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Go Back   SailNet Community > Out There > Racing > Masthead versus Fractional
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Thread: Masthead versus Fractional Reply to Thread
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Topic Review (Newest First)
11-12-2010 08:58 PM
paulk
To get the full answer

you may have to read C.A Marcaj's books on aero and hydrodyanmics. My understanding is that the fractional rig, going upwind, has a better balance of factors that create a spiralling vortex UP, off the mainsail, than a masthead rig does. This enables a fractional rig to have better (more?) airflow over the sails than a comparable masthead rig, would, so the fractional rig therefore goes faster. In a masthead rig, the flow in the vortex may get disturbed by the flow from the jib, slowing the overall airflow, and making the masthead rig less efficient. Everything is relative, of course. A 50' masthead rigged boat will probably sail faster upwind than a 20' fractionally rigged boat. Fractional rigs and masthead rigs both work; what's being discussed is their average efficiencies under certain conditions.
11-12-2010 11:40 AM
LeftCoast Wait a sec..

RichH : "The simple answer (from the mind of a fluid dynamicist) is: fractional boats are more efficient aerodynamically when close hauled ."

Why? I don't understand why this is.

and..

"Large overlapping headsails"

What my question was that -if- the boat was designed to use non-overlapping sails, then why not just run the headstay to the top of the mast?

I don't see why running the headstay to the top of the mast could hurt anything.

Sorry, this fractional thing has been a pet peeve of mine for a long time. I went around and around with our designer on it and never got an answer that I was really happy with.

-jim lee
11-12-2010 10:29 AM
RichH The simple answer (from the mind of a fluid dynamicist) is: fractional boats are more efficient aerodynamically when close hauled .... AND with a masthead spinnaker are ~equal to the SA of a downwind masthead rig.

Large overlapping headsails are simply aero-inefficient ... as most of the 'suction peak' due to aerodynamic flow is near the luff of a headsail. Extreme overlapping headsails retard the 'dumping velocity' or 'reposition it' too far aft of the point of max. draft in the main and thus decrease the aero-efficiency (circulation flow) of the interaction of the 2 sails.

The fract rigs. (with mast head spinns) for *windward-leeward racing* are simply 'more optimized'.
If races were limited to *close reach or below* .... you'd all would be frantically selling/touting masthead boats.
Long distance cruisers try to avoid 'beating' and thats why cruisers prefer non-frac. boats. - the benefit of larger SA when 'off the wind' and not having the *almost-requirement* of needing to use a spinn. when going 'down'. Nothing is worse than going 'down' on frac. boat with NO spinn flying.
11-12-2010 09:38 AM
Faster
Quote:
Originally Posted by LeftCoast View Post
I've not heard an argument yet that really convinces me that a fractional is better than a mast head.

The two I hear most..

A) You can bend the mast to depower the sailplan. - The J35 is a masthead rig, and the mast bends wonderfully for depowering the sailplan.

B) Masthead rigs need large overlapping jibs to work. - Thats not because they are masthead rigs, that's just how they were designed. You could just as easily design the boat for small non-overlapping jibs and just run the forestay to the top of the mast.

As for chutes.. Seems no one wants them anywhere but at the top of the mast now.

-jim lee
What you've said is true, of course, though the masthead kite issue is primarily a racing one. And in the J35, the rig is not exactly the same section and the controls are more involved than the typical run-of-the mill masthead cruiser-racer.

What I like about the frac rig is that jibs, genoas and standard hoist kites are smaller, lighter loaded and easier to tack, gybe and winch in than a masthead for a given sized boat. Not an issue until you get up into the 30+ footers, I suppose, but while I do find our 3/4 rig a tad underpowered at times, I like that the boat handles like 30 and lives like 35....
11-12-2010 12:35 AM
LeftCoast I've not heard an argument yet that really convinces me that a fractional is better than a mast head.

The two I hear most..

A) You can bend the mast to depower the sailplan. - The J35 is a masthead rig, and the mast bends wonderfully for depowering the sailplan.

B) Masthead rigs need large overlapping jibs to work. - Thats not because they are masthead rigs, that's just how they were designed. You could just as easily design the boat for small non-overlapping jibs and just run the forestay to the top of the mast.

As for chutes.. Seems no one wants them anywhere but at the top of the mast now.

-jim lee
09-05-2010 04:12 PM
JohnRPollard
Quote:
Originally Posted by GeorgeB View Post
...Fractional boats allow for jibs to be sheeted closer to the centerline giving the boat more pointing ability. ...
George,

That's one point that strikes me as not necessarily correct. In general, I'd say the opposite.

The truncated foretriangle of a fractional rig usually makes achieving tight sheeting angles trickier. Whereas, generally speaking a longer foretriangle shrinks the acute angle between the centerline of the boat and the imaginary line from the tack to the sheet leads.

There are ways to work around this, but in general I'd say the masthead rig has the advantage here -- simply due to the geometry of the foretriangle.
09-02-2010 11:17 AM
blt2ski Paul,

A couple of boats in my YC, a Farr 1020, and a hotfoot 32? might be a 30, anyway, they have gone to a mast head spin, gained lots of speed down wind, penalty was not enough to make them non competitive, along with they now use the smaller fractional kite in upper winds to there advantage too.

Marty
09-01-2010 09:29 PM
paulk I seem to remember reading somewhere (CA Markaj?) that fractional rigs create a better or more effective vortex for their size than masthead rigs. It seems their overall shape with the leading edges of the forestay and then mast more closely resembles the curve of the vortex you want to create in order to move the boat upwind. Downwind... it's another story. I keep toying with the idea of putting a masthead chute on our boat. Maybe I can borrow one from a Swan 48 and see if it works.
08-30-2010 05:48 PM
COOL
Quote:
Originally Posted by puddinlegs View Post
f

Fractional boats don't fly genoas from the masthead... they're typically not designed for the load going to weather. On the other hand, some can carry a masthead spinnaker and something like a Code O within certain wind ranges. But even that depends on the particulars of the rig design.
Actually, many modern Offshore race boats do fly masthead genoas
from frac rigs. They function very much in the manner of a upwind
Code Zero, but with subtle differences. A Code Zero must measure
as a Spinnaker, therefore the midgirth must be at least 75% of the
foot length. This makes it difficult to design an extremely efficient
close winded sail. While it will incur a rating penalty, a masthead
genoa can be a very effective light air sail on a boat that
otherwise carries non overlapping frac rigged headsails.
08-28-2010 09:21 PM
smackdaddy
Quote:
Originally Posted by Cruisingdad View Post
I can talk more about this tomorrow hopefully, but we had a Tiger and a Melges with a Asym that KICKED OUT BUTTS! Very light air, but shifty, which made the Asym awesome. It was a masthead. Manhy of teh racers were saying that fractional was a thing of the past. Me, I just lusted after the Melges...

But good food for thought and makes a good discussion.

B
I had a run on a Melges a couple of months ago. Flew the kite (and trimmed for a bit) in 20+. HAD A BLAST! That was a damn fun boat.
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