Regarding mainsails, some cats have loose-footed booms where the foot of the mainsail is not secured to the boom, ie Prindle 18, whereas other cats have secure-footed mainsails where the sail is connected to the boom by sliding the foot of the sail up a track on the boom using a bolt-rope, ie Hobie 16.
What is the major purpose of these differences?
Does it make any difference in technical terms?
Is one rig better than the other?



--
TurboHobo
H14T
H16
P18
G-Cat 5.0
P16
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My understanding is the bolt rope foot mainsail is simple. The loose foot mainsail allows for more adjustments. Making the loose foot "better" if you was a performance boat.

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Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
--

I hear you Scott, on my H16 the mainsail was permanently attached to the boom, made for easier rigging/derigging of the mainsail, on the P18, it takes 5 min longer to rig mainsail, then attach boom to mast, then attach mainsail to boom outhaul car, then tension everything up with both outhaul and downhaul. But if I was performance orientated, I would have a performance cat like a F-18, I'm not, I do have the need for speed but want to balance that with setup/breakdown time.



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TurboHobo
H14T
H16
P18
G-Cat 5.0
P16
--

Loos foot sails force you to pay attention to the set and make you a better sailor in my book. The pay off is speed.

So the big question is does anybody KNOW? I would like to know the answer.

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Nacra 5.2
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Quote

So the big question is does anybody KNOW? I would like to know the answer.

Know what? Which one is better? What do you define "better" as? It appears Turbo was looking for simple, so the fixed foot would be better for him.

If you are talking performance the loose foot rig must be better since every modern performance cat I have seen has a loose foot. All those engineers and designers can't be wrong about which is higher performance.

So:

Quote

What is the major purpose of these differences?

Performance vs simplicity

Quote

Does it make any difference in technical terms?

Performance

Quote

Is one rig better than the other?

Performance better with loose foot. Simplicity better with fixed foot

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Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
--

i find that with my old sails on my H18, the loose footing provides more adjustment to make up for the age of the sail. making an old sail still quite usable, albeit not for racing. thus i don't need a new set of sails... this is "better" for me...

Yup! pretty much what I had already figured out even with my limited experience, the H16 mast base had a stopper and limits the amount of mast rotation but most performance cats have the nylon ball sitting on top of a rod and there is no limiter, in fact most performance cats even have a mast rotator bar to manually rotate mast. And then there is the 2:1 downhaul, which most racers upgrade to even 6 or 8:1, I can only imagine the forces exerted on a mast with a 8:1 downhaul; and then you have the 2:1 outhaul, I'm sure that is upgraded as well, all in an effort to stretch the sail as tight as possible to increase airflow over the wing/sail, so as to increase overall boat speed. But unless one is schooled in airflow properties and how to read "telltales" on one's sail, one will reach a plateau with sailing, which can only be overcome with the help of someone more experienced.

I think I have just about reached that stage.



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TurboHobo
H14T
H16
P18
G-Cat 5.0
P16
--

between a full set of teltales and a bridal mounted wind vane, thats your most useful set "gauges" on a cat...i love that hobie bridal mount wind vane, your eyes are already looking that way, no looking up at the masthead all day. i have got three teltales on the main and jib...you get a real clear picture of whats happening that way. yup..take your heading, fly the teltales, and sail the wind...petal to the metal.

--
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bill harris
hattiesburg, mississippi
prindle 16- "BLUE RIBBON"
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Don't con a con man. What makes it simpler? I have had 2 h16's and threading the boom on the mainsail was not simpler than attaching my 5.2 main with a quick release pin. Now as for performance, there has to be a reason for the loose footed main to be better performing. It all has to do with air flow, but I cannot figure out why air flow would be more efficient with loose footing, as a matter of fact I would think the loose foot would be a more disturbed air flow.

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Nacra 5.2
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With my old H16 I left the boom on the sail all the time, just rolled it up in the sail. So I just slid the front of the boom down into the mast track once the sail was raised and put a little tension on the outhaul and was done. No mast rotator to connect and not much of a downhauler or outhauler to adjust. My H18 has a loose foot and probably takes slightly longer. But personally like the ability to disconnect the boom and let the main flap in the wind without boom attached on the beach, versus having the boom swing around on shore.

Performance wise you are able to make a wing shape in the sail all the way to the foot with a loose foot, or flatten out the sail, depending on conditions by adjusting the outhaul and having that batten at the bottom of the sail, it allows you to shape the bottom of the sail. With a bolt rope bottom you have either a straight foot (boom shaped) or a pocket once your sail is old and blown out. No good way to adjust the foot of your sail as I recall.

With the fixed foot sail, the only thing the outhaul does is determine how tight the foot is pulled, it doesn't really change shape since it is fixed in a track on the boom.

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Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
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That was very lucid and makes sense. Now my question is why is a wing more efficient because the bottom cannot be shaped or does the rest of the wing make up for the inefficency of the bottom edge.

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Nacra 5.2
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Quote

Now my question is why is a wing more efficient because the bottom cannot be shaped

Guess I don't understand the question. Are you talking about the Americas Cup wing sail?

More efficient because the bottom can't be shaped? I don't follow....

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Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
--

No i meant that a wing sail is more efficient and the bottom cannot be shaped. Now does it follow that a fixed foot is more efficient? I am just trying to keep the conversation lively.



Edited by golfdad75 on Dec 18, 2010 - 07:15 AM.

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Nacra 5.2
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I get it, with the sail rigged loose-foot, you are able to curve the sail more by easing off on both the downhaul and outhaul, more so than on the fixed-foot sail, but without knowledge of airflow properties, it don't mean squat. I don't know if more curve to the sail is better than less curve, or under what circumstances, ie, more curve on more wind, or less curve on less wind, or vica versa. That is why I said that one would reach a level in sailing which could only be overcome with more knowledge, knowledge gained from a more experienced sailor.

I guess one could educate oneself using the internet...........!




--
TurboHobo
H14T
H16
P18
G-Cat 5.0
P16
--

golfdad75

No i meant that a wing sail is more efficient and the bottom cannot be shaped. Now does it follow that a fixed foot is more efficient? I am just trying to keep the conversation lively.Edited by golfdad75 on Dec 18, 2010 - 07:15 AM.

the questions were "What is the major purpose of these differences?
Does it make any difference in technical terms?
Is one rig better than the other?"

The reasons H16's come this way, is this is an old design, and not the most dynamic to fit in different wind ranges.
Modern design include the ability to change the shape of the sail at many different locations (luff, leach, out/downhaul, mast shape effecting sailshape along the mast axis to open or close the pocket, rotation limits or inducement, etc all to get the correct chord, depth, pocket shape, draft, etc)

ok to the wings: how many birds have wings that don't change shape at all? (none, they all can change the shape of their wing / sail depending on its needs).
Look at f-14 (jet), it has adjustable wings for different conditions (speeds). also modern testing in high end wing design include the ability to morph shape

Quote

the bottom cannot be shaped

are you kidding me?
The main trim parameters are: master wing rotation (similar to mast rotation on a conventional rig); master camber control (general rotation of the flap element); flap twist control (each flap can have a specific angle of rotation).

Quote

Don't con a con man. What makes it simpler?

it is a lower tech system, it has a simpler out haul, minimal downhaul, jib blocks on the beam, 2 way jib adjuster out of the way (beam), less lines, less fuss, shorter.. lighter mast, no boards, simpler rudder systems, no spinnaker, stock non collapsing tiller extension, simpler design (deck lids),

turbohobo

I get it, with the sail rigged loose-foot, you are able to curve the sail more by easing off on both the downhaul and outhaul, more so than on the fixed-foot sail, but without knowledge of airflow properties, it don't mean squat. I don't know if more curve to the sail is better than less curve, or under what circumstances, ie, more curve on more wind, or less curve on less wind, or vica versa. That is why I said that one would reach a level in sailing which could only be overcome with more knowledge, knowledge gained from a more experienced sailor.

I guess one could educate oneself using the internet...........!

Typically, the SIMPLE rule of thumb is, the harder it blows, the tighter she goes

this means as the wind picks up, you want a tighter downhaul, tighter outhaul, restricted rotation, and more mainsheet to flatten the sail (upwind). U will probably need to travel out a little bit or you will capsize with a tighter main.. but if you have ever sailed with a loose main in heavy air... you know that your sail will not behave .. it will invert, flop, pop, fart and burp... never good sounds or sights on a boat. This is even more dramatic on less stable boats (mono's) and inverted sails can lead to inverting masts.... never fun to see and bad things can happen...(snap)

again, the SIMPLE idea is that in lighter air... you want a bigger pocket shape to your sail.
Technically, this will create a more powerful low pressure zone and increase the speed of the air behind your sail (due to the curve in the leeward side of the sail).

completely irrelevant but true is the fact that this low pressure actually sucks you forward, you are not being pushed upwind... but being sucked :) sounds dirty and i like it that way :~p

Downwind is a different creature... typically.. you will want to be traveled out downwind regardless of wind speed, and your down and out haul loose... if you have rotation control... you want to let it rotate all the way. Downwind you are being pushed by the sails, so you want a big pocket in both light and heavy air... however you will want to downhaul to open the top and let some wind/power bleed off if your are overpowered.



Edited by MN3 on Dec 18, 2010 - 07:23 PM.

oh

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Nacra 5.2
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Quote

completely irrelevant but true is the fact that this low pressure actually sucks you forward, you are not being pushed upwind... but being sucked :) sounds dirty and i like it that way :~p

Sorry Andrew, you have it mixed up. Nothing is sucked, or pulled. We don't actually "pull" a wagon, it is pushed. If you analyze the loads involved you realize that the palm of your hand is pushing on the rear edge of the wagon handle. This increased pressure forces the wagon forward.
Our shrouds are under tension, follow the loads from the hound to the pin at the hull & you find the same thing happens. There is no sucking that shroud downwards.
A sail is much the same as an aircraft wing, which operates on Bernoullis Therom. Airflow meeting the leading edge of a wing, or sail, is forced to split. To simplify greatly, lets think of two air "packets", "A" is forced to travel around the front side, "B" the rear. The two packets want to meet at the same time at the trailing edge.
A wing generally has a degree of camber,(again simplifying, the greater the camber, the greater the potential lift) this camber results in a greater distance along the top edge. "A" is forced to speed up to go the greater distance in the same time. We never added any energy to packet "A" or "B", so the total energy must not change. To achieve this, the pressure of faster moving packet A decreases. We now have a low pressure on top of the wing, & a relatively higher pressure underneath. You have heard since grade 4 science that Nature abhors a vacumn. The higher pressure area tries to get into the lower pressure & equalize it. Result, the wing is forced upwards,(lift) by this higher pressure. We need the higher pressure to obtain results.
Our cars engine does not "suck" air, it is pushed in by higher outside pressure, hence the decrease in power at higher altitudes as atmospheric pressure decreases. To regain that power we turbo charge, or super charge. TC or SC do not create more "suction", they create more pressure on the intake side. It is important to understand this principle, it is not just a matter of semantics.
This differential of pressure is responsible for quite an amazing repertoire in our everyday lives, carburetors, forcing solvent up the tube (notice I didn't say "suck") of the gizmo we attach to our air compressor for cleaning things, & most importantly breathing. Take a breath, our diaphragm drops creating a lower pressure in the lung cavity. Higher atmospheric pressure pushes air into our lungs. Jump off your Cat with a piece of hose in your mouth, then use it as a snorkel. Floating on the surface it is easy to breathe, about 16" down, you die. At that point the water pressure squeezing your chest cavity has equaled the air pressure trying to push into your lungs. No amount of "sucking" can refute the laws of physics and allow you to draw breath. If you could increase the atmospheric pressure it could overcome water pressure to a greater depth & allow you to breathe. This is one reason a SCUBA tank must be pressurized.
Back to sails, now that we understand it is a higher pressure driving the sail,(and skegs or boards converting this push into forward motion). To get more lift(power)we need greater total pressure,which can be obtained by more area, higher wind velocity, denser air, co efficient of lift that is determined by wing shape (CL), or some combination of these. Air velocity has a "squared" function in the equation, & so has the greatest effect. We can't do much with air density, but we can alter the CL by changing sail shape.
A wing,(aircraft or sail) can increase/decrease camber several ways, flaps being the most common way to increase. As they move rearwards & down, they increase both chord & camber. Same is true for leading edge devices,(slats). Increasing chord results in an increased wing area. If you couple the larger area with an increase in CL ie greater camber, the result is greater lift, (power). Of course nothing comes free, greater lift results in greater induced drag. As long as we increase lift more than the resultant increase in drag, we are ahead of the game. A tall narrow sail results in what is called a high aspect ratio,(chord vs span) very efficient...think of a gliders wing, or any soaring bird. This is where the sail trimmer comes in. Modern rigs allow us to change the sail shape to suit the conditions. A hard wing sail will be more tunable, & powerful, but the cost & complexity will also be greater.
And you thought college physics was a waste of time



Edited by Edchris177 on Dec 19, 2010 - 01:54 PM.

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Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
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Edchris177

Quote

completely irrelevant but true is the fact that this low pressure actually sucks you forward, you are not being pushed upwind... but being sucked :) sounds dirty and i like it that way :~p

Sorry Andrew, you have it mixed up. Nothing is sucked, or pulled.

----- Gigantic Quote Snip ------

I knew this would happen as soon as the mysterious MN3 mentioned how much he sucked.

This reminds me of the heated arguments among various engineers and scientists we used to have on the old hobielist, awesome stuff!

For anyone following along and is confused, don't worry too much about the theory and whether the wind sucks or blows, just try to understand the sail shape required for various points of sail.

As simplified above:

Loose for downwind.

Tight for upwind.

How tight? How loose? What position should each line and adjustment be for a particular point of sail, with a particular:
crew weight
sail age and condition
wind strength
wave height
etc.

That's what you learn by "time on the water", watching others, and getting coaching.

--
Damon Linkous
1992 Hobie 18
Memphis, TN

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Quote

we need greater total pressure,which can be obtained by more area, higher wind velocity, denser air, co efficient of lift that is determined by wing shape (CL), or some combination of these. Air velocity has a "squared" function in the equation, & so has the greatest effect. We can't do much with air density, but we can alter the CL by changing sail shape.

That's what i meant.. thanks

Thanks Andrew, Chris & Damon, I do know the basics of airflow over a wing surface, the wing is cambered on top, flat surface on bottom, airflow accelerates over top surface creating low pressure, air slows on bottom surface creating high pressure and that is how we get 'lift' on a wing. However, a sail because of the thinness of the cloth, is curved on both surfaces so airflow properties will change, and I am fairly certain, will change dramatically. But I like simple :-

the harder it blows; the tighter she (the sail) goes
loosen sail downwind - tighten sail upwind

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TurboHobo
H14T
H16
P18
G-Cat 5.0
P16
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Edchris177

Quote

completely irrelevant but true is the fact that this low pressure actually sucks you forward, you are not being pushed upwind... but being sucked :) sounds dirty and i like it that way :~p

Sorry Andrew, you have it mixed up. Nothing is sucked, or pulled. We don't actually "pull" a wagon, it is pushed. If you analyze the loads involved you realize that the palm of your hand is pushing on the rear edge of the wagon handle. This increased pressure forces the wagon forward.
Our shrouds are under tension, follow the loads from the hound to the pin at the hull & you find the same thing happens. There is no sucking that shroud downwards.
A sail is much the same as an aircraft wing, which operates on Bernoullis Therom. Airflow meeting the leading edge of a wing, or sail, is forced to split. To simplify greatly, lets think of two air "packets", "A" is forced to travel around the front side, "B" the rear. The two packets want to meet at the same time at the trailing edge.
A wing generally has a degree of camber,(again simplifying, the greater the camber, the greater the potential lift) this camber results in a greater distance along the top edge. "A" is forced to speed up to go the greater distance in the same time. We never added any energy to packet "A" or "B", so the total energy must not change. To achieve this, the pressure of faster moving packet A decreases. We now have a low pressure on top of the wing, & a relatively higher pressure underneath. You have heard since grade 4 science that Nature abhors a vacumn. The higher pressure area tries to get into the lower pressure & equalize it. Result, the wing is forced upwards,(lift) by this higher pressure. We need the higher pressure to obtain results.
Our cars engine does not "suck" air, it is pushed in by higher outside pressure, hence the decrease in power at higher altitudes as atmospheric pressure decreases. To regain that power we turbo charge, or super charge. TC or SC do not create more "suction", they create more pressure on the intake side. It is important to understand this principle, it is not just a matter of semantics.
This differential of pressure is responsible for quite an amazing repertoire in our everyday lives, carburetors, forcing solvent up the tube (notice I didn't say "suck") of the gizmo we attach to our air compressor for cleaning things, & most importantly breathing. Take a breath, our diaphragm drops creating a lower pressure in the lung cavity. Higher atmospheric pressure pushes air into our lungs. Jump off your Cat with a piece of hose in your mouth, then use it as a snorkel. Floating on the surface it is easy to breathe, about 16" down, you die. At that point the water pressure squeezing your chest cavity has equaled the air pressure trying to push into your lungs. No amount of "sucking" can refute the laws of physics and allow you to draw breath. If you could increase the atmospheric pressure it could overcome water pressure to a greater depth & allow you to breathe. This is one reason a SCUBA tank must be pressurized.
Back to sails, now that we understand it is a higher pressure driving the sail,(and skegs or boards converting this push into forward motion). To get more lift(power)we need greater total pressure,which can be obtained by more area, higher wind velocity, denser air, co efficient of lift that is determined by wing shape (CL), or some combination of these. Air velocity has a "squared" function in the equation, & so has the greatest effect. We can't do much with air density, but we can alter the CL by changing sail shape.
A wing,(aircraft or sail) can increase/decrease camber several ways, flaps being the most common way to increase. As they move rearwards & down, they increase both chord & camber. Same is true for leading edge devices,(slats). Increasing chord results in an increased wing area. If you couple the larger area with an increase in CL ie greater camber, the result is greater lift, (power). Of course nothing comes free, greater lift results in greater induced drag. As long as we increase lift more than the resultant increase in drag, we are ahead of the game. A tall narrow sail results in what is called a high aspect ratio,(chord vs span) very efficient...think of a gliders wing, or any soaring bird. This is where the sail trimmer comes in. Modern rigs allow us to change the sail shape to suit the conditions. A hard wing sail will be more tunable, & powerful, but the cost & complexity will also be greater.
And you thought college physics was a waste of time Edited by Edchris177 on Dec 19, 2010 - 01:54 PM.

See that is what I meant. Us Nacra sailors are just smarter

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Nacra 5.2
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Ok, i feel i am vindicated here. I posted a simple question on another site, where there are many engineers, pilots, expert/pro cat sailors, even NACRA (Europe) developers... and the concensus is

YES, UPWIND you are being SUCKED forward.... :)

http://www.catsailor.com/…er=225671&gonew=1#UNREAD

Now i do accept Edchris177's theroy that you aren't pulling a wagon, you are pushing on the handle while dragging it backwards, and in fact the "suction" is actually a factor of low pressure vs high pressure... but we (non physicist) accept there is a function called suction in the real world... i.e. when you drink from a straw, you are sucking/pulling the liquid up the straw, not increasing the pressure in the rest of the universe.

golfdad75 - you are entitled to your own wrong opinion



Edited by MN3 on Dec 20, 2010 - 10:10 AM.

Andrew I resemble that remark

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Nacra 5.2
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Quote

i.e. when you drink from a straw, you are sucking/pulling the liquid up the straw, not increasing the pressure in the rest of the universe.

You nearly have it Andrew! You do not NEED to increase pressure in the rest of the universe, you merely use the atmospheric pressure that is already surrounding you. It is that pressure,(roughly 15 psi where you live) that forces the water up the straw.
Try this, find a long tube & try "sucking" water from a container on the ground while standing on a ladder. Once the pressure in the water column equals the atmospheric pressure, no amount of sucking will draw it any higher.
IF you could increase the atmoshperic pressure, the water would be pushed higher, but understand it is being pushed by the higher pressure.
You are correct in that we can use a vacumn gauge & measure "suction", but in reality this suction is really a pressure differential, & it is ALWAYS the higher pressure trying to push into the lower pressure that does the work. If you placed your glass of water in a sealed container, evacuated the air, you could suck all you want, & no water would go up the straw, because there is no pressure to push it up.
On another note, I hope it warms up down there. Pictures of orange growers chiseling through solid fruit can't be good for a lot of peoples incomes. My buddy said he took a long delay in Atlanta for deicing, where is global warming? Bring it on.



Edited by Edchris177 on Dec 22, 2010 - 08:10 AM.

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Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
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That is what I have been saying

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Nacra 5.2
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wow i did not that it was that complicated

Meh..this is like arguing which direction the current from your car battery flows..it's of no consequence as long as you understand how the power is created.

However, Andrew was wrong about something. You're not being pushed by the wind downwind. The apparent wind should always be in your face, it's just at a much lower net velocity so you need more powerful sails, instead of faster sails.

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Rob
OKC
Pile of Nacra parts..
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Now let me get this straight, a spinnaker is ballooning out in front of your boat, does that mean you are being pulled by the sails resistance to the wind or are you being pulled by the low pressure in front of the spinnaker and the high pressure is trying to fill the vacuum. Or are you being pushed by the wind which has to be of greater velocity of the apparent wind you create or are you being pushed by the wind filling your spinnaker. But if the spinnaker is in front of the boat how can it be pushing you. Now another question to ponder if the same circumstances are in the southern lattitudes is the situation reversed hence pull becomes pushed. Remember Benoulli lived in the Northern Europe. All very confusing.

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Nacra 5.2
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I’m back from Christmas in the west, & off tomorrow for some sailing in Antigua.
I followed the other thread Andrew linked to, out of nearly 40 entries, one guy, (who posted no credentials) said you get "sucked". One other pilot posted a Wiki of Bernoulli’s Therom, & another fellow posted the lift formulas , which was exactly my point, the sail generates lift.
You simply cannot be taken seriously by quoting Wiki during a science argument. As my med school daughter replied when I asked her the result of using Wiki as a reference..."Fail", followed by, "actually, epic fail would be more correct“.
Most of the other posts were jokes/innuendo. One or two out of 36 is hardly a consensus or vindication.
Who is correct is not really important though, what is correct is the target.
The chap who posted that it was a difference of pressures was on target, & notice he took a verbal lashing for using "sucked." The post of the egg being pushed into the bottle, an experiment we all did in grade 5 science, also correctly demonstrates the concept, the egg is forced in by the greater pressure outside the bottle., it simply is not correct to think it gets sucked in.
The fellow who posted the lift formulas was agreeing with what I have tried to show, that a shaped sail develops lift in much the same way as a wing, & by manipulating the shape we can increase lift. The post referred a couple of times to increasing lift, exactly what I have been saying. Reynolds numbers & NACA forms are familiar fare in the aeronautics world, & you won’t find those engineers or scientists speaking of “sucking” to get airborne. It is the higher pressure that does the work, & modern wings are quite innovative in preventing this pressure from “leaking through.”
.I think the confusion originates from the two different ways a sail can create propulsion, & that we can use combinations of the two on different points of sail.
The first is simply by providing resistance to the wind, think of holding up a sheet of plywood facing the wind. You get blown backwards, pushed so to speak. A boat can use this force to make way downwind. We do not have to travel dead downwind, the resistance of skegs or boards can translate some of the drift into lateral motion. I believe this is what is referred to as being “pushed downwind”.
We rarely sail this way as we know it is slow, instead we add the second method of propulsion, lift, generated by a shaped sail to the direct pressure of the wind.
Visualize your cat with a strong wind blowing from directly abeam. By itself the boat would drift dead downwind, impeded by the resistance of skegs or boards. Now centre the traveller & align the sail fore/aft. What happens? We get pushed over. Did some mysterious force of suction pull us over? No, I think we all understand the higher pressure on one side pushed us. We are now lying on our side drifting like hell downwind. Any suction happening, or is it the pressure of the wind on the tramp pushing the boat downwind?
We get up & on our way again, this time pointing more into the wind & easing the traveler. Our boat goes like a spit watermelon seed because we are now utilising lift as the motive force. Suddenly a good gust hits us, but we are slow to react & go over again. Did things just change, & we got “sucked” onto our side, or were we again pushed over by the pressure of the gust? I think we all agree from experience that even though we were developing lift, & sailing upwind, it was the increased pressure on one side that toppled us. This is just an increase in the same force that drives the boat upwind.
There is one more very important concept of lift that needs to be considered. That is that the lift generated acts roughly 90* to the chord of the wing., (chord is a line from leading edge to trailing edge)ie a horizontal wing generates lift that is vertical. From this concept we can visualize how the lift vector drives our boat..
Beam wind, & the sail way out, 90* to the hulls. The sail will flog til we travel in enough to develop shape & create lift.(a “hard” aircraft wing will develop lift in this condition & drive the boat forward) The lift vector is nearly aligned with the hulls, most of the force is translated to forward motion, we sail fast.
As we head up, & pinch, the sail is still developing lift, and the lift vector is still appx 90* to the chord of the sail (wing). But where does this vector now point with regards to where the bows are pointing? Sheeted & traveler close, the sail chord is nearly aligned with the hulls, the lift vector, at 90* to the apparent wind has quite a sideways component, & wants to topple us.
We know this is true from watching the monohulls lean. Is this “lean” the result of being “sucked over”? I think we all agree that, again it is the higher pressure pushing the boat over. The boat is still moving forward because we are still developing some lift & the skegs or boards are transferring some of the leeway into forward motion. The closer the vector is to 90* the less effective the keel/skeg/boards become, our speed dies off. Non believers should pull the boards during a pinch & observe the results. Point too high, the sail loses shape, no lift gets generated ,the driving force pushing us forward ceases, & we drift to a stop..
In conclusion I believe many sailors understand that a sail can propel a boat two different ways, pure resistance to the wind or lift. On some points of sail we can use both. Upwind, it is the keel/skegs/boards, in conjunction with developed lift that propels us. They are confusing the term “suction” with “lift” & how lift is generated,, but they are correct in that upwind, lift is the motive force, whereas downwind you can sail on “push” alone.
I would also suggest that we know dead downwind sailing, (using only resistance to the wind, what some call being pushed) is slow & inefficient. That is why we rarely sail dead downwind. We sail at an angle that will also generate lift & increase our speed. Visualize where the lift vector points sailing downwind to see the added effect it has on our boatspeed. Modern spinnakers are not hoisted solely to resist the wind. By keeping the air flowing on both sides they can also generate lift.
One of the best texts I have used on the subject is by A.C. Kermode, “Mechanics of Flight, an Introduction to Aeronautical Engineering.” Borrow a copy from the library, you will read science & logic, and gain a true understanding of lift & why the new wing sails with flaps will rule. The math is easy to follow, & only used when a point needs to be proven, to show how "A" is superior to "B".
This post is longer than I intended, but it is difficult to cover the subject with only the written word, diagrams would have helped immensely.
Here is hoping everyone enjoys good health & has a great season in 2011, mine starts Jan 1 in Antigua, & many thanks to Damon for making this a site where opinions are argued with a modicum of decorum.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

PS
Andrew, the old joke of suck,squeeze, bang, blow, is just that, a joke to describe a 4 stroke internal combustion engine. Everyone understands that air enters on the intake stroke as a result of atmospheric pressure, not suction, & that to restore power lost at altitude (due to decrease in atmospheric pressure)we use turbo charging or super charging, "a blower" to create more pressure on the intake side.
The jet engine also uses the same four cycles, they just occur continuously & simultaneously. Compression is achieved aerodynamically through acceleration then diffusion at each stage through the turbine, a stage consisting of a rotor & stator disc. I operate the most powerful turbine engine in commercial production, the GE 90-115B, output 115,000lb of thrust. I would not agree with the poster who said they "suck, squeeze,bang,blow, with less moving parts. Compression alone is 13 stages,(appx 45:1)& each stage has a hell of a lot of blades. Add in the fan, thrust reversers etc & your parts count is getting up there. The front fan of a modern jet engine is a glorified propellor, & is shaft driven by the low pressure turbine. This fan moves a large volumn of air backwards at a relatively slow speed, utilising Newtons law of action/reaction. The "propellor" also uses the high pressure on the back side of the blades to push the aircraft forward, it does not get sucked forward, ditto for the rotating blades of a helicopter.
Ask your GF for Mechanics of Flight for your birthday, you will find many days of enjoyable & educational reading



Edited by Edchris177 on Dec 31, 2010 - 11:14 PM.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

Holy EC, good long writeup.

The main concept that everyone seems to miss is that lift, isn't suck or push but a combination of the two that is generated in a way that is more complex than either of those. It's exactly the same concept as a wing (hence rigid wing sails being used in various forms). The air on the leeward side of a sail travels faster than on the windward side and the pressure differential causes a the force. The best sail shape that you are trimming to is to get the air moving smoothly on both sides of the sail (laminar flow to those in the know) without turbulence. That is why you try and trim until the telltales flow evenly backward. The main job of the jib is actually in speeding up and straightening the flow over the leeward side of the Main not just providing extra forward force. Hence the reason why jib's have reduced in size in recent years.

The beauty of a fabric sail is that is flexible, so you can shape it in two axis to get what you want and tailor it to your wind, the downfall is that a flexible sail cannot impart as much force to the boat as a rigid one. Hence the Americas cup and little americas cup boats experimenting with trimable wing sails.

Even going fully downwind you aren't usually just catching the wind in the sail, without any flow over the leeward side. If you do you will only be using half the power available to you. That is why it is more efficient to do the mild thing or wild thing by sailing a couple of points off of the wind, once you get that air flowing on both sides of the sail you can build that extra momentum and you get MUCH more forward momentum.

I'm a darn hydraulics engineer and my bud who coaches sailing out of the Royal Victoria Yacht Club knows way more about lift and drag than I will likely ever want to. He needs to to win races.

D.

--
Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
--

Yeah, a couple hours reading aeronautical engineering would do much to advance ones knowledge base.
The hard wing sails can develop lift at only a few degrees angle of attack, they don't need air pressure to shape them. It will take some years, but I'm sure we will see that technology trickle down. Just think what leading edge slats, multiple slotted flaps, along with vortex generators to hold laminar flow at high angles of attack could do, in even light wind.
I just found out the people who run the sail shop I use have raced in the Little Americas Cup, & won a gold medal at the Olympics, sailing Tornados, so they know a bit about Cats.
There is a thread on SA about a C cat, built by the family that is being repatriated to Canada. I hope to see it sail next summer.
Not doing much to mine this winter, the hulls & rear beam are outside. The rest is inside for a few tweaks. We are headed way south early a.m. will sail some Hobies there. I almost bought another Cat, actually two...I think I might be sleeping alone if I put two more on the dock next summer!



Edited by Edchris177 on Dec 31, 2010 - 11:30 PM.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

Planning on boning up on my drag and lift theory for incompressable flow for work, just need to find the time. Not to mention advanced sailing theory, for the most part all you really need to know are the rules of thumb found in the basic books but I like to know about these things. Lots of cool things for the future, if they can just figure out how to keep the wing sails from breaking when you capsize. My favorites are the adjustable foil daggerboards that reduce drag by raising the hulls out of the water.

My wife is expecting me to pick up a second boat at some point. Problem is finding one I want within a reasonable distance or getting it shipped here. I really would like a boardless boomless boat like a 5.0 or 5.7, but there just aren't any within a days drive of here (out and back). If I wanted a laser though, I could have them by the bushel, sigh.

D.

--
Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
--

Quote

out of nearly 40 entries, one guy, (who posted no credentials) said you get "sucked".

I am so easy going, thats all i need :)

great info. i learned a lot. Mostly:
Math sucks. and reading long posts blows

(just kidding thanks)

If you got sucked (or blown) there would be MANY MANY more sailors than there are now. At least male ones. :)

--
Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
--

See there is hope for you Andrew, hell my Dad went back to school at age 55!
You need to be down here in antigua, the wind is blowing every day at 40 clicks, gusting to 55.(24-33mph) They have a well used Mystere, but won't let it out til the wind goes down.
I can say that a Hobie Wave will pitchpole! I nearly did it, wife wasn't impressed. I watched two English couples PP it 3 times! The rescue boats were getting a little peeved. I'm amazed they still let them out. I thought the English were used to big winds.
The Wave isn't a Nacra speed wise, but I'm learning to get out through surf, not big stuff compared to what you Florida & Great Lakes people do, 4' break max, but i've never dealt with it before. Coming down off the swells at max speed is also a rush.
Coming in they want you to drive them full power right up onto the beach, watched a couple ignore that advice & nearly get their legs broken as the boat went nuts in the shore break. I asked about the bottoms, he claimed they wear through & that they fix them with "cloth & plastic". I didn't think that would stick, but upon close observation of one laying on its side,(not mine) there is definately something added there.
Antiguan beer isn't Grolsch, or Dab, or Gosser, but it's cold & flows freely, so what the hell,I'll drink it.

Andrew, if you promise te send it back, I'll lend you my only copy of Mechanics if Flight

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

~~ Hey Andrew.... Like I said on the other site~~~~ "I don't care if I'am SUCKED or BLOWN,,, if its WET its FUN~~

--
~ Vietnam Vet 69-71~ 17 Hobie w/big jib, ~18 Hobie mag,~DN Ice sailor,
and other toys.......
~~ I live in NY state on the north shore of Oneida lake in
Bernhards Bay. ~~~~~~
--

So who wants to explain how a jib interacts with the main to improve boat speed?

--
Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
--

The pocket between the jib and main pre-compresses the air. So the air going towards the leeward side is already going faster prior to it hitting the mainsail. Then the shape of the mainsail causes the sail to speed up even more making the pressure differential between the windward and leeward side of the mainsail higher than it would be without the jib and causing more force to be imparted to the sail and hence greater speed. The other thing the jib does at the same time is it takes the air currents which are generally turbulent and swirl around a bit and straightens them out a bit prior to them hitting the main, this creates a more laminar air flow and threfore more efficient air flow over the main.

Does that help?

--
Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
--

That is also my understanding, the jib accelerates the air. I'm not 100% sure, but it may also work the same as leading edge slats on a wing. The gap energizes the airflow & allows it it to remain laminar over the main wing at a higher angle of attack, & remain attached for a greater distance along the wing.
The jib also creates a lifting surface with its curved shape. Biplanes were an eveolutionary dead end, though both wings created lift. It may be that builing a taller mast, longer boom creates more penalties than the double wing effect.
I believe the jib adds a significant amount of extra lift. I sailed my 5.7 on jib alone one windy day, & if a brisk walk is 3 mph, I had to be getting close to 10 mph on a reach under jib alone.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

the "slot effect" of the jib is hugely debated

the theory looks quite pretty but when cats have a limit on their sail area, like a-cats and moths, they just seem to point higher, more lift? and go faster, less drag? WITHOUT a jib

it's only when you have unlimited sail area that you might as well bang some more cloth up in the free space between the mast and bow to catch more air

this report from 1973 shows it's pretty much always been debated

http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf#search=%22jib%20slot%20effect%22

but my boats were designed with jibs so i'll leave them on, they seem to work;o)



Edited by erice on Jan 05, 2011 - 06:16 AM.

Yeah not sure about how the jib affects pointing, except that the extra force forward of the center of gravity provided by the jib would tend to pull you away from the wind.

The only thing that I can say about the slot effect is that it may be debated for flexible sails but I have seen many simulations and models of ridged structures very similar to a jib that are intended to do the same thing and work very well. Specifically I'm talking about hydroelectric turbine stay vanes and wicket gates and they certainly increase the efficiency of the turbine upwards of 20%, without the effect of adding more "sail" into the situation. But like I said nothing else works quite the same as a flexible sail, as long as it helps me go faster the physics don't really matter.

--
Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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