Which points better

[LadyInBed]

Given the same underwater profile, so it can be ignored, which mast position will beat to wind better.
A quarter fore triangle / three-quarter aft; a one third / two thirds; or as seen in some cats and a Fjord 33, a large fore triangle and short main boom?

 

[jimi]

Does it not depend on wind strength .. I'd imagine that an overlap with a slot is going to be the most efficient to windward?

 

[LadyInBed]

OK, if wind strength has a different effect on the three profiles, which is better for beating into which strength of wind?

 

[Richard_Peevor]

not much help but...

you do want an overlapping genoa for the slot effect not sure about the mast position though. Isnt it further back when the wind is stronger helping to depower the rig to keep control (is with dinghies with mast rake to get the cnere of effort of the sail behind the centre of gravity but for yachts you would just reef down in the usual way rather than mess about with the rig...)

what is your scenario though? Not many of us have the option of moving the mast? Are you building/desiging a new boat?

 

[jonjo]

If the under water profile is fixed but I can vary the ballast my preferance would be a high aspect 50/50 sail plan.

 

[Rowana]

Why do you ask?

Not thinking of chopping off one of your masts, are you ? ?

 

[LadyInBed]

No Jim, I ask just out of curiosity about design. Some boats are built with the mast up near the bow with a long boom, often gaff rig but not always. Most have the mast one third back along the LOA, and a few (some cats and a fjord 33) have the mast two thirds back along the LOA.
The position of the mast must (I think) affect the boats ability to beat so I would like to know which does it better.
The area of the foresail will have an effect, but given the same foresail area, depending how far the mast is forward along the LOA will give different overlaps on the mainsail. That then brings wind strength into the equation, which all starts to get a bit complicated for my mind, so I am hoping someone here can explain things in simple terms.
/forums/images/graemlins/confused.gif

 

[boatmike]

Actually there should be no real difference in windward performance. The position of the mast is driven by several other factors though that are interesting to consider. Firstly some old boats have the mast well aft with a big genoa as it used to give an advantage under the old IOR rules. Then as you mention Catamarans, some like my Prout Snowgoose have the mast well aft and a very small mainsail for structural reasons originally. That is it put the mast directly in line with the main structural crossbeam or bulkhead. Neither of these things have any affect on windward performance to any real extent apart from allowing a cutter rig forward by hoisting a genoa (or jib) and inner forestay together which if the mast were forward would require a bowsprit. This gives 2 slots to windward rather than one (Jimi's point about overlap is therefore valid)
A smaller jib and larger main is certainly easier to handle when tacking, so the modern tendancy is to go that way. Actually the sail plan must still be designed to have the centroid of area of the sails pretty well in the same place relative to the centre of keel resistance to avoid heavy weather helm or lee helm so the performance of both should be the same to windward assuming the sails are cut right and set right. Arguably this is easier if neither the main or genoa are too big.
There are two other effects though of a big foresail set on a mast which is mounted aft. The first is that the main will not blanket the genoa to the same extent when going downwind, The second is that the high angle of incidence of the forestay provides lift to the bows when on a broad reach, especially on a cat which does not lean over like a monohull. This stops the bows burying in a gust and increases speed. As cats sail faster, are lighter, and prone to burying their lee bow in a gust this is a useful side effect. Even with my boat which is 37ft OAL and 6 tonnes in weight (so heavy for a cat) I have been hit by a 35 knot squall on a beam reach with full genoa up and it picked the bows clean out of the water pointed me downwind and plonked me down again doing about 9 knots! A very useful thing to know as many cats bury the lee bow and trip under these conditions.
THERE IS NO SUCH THING AS A PERFECT BOAT......

 

[Guest]

[ QUOTE ]
the main will not blanket the genoa to the same extent when going downwind

[/ QUOTE ]

Lots of interesting thoughts in your posting. Just one issue with the above quoted bit. Well, yes, it's true that the genoa won't be so blanketed, but at the same time the principal pulling sail in that situation (principal because unblanketed and therefore most of teh drive comes from it), the mainsail, is obviously smaller which cancels out the advantage with the genoa.

 

[Guest]

It seems to me that a design with the mast well forward ought to suffer from the following effect: that a greater proportion of the drive comes from the mainsail, but that the airflow over the luff of the mainsail suffers from turbulence caused by the mast, making it a less efficient sail.

On the other hand, in confined waters where frequent tacking is necessary a small foresail means it can be drawing straight out of the tack, minimising the loss of boat speed through the tack. I noticed this effect very strongly this summer tacking against various other boats through an archipelago in strongish winds (F5/6). My boat has a large foresail, the other boats had small foresails, but broadly similarish hull forms. When there was a long space, we could pull ahead quite fast, but in the narrow gaps between the islands, no matter how hard we tried we couldn't sheet the big genoa home as fast as they could whip in their jibs, so it took us more time to pick up speed again.

 

[boatmike]

Mainsail being the principal drive downwind?

Not really. Otherwise why would racing boats hoist a spinnaker with the mainsail still set? A smaller main need not have a serious blanketing effect if the profile of the headsail is bigger especially if the main is carried loose footed. The other point is that an involuntary jibe if gull winged is less problematic if the sail is small. Depends on your boat and the relative sizes of sails. I often see people struggling dead downwind with a flapping genoa and full main when it is obvious that the genoa would be more efficient on its own.

 

[boatmike]

Tell me a story Simon. Try tacking a catamaran with a Genoa that overlaps like mine does and a boat that stalls in irons half way around! Not everything is easier on a catamaran............. I can point quite high and often surprise monohulls with my angle and speed to windward but when I have to tack they usually wonder why I have stopped and am running about on deck like a aerobic chicken!
Sometimes I confess I even start my engine and motor round on the other tack while I re-set my sails (don't tell anyone!)

 

[Guest]

Re: Mainsail being the principal drive downwind?

Spinnaker has a pole to pull it out on the windward side to ensure a large part of it is pulled out of the main's shadow. Also a shape designed to maximise it's ability to bag the wind, and it's cut at the tope to ensure it catches wind spilled over the top of the mainsail.

My boat has a large genoa and small main, but even in the strongest winds the main causes the genoa to collapse direct downwind unless it is goosewinged. As you say, the easiest solution is often to drop the main (or goosewing, or broad reach). But the fact that the main's small doesn't help much.

 

[jamesjermain]

Re: Mainsail being the principal drive downwind?

The important thing is to get the centre of effort of the rig properly balanced with the centre of lateral resistance and to have well-cut sails. However, in general, a non-masthead rig, whether half three-quarters or 7/8th rig. will be quicker assuming a headsail with some overlap and reasonable luff length. The rig is more flexible and the main can be flattened as the wind increases by bending the mast.

A three-quarters rig with a full-height, long-luffed genoa with a small amount of overlap (10 per cent) is incredibly efficient in moderate to strong winds since both sails will be full and drawing strongly and the overlap effect will be at its maximum without causing any backwinding of the main.

On a masthead rig the main is very largely ineffective in moderate winds because of backwinding along the first third of its length while the large overlapping genny is not a very efficient shape to windward and has a large drag component.

Off the wind and down wind, different factors apply, of course.

PS. In answer to the original question, the position of the mast will depend on the chosen balance of areas between main and foretriangle