Mast head or Fractional rig

Discussion in 'Sailboats' started by Bruce46, Nov 22, 2010.

  1. Bruce46
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    Bruce46 Junior Member

    Starting from a clean sheet of paper, which is better Mast head or fractional rig. Correct me if I'm wrong, however, I thought the fractional rig was a way to beat rating rules. Thanks.
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    Depends.. Mast head is considered stronger so it's generally better for cruising. Fractional is easy to adjust for better performance so choice for racing..
     
  3. jehardiman
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    jehardiman Senior Member

    Concur.

    In reality Bruce, you are not (and should not be) starting from a blanks sheet of paper. What you should have in hand is a list of design requirements and objectives. These will steer the design. A short handed cruising boat will by necessity have simplier, less costly, and more robust rigging than a racer full on racer with a bendy mast controled by runners/jumpers/baby stays which will have constant maintinance and sacrifice robustness and costs for less weight aloft in order to wring that last tenth of a knot out of the rig.
     
  4. gggGuest
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    gggGuest ...

    Other way round. When there's nothing in the rating rules either way fractional rigs are universal. IIRC the masthead rig came about because on rating rules 50 years ago jib overlap was unmeasured, so the bigger the jib the more free area.

    But maximum performance per square foot may not be your best design criterion. On a small boat that can be rigged with just forestay, shrouds, spreaders, no support above the hounds and no backstay or runners and no support above the hounds then IMNSHO fractional rig is the only rational choice. Once you are into the size where you need backstay and/or runners to keep the stick up then there are a huge number of pros and cons, depending on application.
     
  5. gonzo
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    gonzo Senior Member

    Fractional rigs help prebend the mast. Herreshoff observed that the top of the mast tended to bend backwards, and the forestay attached a bit lower gave it an even curve. That is from the days of evenly tapered masts.
     
  6. michael pierzga
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    michael pierzga Senior Member

    Each rig has its value. Masthead rigs hang big sails. I like them. well suited to cutter rigged. One modern detail to incorperate in a masthead rig is dont take the forestay fitting all the way to the mast head. Design your rig to be 15\16ths, with the spinnaker halyard coming off the masthead and the headstay and jib halyards lower. This is important because on a modern rig, with rollerfurling, the masthead become very cramped. When you sail with a spinnaker...particularly a spinnaker in a sock, the sock will ride on top of the roller headstay swivel and may chafe of become fouled. Not good. Also when jibing a spin naturally falls back towards the headstay. Very much a possibilty that on a true mast head rig the spin gear may get caught on the masthead roller furler.
    The mast head in the picture represents a modern rig. About 15\16th with good room above the headstay for spinaker gear. You will see the same detailing on many good yachts.
     

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  7. RHough
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    RHough Retro Dude

    Not to argue, but ...

    Look at old working boats like a gaff cutters. The working sail plan of jib and main has about the same area distribution as a fractional rig with a leg o' mutton main.

    Most people that have sailed both masthead and fractional rigs like the fractional rigs better even outside a racing environment.

    I can't think of many/any reasons to prefer a masthead rig over a fractional. Fractional rigs provide more power for the given sail area and make sail handling easier.

    R
     
  8. Paul B

    Paul B Previous Member

    Modern fractional rigs, for racing and cruising, do not use runners, jumpers, babystays, etc. They are just as "strong" as a masthead rig.

    The modern fractional rig is the easiest of all sloop/cutter rigs to use.

    It is more aero-efficient than a masthead rig.

    The headsails are smaller and easier to handle than a masthead rig. This is especially true when using the modern ideal of no overlap on the headsails.

    The rig can be easily depowered in puffy conditions.

    It can be sailed well under main alone.

    A masthead downwind sail can be flown to increase speed when preferred.
     
  9. Bruce46
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    Bruce46 Junior Member

    Thanks for everyone's input. Years ago I was intune with sailboats now I'm trying to figure out what I forgot, so I don't go off in the wrong direction.
     
  10. jehardiman
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    jehardiman Senior Member

    Absolutely not. There is no appreciable areodynamic difference between between a fractional rig and a masthead rig of the same area and aspect ratio, and the whole jib issue is absurd as relative jib size is dependent on stability issues, not rig design. For two rigs of identical area and mainsails, the jib on the fractional rig is less efficient because it has a lower aspect ratio and more overlap than the masthead rig while the fractional rig has a slightly lower CE and weight aloft. Making the fractional main larger to increase jib aspect raises the centers again so it becomes a general wash between the two rigs styles.

    What efficiency a fractional rig brought to the table is the ability to easily put more bend into the mast which allowed the strech sails to taken out giving more control over power. With modern low stretch materials and engineered masts this capability might be rendered moot or more important depending on how the sail is cut to interact with mast design. This capability is best seen in the unstayed "zero fractional" cat rigs where mast bend is all controled by sail tension.

    Absent of rule bias, there is no reason to select a fractional rig over a masthead rig or even an unstayed cat rig for that matter. It just depends on how many strings you want to pull.
     
  11. Steve W
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    Steve W Senior Member

    A nicely proportioned mashead rig like those used on the J35 and J29s work very well.The J29 is one of the few boats that was offered with the choice of a low aspect masthead rig or a taller fractional rig with the same sail area on the same hull,the masthead version is the faster and it is reflected in the rating.I mention this only because it is the ONLY boat i know of where masthead and fractional rigs can truly be compared on the same(not just similar) platform with many examples and many different crews with conclusive results. I like the proportions of the masthead J boat rigs as they are much lower aspect than the IOR rule rigs. jmho. That said a couple of years ago i put a J24 fractional rig on a C&C24 which was originally masthead and hugely improved it imo,(turned it into the highest rated C&C24 in the country)
    Steve.
     
  12. RHough
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    RHough Retro Dude

    Aspect ratio is span and non-overlapping area. The distribution of area between main and jib has no effect on AR. The jib on a fractional rig cannot be less efficient due to low AR unless you are sailing on the jib alone. The wind "sees" the two sails as a single foil at upwind sailing angles. Off the wind you set flying sails from the masthead.

    I don't think you can name one rule set that measures all sail area equally or has a sail area limit where the rig has evolved to a masthead configuration with overlapping headsails. Masthead rigs with overlap come from rules that did not rate the overlap and from the fact that the bigger J measurement compared to E allows larger downwind sails under those rules. It was rule bias that created the big jib, little main masthead rig. You have that backwards.

    Unstayed cat rig masts can also fall off to leeward in gusts and de-power the rig, just as the top of a fractional rig can. This reduces sail trim requirements. There are no more strings to pull on a modern fractional rig than on a masthead rig, the strings you do have to pull (the sheets) require less attention on a good fractional rig than on a masthead rig.

    A fractional rig does not depend on headsail area for performance. As the wind builds increased leech tension also flattens the sail, you don't have to reef as often since the basic sail controls can change the sail shape to a greater extent. When it is time to reef the CE comes down and the weight aloft is reduced at the same time. The boat still balances well. The power from the full jib<>main interaction is maintained. When the main is reefed so the head of the main is at the hounds it becomes a short masthead rig and the efficiency stays high since the leech of the jib is still close to the mast helping the rig work as a single element.

    Once you decide to use a fractional rig, you don't have to worry about sheeting angles and spreader tips poking holes in Genoas. The chainplates can move outboard and compression loads can be reduced. Lower loads can reduce weight in both the rig and the hull structures. The weight saved in the hull and deck can be used as ballast so the came hull can use a taller fractional rig, increasing span and efficiency again.

    R
     
  13. Chuck Losness
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    Chuck Losness Senior Member

    SteveW
    The Olson 30/Olson 29 were the same hull. The 30 had a masthead rig and the 29 was fractional. The only difference between the hulls was that George Olson cut about 1 inch off the very point of the bow on the 30 to make it a 29. The deck layouts were different also. They rate the same in So Cal PHRF. I always thought the 30 was faster. But then I might be prejudiced. 3 of my best friends bought Olson 30, Hull 0, at the Long Beach Boat Show back in 1978 and named it Mas Rapido. I raced more than a few thousand miles on their boat including the 1979 Newport Beach to Cabo race. We hit speeds of just over 20 knots in the later part of the race. Didn't do too well on corrected time because the first third of the race was hard on the wind into a 25 to 35 knot Southwesterly. But we sure flew once the wind turned around.
    Chuck
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    This was the period when the fractional rig was introduced by Bruce Farr into the IOR and shook the sailing world; the same message remains true today ... although really you should be thinking of rotating wing masts/fractional headsail/with masthead reaching/downwind sails. that is, If you want to get up to speed in this century.
    In 1972 the ¾ rig was viewed with alarm; considered bendy, fragile and dangerous. The preferred rig was heavy masthead with small main and large numbers of headsails. Each one had to laboriously changed in varying wind strength. Farr changed that rigid attitude with 45 South and proved that big dinghy fractional rigs on an 18 footer-type bendy mast was not dangerous and easy to handle. Everything was controlled from the cockpit and in heavy weather the rigs were self regulating. Instead of dragging different sized headsails forward on a masthead yacht, the New Zealanders either allowed the mast to bend, flattening the big main or put in a reef. Also reduced loads aboard a ¾ rigged yacht allowed the deck gear to be lighter and cheaper.
    Anyone dabbling in light displacement and bendy fractional rigs was dismissed as eccentric. Sparkman &
    Stephens had experimented with fractional rigs in 1973 on Prospect of Whitby but had kept ratios between main
    and headsail similar to masthead designs – and achieved nothing. Farr was more extreme with large main/small headsail setup and revealed the IOR could be different – even though he designed and developed his concept first and then made it fit the Rule.
    Light boats lift over big seas whereas heavy boats plough. Sparkman & Stephens boats of the late 1960’s were heavy and had the centre of effort well forward to offset rounding up to weather as they plunged deep into waves and buried bows. Offwind heavy boats required more than one person on the helm and to gain speed large areas of sail had to be carried - but because of their designs and weight, they could never go past theoretical hull speed. The Antipodean dinghy/yacht was a complete contrast; so simple and basic New Zealanders found it difficult to comprehend why the rest of the yachting world considered them extremists (it took a decade before the roles were universally and completely reversed). At Deauville, crews on conservative Quarter Tonners were often frustrated by light boat performance and US designer Bruce Kirby sailing on his own design Fred Jnr. wrote of this frustration in a US yachting magazine: “A couple of boats had laid off across our stern while we were being held high and now were on our lee beam with a much better angle to the line. One of them was Genie, sister to 45 South. In the brisk beam reaching she was moving fast with her light, dinghy-like hull picking up every wave while the medium displacement boats like ours, only managed to grab every second or third one. The annoying part was that at the finish, the two New Zealand boats were first and second even though they had been buried at the last mark before the wind filled in.”
     
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  15. RHough
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    RHough Retro Dude


    J29 Dimensions
    TYPE/CLASS I J P E
    J-29 OB FR 35.00 11.50 38.80 13.00
    J-29 OB MH 40.00 12.00 35.00 12.00

    J29 Ratings (Hi - Lo - Avg)
    J-29 OB FR 108 125 114
    J-29 OB MH 108 114 111

    The average PHRF rating for J29's rates the FR 3 sec/mile slower.

    Look at sail areas:
    J29 FR
    Main 38.8 x 13 x .59 (std roach) = 297.6 ft sq
    Genoa 35 x 11.5 x .775 (155%) = 311.9 ft sq
    Spin 35 * .87 * 11.5 * 1.8 = 630.3 ft sq
    Total Upwind area = 609.5 ft sq
    Total Downwind area = 927.9 ft sq
    100% SA = 498.9 ft sq
    AR = 3.84:1 (43.8^2 / 498.9)

    J29 MH
    Main 35 * 12 * .59 = 247.8 ft sq
    Genoa 40 * 12 * .755 = 362.4 ft sq
    Spin 40 * .87 * 12 * 1.8 = 751.7 ft sq
    Total Upwind area = 610.2 ft sq
    Total Downwind area = 999.5 ft sq
    100% SA = 487.9 ft sq
    AR = 3.28:1 (40^2 / 487.8)

    With the AR of the FR rig of 3.84:1 compared to 3.28:1 for the MH rig, you would predict the FR rig to be faster, but it rates 3 sec/mile slower.

    Why?

    The MH boat has 19% bigger Spinnaker.

    This is a -6 sec/mile rating change in most areas. The MH boat should rate 6 sec/mile faster. The conclusion (if one can be drawn) is that the difference in rating is not because the MH rig is better, it just has more downwind sail area.

    The FR rig should be faster with a full hoist Spinnaker. The downwind sail area would then favour the FR boat (1086.4 to 999.5).

    For a design that is not restricted by a rating rule, the FR rig has a better sail plan upwind and with a masthead kite, the FR rig's larger main should make for a better balanced boat off the wind.

    R
     
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