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Old 18-03-2010, 09:58   #16
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Originally Posted by GordMay View Post
According to Bowditch, the distance to the visible horizon in nautical miles can be calculated using the formula:
OK ... where's the rule of thumb for how high my eye is when I'm standing on the deck looking to the horizon
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Old 18-03-2010, 10:24   #17
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I'd modify the hull speed equation to be the K value instead of 1.34 ...
... because I've seen many threads where people think the hull speed formula is wrong, but they are not using the proper K value for their hull.
See Chris Price’s interesting discussion about the Form Factor Constant (K, or C):
Data - Marine Formulae #1

More good stuff from Chris Price:
Sailboat Design Data
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Old 18-03-2010, 15:46   #18
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"A third of the watts of your solar panels is the output in amps for a 12 volt system in good coniditons"

I don't know if this is true or not, but could be a useful quick and ready reckoner.

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I'm afraid it ain't true at all, wish it was ;-)

My 660W array gives me max. 50A at 12V so it's less than 1/10 of the watts.

cheers,
Nick.
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Old 18-03-2010, 16:15   #19
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Quote:
Originally Posted by Pete7 View Post
"A third of the watts of your solar panels is the output in amps for a 12 volt system in good coniditons"

I don't know if this is true or not, but could be a useful quick and ready reckoner.

Pete
Should be 1/12th, right (rather than 1/3rd)? Since Watts = Volts x Amps by definition...
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Old 18-03-2010, 17:53   #20
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I'm afraid it ain't true at all, wish it was ;-)

My 660W array gives me max. 50A at 12V so it's less than 1/10 of the watts.

cheers,
Nick.
Nick 12V time 50A that 600W.
I told you it is all depending of some one thumb size.
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Old 18-03-2010, 18:11   #21
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The precise length of any line is exactly twice the distance from the middle to either end.
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Old 18-03-2010, 18:13   #22
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In reality: when I see 50A the voltage is at 13.5 or so... that would be 13.5 * 50 = 675W so either my panels are better than spec'd or my amp meter is off. I must admit that I never calibrated it. I am currently installing an Outback 60A MPPT controller that will report every little statistic I can imagine so we'll see ;-)

ciao!
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Old 18-03-2010, 18:20   #23
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Nick, Can't believe your output would be that low. We get more than that with 170 watts. Tilting and a MPPT controller.


Jim
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Old 18-03-2010, 18:57   #24
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OK ... where's the rule of thumb for how high my eye is when I'm standing on the deck looking to the horizon
Quiet simple.
Thumb = Pouce = 27.07mm, find how many thumbs between eye and sea, apply Gord formula and bingo another rule of thumb.
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Old 18-03-2010, 20:04   #25
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Think Amp-hours.

660/3 = 220ah,

170/3 = 57ah.

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Old 18-03-2010, 20:29   #26
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Nick, Can't believe your output would be that low. We get more than that with 170 watts. Tilting and a MPPT controller.


Jim
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I find it very curious that with 170W you get more than 675W. I think you found a source of free energy! ;-)

But my output is already more than the panel specifications so I guess with my MPPT controller and tilting I might get 1 kW?

cheers,
Nick.
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Old 19-03-2010, 13:21   #27
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Think Amp-hours.

660/3 = 220ah,

170/3 = 57ah.

Ah ha, so there was a rule, just got the units wrong.

Nick, can you confirm the rule, do you get 220 amp hours on a good day?

Talking of solar, I fitted our first 45w panel this morning, second hand bargin off e bay. Sadly it was so misty in Portsmouth Harbour this morning the commerical shiping even had their fog horns going with vis down to 400 yards. Wet T towel (it started off dry) over the panel so I didn't short anything whilst fitting the wiring. All connected up lights on the regulator looked good, just need some sunshine now to see how effective it is.


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Old 19-03-2010, 16:53   #28
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Ah ha, so there was a rule, just got the units wrong.

Nick, can you confirm the rule, do you get 220 amp hours on a good day?
Hi Pete,

Yes that rule sounds good. We will beat it on a good day in the tropics when we get 260-280Ah but that is exceptional.

cheers,
Nick.
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Old 19-03-2010, 17:09   #29
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Nick hi,

Excellent, now then what to do with my free 15 amp hours each day

Pete
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Old 19-03-2010, 19:36   #30
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The 1 in 60 rule encourages people to steer accurately.

"After 60 miles, we are one mile off course for every degree off course."

So five degrees (or so) is another hour at sea!
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