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Old 04-05-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Brehm62 :

Let me see if I can explain this.
Wind Power = 1/2 * Rho * Velocity^3 * Area

Rho is the density of air which is about 0.0807 lbs / cubic ft.
If we are using cubic feet then we need Velocity in feet also.
Let's say the wind is blowing 6 knots.
That is about 6.87 mph or 10.08 feet / sec.
Let's just round that down to 10 ft/sec.
Let's say we have 200 square feet of sail area.

Okay: 1/2 * 0.0807 lbs / ft^3 * (10 ft/sec)^3 * 200 ft^2
0.04035 lbs / ft^3 * 1000 ft^3/sec^3 * 200 ft^2
8,070 lbs (ft^3 / ft^3) * ft^2/sec^3
8,070 lbs ft^2/sec^3
Now we have to divide by gravity which is 32 ft/sec^2
8,070 lbs ft^2/sec^3 / 32 ft/sec^2
252.1875 lbs (ft^2/ft) /(sec^3 *sec^2)
252.1875 lbs ft /sec
1 Horsepower is 550 ft lbs /sec so this is a little less than half a horsepower
0.46 HP


Well explained that man!
A word of caution.
My High School maths teacher woulde be jumping up and down like a hairdresser at THREE horrible sins within your explanation....

1 : Never, but never put units in the middle of equations. Write them above and below the equations if you wish but never within the equation.

2 : The unit you use for force should be the lbf ... do not forget the "f" after it.

3 : Worst sin of all, NEVER pluralise a unit. Units have, by definition, no plural. If you put an "s" after a unit, it denotes "second", so let's have none of this "lbs" stuff ever again.


Apart from three sins, it was a good explanation.

The thread has become rather confrontational. For me, I just put the sails up, choose a course, and trim the sails until they just stop flapping. My ship is fastest when the relative wind is ahead of beam.

One of the beauties of sailing, methinks. I am fastest when the wind is slightly against me.

Now there is a pleasant thought.

On my beloved Loch Ness, the wind is either right on my nose, or right on the back of my neck, as the land masses either side channel the wind right at me, or behind me. Unless I choose to sail across the Loch, that is. It's only about a mile wide.
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