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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.

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 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.

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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I think the future will bring us back to full on mh rigs, giving up I is giving up power, with a row of window shades. Pick the power you need, be it a frac or mh, upwind or downwind. The open 60's have moved the butt back to allow enough j for mh upwind headsails tacked to the deck, not the sprit.







New cruising boats are following with multiple headsail options, Tartan being one. But I'm curious how many cruisers really want to work that hard?

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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Flex? The backstay? Or the over anxious guy on the checks trying to drive the spar through the bottom of the boat? :)

I like in line rigs, even with their short comings, versus swept spreader rigs. In line rigs have a larger range of movement allowing better draft depth control but for a cruiser a swept spreader rig is probably better.

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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