[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.

 

[Omatako]

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

Bob, whilst I unreservedly bow to your superior knowledge, it would be great for me to share some of it. Nothing in the post above leads me to an understanding of why/how a frac rig trumps a masthead rig.

 

[Omatako]

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.

Ditto my response above. Please give me something I can believe in.

 

[Omatako]

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?!

And finally someone comes up with a possible explanation. Thanks HS.

But I still have a problem believing that the mingled vortices at the top end of the main will create sufficient drag to over come the additional sail area of 140%, low footed, masthead genoa.

Strangely, I actually believe that a frac rig is better because I cannot forget marvelling at the giant AC trimaran back in Valencia going to windward and dropping the headsail altogether and going faster. Or the relatively tiny jibs on the AC foiling cats.

But still I haven't heard any compelling evidence to suggest this is true of a non-wing rig.

Maybe someone who designs and builds boats will take a frac rig and sail it for a month then change it to a masthead rig and sail it for a month and provide some real comparative data to prove the point. Then we would have something to believe in.

I will follow this thread with interest even though I have no intention of ditching my masthead rig.

 

[Omatako]

I think mastheads are fine, but they are a compromise. Beating upwind a fractional is always going to point higher and be faster.

Yes you may be (and probably are) right but why? And please don't do like other posts and show me a picture of a boat that won a race and say "This is why". Geez

Reaching a asymetric is always going to be faster, downwind a spinnaker is always faster. The upside to a masthead jib however is that it does them all well on one sail. If you will never consider a furling genniker then a masthead is likely faster all round. But not on any one point of sail, it's more of an 'on average' rig.

What reason is there to assume that a masthead rig will not use a code sail, gennaker or spinnaker off the wind? If a frac rig furls the headsail and a masthead furls the headsail and both fly similar downwind sails why would the frac be faster off the wind? Maybe a taller rig, maybe a bigger main, I'm not saying it isn't faster, all I'm trying to do is understand why.