Fraction vs. Mast Head

Can anyone shed some light on the advantages and disadvantages between the two different rigs. I am looking for a general idea, not nec. related to RC sailing. Walking through the yard the other day I saw many dedicated race boats and it was split about 60% fraction rigged and 40% mast head. Juts curious about the pros and cons. Thanks for the help in advanced.
Andrew Miller

most boats sail better with a large main and small jib or vice versa. Having two equal-sized sails isnt that efficient-witness the old CCA boats with the huge overlapping genoas and the modern day ACC and supermaxis with the large roach in main and small genoas. In light air big boats now use Code Zero asymmetricals upwind-so even if your boat is fractionally rigged you are allowed to fly a masthead Code Zero. On RC boats the balance often isnt that good in moderate winds. You get a lot of lee helm out of a huge masthead genoa or jib.

Mast head rigs rated better under IOR, so older race boats from when that rule was in use were often masthead rigged.

Luff 'em & leave 'em.

Forestay attachment points affect the way that the mast bends under load from sails, as well as being shaped by the stays. Class rules often determine whether the boat is allowed a mast head rig, most I have noticed are fractional.

Classes that allow more sail area, or even unlimited sail area (basically more area than can be normally carried) will often see Genoa’s going all the way to the mast head in an effort to squeeze on every inch of sail possible.

From my understanding, you want a frac if your boat uses its main to drive off of. This usually allows the boat to optimize its pointing ability and does better upwind. Whereas, many overlapping mast head jibs end up back winding your main and don’t allow boats to drive up wind as well.

Downwind is the reverse. Having a masthead allows for the spin to get out away from the main and allows the boat to take a deeper angle.
There are always execeptions of course :).

I notice most of the newer boat designs use huge mains with frac blades for the head sail.

I’ll assume we are talking racing and not cruising.

There has always been a fundemental truth with sloops - the mainsail is connected to the rig along two sides (the luff and the foot) while the jib is connected along one side only - the luff. (Of course in our world of RC sailboats - we have a jib boom too). This simple truth means (for big boats) that it is possible to exert a greater range of trim/shape controls over the mainsail, than it is over the headsail. Note that this doesn’t mean that a mainsail is necessarily more efficient than a headsail - otherwise - rating rules aside - we’d all be sailing around in cat boats.

There has been some interesting work done in the field of “mast-aft” rigs - where the mainsail is dispensed with and the boat driven by a “headsail” placed much further aft than would be the case with a sloop rig. I’m sure there is some stuff on the web about this somewhere.

Years ago (back in the late '70s) I recall an exercise where two Ron Holland design three-quarter tonners, from the same mould, were compared - one with a masthead rig and one with a fractional. The results were not really surprising. If I recall, upwind in a straight line, the boats were comparable. But upwind, with lots of tacking, the fractional came out ahead. If you think about it, as you come through the tack, the mainsail is pretty much set as soon as it fills, and it’s available to drive the boat. The headsail has to be trimmed in - which on a large boat is slower with a masthead genoa than a fractional genoa.

Reaching and downwind, performance comparisons will vary (for the same hull) depending on point of sail and wind strength. The masthead rig carries more sail area downwind (not counting a modern fractional with masthead spinnakers). A conventional fractional not only has a smaller spinnaker, but has a greater problem with blanketing from the mainsail when running square. On the other hand,the fractional has fewer control problems as the breeze gets up. So, other things being equal, you’d prefer the masthead rig when the wind is light and aft of the beam, but the fractional when the wind is fresh and aft of the beam.

When Bruce Farr and others made fractional rigs popular within IOR racing, they introduced bendy rigs. Although fractional rigs had been around for years (look at any of the J boats or old meter boats) the mast had been kept in column and not designed to flop around while sailing. Bruce Farr’s designs could de-power by having the upper part of the rig fall away to leeward as the breeze freshened. This allowed a given sail combination to be carried through a wider range of breeze cf a masthead rig. So around a short course - fewer sail changes, and faster tacks, meant a quicker boat - even if the straight line boat speed was the same.

As usual, this is an over-simplification and just part of the story.

I could go on and on…


fractional rigs beat upwind better than masthead rigs

With respect Ed - I’d have to say “not always so”.

This article make interesting reading - and comments on some of the negative of mainsails. Items a) b) and e) (and to some extent d)) are relevent to this discussion. I’m not necessarily agreeing with this article - just putting it forward as a contribution to the discussion.


hi muzza,
why do cats like the hobie 16/18 have frac rigs? an instructor once told me that because cats cant point as high as a mono, a frac rig is necessary.


I’m not sure that I’ve ever seen a dinghy or small cat of any variety that did not have a fractional rig. Almost all dinghies, if they have a jib at all, have a non-overlapping jib. The exceptions that come to mind are the Flying Dutchman and 505 classes.

It’s a good questions Ed. I started writing an answer which talked about lift v drag in over-lapping and non-overlapping headsails, and why non-overlapping headsails generally result in better pointing ability (whether masthead or fractional) - but the answer was getting very long. Perhaps I’ll try again when I have a little more time.

The bottom line is that fractional rigs are practical on small boats for many reasons - lighter weight, usable in a wider range of wind strengths, lower CoG just to name a few. Non-overlaaping headsails are practical too, and generally point better than the overlapping equivalent (with the emphasis on “generally”).

I think however that the comment about cats not being able to point as well as monohulls might attract some comment from those racing cats - especially perfomace cats like the A Class and the Tornado. In such boats, the angle they sail to windward has as much to do with maximising VMG as is does pure pointing ability. A good cat can point very high - but this may not be its fastest course to windward.

And yes - the headsail definitely has a role in imporving the pointing of a boat compared to a mainsail-only rig.

Sorry - I don’t think I’ve addressed your question fully.


that is why most cats have rotating masts to present a better sail angle/shape relative to the apparent wind.

fractional rigs almost always go better upwind in light air. In heavy air the fractional boats seem to do better. Of course, there are always exceptions. In light air boats used to have huge 150+ percent overlapping genoas.

Ed & Muzza -

my big cat is uni-rigged, much like the “A” Class cat. It is an 18 Square (18 x 12 x 31 foot mast and 194 sq. feet of main only)

It seems from experience, I can sial and point much higher than a jib-equipped cat, but once we crack over on a downwind leg, it seems the jib equipped boats can go deeper and a bit faster. Always was a challenge - I’m first to the weather mark, and then had to fight off all the bigger sail area boats downwind. Never got really good at that, but some ofour top-flight Square sailors were.

Regardless, there is never a good substitute for too much sail area or too light of a boat. One can always reduce sail or add weight.

Interesting subject since many classes (R/C) have gone out of their way to specifically restrict “main only” rig configurations. Anyone else ever wonder why? Check it out - some are our “development class” boats and makes one ponder.

Interesting comments Dick. This is becoming a fascinating thread. Ed - I would have thought that the purpose of the rotating mast was to get a better airflow over the sail (especially leeward side) - in much the same way as a wing mast would. Some of the reasons why we often see them on cats is that:

  1. they are free of the rigging paraphernalia that goes with extras (i.e. they do not need backstays, runners etc - nasty bits of wire that usually prevent use of rotating masts)
  2. they sail at higher apparent wind speeds than we slow monohull sailors. If you think of the mast as a foil (albeit rather an inefficient one) then it is operating at higher Reynolds numbers than it’s monohull cousins (but nonetheless very low) - and so benefit more than slower boats from reduction in drag offered by the rotating mast.

There is an interesting article (linked below) which touches on many of the things in this thread. Have a read if you have a few minutes:

There’s another twist we haven’t mentioned. Other things being equal, a bigger boat can outpoint a smaller boat. Just another ingredient to add to the recipe.


I think you will find that cats cant point as high because of the speed they are going. compare a laser & an A Class Cat (because it happened yesterday & the day before, me on laser my boss on his A class), both cat rigged boats, in 10-15 knots, the laser outpoints the A Class. both boats are overpowered… but the A Class was sailing with both telltales streaming aft, on the laser, if the main had of been full, then the tell tales would have been streaming aft as well. it also doesn’t help that my laser is like over 23 years old & the sail is prob 20 years old, but i wasn’t that far behind the other lasers in pointing.

I see said the blind man to the crippled nudist who put his hands in his pockets & promptly walked away.

Man Yachtie you relly know how to make someone miss their laser. LOL, i guess i made out though, bought it over 12 years ago for $450 from a woman that really had no idea what she had. Sold it last spring for $1200 after doing nothing more in the time I owned it than replacing the through hull plug in the cockpit and the stern and putting new paint on it. I also tend to believe that racing dignhy prices in Annapolis are somewhat inflated due to the demand. I think a new 2005 laser from Annapolis Performance Sailing is close too $4600. Craziness, anyway Im done rambling.
Andrew Miller

Fraction beats masthead. Especially downwind. Morover, upwind you loose control over the upper parts of the jib in a breeze, in the case of masthead. It is better to have this sail area in the main, which you can control. The scoweling phenomenon (Runge-Kutta 2nd order circulation) over mainsail at the “above-jib” part of the mast, is another benefit of fraction-rig. Finally, Cougar, maple-sirup can be more easily handeled in a breeze, since you don’t need to sheet the jib so frequently!

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by booster

The scoweling phenomenon (Runge-Kutta 2nd order circulation) over mainsail at the “above-jib” part of the mast
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Uh, ok Booster. Please inform the “rest of us” what in the heck this is. It’s a new one on me[:D][:D][:D]

I Googled it and came up with a lot of impressive looking formulae that have something to do with vortices. I took a quick look at it and decided I wouldn’t go there.


<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Millrtme

I also tend to believe that racing dignhy prices in Annapolis are somewhat inflated due to the demand. I think a new 2005 laser from Annapolis Performance Sailing is close too $4600. Craziness, anyway Im done rambling.
Andrew Miller
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
mmmm, they have increased the prices this year by 10% on lasers, takin the Aussie Laser cost up to AU$8200 if my mate tells it correctly (which he does, he is gettin a new one very soon).

I see said the blind man to the crippled nudist who put his hands in his pockets & promptly walked away.