An interesting boat : matinbleu (free standing "wing sails" goélette)

[sharpii2]

Originally Posted by yves

Below an interesting boat with 2 free standing masts and kind of wing sails :

http://www.voilesetvoiliers.com/gran...e-en-question/
...

We should all learn how to speak French. That is the language of sailing!

At least when it comes to innovation.

Here, they have dealt with an ancient problem in a very interesting way.

The problem being the ugly bump the mast makes at the leading edge of a fore and aft sail.

This ugly bump disrupts airflow around the sail, often killing its drive.

There have been a number of methods, ancient and modern, to deal with this problem (though the ancients didn't understand aerodynamics in the modern sense, they were keen on what worked well and what didn't). I'll state them in the order they most likely appeared:

1.) move the leading edge (luff) of the sail forward of the mast. The mast is still providing drag, but is not disrupting airflow over the lee side of the sail, the side that counts.

Two examples of this are the northern European dipping lug and the more southern European latteen sail. (other cultures had developed these rigs as well, but they are well known in Europe)

The dipping lug sail was always set downwind (lee) of the mast, so the luff was always in clean air, and the lee side could form a nice fore and aft curve.

The lateen sail had a long yard along its entire leading edge, but since it was supported by the mast at one third to one half its length, The yard could be much thinner than a mast would be if it went up the entire leading edge itself, and so provided far less disruption.

2.) Increase the fore and aft length (foot) of the sail, so the mast thickness divided by the sail foot is smaller. This way, the air flow disrupted by the mast has a greater chance to reattach itself to the lee of the sail, restoring most of the drive.

A good example of this is a Cape Cod cat boat, with its short leading edge (luff) and its long chord. This may be a main reason early gaff rigs somewhat squat in their appearance.

3.) Set a smaller sail ahead of the larger 'drive' sail to smooth out disruption caused by the mast.

This sail was more often than not a jib. The jib directed clean airflow past the lee side of the mast, smoothing out disruption there. (later, when high tension luffs became possible, jibs were discover to produce excellent drive themselves)

4.) Streamline the mast by making it thinner, side to side.

This was done by turning the mast into the compression member of a truss system, with stays, shrouds and spreaders on either side. This worked best for upwind sailing, as when the boat was sailing across the wind, the wider fore and aft side of the mast ended up in front of the sail's luff. But since boats were judged primarily by their upwind ability, by that time, this seemed a small sacrifice.

5.) Streamline the mast and allow it to pivot with the sail. This way the fore and aft side of the mast almost never presents disruption on the lee side of the sail.

6.) Make the sail into a symmetrical airfoil, with cloth on both sides, and put the mast inside the airfoil.

This is what has been done here, except the symmetrical airfoil does not not extend the entire length of the foot.

I'm surprised the 'symmetrical airfoil' approach is not used more often. I suppose there is a worry about weight, or the symmetrical airfoil does not provide as much lift as a single cloth sail (forming an asymmetrical airfoil) does.

Maybe the second reason is why they took this approach. This sail, in its entirety, can still form an asymmetrical airfoil.

[Eric Sponberg]

I'm surprised the 'symmetrical airfoil' approach is not used more often. I suppose there is a worry about weight, or the symmetrical airfoil does not provide as much lift as a single cloth sail (forming an asymmetrical airfoil) does.

Maybe the second reason is why they took this approach. This sail, in its entirety, can still form an asymmetrical airfoil.

In my opinion, the reasons are obvious. Any two ply sail arrangement seems to have a lot more parts than a single ply sail including double the amount of sail fabric. That adds complication and weight and more than doubles the cost of a sail. Also, it is my belief that in general people devote way too much attention to 2D streamlining shapes. The 3D planform shape of the sail and the shape of the leading edge have far more effect on lift and drag on all points of sail than do the streamling characteristics of 2D section shapes. And for those factors, that is why I advocate the option #5 above--single ply sails on a rotating mast.

Eric

[capt vimes]

eric

you mentioned in this thread, that "it would be a cool little boat" when designing a mini transat with a rotating, freestanding mast.

now i found some very strange paragraph in the mini class rules:

J-18-a Masts passing through the deck must be watertight internally from the mast foot up to the first halyard exit, otherwise, mast openings inside the boat must be less than 300mm from the mast foot.

from: http://www.classemini.com/?mode=jauge-classe-mini

to me, this rule makes absolutely no sense at all...
"watertight internally up to the 1st halyard exit" means that it has to be sealed at the height of this exit - doesn't it?
if you have to seal it off, then there could not be an exit, because there cannot be anything to exit...

but then comes the second part of the rule:
"otherwise all mast openings must be at the mastfoot"...
so - either you seal off the whole thing, or you leave it open all the way down to the maststep at the keel...
this is nonsense!
your wingmast design with the stubmast sealed of at just above deck level is a safer configuration!

is it my bad english or am i just to dumb to understand that rule correctly?

[capt vimes]

oh my... forget my rant...
when somebody says "exit" - to me it is the top most "exit" where a halyard is then attached to the sail and not where it "enters" the mast...
took me some time to get my head around it... and it is probably due to some translation issues (french - english) as well...

[Eric Sponberg]

Originally Posted by capt vimes

oh my... forget my rant...
when somebody says "exit" - to me it is the top most "exit" where a halyard is then attached to the sail and not where it "enters" the mast...
took me some time to get my head around it... and it is probably due to some translation issues (french - english) as well...

Roger that.

[Tanton]

66' Schooner with free standing, rotating masts and wing sails.

[sharpii2]

this means - if i got it right - you either run all the lines not in but outside of the mast, or you bring them down to the mastfoot and from there back to the cockpit...
why isn't it enough to bring the lines out just above deck and seal the mast from deck to the mastfoot?
you do not have any leaks into the boat in the case of a capsize either...
why has the mast to be watertight up to the 1st halyard?
deck stepped masts are also not watertight, nor do they have to be...

this is a stupid rule - makes no sense to me and probably one of the reasons why nobody tried this approach before...

The rule maker(s) probably didn't think of that possibility.

It seems quite clear that the intention of rule was to keep the cabin from flooding through the mast, if the boat ever turtled.

[capt vimes]

thank's but it was a misunderstanding on my side...

"exit" is to me the point where the halyard leaves the mast to the sail... meant was where it "enters" the mast to go up to its "exit"...
awkwardly described in the rule but if you get your head around and replace "exit" with "entry" it makes all the more sense and i know that this rule is meant to prevent flooding the cabin in case of a capsize...

[capt vimes]

i was wondering - looking at gary's bay skimmer - what would be the most efficient aspect ratio for the sails on a free standing, rotating mast?

the sails should always and at every point have a clean airflow, they should never operate in a stalled mode, so the higher the aspect - the more efficient they should be - isn't it?
when i look at birds or aeroplanes it becomes somewhat obvious that the thinner the wing, the more efficient they become...
swallows (some of them stay airborne for years!), albatrosses, sailplanes (heck - they are so even named ) all have very thin and long wings...

but what would be the optimum aspect ratio?
depending on the width of the wing?

[Perm Stress]

Originally Posted by capt vimes

i was wondering - looking at gary's bay skimmer - what would be the most efficient aspect ratio for the sails on a free standing, rotating mast?

the sails should always and at every point have a clean airflow, they should never operate in a stalled mode, so the higher the aspect - the more efficient they should be - isn't it?
when i look at birds or aeroplanes it becomes somewhat obvious that the thinner the wing, the more efficient they become...
swallows (some of them stay airborne for years!), albatrosses, sailplanes (heck - they are so even named ) all have very thin and long wings...

but what would be the optimum aspect ratio?
depending on the width of the wing?

birds, aeroplanes, swallows, sailplanes ... each one of them work in quite narrow angle of attack (1...5 degrees) and speed range.
Most of them work in more-or-less steady flow.
They are optimized for high L/D ratio at cruising speed.

Totally unlike sailboat sails, which have to work in angle of attack 15...90 degrees. Not to mention stability issues; they effectively prohibit rigs of aircraft aspect ratio.
Boat rig have to deal with pitching-rolling-having- ... .

In short, with all the restrictions, rig aspect ratio of well proven boats should be quite close to optimum.
*Squat sails for heavy keelboats, where more brute force is often more or equally important as decent (not High!) L/D ratio.
*High aspect wings for A-class, America's Cup catamarans, where relative importance of L/D against brute force is higher.

[Eric Sponberg]

Originally Posted by capt vimes

i was wondering - looking at gary's bay skimmer - what would be the most efficient aspect ratio for the sails on a free standing, rotating mast?

the sails should always and at every point have a clean airflow, they should never operate in a stalled mode, so the higher the aspect - the more efficient they should be - isn't it?
when i look at birds or aeroplanes it becomes somewhat obvious that the thinner the wing, the more efficient they become...
swallows (some of them stay airborne for years!), albatrosses, sailplanes (heck - they are so even named ) all have very thin and long wings...

but what would be the optimum aspect ratio?
depending on the width of the wing?

I don't know if you could precisely say what is the "optimum" aspect ratio for a free-standing wingmast, and that is because "optimum" suggests that there is one point or region of aspect ratio where the rig works best, and at values greater or lesser than that, the rig works not so well.

In general, no matter what rig shape or wing shape, be it an aeroplane or a bird's wing, a sail, or a keel or a rudder, the higher the aspect ratio the more lift and less drag you get at any given angle of attack. This improves to the stall angle--all wings will stall at some angle of attack--and higher aspect ratio wings will stall at lower angles of attack, and lower aspect ratio wings will stall at a higher angle of attack.

Also, as aspect ratio goes up, so does the height of the center of aerodynamic forces, the center of effort. The more lift for less drag will improve driving force, but the taller rig and higher the center of effort will also increase heeling force. Heeling force is countered primarily by the boat's stability, and stability varies from boat to boat.

Which is all to say that you cannot say, in general, what is the optimum aspect ratio for a free-standing wingmast. All rigs are different, all hulls are different, and you have to tailor all the elements of the boat design to arrive at the best compromise for the rig and design at hand.

Wing thickness also plays a part, as you surmise. Thin wings have a very narrow range of angle of attack and generating lift before stall, and thick wings have a relatively wider range of angle of attack before stall. When designing a free-standing wingmast rig, I almost universally use a 2/1 chord to thickness ratio in an elliptical section shape of the structural wing itself, not including the chord of the sail, as I have found this to be a forgiving shape aerodynamically, meaning that it is relatively easy to trim, plus it is very easy to engineer. I've talked about this before. By trimming the mast, I mean setting the sail and wing at the right angle of attack and trim so that you get good smooth airflow over both sides of the wing and the sail, as evidenced by telltales, as the boat moves through the seaway. A boat with a narrow wingmast, say 3/1 chord/length ratio, is going to stall more often as the boat sails through waves--and the window of acceptable mast trim is much narrower than on a thicker section.

There are structural and windage issues as well, and basically what you are trying to achieve is the smallest possible mast profile that is consistent with strength and stiffness. You want just the right amount of strength for the boat at hand, plus the right amount of mast deflection. Some deflection is good, but too much deflection is bad. Deflection is governed primarily by mast section width, which in turn determines mast section length. Wall thickness and the amount of carbon fiber you must have to build the mast is dependent on section shape and the bending load at any given height. Carbon fiber is expensive, and you want to use the least amount possible. As you make the mast section bigger, interestingly, you use less carbon fiber. Smaller mast sections use more carbon fiber. This is because you get more strength and stiffness due to section shape than you do from wall thickness. And if you go too thin on the wall thickness, the mast is going to buckle too soon, before it reaches the target strength. So, you're trying to hit a target where you get the right strength and the right stiffness, with the right wall thickness, that isn't too big in overall profile. Remember, the boat sits still for the vast majority of its life, either at a dock or at a mooring, and too much wingmast area is a bad thing. You want the minimum amount, consistent with strength and stiffness.

So, in summary, there are a whole host of factors that go into designing a wingmast rig, and aspect ratio is only one of many factors. Quite frankly, I pay little attention to aspect ratio at first. I know I have to get a certain amount of sail area onto the boat, I know the number of masts, either one or two, and I know that I am restricted on boom length--you need space between mast and boom on a 2-stick boat, and generally you don't like the boom overhanging the stern (although I know of situations where that is OK). So mast height kind of falls out of the equation somewhere in all of that. In my latest design, the Globetrotter 66, aspect ratio as defined by P/E = 2.64, and as defined by span^2/area = 3.06. That's generally where my aspect ratios fall, and I don't necessarily try to optimize that--it is what it is, all things considered.

Eric

[sharpii2]

Originally Posted by capt vimes

i was wondering - looking at gary's bay skimmer - what would be the most efficient aspect ratio for the sails on a free standing, rotating mast?

the sails should always and at every point have a clean airflow, they should never operate in a stalled mode, so the higher the aspect - the more efficient they should be - isn't it?
when i look at birds or aeroplanes it becomes somewhat obvious that the thinner the wing, the more efficient they become...
swallows (some of them stay airborne for years!), albatrosses, sailplanes (heck - they are so even named ) all have very thin and long wings...

but what would be the optimum aspect ratio?
depending on the width of the wing?

That's a good question.

It has a bit of a complicated answer.

The answer is:

it should have as high an aspect ratio as possible, given the materials used in making the mast, the initial and ultimate stability of the boat, and other practical considerations, such as necessary mast thickness and ease in raising, lowering, and stowing the mast.

As a mast gets taller, its section thickness almost always has to increase. The usual two exceptions to this rule are;

1.) materials for the taller mast having greater strength to weight ratios, and 2.) the taller mast using more of a truss like arrangement of: stays, shrouds, and spreaders. (This may be one reason we don't see many un-stayed masts in the 'Open' classes)

It should also be kept in mind that if you if you double the height of your mast, you must reduce it weight by half, in order to have a similar effect the boat's Vertical Center of Gravity.

Since I usually stay away from racing, in my design concepts, I find going to a higher aspect ratio usually not worth the trouble. The performance gain vs. the extra cost and complexity is simply not worth it.

[Eric Sponberg]

Originally Posted by sharpii2

2.) the taller mast using more of a truss like arrangement of: stays, shrouds, and spreaders. (This may be one reason we don't see many un-stayed masts in the 'Open' classes).

Personally, I think that the reason we don't see many unstayed masts in the open classes is that few people accept them, understand them, know how to design and build them, and sail them. I am convinced that a free-standing wingmasted cat ketch would every bit as competitive as the lot of what's sailing today, would be very much safer by virtue of the fact that such boats are much easier to control and keep running, particulalry on off-the-wind sailing angles where they excell in speed.

Eric

[Gulf Islander]

Shearwater was designed and built by Richard Black and I believe her freestanding carbon fibre masts were designed for Richard by Eric Sponberg around 1980. She has two-ply wing sails which wrap around the mast. I bought her from a friend on Orcas Island in 1999 and use her for daysailing. She is stored in the winter on her trailer. She is great fun to sail.
This year I am undertaking a refit and considering improvements. Her sails are about 5.5 oz and two ply means 11 oz. As a result of their weight they have hardly worn in 30 years. My question is what weight of sail cloth is appropriate for two ply sails? Can I go for 3 oz as the fabric is two ply?
I would also change from the older triangular profile to the current blunt ended sail profile.
Thanks for any suggestions.

[Gary Baigent]

Originally Posted by capt vimes

i was wondering - looking at gary's bay skimmer - what would be the most efficient aspect ratio for the sails on a free standing, rotating mast?

the sails should always and at every point have a clean airflow, they should never operate in a stalled mode, so the higher the aspect - the more efficient they should be - isn't it?
when i look at birds or aeroplanes it becomes somewhat obvious that the thinner the wing, the more efficient they become...
swallows (some of them stay airborne for years!), albatrosses, sailplanes (heck - they are so even named ) all have very thin and long wings...

but what would be the optimum aspect ratio?
depending on the width of the wing?

Capn. The ratio is around 3x1 including the wing masts. The two sails have to fit on an 18 foot boat so therefore the foot of each sail has to be quite short; this is compensated for by the square tops to produce a larger sail area. 3x1 is pretty low aspect ratio but the CBSkimmer has truly excellent windward performance, beating many larger yachts. Lazers (good windward performers, imo) it eats for breakfast. Not gloating but the medium low, but large sail area works way better than I had expected. The rigs had to be reasonably low to keep the CoE down because the skimmer is unballasted (except in heavy winds when you can fill the water ballast tank). The rotating wing masts are definitely worth building for their efficiency. There is no problems in handling them, no danger. However the skimmer's are now stayed rigs, not freestanding. My failure of my earlier setups was my fault but if I was to do the masts again, would have them sorted correctly.
Also I have no problem with the so called stalling of the main rig from the fore one ... because it doesn't happen. The wind indicator of the fore rig always points higher than the after rig - so there is no indication of backwinding on any point of sail. Maybe if you ran completely flat off there would be some disturbance on the fore main - but I never sail that way, preferring speed.
The wing mast thickness is also no problem. This sounds like some brochure blurb but really, such light planing boats with lowish, powerful rigs actually work perfectly. The efficiency of rotating mast rigs on a big dinghy allow no dumbing down of performance with ballast and heavy keels. But of course you have to sail attentively; keep the sails cleated in gusting winds and you'll go over, just like on any dinghy.