another point worth making is that I found it very difficult to get a good estimate of how much speed is lost. Because my boat is very light (about 3000kg) but carries a lot of sail (75 sq m), it is very lively and changes speed easily with slight changes in wind speed. After pulling the motor in and out if the water a few times I guessed that the drag was in the range of half to one knot, but even that range has a low confidence rating.
Dave K's comments confirm that on heavier boats the effect is less. So its likely my estimate of cost of drag is at the high end of the range. I also suspect that the closer to hull speed you are sailing, the less the effect would be as well.
What I think would be nice would be a controller that would allow you to dial up as much or as little drag/power as you want at the time. Some days when the batteries are pretty full, it would be nice just to have a trickle to keep them topped up and if it cost a quarter of knot to get say 50 watts that would be fine for me. On other days I'd want every watt I could get and wouldn't mind how many knots it cost me. If I could get 500 watts at a cost of 2 knots of boatspeed, even that could be acceptable when I have a big amps deficit.
So even on the same boat the needs would change from day to day. Where I am right now in Ku-ring-gai chase national park, I've been cruising around here for the past 10 days. Many of the days have been overcast and rainy and the solar panels have barely provided for my domestic use. The inlets are steep sided and narrow, and difficult to sail against the wind because so much tacking is involved, as well the wind being very fluky. So we often chose to motor when the wind was contrary. So one day I ran the generator to top up the engine batteries while we were motoring. If I could have had some regen to do this it would have been nice - and because we are just cruising around the inlets here, there is no rush, and it would be inconsequential whether it took 2 hours or 3 to sail to another inlet.
Cheers
Chris
On 12/03/2010, at 3:35 AM, gramplarry wrote:
chris, dave, myles.
cris you mention 200 watts per hour and being nirvana and 100 wats per hour as being acceptable. There is no free lunch so what drag sacrifice would be acceptable to you. e.g.1/4 to 1/2 knot?
I have to agree with Myles. Originally i thought that only racers would be interested in feathering or folding propellers. The around the world cruisers i have talked with(The company has a booth at the Annapolis boat show every year)quickly sorted that out for me. Cruisers have even more interest in low drag. A 1 knot difference can add an extra day or two to a week long passage. It makes sense if they can average say 5 kt then a 1 kt drag will add 20% more time to the passage.
But is there a break even point for watts vs speed? and what about the comfort and convience factor. Many cruisers enforce very strict power conservation rules while making a passage. how much is it worth to relax those rules?
larry
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