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Old 17-03-2010, 17:27   #1
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Question Rule Of Thumb Formulas...

Just how far is the apparent horizon. Square root of the Height of eye above sea surface in feet times 1.17 Found that in a Julian Stockwin book.

Approximate possible speed of a displacement hull of course is square root of waterline times 1.34

After finding the one on distance to horizon I thought to ask for replies listing other useful rules of thumb.

Michael D
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Old 17-03-2010, 17:48   #2
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A wave's speed in knots is 1.6 times it's period in seconds.
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Old 17-03-2010, 17:59   #3
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Rule of Thumb, You can stop by a buddys boat for a shot of Rum on the way home.. and the Wife knows about it before you climb back on your own boat...
Rule of Thumb,
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Old 17-03-2010, 18:00   #4
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My favorite rule of thumb: With your arm outstretched, hold your thumb sideways. Each thumb width equals 20 minutes of sunlight measured from the horizon. In other words, if the sun is two thumbwidths above the horizon, you've got about 40 minutes until sunset. Kinda fun.
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Old 17-03-2010, 19:00   #5
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I love the sunlight rule of thumb Geo mentions. We used to use that to determine when we had to quit waterskiing and head home when I was in college. It always held true(ish)! Another rule is any project will cost twice as much as you plan, and take 3 times as long. I've tried to disprove this theory several times, but it's a stubborn rule that always wins.

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Old 17-03-2010, 19:15   #6
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I love the sunlight rule of thumb Geo mentions. We used to use that to determine when we had to quit waterskiing and head home when I was in college. It always held true(ish)! Another rule is any project will cost twice as much as you plan, and take 3 times as long. I've tried to disprove this theory several times, but it's a stubborn rule that always wins.

Jeff
I can vouch for the extremely high accuracy of the three-times-as-long rule. It ought to let me estimate how long quite accurately as three times my best estimate, yet I always feel "This time will be different" and get proven wrong again. Insanity? Probably.
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Old 18-03-2010, 04:11   #7
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See Mitchell Charity's excellent (& fun) pages on approximation.
Rules of thumb (and other body parts) every coastal pilot should know (or have available).

Your Body Ruler - A User's Manual
Your Body Ruler - A User's Manual

Measuring Angle and Distance
Measuring Angle and Distance with your Thumb
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Old 18-03-2010, 04:22   #8
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Arm outstretched, fist closed, the angular difference is approx 10 degrees.

Arm outstretched, hand spread open fully, the angular difference between tip of thumb to tip of little finger is approx 15 degrees.
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Old 18-03-2010, 05:44   #9
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Rule of Thumb, You can stop by a buddys boat for a shot of Rum on the way home.. and the Wife knows about it before you climb back on your own boat...
Rule of Thumb,
man, ain't that the truth!
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Old 18-03-2010, 05:45   #10
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At my marina we like to say: Give a man a beer, he's drunk for a day; teach a man to fish and he's drunk for a lifetime.

(we have several cases of beer that have been dropped overboard over the years).
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Old 18-03-2010, 06:04   #11
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Just how far is the apparent horizon. Square root of the Height of eye above sea surface in feet times 1.17
Michael D
1.17, 1.15. They must have different size thumb.
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Old 18-03-2010, 06:57   #12
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Different sized thumbs, should you have one such, can be accommodated by “calibrating yourself” per: ➥ Your Body Ruler - A User's Manual

According to Bowditch, the distance to the visible horizon in nautical miles can be calculated using the formula:

Distance = the square root of the eye height in feet x 1.17
D = 1.17 x Hf
-or-
Distance = the square root of the eye height in meters x 2.07
D = 2.07 x Hm

* depending upon whether the height of eye of the observer above sea level is in feet (hf) or in meters (hm).

Here ➥ http://www.irbs.com/bowditch/pdf/chapt22.pdf
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Old 18-03-2010, 08:59   #13
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I'd modify the hull speed equation to be the K value instead of 1.34. While 1.34 may be a typical K value for many cruising monohulls, it can vary considerably. I mention this 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.
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Old 18-03-2010, 09:52   #14
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At my marina we like to say: Give a man a beer, he's drunk for a day; teach a man to fish and he's drunk for a lifetime. (we have several cases of beer that have been dropped overboard over the years).
"Give a man a fish and you will feed him for a day. Teach a man to fish and you ruin a good business opportunity".

Pete
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Old 18-03-2010, 09:55   #15
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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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