Talador
I will assume the question is directed at me.
1 watt = 1 Newton x 1 metre/second
Watts = 800 x 4 = 3200W
1 HP = 746W
HP = 3200/746 = 4.3HP
The major loss is with the prop. Big prop with high aspect blades rotating at low rpm is more efficient than a small prop spinning at high rpm. A pod drive at 1200rpm is high rpm so prop will have low efficiency - maybe 65% or worse. Then there are the electrical and mechanical losses to account for.
The 800N is for a 4 tonne proa with 18m lw hull at 4m/s; in calm sea conditions, no windage, perfectly smooth glossy hull, no in-water appendages like pods or rudders. Basically it is what you would get in a tow test with a 4m/s tail wind.
Rick
On 26/03/2015, at 4:33 AM, "taladorwood@yahoo.com.au [harryproa]" <harryproa@yahoogroups.com.au> wrote:
Peter - "Power is a function of speed times drag. So 800N at 4m/s (say 8kts) equates to a power input of 3200W. At 60% efficiency that becomes 5.3kW"
I have tried to convert that to hp, but I am totally confused. A Newton = a watt so 800 N = .8kW so where does the 3200W (4.26 hp) come from? It seems low, even the 5.3 kW seems low.And why are you using a 60% efficiency? I thought you said the Electrical/motor system was apx. 90% efficient?Also these figures are for the boat not each individual hull? Correct?Thanks in advance,Talador
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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