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Split Junk Rig on Westerly Windrush

  • 27 Mar 2013 20:26
    Quote
    Reply # 1253143 on 1250251
    Deleted user
    Chris Gallienne wrote:Slieve, thanks for your detailed reply - it would be great to have a chat about this some time, once I've got my head around it well enough to be capable of reasonable coherence.

    I am old enough to have made enough mistakes to realise that the right answer is very much preferable to the answer I would like to be right.

    I have uploaded a simple sketch I have made of the Windrush as an aid to thought on this subject. s It have marked the approximate positions of the CLR and CE of the original Bermudan rig, together with position of bermudan mast. I have superimposed what is basically your sketch of Poppy's rig, scaled to what I envisage, with the new CE marked on it.

    If a fat hollow wooden mast is not part of the right answer, presumably you would recommend a smaller aluminium section? Maybe in a tabernacle?

    Edit: the sail as I have drawn it has 33% balance.
    Hi Chris,
    I like your sketch, though it seems to my untutored eye that you have moved the mast a long way forward.  On my Splinter we only moved the mast about 12" (1') forward.
    As the most westerly split junk in the UK, therefore the nearest to Cornwall, perhaps you would like to come for a sail in Amiina some day soon.  She is in the water at Parkstone Sailing Club, and ready to go.  If you ever get anywhere near, then give me a call.(01202 761774)
    I would be delighted to show you the merits of the rig.
  • 26 Mar 2013 22:18
    Quote
    Reply # 1252416 on 1252054
    Anonymous member
    Chris Gallienne wrote:Arne
    OK, the Windrush is a fairly narrow hull 7.65m loa and 2.26m beam, with a 33% ballast ratio on 2190kg displacement. Taking beam/5 would give a righting moment of 990kpm, a little lower than my guess.

    Chris

    Edit: The grade I have been discussing is 6082T6

    Just for the sake of comparison: Fantail is 8m loa and 2.6m beam, with a 50% ballast ratio on a nominal 2500 kg displacement.  In reality, I think the boat weighs more like 2800 kg and probably has done so for a very long time.  I'm afraid the sums to work out her righting moment are too difficult for me, but I'll happily accept Arne's best guess.
  • 26 Mar 2013 20:38
    Quote
    Reply # 1252353 on 1248976
    Deleted user
    David,

    yes you are right - in fact it's rather less than half as around 1.5m of it will be below the partners, leaving only 3.5m above deck where the weight is most significant. if I can't find 3/16 tube I think I'll go with the 1/4" wall.

    Chris
  • 26 Mar 2013 20:10
    Quote
    Reply # 1252329 on 1252262
    Anonymous member
    Chris Gallienne wrote:Arne
    Thanks for doing all this research for me. I'm very tempted by the idea of using 3.25 section - less than half the cost in the UK and half the weight - but it's a 'safety factor' of lless than 1.5 ... and if I have to go to the additional trouble/expense of beefing it up iinside, the 6.3mm wall tube might be a better option, don't you think. Anyhoo, I have another supplier to ring about the possiblilty of 4.76 section - maybe i'll find some.
    Chris,
    Since the tube only constitutes the bottom half of the mast, and the weight of it is not carried through to the top half, using a 6.3 wall wouldn't be such a bad thing.

    I used a 5" x 10g mast on my Sadler 25, and bent it. 6" x 10g should be 
    OK, but perhaps it would be more confidence-inspiring to go for the 1/4".
  • 26 Mar 2013 19:03
    Quote
    Reply # 1252262 on 1248976
    Deleted user
    Arne
    Thanks for doing all this research for me. I'm very tempted by the idea of using 3.25 section - less than half the cost in the UK and half the weight - but it's a 'safety factor' of lless than 1.5 ... and if I have to go to the additional trouble/expense of beefing it up iinside, the 6.3mm wall tube might be a better option, don't you think. Anyhoo, I have another supplier to ring about the possiblilty of 4.76 section - maybe i'll find some.
  • 26 Mar 2013 18:30
    Quote
    Reply # 1252234 on 1248976
    Anonymous member (Administrator)

    Chris, that 6"/ 152.4mm section with 3.25mm walls should stand up to a bending moment,

    M=1418kpm if I got it right (6082 T6 grade).

    It could be a bit on the low side, but that depends on where you are to sail the boat. Would it be possible to find a 2-3m long well-fitting ‘inner tube’ to beef up the lower section? Even if that tube is a bit roomy (say 5.5"), it could be used just by building up its diameter with stout tape on 5 places until it just fits. That may help you out.

    Arne

    Last modified: 26 Mar 2013 18:30 | Anonymous member (Administrator)
  • 26 Mar 2013 14:26
    Quote
    Reply # 1252054 on 1248976
    Deleted user
    Arne
    OK, the Windrush is a fairly narrow hull 7.65m loa and 2.26m beam, with a 33% ballast ratio on 2190kg displacement. Taking beam/5 would give a righting moment of 990kpm, a little lower than my guess.

    Chris

    Edit: The grade I have been discussing is 6082T6
    Last modified: 26 Mar 2013 14:39 | Deleted user
  • 26 Mar 2013 13:56
    Quote
    Reply # 1252035 on 1251993
    Anonymous member (Administrator)
    Chris Gallienne wrote:Thanks, Arne, fascinating stuff. Very hard to find such information on a 40+ year old boat, but I guesstimate a righting moment for the Windrush of around 1000-1200kpm.

    Should really be looking at an aluminium section similar to that of Fantail, but most of the suppliers in the UK hold 6" (152.4 mm) alu tube with wall thickness of 3.25mm (too light) or 6.35mm (too heavy). 127 x 4.76 is available but this section will be too small. I'm sure 152.4 x 4.76mm is available, which would be about right so will keep looking.


    What kind of alloy and temper grade can you have? This plays some role as the yield strength varies a lot with choice of alloy

    As for finding the boat's max righting lever, it seems to vary between 1/6 of the beam for unballasted boats (43cm on Frk Sørensen with a beam of 2.45m) and up to 1/4.5 of the beam on narrow , very heavily ballasted boats. Your guessing on your Windrush may be righ, or a bit on the high side, I guess.

    Arne

    PS: Luckily I live in a country which produces aluminium so there are many sections and alloys to choose from. It's a way of exporting cheap hydro-lectric power in Norway. It takes 12 - 14kWh to produce a kg of aluminium but only 5% of that to re-melt it...

    Last modified: 26 Mar 2013 17:15 | Anonymous member (Administrator)
  • 26 Mar 2013 12:30
    Quote
    Reply # 1251993 on 1248976
    Deleted user
    Thanks, Arne, fascinating stuff. Very hard to find such information on a 40+ year old boat, but I guesstimate a righting moment for the Windrush of around 1000-1200kpm.

    Should really be looking at an aluminium section similar to that of Fantail, but most of the suppliers in the UK hold 6" (152.4 mm) alu tube with wall thickness of 3.25mm (too light) or 6.35mm (too heavy). 127 x 4.76 is available but this section will be too small. I'm sure 152.4 x 4.76mm is available, which would be about right so will keep looking.
    Last modified: 26 Mar 2013 12:34 | Deleted user
  • 26 Mar 2013 10:08
    Quote
    Reply # 1251944 on 1248976
    Anonymous member (Administrator)

    Stavanger, Tuesday

    ...Numbers, numbers, numbers...

    Like Annie I think that the hybrid wood-aluminium mast is a great idea in this size range. The fine thing with it is that one can use an aluminium tube right from the shelf and then some spruce planks right from the sawmill. No expensive welding etc. of the aluminium (which would ruin its tempering) and no need for waiting for a tree trunk to dry (like on Malena’s and Johanna’s masts).

    The new  7.5m JR mast for my 6.5m Jollenkreuzer, "Frøken Sørensen" will also be of the hybrid type. I will probably go for a tube, 5 or 6m long, 110 x 3.0mm in section. The alloy is 6082 T6 with a yield strength of around 250MPa (= 250 N/sq mm = 2550kp/sq cm) . This should give a bending moment of 669kpm. The boat’s max righting moment should be around 387kpm with 2 people on board. Priority is on a light mast on this boat: When sailing in protected waters and with an engine, it is no disaster if the mast breaks.

    Now I found data about the 6261 T6 alloy on the web. It appears to be similar to the 6082 T6, just a little stronger at around 270MPa. If my calculations are right, Fantail’s mast should take a bending moment of 2250kpm. With the Raven 26’s max righting moment guestimated to be between 1300 and 1700kpm, that mast should cope  well (remember, the ultimate strength of the 6261 T6 alloy is typically around 330MPa).

    It took some head-scratching to handle these numbers - mechanics is not the main branch of my engineer’s training. I definitely don’t guarantee that my calculations are right!

    Cheers, Arne

    (PS: 1kp = 1 kilopond = 1kg force = 9.80665Newton. 1Pa = 1 Pascal =1N/sqm, which means that 1MPa = 1 MegaPascal = 1N/sq mm)
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
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