[MedSailor]

There's a concept that's been bugging me about the factional rig. I understand the advantage of mast bend, but the smaller foresail is also touted as an advantage with the main being "the sail that does all the work anyway".

But, wasn't the big deck sweeping and overlapping headsail supposed to be the thing that really mattered? All the club racers have huge overlapping headsails and the "slot effect" is supposed to help with speed. I've heard people say that the headsail is the one that does all the work and pointing...

These guys are fast right? How come they look like masthead boats with big overlapping gennies?

MedSailor

 

[Stumble]

The jibs are all fractional in that shot, Puma is sailing on a Code 0.

 

[manatee]

Interesting discussion about the rigs here.

 

[paulk]

I read something, somewhere (Marcaj?) about how the shape of a fractionally rigged mast and sail was closer to the optimal leading edge shape than a masthead rig. The fractional rig supposedly creates a "better" vortex off the sails, which means going faster with the same sail area. It seems that a curved, (or "jointed",as it is in a fractional rig) leading edge works better aerodynamically than a single straight line (as a masthead rig would have). Airplane leading edges are often swept back, perhaps for this same reason?

 

[bobperry]

Med:
You have a lot of catching up to do. A lot.
Bend has almost nothing to do with it. Bend is more important on a masthead rig where you use bend to increase headstay tension and flatten the main. With a frac rig you get headstay tension with runners or swept spreaders. And, if you sweep your spreaders 20 or more degrees you are not going to be bending the mast much.

This of the rig as one big foil. The jib is just the leading edge.
With a big main and a small headsail you no longer have to change headsails for optimal performance, theoretically anyway. There are heavy weight jibs and light weights jibs but the LP will remain about the same.

The photo you posted is misleading. Only one boat is flying a masthead foresail and this sail is a code zero. It is not their working headsail. It is a reaching sail. The other boats in the pic clearly show frac rigs. Look closely.

Tomorrow I race FRANCIS. Big, tall frac rig and I will make it sing.

In the end we know that one, articulated wing sail is the best sail for speed. The frac rig comes close to replicating the "one foil" approach than does the masthead rig.

 

[MedSailor]

From poster Ian McColgin on the wooden boat thread linked to:

-----------------------
Rating rules and fashion favor bigger jibs. Pity there's even such a thing as "free" sail area in the overlap past J and the high roach of some abaft the mast sails with huge full battens. While the fractional rig with small jib is probably the most efficient way to carve up some fixed pile of canvass, not to mention the fun of bending the mast to shape the sail a bit, there is much to be said for keeping a dead straight column and the structural simplicity of a masthead rig.
--------------------------------

Does this help explain the deck sweeping 140+ genoas and masthead rigs that seem to dominate the racing I've been a part of?

MedSailor

 

[paulk]

Good luck, Bob, though have you considered getting some new sails? Carbon ones might scare a lot of competition into making mistakes.

 

[bobperry]

Med:
That guy has never sailed a modern frac rig. As I said in my post, with spreaders often swept to 30 degrees there is no mast bending to the degree it was some in old masthead IOR rigs.. You can bend the small top proportion if you want to but below the hounds the mast will be dead straight. I know becuase I design and sail these boats. I don't hypothesize or speculate. You can tune in a little pre-bend if you like. But once sailing there is little you can do with bending the mast. That doesn't always stop us from trying.

My Flying Tiger 7.5 meter.

My Flying Tiger 10 meter

ICON my all carbon 63' frac rigged lifting keel racer/cruiser

 

[bobperry]

Med:
The other huge variable that I forgot to mention is modern sail fabrics.

The modern racing matrix type sail does not stretch the way dacron stretches. The shape you want is there from the get go. You can adjust halyard tension, outhaul and cunningham to increase the draft of the sail but you can;t stretch a modern sail with mast bend the way you could with a dacron sail.

Take a look at a modern racing boat in light air. You will most probably see creases in the luff. Years ago this would have been unheard of. But with modern fabrics that one of the few ways you can play with draft.

Like so many "advancements" it takes a combination of factors to make it all work together.

 

[MedSailor]

Bob,

I definitely see the merits of the fractional rig, and while I've heard a lot of people talk about mast bend on fractional rigs, I get what you're saying. Actually, the only sailor that I've ever sailed with that REALLY seemed to know what he was doing with mast bend and make it work in his favor is Doug on Night Runner. Damn I love that boat.... Skipper too. I've learned a lot from the few times I've sailed with Doug.

I can see now about the flying wing shape of the mainsail, and how the rigid wind of some of the americas cup boats resembles one quite closely. Small headsails sound like a great thing to me too..... Basically, I'm sold on the factional rig concept in every way.

To phrase my question another way, what ARE the advantages of masthead rigs and big overlapping headsails? It seems like most of the "arguments for them", like the slot effect, and them being stronger, just don't hold up. Are they outdated tech? Racing rule beaters? Is the buying public just more "comfortable" with them?

MedSailor

PS I am very familliar with ICON. I used to live across the fairway from her at Shilshole, and when I moved up to Anacortes, she followed me up here. I admire her often, though usually from the stern....

 

[bobperry]

Med:
I think we need to get a real sailmaker involved in this chat. We need to divide "efficiency" from "rule advantage".

Time marches on.
Things change.

 

[Shockwave]

Then why is the mast head rig J29 faster then the frac rig 29, same with the Thomas 35 or even the J35 vs the J36? Hard to figure sometimes.

 

[christian.hess]

hmmmmmmmm I like frac rigs...I like sailing and working the main

maybe its cause I like sailing dinghies...like lasers

hmmmmm

 

[Stumble]

The downside to fractional rigs is that they often also have fractional kites. Which drastically reduce available sail area downwind and when reaching. Ideally you have a fractional upwind and a masthead down. But it takes a different mast than pure fractional since the runners don't support the upper tip, it's gets tricky to jibe and risks the mast

For cruising there is an argument to be made that a big overlapping jib is nice to have since they are better on a reach. But with asymetrics hitting the cruising market this is falling away.

 

[bobmcgov]

I've read the claim that a fractional rig is not a masthead rig with a short jib; it is a masthead rig with a tall main. That is, think of the hoist above the forestay tang as free mainsail area. Plus, the taller rig buys you more roach area ahead of the backstay, all the way down. The stick on our Albin Ballad looks stumpy compared to many of today's frac-rigged boats -- because it is.

I wonder if having the top of the main operating in clean air isn't an advantage, too, close-hauled. Esp. in light to medium winds, the mainsail needs to be twisted off quite a lot to keep the angle of attack the same from bottom to top, when the wind at your Airex may be twice what it is at deck level and AWA is maybe 10 degrees farther abeam. The top of the main can be twisted off more than the top of the jib; on a masthead rig, that may choke the slot & cause lots of drag. Just speculating.

 

[RichH]

The problem is that most of what has been published and routinely discussed, (even in US high schools) etc. on how sails (wings, etc) work has been wrong since the time when the Wright Brothers empirically discovered an entirely different reason for 'lift' (and such applies to all fluid dynamics - hydrodynamics as well as aerodynamics). Suffice it to say that such sciences are definitely NOT intuitive; yet, when using tell tales on sails or combinations of sails one can under exact conditions see the effect of air apparently 'circulating around' such sail and sail combinations.

Without getting into discussion of fluid dynamics and losing everyone via 'glassy eyed stares and nods', sails 'work' aerodynamically (going upwind) simply because the flow air has 1. 'friction' and 2. the apparent oncoming boat/sails causes some proportion of that airflow to go FORWARD on the windward side of the sails which in turn cause an 'upwash' way out in front of the boat causing a circulation effect of air flow. This is not dependent on whether the boat is mast headed or fractional rigged - its just the total area of the sails moving forward that causes effect ... and the effect is air circulating 'around' the sails/wing/propeller/turbine, etc.
The seminal articles that 'unveiled' such strange behavior to the sailing community was introduced by Arvel Gentry (aerodynamicist and sailor) in the early 1970s. Do websearch for: airflow+sails+circulation
The same applies to all fluid flow about 'foils'.

http://ljjensen.net/Maritimt/A Review of Modern Sail Theory.pdf
https://cld.pt/dl/download/10d8878b...ht - The Aerodynamics Of Sail Interaction.pdf

Again, such fluid flow is NOT 'intuitive' yet once you begin to understand exactly what is happening, you can under the 'right' conditions see these effects when using a full set of tell tales (or smoke tracers) on sails/wings, etc. To reach such understanding you will have to ignore everything you previously ever learned about how wings and sails 'work'. Under the 'right' conditions when sailing aerodynamically, usually when the air speed flow is exactly matching the forward speed of the boat/sails, you will notice that the windward side tell tales will not be streaming aft but will be streaming FORWARD ( a vectorial SUM) !!!!!! ... streaming forward on the windward side and streaming aft on the leeward side --- the 'total' flow is 'circulating' around the sails and in so doing is causing an optimum amount of 'upwash' well out in front of the boat/sails; hence the ability to 'point' (and point higher, etc. than when attempting to explain or rationalize by the 'wrong' and long held theories). The 'upwash' is simply air moving out of the way of the oncoming boat/sails and moving aside towards the apparent 'lower pressure' (leeward) side and long before it gets even close to the boat/sails. The windward side of the sails moving forward is causing 'some' of the air on the windward side to flow FORWARD .... its simply caused by the square foot of sail / wing material moving forward and doesnt matter if its all held up by a masthead rig or a fractional rig ... its 'just' the amount of sq. ft. of area moving forward. The 'keys' here are twofold: 1. air flow is 'circulating' around sails and 2. air has viscosity (friction).

A lot of articles and reply commentary have been written about 'sails and their recirculation flow regimes'. Do a websearch but be warned that some of the aerodynamic theory will be difficult to understand, .... at least on the first few readings. If interested, do read and reread and reread again such articles several times and then transfer that knowledge to YOUR sails with telltales installed .... you will eventually state "oh yeah, I think I can SEE what is happening; but, you have to have a full set of tell tales and 'gentry tufts' installed to see this.
Your high school science teacher was entirely WRONG about wings and sails and propellers, and what causes 'lift' etc.

;-)

 

[christian.hess]

Ive always had a bad time with tell tales...or better put I never completely use them, rather I go by speed and and wether Im luffing or not to get best angle and speed...etc...

its interesting when you say all this rich and It kind of resonates with me cause I pay too much attention to certain things...and just going by telltales alone never worked for me...

I too have noticed the windard tell tales point STRAIGHT up and Forward on many occasions trying to determine what was actually happening...juduging speed, heel, angle etc...


in the end I use many ways to trime sails versus just one way, use all methods to be better etc...

peace

 

[capt vimes]

RichH - i think you missunderstood the article...
I have not read the full article, but mr gentry speaks clearly of faster and slower airflow around an airfoil and thus creating a pressure difference which in turn produces lift...
Point 2.5 of the linked article.

What he did was introducing an circulation flow to simply model the airflow around an airfoil because the standard theories and models did not produce the right figures...
Pernouille is still valid and telltales on the leading edge of a sail may point forward because the airflow seperates there, producing vortices...
There is no circulation flow around a sail!

 

[hellosailor]

Med-
Oddly enough I asked a similar question just yesterday. Logically, there should be no innate advantage in having a smaller shorter foresail, which suggests that all of the "logic" in favor of fractional rigs is based on bubbameisers. (Nonsense tales perpetuated by grandmas.)

So let's look at physics and ignore all the bilge myths.

What is different about the interaction of the sails (main and fore) when you have a fractional rig? Well, there's a vortex coming off the end of every wing and that turbulence creates drag. With a masthead rig, and the main and foresail both having their top ends in the same place, wouldn't those two vortexes reinforce and create extra drag, and a loss of lift on the top of the main?

Aha. Maybe, maybe not, but has anyone ever bought time in a wind tunnel and actually examined that? I don't know, I've never heard mention of it but I'm not into heavy reading of aerodynamic testing.

Now take the same two sails, and stagger them so the two tip vortexes do NOT intermingle and reinforce each other. What do you have? A fractional rig!

Maybe it could be that simple. Simple physics, which no one has bothered to document or examine in detail because "everyone knows" the fractional rig with less sail area magically is faster. Magically. Hmmm, maybe that way the vortex coming off the tip on the foresail actually increases the flow below the tip of the main, actually boosting the performance of the main as well?!

Or can anyone point us to some objective wind tunnel work on this?

" Argh, yes, cod be faster than whales, so that must be the right way to shape a hull."
Ahuh. And rotting meat generates flies. Nothing new under the sun.

 

[Omatako]

I got glassy -eyed about here

This is not dependent on whether the boat is mast headed or fractional rigged - its just the total area of the sails moving forward that causes effect ... and the effect is air circulating 'around' the sails/wing/propeller/turbine, etc.

and here

and doesnt matter if its all held up by a masthead rig or a fractional rig ... its 'just' the amount of sq. ft. of area moving forward.

So none of this scintillating reading leads me to an understanding why a frac rig is better than a masthead rig.

Or I missed something.