Junk Rig in the Malacca Strait

  • 11 Jul 2024 05:31
    Reply # 13380647 on 13379850

    Hi Arne,

    Appreciate your help. Saving the endless hour trying to figure out the whole boat design from scratch and ended up designing a floating suicide vessel :)

    Now I can pull out the Johanna 60 Master plan @AR=1.98 and work out the aluminum pipe details and fabric procurement.

  • 10 Jul 2024 20:36
    Reply # 13380480 on 13379850

    Yes, this looks well proportionate visually. Will study in-depth in the morning.

    Thousand thanks.

  • 10 Jul 2024 19:51
    Reply # 13380461 on 13379850
    Anonymous member (Administrator)

    Something like this?


    (Members album Arne's sketches, section 8-3)

  • 10 Jul 2024 17:26
    Reply # 13380367 on 13379850

    Thanks for your speedy reply and the complement.

    Part of my plan is to set up OpenCPN with an opensource wind instrument and speed log to capture the performance of both rigs. I have too many hobbies LOL.
    I wish to have a sail area equal to my current set up,  Main 11.5m2, furler Genoa 10.8m2. total 22.3m2 which is about SA/disp.=25.5 to facilitate direct comparison between the pointy rig and JR. I hope this is not too much to ask. 
    As for the sail cloth, I am eyeing on some acrylic fabric for kite-making, weights between 100 and 150g/sqm. I would have to order some samples to have a look.

  • 10 Jul 2024 15:49
    Reply # 13380306 on 13379850
    Anonymous member (Administrator)

    you appear to have a very good grasp of the numbers.
    Considering  the waters and wind conditions you are mentioning, I have now enlarged that sail a little.
    This gives a SA/disp.=23.8, which is not extreme in any way for such a little boat.
    Remember not to use too heavy sailcloth. I used 220g/sqm on a 20sqm sail, and that was on the heavy side. I suggest something between 100 and 150g/sqm  -  but make sure it is sun-proof!


    (members Album Arne's sketches Section 8-2)

    Last modified: 10 Jul 2024 17:16 | Anonymous member (Administrator)
  • 10 Jul 2024 13:18
    Reply # 13380224 on 13379850

    Hi Arne,

    Great to hear from you. Wish you health.

    Yes, Port Klang is a coastal port city next to Kuala Lumpur, the main harbor of Malaysia, 2nd to Singapore in South East Asia, 12th world ranking. Enough sheltered water for a whole day sailing and some.

    Attached is the sail plan of yours showing the best mast position which is through the hatch. 

    Following is my interpretation of your mast scanting calculation for straight aluminum mast.

    Displacement of the boat is 840kg, hence 3% is 840*.03=25.2kg.

    Righting moment = Mr = Displacement, kg x 9.81  × LrB, m

    Where Lr  =20% of beam, Boats with ‘normal’ ballast keel of 30 to 40% of the displacement: Choose any between 19 and 21% of the beam.

    Mr = 840kg x 9.81 x .2 x 2.42m  = 3988Nm #

    For coastal cruising and no wild racing,  breaking moment at the strength of the mast at the partners
    Mb≥ 2.0 × M7976Nm #

    For serious offshore cruising or for racing, I recommend to be beefed up to ≥ 2.5 & 3.0 the breaking moment of the mast at the partners should be between 9970-11964Nm #

    Determine the yield moment of 6061-T6, Dia.127mm x 3.175mm

    #6061-T6, Dia.127mm x 3.175mm extruded pipe is the size I might possibly source from my friendly local hardware store.
    where, Yield stress of 6061 = 240N/mm2

    Mb yield= 240N/mmπ/32 (1274−120.654)/127
    =8952649Nmm=8953Nm#≥ 2.0 × Mr7976Nm (2.24xMr to be exact)

    6061 Aluminum Density = 2.7g/cm³

    Dia.127mm x 3.175mm thk weights 3.33kg/m 

    Max Mast Length
    @2.5% Displacement.=21kg/(3.33kg/m) = 6.31m
    @3% Displacement. = 25.2kg/(3.33kg/m) = 7.57m
    @3.5% Displacement.=29.4kg/(3.33kg/m) = 8.83m

    Hence the mast height in your design 7.82m is right about in the safe ballpark with 6061-T6 OD127mmx3.175thk A.K.A. OD5"x1/8" thk
    Note# 6061-T6 OD100mm x 4 thk @3.28kg/m, is negligibly close to 3.33kg/m

    Step-taper mast with Dia. 88.9mm 3.175mm thk straight top section with breaking moment of 4246Nm will allow upto 8.26m total mast height. within 3% of displacement.

    This is where I start asking what is the safe sail area.

    1 file
    Last modified: 10 Jul 2024 14:43 | Anonymous member
  • 10 Jul 2024 10:07
    Reply # 13380182 on 13379850
    Anonymous member (Administrator)

    Ken, welcome to the JRA!

    I had to look up Port Klang on Google Maps  -  is that the harbour of Kuala Lumpur?
    In that case, it appears to have some sheltered waters around it, suitable for your boat  -  if you are a keen reader of tide tables.

    The limiting factor when deciding for sail area is how tall or heavy mast it is wise to fit the boat with. I try to keep the mast weight below 3% of the boats weight.

    Where do you plan to position the mast? It could be through the fore hatch or through the fore deck.

    Anyway, good luck.


  • 09 Jul 2024 16:55
    Message # 13379850

    I am completely new to sailing, This year,  I finally ticked off 2 lines on my bucket list, i.e. learn to sail on 1st of March & bought a sailboat  which I can afford on my birthday.

    Presently, I am working on making good Coddiwomple, a 1985 Jeanneau Bahia 22 lifting keel trailer sailor with 11.5m2 main, 10.8m2 Genoa (totaling 22.3m2} while gaining as much sea mile as quickly as I can manage. Also, to investigate my home water and to ponder over what type of sailing I would end up doing long term.

    My higher bucket list is build/convert a Junk Rig which I am preparing for next haul out for a proper hull repair coming March when the dry season sets in.

    I Joint JRA as suggested by Arne Kverneland when I posted on the FB group International Junk Rig : Sailing Old and New before I made the boat purchase. 

    Currently, I do know some "must have",

    1. Cockpit Bimini, We who live 180nm (3 degree) off the equator prefer shade over sun burn all year round.
    2. Carry the biggest safe sail area possible for predominant light wind condition 3-5kts wind & huge tidal ranges at Port Klang. Tide vary between 2 metres during neaps and 5.5 metres during springs. Lowest Astronomical Tide recorded is 0.0 metre while the Highest Astronomical Tide recorded is 6.1 metre with typical 3-5kts current. 
    3. Definitely not going to live aboard. I am 186cm and there are no healthy long term living space in the 22 footer cabin. Should be fine for occasion month long camping/cruising/island hopping.
    4. Racing is the least of my concern.
    5. Solo Sailing.

    As for Junk sail platform, I am overwhelm by the treasure trove of information shared by selfless contributors. As a project person all my career, I tend to prefer Arne's HM variation for the comprehensive build literature. Also, since this will be the first Junk Rig in my yacht club history, I must make it work as I wish to use my conversion as a re-introduction of Junk Rig in this waters.

    My 1st task is to determine the biggest safe sail area. I want to show off the junk rig simple reefing ability.
    Spinnaker 25m,Main 11.5m2, Genoa 10.8m2.
    I wonder if the total of Spinnaker + Main. 36.5m2 , is a good starting reference? 

    1 file
    Last modified: 09 Jul 2024 17:06 | Anonymous member
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