If you plot the power consumption curve with power on the vertical axis and speed on the horizontal axis the curve is not a straight line i.e. a constant rate of change of power to speed. At the lower speed it is linear but once you get to speeds approaching the square root of the waterline length the power consumption becomes exponential. This is the dilemma with displacement hulls. Thanks. Steve S.
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electricboats@yahoogroups.com, chris Baker <chris@...> wrote:
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> Hi Steve
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> Its true that the extra knot comes at quite a cost of doubling of batteries and solar panels...
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> but is that all there is to it?
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> If I knew the boat was always going to encounter no current and calm conditions a motor of 1 kw would be fine and give a good result. But to allow for adverse rather than ideal conditions we have to be able to handle a strong breeze and contrary current and this why i would still choose this motor over a smaller 1kw model.
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> And as part of this picture I have to deal with the extra cost of batteries and solar panels.
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> Last weekend we took it on the Brisbane River with three different groups of people. On Saturday we were out from 9 am till about 3pm and at the end of the day the batteries had 70% state of charge. On Sunday we were on the water for about 3hrs and explored up Breakfast Creek where there were bat colonies and various waterbirds to catch our attention. Most of the time we were travelling at around 2 or 3 knots, at which speed the solar panels can provide more power than we need. Then for a while on the Brisbane River she was travelling mostly at around 1.5kw to 2.5kw At the end of the trip the batteries had 64% capacity.
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> The battery pack is a 15kwhr thundersky lithium pack and the solar panels are Solara thin panels of 560 watts total capacity. Say we got 1kwhr from the sun each day, and we used about a third of the pack. So about 7kwhr of power was consumed. We were boating for about 9 hours total so that works out to be less than 1 kw average power.
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> On the whole I'm very pleased with the balance of motor/batteries/solar panels.
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> Cheers
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> Chris
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>
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> On 12/07/2012, at 5:52 PM, Steve wrote:
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> >
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> > As the speed (knots) of a displacement hull approaches the square root of its' waterline length (feet) the slope of its' power consumption curve (speed/power) goes from horizontal to vertical. The first KW gave him 4+ knots; the second KW gave him less than one knot. To provide the luxury of that extra KW would require doubling his battery bank capacity and doubling the number of solar panels to keep the larger bank charged. The extra expense is not providing a very good return for the sake of less than a one knot gain. Thanks. Steve S.
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