Transitioning to junk rig on Cheoy Lee 38

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  • 11 Mar 2021 10:39
    Reply # 10184765 on 9393819
    Anonymous member (Administrator)

    Scott,

    to find a useful diameter of that mast, I used the modified Hasler-McLeod (PJR) method described in TCPJR, Chapter 6, and came to D = 31.9cm. The diameter at the step is 50% and at the top it is 43% of D. Note that I have kept the diameter constant between C and E, that is from 20cm below Partner to 123cm (= 0.1 LAP) above it. The mast taper from E to I and from A to C is straight.

    When recently toying with the idea of designing a 6m pram, I found that the (2D) QCAD program let me find the displacement by drawing up an area curve of the sections and then simply find the area of that area curve.
    Now I used the same method to find the volume of your mast.
    I double-checked the result by using maths, and the result was 686dm3  -  close enough.

    If we guess the density of Douglas Fir to be 0.55kg/dm3, and that you by building a hollow mast, remove 30% of the ‘solid volume’, that mast should come out with a mass of
    666 x 0.55 x 0.70 = 256kg
    If the boat weighs in at 8000kg, the mast should add about 3.2% to it. If the boat were mine, I would go for it.
    An extra advantage with the graphic way of finding the mast volume is that the one can print and then cut out the area curve. By balancing the curve on a ruler, the vertical position of the CG can be found. You will be surprised at how low it sits  -  about 3.75m above deck.

    As for mast rake, I see no reason for raking the mast this or that way, in this case.

    Good luck,
    Arne


    Last modified: 12 Mar 2021 16:14 | Anonymous member (Administrator)
  • 11 Mar 2021 05:56
    Reply # 10184378 on 9393819

    One more question before building the mast, I read an article that said the mast should be angled forward, do you have experience with that? If so do you recommend it? How much angle?


    thanks!

  • 10 Mar 2021 04:01
    Reply # 10181076 on 9393819

    I’m back!  Almost ready to install the mast, just wondering if yo have any suggestions on the diameter mast to use.  I’m building it out of Douglas Fir, unless you have a better idea there as well. 

    thanks for all your suggestions!

  • 31 Dec 2020 00:24
    Reply # 9522853 on 9393819

    Great information!  Thank you both!



  • 30 Dec 2020 11:14
    Reply # 9509240 on 9393819
    Anonymous member (Administrator)

    Scott,
    I would hesitate to go any larger with that single sail. The resulting SA/disp. of 15.4 may not be astronomic, but I still think it should move the boat quite well.  A single sail makes better use of the sail area, in particular when running before, than the same sail area divided into 2-3 sails. If you are mainly planning to glide around on flat water in light winds, you could make the sail taller (with the same chord), but then there is the mast issue. It depends on how much money you want to spend, or what skills you have as a mast maker. Personally, I would rather suggest the shown rig, and then have a lightweight jib as a possible Plan B. It could also be an idea to increase the camber/chord ratio of the junk sail from my 'standard 8%' to 10%. This will add muscles to the sail.

    Arne

    (I have once added a lightweight jib from spinnaker cloth to a Nordic Folkboat, and it did wonders when in ghosting conditions, despite its modest sail area.)


    Last modified: 31 Dec 2020 09:13 | Anonymous member (Administrator)
  • 30 Dec 2020 08:47
    Reply # 9506765 on 9393819

    As one of the few users of junk-based wing sails, I would have to advise against its suitability here. I've now managed to build a reasonably sound rig for my 23ft, 1 ton boat, after failing to do so with a good balance of performance, robustness, longevity, convenience and economy on my previous 35ft boat that sailed at around 8 tons displacement. I have no doubt that it's possible, with enough engineering skill, resources and hard cash deployed, but we need to work up to the point where it seems reasonable to try to build a rig for this size of boat, with an intermediate step at around 30ft and 4 tons displacement. And there needs to be a strong reason for doing so, such as entry in a single handed ocean race. 

    I think that Arne has sketched a good and suitable rig for the Cheoy Lee 38, and it will be manageable for a someone with enough strength and fitness. I wouldn't want a larger single sail than 60 sqm on any boat, unless you're young, strong, big and fit, whatever mechanical aids are used. Especially if this is the first junk rig you've been shipmates with.

  • 30 Dec 2020 04:14
    Reply # 9502104 on 9393819

    Thank You Arne!  

    Your post is very encouraging!  What are your thoughts on a wing sail JR?  Could I go with 75 instead of 60 on the size? Or do you believe that would unbalance the boat?


    thanks!

    Scott

  • 04 Dec 2020 18:44
    Reply # 9405057 on 9393819
    Anonymous member (Administrator)

    The question when to switch from one to two masts, is not that easy. The answer depends both on the boat design, the crew, and the intended style of sailing.

    The Cheoy Lee 38 should take a quite tall sloop rig better than most. She is big, with almost  eight ton displacement and with 40% of that as ballast. Plenty of beam and draft should ensure power to carry a healthy rig. Even more, there is a good mast position waiting for a sloop JR mast  -   through the foredeck hatch and between the berths. This position calls for fairly short battens and thus a moderately tall rig. Such a rig is the easiest to deal with, area for area, as long as the boat has sufficient stability. As can be seen, the foredeck will have plenty of free room for anchor handling.


    Now I had a go to see how it looks. The short chord of the sail is only 63% of the waterline, which indicates that downwind steering should be easy enough. In addition, the moderate length of the battens reduce the load on these quite a bit. At 5.95m length, they will make good use of 6m or 20’ tubes.

    As for the workload on the crew, the hardest bit is no doubt hoisting the sail. However, my friend Svein Magnus in his Samson has hoisted his 70sqm mainsail by hand for the last 20 years, so it is doable. Even so, I recommend fitting an electric capstan of some sort. They are much cheaper than the extra cost of fitting a second mast and sail.  I recommend dividing the sheet in two, with an upper and lower section.

    If one is about to round 70, a big sloop JR may not be a good idea, but then it may not be a good idea to take on an 8 ton boat either...

    The ultimate stability of that vessel should be around 6100kpm (Disp. x beam x 0.21). The mast should thus be built to take a bending moment of between 12200 and 18300kpm.
    Something like that...

    Arne


    Last modified: 05 Dec 2020 14:10 | Anonymous member (Administrator)
  • 02 Dec 2020 20:32
    Reply # 9400341 on 9393819

    Thanks!  I’ve got the boat profile, it’s on “sailboat data”. And I believe I have the necessary  skills to do the majority of the work myself. Now just need a good plan to feel confident making the changes 

  • 30 Nov 2020 05:03
    Reply # 9393955 on 9393819
    Anonymous wrote:

    I would like to go with a junk rig on my Cheoy Lee 38, it’s currently deck stepped.  What’s my first step?

    There are greater experts than me in the JRA but it seems to me that your first step is to assess the practicality of a junk conversion for your boat. A 38 foot boat will really need a two sail rig because a single sail to match your current sail area will make for a very large sail which equals all sorts of design and engineering challenges, and it will be a difficult sail to handle. For practical purposes a sail of up to about 56 sq m is about as large as you want to go, although there are junk rig yachts with larger sails.

    So your first stage is to obtain a lines drawing of your boat which shows the underwater profile including the position of CLR which will be required to determine the CE of the new sail plan. And then armed either with a copy of Practical Junk Rig, or the expert knowledge of some of our members, you need to come up with a draft sail plan. From the draft sail plan you need to assess the practicalities of installing the rig on your yacht. This includes mast positions, strengthening of the cabin top for the mast entry, mast step design, because the mast will need to be keel stepped, and impact of the rig on the accommodation of your yacht.

    From this stage you need to look at how you are going to make it actually happen, and one of the big issues here is obtaining suitable mast/masts. Another big question is 'who is going to do all the work?' If you have the necessary skills DIY is the best way to achieve a conversion, otherwise you need to find suitable skilled help and assess the cost of this.

    Good luck.

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