sail aerodynamics

Hi Retro dude, " RM ", ??, rolling moment !?.
I'd love to be able to draw a whee picture for you, only I don't have the means so I'll try to "draw " one with words.
The Top mast section would needs weigh no more than a standard mast, as it's stayed in the same manner .When extended it would have a reasonable central "bury" inside the A frame - Bipod - [ whatever you'd like to call it] triangle section of the wishbone mast .
This bury would be necissary to initially support the extended top section, with a somewhat "sloppy " fit - for & aft only as the shrouds support sideways - untill the forstay and triatically stayed back stay were tuned.

Viewing the rig from aft, the lower section of the mast/s are a rectangle with a triangle on top. [ all foil sections].
The rectangle, hinged at the gunnels, is naturally braced diagonally, corner to corner and there is a small horizontal cross member about a meter from the top of the triangle section.
Across the very top of the triangle there is a crossmember equal to the width [ though not necisarilly ] of the rectangle section . That "cross member" is the hinged spreaders supporting the extended top section .
It appears to look like a cross member only when the top shrouds have "picked them up" and they in turn " pick up two shrouds that connect to the two " corners" of the rectangle .
The 4 shrouds are all a given leangth so must return to the same position every time.

Retracted, the most likley circular alloy mast section [ could be foil ] would be flush with the top of the triangle so as to be flush with the bow when the rig is lowered. The spreaders, with no tention on the shrouds to hold them up, will now be flush with the two downward chords of the triangle.


Extended ,the top sections "bury" starts from that small cross member near the top of the triangle.

Halyards & electronics ?
I am a rigger, the type that works with cranes and have owned several.
Most cranes, if the boom is extended and you don't winch down the hook, the hook will "travel" all the way to the top, as the boom gets longer but winch wire remains the same leangth. [ it's called double blocking and will pop the hook off - and anything hooked to it !!]
To combat this, there is a way to reave the rope internally [ in the boom] so that it remains static - in relation to the extending boom .
The same could be done for both halyard/s and electroincs. remembering one wouldn't raise & lower the top section much at all, it'd be simpler to just give the halyards some slack .
Gaffers are great, but I'm my own worst enemy in that I like to think & build outside the square and the aft mast really appeals to me as it seems to make logical sence to me and have a number of advantages specific to our needs.
Yeah yeah , everythings a compramise darn it !!.
Hope you can envisage it.
Globaldude

 

I havn't read all of this thread yet. Just upto page 7.
If the distance between the trailing edge of the jib and the leading edge of the mainsail is large can you get extra performance for a set sail area. If you measure the chord as the whole sailplan. Kind of like extending flaps on a plane?

Im sorry im a bit of a noob

Thanks Alex

 

COE of a Square Sail?

Does anyone have any references that address the question of where is the most likely true Center of Effort of a rectangular sail as used on a square-rigged vessel.??

Take for instance the sails on Maltese Falcon, best guess, where is the CEO of each individual sail

 

Brian
You may find this site of interest:

www.weatherlysquareriggers.com

where the square rigger that points is considered, and the fact that the spacing between the masts makes a considerable difference!

 

Weatherly Square Rigger

Thanks Percyff,
Yes I am aware of that site...in fact I recommended it to a few friiends. Just recently I wrote the author Philip Goode about several questions I had on his 'horizontal upper stays'...I did not understand his descriptions at first. I subsequently found them running to the forward mast.

And yes, I did note his interesting finding about the mast spacing.

I found his findings about the 'leading' headsails of interest as well. I've always maintained that the headsails of a rig are extremely important and productive. Took note of his 'sky link assembly' to provide forward staying unaided by the addition of a forward mast. Since the whole mast of the DynaRig rotates there would have to be some modification of this idea, but I think its possible. I'm looking to fly a headsail on my uni-rigged Dynarig motorsailer...I've got a few ideas myself, but I'm willing to except other ideas from anyone who wants to contribute:idea::idea:

 

Very interesting design Brian. For what will be in effect a single foil, I would think that the C/4 point at the mean aerodynamic centre should give you a very close approximation of the CE when sailing upwind and the flow is chordwise.

It will be interesting to see if the rig is dynamically stable. Have you looked at the moments? With the pivot/support point at C/2 I would expect the rig to try to twist the leading edge to leeward, creating wash-in at the Royals. Kind of exactly opposite to what you want for wind shear and gust response.

I understand your reasoning behind the Gallant/Course areas. Have you thought about bigger Courses and smaller Gallants? Striking the Courses and using the Gallants for higher winds and sea-states means that your working sail is less effected by calms in the troughs and they should see less variation (although higher average) wind speed. Using the Gallants for working sail also increases visibility and reduces the chance of waves hitting the sail.

As far as head sails go, I'm not sure I see much benefit in adding a fore and aft sail to a DynaRig for upwind work. Stanford Yacht Research did a workup of MF's rig. IIRC, the pressure distributions were just about what you would expect from three wings more or less in line. The trim angles on each mast were lower on the centre and aft mast, much like the trim angle difference between the jib, main and mizzen of a ketch or schooner. Each leading element is in the upwash ahead of the element behind it. Adding a triangular sail to the rig will lower it's AR and may reduce windward performance due to the increase in induced drag. If you make the rig big enough to have good light air upwind performance and count on not flying the Royal in a breeze, you might have a real winner.

Off the wind, there is probably a good case to be made for a "Code 0" type flying sail for close reaching in light air and other flying sails for off-wind work.

I like what you've done with the concept. I find it much more attractive than the mast-aft designs. I see the mast-aft rig as a riggers nightmare for loading and the DynaRig as simple and elegant. Very, very well done.

 

Uni-rigged DynaRig discussions

I understand what you are getting at...the actual CE being located approx 25% from the leading edge. So far I am hearing 25-35% range. But you know this is true for the Burmuda rig sails, and water foils (CLR) as well, yet we by convention first seek out the geometric centers to determine 'balance', and somehow that seems to work, even to my amazement

You are correct, the rig will try to twist, and this was pointed out as one of the reasons that the Dynarig built with older materials was going to be not so successful. It is only the recent carbon fiber materials that can resist the twist, that makes the DynaRig feasible.

I've adressed that 'wind gradient' question on several of the Maltese Falcon discussion threads. It was decided that to account for the gradient would add too much complication to the rig. Gust response might be the same.

Two items were brought to my attention. First I orginally mislabeled the Gallants as Topsails and vice-versa. The Gallants flew over the Topsails which flew over the Courses...sorry about that. I corrected it on my website dwg. now.

This striking of the Courses in big seas was brought to my attention by the author, Philip Goode, of www.weatherlysquareriggers.com. Thanks for bringing to my attention though, in case I had not heard from Philip.

I guess I'm partial to the effectiveness of headsails, having preeched it so long. I definitely think they help the uni-rig point higher. Maltese Falcon (3 mast) and the 2 mast version proposed by Perini Navi have the benefit of 'interaction' between the fore-aft sails. The uni-rig does not.

Besides I really want that 'storm support forestay'

In reading 'between the lines', Philip's weatherly squarerigger, I definitely get the sense from his model studies that he appreciated the headsails in addition to the forward mast to a great extent

One quote from his paper that really caught my eye, "Close hauled at 45 degrees to the wind the effect of increasing the traditional mast spacing by 22% was astonishing, with the sailing force going up by 84%"

Thanks very much. I don't find it 'more attractive' in the visual sense, but it has its own appeal. I think this would be an exciting project that could build further upon the pioneering work done by the Maltese crew

 

america cup alinghi likely to sail Inflatable sail battens which if successful are likely to be seen in the main stream of sailing in the future. http://www.alinghi.com/en/32ndac/rules/

 

visualise an aeroplane landing, with flaps fully extended. ın modern commercial planes you would normally see two sets of flaps with a considerable gap between each of them.
I believe those slots in between are there not because the designers could not make them a one continous curve touching each other.
The curve of the entire wing in that configuration is just too great for the
air flow to remain attached to the upper wing surface without seperation and stalling.
The slots in between help the flow to remain attached to the adjacent wing section, creating a series of deflected flow patterns rather than just one big surface unable to cope with the total demand.
Perhaps it is not too far fetched to carry this to the mainsail and jib configuration. the jib being the static wing and the mainsail acting like a flap, with a slot in between.

 

You have it basically correct. A.M.O Smith's Wright Brother's Lecture, High-Lift Aerodynamics, fills in the technical details.

 

I'm trying to print out the A.M.O. Smith Lecture, but while my computer tries to do that, to carry Omeron's point a bit further, I'm looking at fig 134 (pgs 228-9) of The Theory of Wing Sections, which has to do with slot combinations, and it seems you get more lift with the major portion of the divided section in front of the slot, which would seem to imply that the old IOR style big jib/ small main combo may be better than the blade jib/ big main combo we see now. And which I have on my 40er. That, and the Gentry fig 17 Streamlines around jib and main sail pic I have in front of me on my bulletin board, which are for a 165% jib, which is about the same area as the main. ?????

Paul

 

Changing tack slightly...

For anyone interested in understanding how wings, sails and foils work, I have found a couple of interesting and simple(ish) to follow websites. They seem to disagree about the precise role Bernoulli plays in the whole affair, but otherwise both sites are pretty thorough.
1. http://www.regenpress.com/
2. http://www.av8n.com/how/htm/airfoils.html

It led me to thinking though, which is always dangerous. Wings and sails may use the same principles, but they have quite different requirements.The forces on an aerofoil can be resolved into lift and drag components. On a glider's wing, the lift acts mostly upwards, with a forward component at the front and backwards component at the back. The magnitude of the forward component is greater than the backwards bit, so the glider moves forward as well as staying up in the air. However, when applied to a sail, the component of lift that is upward on the glider (a Good Thing), is causing leeway and heeling on a boat (a Bad Thing). Only the forward component of lift at the front of a sail is actually beneficial. I guess what I'm saying is that lift, whilst a good thing for gliders, is a mixed blessing for boats.

To look at it in a slightly different way, presumably a glider is designed to be able to maximise the downwash angle at the trailing edge, whilst a boats sails should minimise the downwash angle. That seems a fairly fundamental difference between wings and sails, and one that that is not often talked about in most books, which usually claim that sails are just wings on the side. And that's before you even get to the more obvious differences like (range of) angle of attack, twist, mast interference, sail interaction etc etc that makes a sail so different from a wing.

So, am I right in thinking there would be merit to a sail that looked a bit like the low pressure side of a NACA section, but chopped of at the maximum camber point? It seems to me that aft of that point, the sail is only producing forces that heel and drive the boat backwards, so you may as well do with out that bit.

Does this also explain why wing masts can work so well? A normal mast disturbs the airflow at the front of the sail, which is the very bit you need. By the time the flow re-attaches, the force vectors are pointing in the wrong direction.

 

PI, given the general direction of your last post, you might like this;

http://www.modelresearchlabs.com/hand_launch_glider_airfoils.htm

I've been looking at this one for a while now, and while I'm not quite sure what to do with it as far as sailboats, it is thought provoking....

Paul

 

Perhaps the leeward curve of the jib and mainsail combined should look like
the low pressure side of a NACA wind section, starting from the forestay and all the way to the leech of the mainsail.
This also makes you think what a different size of airfoils we get. Near the top of mast, the chord length is perhaps a metre or less, whereas close to the deck, we may be talking about 10,20,+ meters.
This also requires substantially different section thicknesses, if you want the entire sail to function. I think that is just not possible, and here the sails and wings go their own way.

 

Interesting site Paul. I'm not sure that that most of the lift on a sail is generated in the windward side though, so don't know how relevant to boats that would be, but worth considering.
Hi Omeron, I agree you ought to consider all sails combined. I was sticking with a single sail for now, to make it easier. You're right, for most boats with triangular sails the chord varies massively. However, if you look at something like an A Class cat the sails are nearly rectangular, so the chord stays constant for most of the span.
If minimising downwash is the aim of the game, then flat sails (low camber) must help. Perhaps this is another area where over-rotating wing masts help, their sails tend to be much flatter (may be 8% camber v. 13% on a 'normal' rig).