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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.
 

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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.
Simple, the J-29 MH has more sail area than the fractional downwind by about 10% thanks to a much bigger spinnaker flown from the top instead of at the fractional point. Based on standard PHRF rules if the FH switched to a MH kite it would be penalized 6 seconds making it 3 seconds faster than the MH.

While discussing why is beyond my knowledge base looking at boats that are limited in sail area but not sail plan I think are instructive. Those classes with just a sail area maximum have pretty much all switched over to high aspect cat rigs, those with main sail are maximums have high aspect mains and overlapping jibs, those with max main and max jib area use high aspect mains and high aspect non overlapping jibs.

What this indicates to me is that for the same sail area upwind the ideal seems to be a massive main. If you have to divide it up then add the smallest jib possible, finally if you have extra sail area you tack it on to a bigger jib.
 

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That isn't a jib, it's a code sail. And on your diagram above it is still running a fractional genniker.

I would point out that hydroptere is so weird in its design and configuration I don't know e many iterations of sails it has. It is very possible that they have tried masthead rigs, fractional rigs, cat rigs, ect... The boat is a working test platform and doesn't really have a final configuration. You could probably find all sorts of weird combinations that have little to do with what the program thinks is fast.

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

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Oma,

Are you asking for mandates or realities? Fractional boats have a different rig system and sail profile than a masthead, it isn't just moving the forestay lower on the mast. There are a lot of things that change like main size, lead angles, J leingth, mast bend issues, aspect ratios. These things then change again as other things are decide like backstay vs running backs, shroud angles, ect. All of these things change and interact with each other thus it is very difficult to discuss one in isolation.

Fractional rigs have the mast further forward, which allows for a larger main, along with a the smaller jib means that the sail area stays pretty close to the same. However it also results in a shorter J so you get a smaller spinnaker particularly on a symmetric which is limited typically by the J leingth. On the other hand an asymetric is less driven by the J length since the sprint pole makes up a significant portion of the foot leingth.

A good example of how this works is the dimensions on the J-29 since there is both a MH and a FH boat designed to perform the same.

The MH rig is 3.8' taller, .5' longer J, 1' shorter boom, ect...

The end result is the MH gives up 50 sq foot of mainsail, and gains 50 sq foot of jib sail area. But gains 150sq foot of downwind sail area because of the taller spinnaker. On the other hand if you were to put a MH kite on the FH the total sail area would be much higher downwind.

It gets complicated because there are so many variables, which is why it is instructive to look at what the fastest boats are doing. Simply because they have the time and money to spend playing with the permutations. If you really want to learn why, you need to spend some time reading books on rig design and engineering. Because it is a complicated subject (and way past my expertise).
 

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Your numbers are a little off if the leech is 12% and the SMW is 1.8 on a J29. The mh downwind sail area is 667, the frac downwind sail area is 644. So a delta of 23 sq ft. Is this 23 sq ft enough to overcome the taller rig the frac j enjoys?

Upwind with blades the frac has an additional 3 sq ft, upwind with overlap the advantage goes to the mh rig (sorry, didn't bother to figure that delta) but again the frac has a taller rig and should be able to sail in more breeze and enjoy more shear.

But at the end of the day the mh is rated 6 seconds faster and is simply a better all around boat. My personal belief is the lower ce of the mh rig is a bigger advantage then initially thought. The mh boat stands up better.

Where is the frac an advantage? It's an easier boat to sail provided the extrusion/layup is strong enough to allow mh sails (codes through kites) and the spreaders are swept so runners and checks aren't required to keep the rig in the boat. Where is it a disadvantage? When more power is needed in the light stuff upwind a small j and I just doesn't give enough oomph with overlap. Hence the strong development of the mh codes and sprits. You can't simply add I because the geometry of the sail is structurally unfeasible, the aspect ratio doesn't work. So modern boats add j (sprit) and I (mast head sheave boxes) and essentially become a mh boat with a very big rig.

What's best? MH or frac? Looking at the J 29 the MH is best, looking at other boats, I don't know, there aren't many boats built as both MH and frac.
Shockwave,

The rates size for the J-29 spinnaker is 630sq feet (FH), 751 (MH) for a delta of 121sq feet. It's a pretty major upsizing relative to the boat. If you put a MH kite on the FH boat however the downwind sail area advantage would flip back to the FH.

My numbers indicate the FH is typically rates 3sec/mile slower than the MH, which in my eyes is directly relates to the size of the spinnaker. But I haven't had a chance to see the two boats side by side.
 

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Your right, I transposed a number. The times I've raced on the 29 mh it seemed quicker all around then the frac, it's been a few years though... Not sure if it was the boat, the sails or the sailors.
No worries... And it yet again proves the difficulties in these conversations. I could take either a MH or a FH load it with the right sailors and smoke the other one doesn't matter which. Because the boats really were designed to the same performance ( J boats intended the two to race OD with each other) it is fun to look at how they played with the numbers to try and get that result. What's equally interesting is what people would do today... Larger higher aspect mains, asymmetric chutes, Code zero's (which really change the whole game), MH spins on fractional jibs.

Oh so many fun games to play.

What I think needs to be carefully reserved is that just because a FH may be better on a windward leeward race course may have little do to with how it works out on a cruising boat. The harder you are willing to work the boat and sail inventory the better a fractional looks. If you are just going to fly white sails then a MH starts to average out better.
 

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Shockwave,

I am not sure what IMOCA rules are on sails and rigging so I am not sure how that drives their sail selection. They also have permanent reaching struts which make large jib better in more conditions since they can be trimmed properly.


Outbound,

I don't think the rig matters much in terms of trimming, it is just different. The size of the jib matters more. So a FH 155% vs a MH 155% is pretty much the same trimming the sail (trimming the rig is very different). But a lot of FH don't carry large overlapping jibs, they use a massive main and just a small blade for a jib. Just enough really to balance the boat.

My trimaran for instance can barely sail under main alone because the main is so powerful it will drive the boat head to wind no matter what you do. A little bit of jib even if it's trimmed badly solves this.
 

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Thought one of the big pluses of frac rigged boats was you got a lot more out of adjusting the backstay beyond increasing fore stay tension. ?Is this still true?
It depends on the boat and the rig. Generally the backstay is just to play the top of the main, and it leaves the jib tension alone. But then you have a lot of fractional rigs without backstays with just runners, or some that don't worry about anything but prebend.

This is where rig design gets really fun. Since you can design a boat without needing to absorb all the backstay tension you can design a different hull. The J-35 for instance sailed best upwind with about 3500psi of backstay pressure. But the boat wasn't designed to take this much pressure and the hull would start to flex. On the fastest boats reinforcing the hull became necessary to offset the mast pressure.
 

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So I was doing some reading last night and this conversation popped into my head. Go take a look at the current generation of C class catamarans. These race in a pure development class, and the only restriction on sails is maximum sail area. Right now they are all using high aspect wing cat rigs. There isn't even a jib to be seen. And some of these teams have massive amounts of money to throw at the problem since the. C cats are test beds for large boat programs (Americas Cup, IMOCA, maxi cats, ect).
 

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Shockwave,

On the J-35 I raced on we carried so much backstay pressure (on top of 16" of prebend) that the hull would actually flex. Based on our measurements the boat shortened by about 2" and widened by 4" (I think, I don't remember the exact measurements anymore). To counteract this we rigged in internal wire support system that tensioned the whole boat to prevent it from flexing.

This is an extreme example of how to get enough forestay tension on a MH can cause all sorts of weird spiraling issues.
 
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