Rigging Question(s)
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Posted: Dec 19, 2010 - 09:41 PM
That's what i meant.. thanks -
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Posted: Dec 20, 2010 - 12:42 AM
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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Posted: Dec 20, 2010 - 10:16 AM
See that is what I meant. Us Nacra sailors are just smarter
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Posted: Dec 20, 2010 - 12:09 PM
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. -
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Posted: Dec 20, 2010 - 01:46 PM
Andrew I resemble that remark
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Posted: Dec 22, 2010 - 09:55 AM
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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Posted: Dec 22, 2010 - 10:17 AM
That is what I have been saying
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Posted: Dec 22, 2010 - 04:04 PM
wow i did not that it was that complicated -
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Posted: Dec 22, 2010 - 06:36 PM
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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Posted: Dec 23, 2010 - 08:57 PM
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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Posted: Dec 31, 2010 - 10:55 PM
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.
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Posted: Dec 31, 2010 - 11:55 PM
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.
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Posted: Jan 01, 2011 - 12:28 AM
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.
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Posted: Jan 01, 2011 - 01:28 AM
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.
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Posted: Jan 01, 2011 - 02:27 AM
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.
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Posted: Jan 03, 2011 - 12:21 AM
I am so easy going, thats all i need :)
great info. i learned a lot. Mostly:
Math sucks. and reading long posts blows
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Posted: Jan 03, 2011 - 12:27 AM
If you got sucked (or blown) there would be MANY MANY more sailors than there are now. At least male ones. :)
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Posted: Jan 04, 2011 - 10:26 AM
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
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Posted: Jan 04, 2011 - 12:16 PM
~~ Hey Andrew.... Like I said on the other site~~~~ "I don't care if I'am SUCKED or BLOWN,,, if its WET its FUN~~ 
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Posted: Jan 04, 2011 - 02:39 PM
So who wants to explain how a jib interacts with the main to improve boat speed?
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