The square top mainsail

[Bill Roberts]

I want someone to explain to me how the square top on a square top mainsail works aerodynamically?Is How is it diffweent from a pin head sail aerodynamically? Is the square top sail an improvement over the pin head sail and why? Don't tell me it is because the square top sail has more area. That is not the correct answer.

[DanBerger]

I'll start.

Answer: because they LOOK faster, so they MUST be faster!!

OK, seriously, I can't explain it as well as Bill can, but the way I see it: in big wind, the square top gets pushed to the leeward which opens the leech and bleeds off pressure at the top of the sail. It is like an automatic transmission.

I'm not sold on them in the light stuff. I think the pinheads are better in the light stuff because of the twist in the sail.

OK, let the flogging begin..

[DanBerger]

And, while we are at it, why don't we have square top JIBs??

It seems like the leech of the main opens and changes the shape of the wing and the air that passes over it. With a pin-top jib, it won't adjust the flow of the air to match the new shape of the main. If you had a square top jib, it would better match the new shape.

At least, I've been pondering that for years...

[Mac M]

A friend had Randy Smyth build a set of sails for his I20 and it had a square topped jib. Not sure of the difference it made, ill have to ask.

Now the flogging can really begin

[Bill Roberts]

OK, before we get into the discussing the square top sail aerodynamics, let's get the "twist thing" set to rest.
Twist in the sail is needed to match the twist in the wind so that the sail can operate at the best angle of attack from bottom to top.
We sail on "relative wind speed", not true wind speed. Relative wind speed happens because we sail across, move across, the true wind speed. We are sailing across the true wind speed in any direction we sail except straight into the wind or staright downwind which are directions we don't normally sail.
Why is there twist in the relative wind direction and speed? Remember that relative wind speed is the vector sum of true wind speed and boat speed. Now, true wind speed has a velocity gradient in it. By that I mean that the true wind speed increases in velocity at increasing heights above the water. In a 10 knot breeze, at water level the true wind speed is 0 knots; at 5' above the water the true wind speed is 5.4 knots; at 15ft above the water the true wind speed is 7 knots; at 30' above the water the true wind speed is 7.9 knots and at 100' above the water the true wind speed is 10 knots. Our sails normally operate in the 5' above the water to the 30'above the water height range. The wind speed near the foot of our sails in a 10 knot breeze is 5.4 knots and at the top of our sails is 7.9 knots. The true wind speed is an ever increasing part of the relative wind speed as we move up the sail to the top. This casuses the relative wind direction to have twist in it. So, the sail should have twist in it to match the twist in the relative wind angle so that the sail is operating at "best angle of attack" foot to head. Example: Let's assume we are sailing to windward, 45 degrees off the true wind direction, at 5 knots boat speed in a 10 knot breeze. At the foot of our sail the relative wind is crossing boat centerline at 22 degrees at a relative speed of 7.6 knots. At mid sail height the the relative wind is crosing the boat centerline at 25 degrees at a relative wind speed of 8.7 knots. At the top of the sail the relative wind is crossing the boat centerline at 26 degrees at a relative wind speed of 9.4 knots. So, the "relative wind", which is the wind we sail on, has 4 degrees of twist in it. The sail therefore needs to have 4 degrees of twist in it to have the whole sail, top to bottom, operating at the same angle of attack. The desired, best, angle of attack of the wind ranges form 20 to 25 degrees sailing to windward depending on th cut of the sail; let's call it 23 degrees. So, on starboard tacck, the sail operates at 23 degrees clockwise relative to the relative wind direction and the true wind direction is another 22 degrees clockwise from the relative wind direction.

Another point as long as we are here: In our example the boat speed bent the true wind 22 degrees and the sail operates best at a 23 degree angle of attack. This adds up to 45 degrees which is the angle at which we sail to windward relative to the true wind. When we tack , we turn through 90 degrees to be sailing at 45 degrees to the true wind on the opposite tack. Now we understand the sail twist thing and why we sail at about 45 degrees to the true wind and why we tack through about 90 degrees sailing to windward.

The aerodynamics of the square top sail coming next.

[DanBerger]

Ugh. I just got another Bill Headache.


So, for another related question (or more like observation): I'm finding that down wind, I go faster with my traveler out to about the footstraps (or less) and the main sheeted in about medium? tight. I remember on the Hobie 16s, everyone would put the traveler all the way out and sheet in tight. In light wind, we still kept the traveler all the way out, but let out some of the main sheet.

I set the mainsail the same way on my A cat in light wind, but when it picks up and I can Wild Thing it, I play the traveler. I have a seperate main sheet and traveler line that I tie together in a big knot. It happens to be in a sweet spot so that I can hold on to the knot and play the traveler comfortably-- I think I just got lucky on that one. I sheet the main pretty hard, head up to heat it up some, then when I pop the hull, I let out some on the traveler and carve down wind. It's fun and fast. The A cat goes a lot faster (than the Sc) and has a higher aspect square top main, so why are the two so different down wind??

I have an idea, I just want Bill to encorporate it into his lecture.

[Bill Roberts]

Professional,
Hobie 16 sailors set the traveller and play the mainsheet because the traveller doesn't work, won't move, under laod. So forget all your H16 super fast sailing tips. Another thing, don't trim your sails to a traveller position and or a mainsheet position.
If you really want to know how to trim your sails correctly, put some tell tales on them top to bottom and luff to leech and both sides different colors and go sailing. Learn what the sail looks like when most of the tell tales are flying against the sail pointing aft on both sides of the sail. Now you will gain knowledge the applies to any sailboat. The traveller position basically sets the trim angle of the mainsail relative to boat centerline. The mainsheet tension sets the twist in the sail, the leech. It opens and closes the leech. Boats with jibs trim the mainsail very different from boats with mainsail only. The jib turns the air flow toward boat centerline in the bottom half of the mainsail. The top half of the mainsail sees the airflow entering more from abeam and don't forget the twist in the wind so the mainsail top needs to be more open, more twist. The tell tales will tell you all of this and more. Go for it!

[Bill Roberts]

Back to the square top mainsail: To better undestand the benefits of the square top mainsail, we first need to understand the faults of the pinhead mainsail.
We all know that as the air approaches the mainsail/mast the air flow field splits. Some air goes around the windward side of the sail, the high pressure side of the sail and some of the air goes around the leeward side of the sail, the low pressure side of the sail. It is this pressure difference across the sail that creates the thrust from the sail. Any time there is a pressure difference across a sail or membrane, the air tries to leak or flow in such a way as to equalize or reduce the pressure difference.
Now let's go to the top of the pinhead sail. Consider the top 10% of the sail, the top 2 or 3ft for example. On the leeward side of the sail the air accelerates around the curved sail dropping the static pressure. This envelope of low pressure is somewhat airfoil shaped with a maximum value at about 30% sail chord and tapering off to zero at the leech. The pressure side air flows across the windward side of the sail and reaches the leech. Now remember that the leech has a slope to it as it approaches the head of the sail of approximately 45 degrees. This top 10% or so of the sail has high pressure on one side and low pressure on the other side and the air on both sides of the sail is flowing from mast to leech. When the pressure side airflow reaches the leech it is going to flow to the area of lowest pressure. The area of lowest pressure for some of the windward side airflow is to flow over the leech and vertically down the sail on the leeward side of the sail and into the "low pressure envelop". This reduces the pressure difference across the top area of the sail and reduces the sail thrust. It is the "forward sloping leech" that allows this leakage airflow to happen.
SQUARE TOP: IF THE LEECH IS VERTICAL, THIS LEAKAGE AIRFLOW DOES NOT HAPPEN. WITH A VERTICAL LEECH WHEN THE PRESSURE SIDE, WINDWARD SIDE OF THE SAIL, AIRFLOW REACHES THE LEECH, IT IS NOT EXPOSED TO THE LOW PRESSURE ENVELOP ON THE LEEWARD SIDE OF THE SAIL AND DIRECTLY BELOW THE POTENTIAL SOURCE OF LEAKAGE BECAUSE THE LOW PRESSURE ENVELOP HAS ALREADY COME BACK TO ZERO AT THE LEECH. WITH A VERTICAL LEECH THERE IS NO FORCING FUNCTION THAT CAN CAUSE THE WINDWARD SIDE OF THE SAIL AIRFLOW TO FLOW OVER THE LEECH AND DOWN THE LEEWARD SIDE OF THE SAIL. THE SQUARE TOP MAINSAIL DOES TWO THINGS.
1. IT MAKES THE SAIL AREA MORE EFFECTIVE.
2. IT PROVIDES MORE CONTROL OVER SAIL TWIST.

SQUARE TOP JIB: IT ALREADY HAS A VERTICAL LEECH. THEREFORE IT IS A TWIST CONTROL DEVICE ONLY.

[J Drew]

Bill,
After reading part 1 a few times, and now part 2, this explains alot.
Thank you!

[Kevin Keller]

Thanks for your explanations Bill. Please keep posting them. It helps our understanding of what's actually going on.

I think I asked earlier but why hasn't anyone put a small winglet on top of the mainsail (T shaped) to stop any leakage from the high pressure side to the low pressure side?

Kevin

[havliii]

Here's my question: If a tall pin top sail is cut down and footage added to the uppermost portion of the leech (and the total sail area remains the same) are we essentially making the sail more effective by moving the square footage to a new location? and when doing so can the mast now be shorter and the CE lower on the total SA.

If, I understand this correctly it seems like a triple home run.

[J Drew]

I would think that existing batten location might be a problem ??
When you put the diagonal batten at the top

[Bill Roberts]

When I tested the first square top sails back in the 1980s, I made the square top area up by cutting down the leech, straightening out the leech from the foot up. The total sail area was kept constant. Testing showed the boat to be slightly faster with the square top sail. The improvement was small but it was always there.
To cut a pin head sail down to reach a point where the sail is wide enough to be an effective square top, 40% or so of the foot chord, to much sail area has been lost.
Remember two things: The top of a pin sail is not a total loss in effective area, it is just compromised. It is less effective than the sail area below that has a near vertical leech, but it is not useless. It is producing some forward thrust. The luff of a sail is where the forward thrust comes from. A shorter luff is usually slower than a longer luff sail. The top of a pin head sail is not junk; it is just compromised relative to the top of a square top mainsail.

[Bill Roberts]

Point: I have never seen in print in any US sailing magazine an explaination and understanding of the need for TWIST in a sail.
I have never seen in print in any US sailing magazine an explaination and understanding of the benefits of the "square top sail".
Why is that????
Why isn't this "common knowledge" on Catsailor or the Hobie Hot Line?