Posted by: svwindward | 2016-03-04

Tech: Why do sailboats move with the wind?

Long-time sailor and member John Middaugh explains what makes sailboats move. You can grab the PDF version for easy printing and reference, but here’s an OCRed version:

John Middaugh, January 6, 2016
Many thousand’s of years ago, a fisherman in the North Atlanta discovered that a piece of rag on his boat moved the boat down wind. Thus, what we know as the Square Rigger had its beginning.

England, France, Spain, & Italy were the prime developers of the Square Rigged ships.

The Lateen sail similar to the type used on the Sun Fish, originated in the Mediterranean and in the Nile river area. This sail has the advantage of pointing better into the wind than the Square Rigger.

The Bermuda rig, the type we use, came much later.

Several principles have to be accepted to fully understand, why wind moves sail boats.

The first, is that our atmosphere which extends up to 440 – 6,200 mi above the earth; a one inch square column of this air weighs about 14.7 psi at sea level. It also must be understood that it pushes out in all directions an equal amount. Nature abhors a space without air, and/or a lower pressure area. The atmosphere at 14.7 psi tries to fill & equalize that low pressure area. Example, when sucking on a straw in a glass of water, a low pressure area is created in the straw. The atmosphere senses this, and pushes down on the water in the glass & up the straw into your mouth. If the top of the glass is sealed off from the atmosphere and the glass is full of water, the water will not come up the straw when sucking on the straw. I have demonstrated this at the last children’s Sailing School.

The second principal, discovered by Daniel Bernoulli, is that as air velocity increases, air pressure decreases. And, as moving air is forced to slow, the pressure builds.

The third principal, is that there is no such thing as a negative, or vacuum force. There is only
positive forces created by the 14.7 psi pressure exerted by the weight of the air and movement of the wind moving to equalize low pressure areas.

The fourth principle, is that free air unlike contained air ( air in a tire ) is encompressible. This is most important to understand.

Lets apply these principles to our sail boat using the Bermuda Sail Plan. That is, a triangle jib and main.

Lets assume the sails are set for close haul. The wind coming across the port side into the fore triangle. This is the area bordered by the forestay, mast, & deck. On my boat, it is about 247 sq ft opening. The slot area, the space between the leach of the jib & the main is, (just guessing ), about 75 sq ft. Thus, the air exiting the slot is traveling about 33 times the velocity of the air entering the foretriangle area. Or, if the air entering the fore triangle area is a 10 kt wind, the air leaving the slot is about 33 kts. I doubt if it is that high, but you see what I mean. A pretty good increase. The increase velocity of the air, reduces the pressure in the slot area and on the leeward ( bow side ) side of the main. Bernoulli’s theory.

The wind hitting the windward side of the main is slightly slowed due to the curvature of the main. Thus, the pressure on the windward side of the main is slightly increased. It is interesting to note that, this air pressure force acts perpendicularly to the curve surface of the sail. Thus, some forces point forward & some point to the side. If all the individual forces were combined in to a single resultant force, it would be about a fourth back on the curve of the main & at an angle of about 70 degrees plus or minus several degrees to the centerline of the boat. Only about one third of this force actually drives the boat forward.

Let’s assume that the pressure on the leeward side of the main drops 0.034 psi and on the windward side, increases 0.007 psi, the addition of these two changes equal 0.041 psi. Multiplying 0.041 times one half the main’s area in sq in (15,984 sq in), equals 655 lbs of force. The forward half of the main does all the work.

The aft half just helps shape the front half. In fact, if sheeted in too much, the aft half can actually retard foreward motion. Now only about one third of this force is pushing foreward, most is pushing side ways causing the boat to lean.

Thus you can see, it is most important to maximize the low pressure in the slot area, since it has about twice the driving force than the windward side.

The Jib experiences the same forces as the slot, but not to the same extent.

Remember all of the wind’s driving force is created by the atmosphere’s 14.7 psi air moving to fill a lower pressure area. That is, an area in the vicinity of Watauga Lake & in particular the lower pressure on the leeward side of the jib & main and to a lesser extent the positive pressure on the windward side of the jib & main.

References: Sail Power by Wallace Ross; Sails by Jeremy Howard-Williams.


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