Minimum preferred mast height

<< First  < Prev   1   2   Next >  Last >> 
  • 31 Mar 2019 19:30
    Reply # 7253493 on 7246671

    Thanks folks, especially David and Arne for outlining the principles. I will be filing this thread in my "useful information" folder.)

  • 31 Mar 2019 09:34
    Reply # 7252961 on 7246671
    Anonymous member (Administrator)

    Mark and Ueli,

    I use a combination of what you use. I aim for maximum 30° halyard angle, that is, with the sail moved to maximum aft position. These days I have settled on having the slingpoint 55% aft of the throat on the yard. This prevents the sail from getting tail-heavy when raising or lowering, and also offloads the yard hauling parrel, YHP.

    A couple of days ago, a German mailed me about the possibility of fitting a JR to a Spaekhugger. I imported his (Bermuda) sailplan to my QCAD program and suggested a JR for it. On the shown sailplan, the sail rides near the max forward position (15-16% balance). In case this results in a lee helm, I planned for shifting the sail as much as 25cm aft (to 10-11% balance).  From this aft position did I draw the 30° halyard line to establish the mast height, as Mark describes. In the shown forward position, the angle is only 14°.

    The resulting, generous mast height, gives plenty of halyard drift and also lets one attach the YHP further up (about 2/3) the yard. This again ensures lighter forces in both the YHP and throat hauling parrel, THP.

    Cheers,
    Arne

    PS: Note that the JR mast is still 1.81m  -  almost 6'  -  shorter than the Bermuda mast, while the top of the JR sail is only about 20cm lower than the Bm sail.


    Last modified: 31 Mar 2019 09:49 | Anonymous member (Administrator)
  • 31 Mar 2019 07:57
    Reply # 7252938 on 7246671

    hi mark

    Mark Millward wrote:

    …came across an instruction to draw a line at 30 degrees from the middle of the yard.  Where this cuts the mast line is the top of your mast.  I added a fudge factor of one foot.

    as there is a tendency to move the slingpoint something like five percent upwards from the middle of the yard, you might be on the short side – specially if you need to shift your sail aft for balance reasons…

    ueli


    Last modified: 31 Mar 2019 07:58 | Anonymous member
  • 30 Mar 2019 23:49
    Reply # 7252802 on 7246671

    At last a topic i can contribute to. I have the folios from Jock Mcleod and in  designing my current rig came across an instruction to draw a line at 30 degrees from the middle of the yard.  Where this cuts the mast line is the top of your mast.  I added a fudge factor of one foot.

  • 28 Mar 2019 13:25
    Reply # 7249205 on 7246671
    Anonymous member (Administrator)

    The general rule on any JR (I think) is that the more freedom one wants to shift the sail forward or aft, the taller one should cut the mast. The drift should not be less than 50-60cm on a 30-40sqm sail. In addition, the halyard should not have to point more than 30° aft from the mast.

    On my rigs, where the yard angle seems to be stuck at 70°, and the slingpoint has been moved to 55% from the throat , it means that my sails can be set with about 10 - 17% balance. If I want to reduce the balance to 5%, I must either move the slingpoint or lengthen the mast..

    The shown sailplan of Ingeborg is a typical example. In real life, that sail now sits about 10cm further forward, so the halyard angle is mostly lower than shown. Another advantage with this mast (unlike the too short mast of Johanna) is that the yard hauling parrel, YHP, can be fitted 2/3 up the yard. This leads to lighter forces in both the YHP and the THP.

    Note how the top of the yard is taller than the Bermuda rig. The 70° yard angle is only 1° lower than the forestay of the original rig, so I regard the formal head of that JR to be a part of the leading edge.

    Arne


  • 28 Mar 2019 08:17
    Reply # 7248972 on 7246671

    Planform is only about defining the shape of a sail. It cannot include the mast line, as this can vary widely for some sail shapes. I'm going to stick with my view that the minimum mast height is given by an answer to the question "how close, in terms of potential damage to blocks and halyard caused by twisting as the sail is squared off, can the yard and mast attachment points be?"

  • 27 Mar 2019 16:37
    Reply # 7247720 on 7247172
    David Tyler wrote:

    I would suggest that the question should be "what is the minimum permissible halyard drift?" and that the answer is "it depends on several factors".

    Ah, I would never have thought to look for the word "drift" in this context. Thank You. Maybe "The Junk Rig Glossary" Definition needs to be changed as well, because I have a hard time putting that and this together.

    It is independent of the type of planform, but is dependent on the type of block used, the type of rope used, and the type of attachment to mast and yard.

    Independent of planform? Does planform include the position of the mast within the planform? That is the angle of the halyard to the yard with respect to the mast. For example with a PJR rig, the yard attachment point is to the rear while a split rig has the yard attachment point much closer to the mast. I would have thought this angle would have made some difference. (I can see how it may not too)


  • 27 Mar 2019 09:13
    Reply # 7247172 on 7246671

    I would suggest that the question should be "what is the minimum permissible halyard drift?" and that the answer is "it depends on several factors".

    It is independent of the type of planform, but is dependent on the type of block used, the type of rope used, and the type of attachment to mast and yard.

    PJR gives a drift of 0.3B, which for a batten length of, say, 4m would be 1.2m. This is generous to the point of being excessive, but allows for indecision on yard sling position, sail AR, need to hoist the sail higher for convenience at deck level and suchlike factors.

    As a rule of thumb, I would suggest that modern yacht blocks are comfortable with a distance between them of twice their length, provided that each is free to rotate on its mounting by about +/- 45˚, this being achieved easily enough with a shackle through a hoop or eye, or a lashing, or a soft shackle. That would indicate a minimum permissible drift of four times the length of one of the blocks, or about 500mm, where two blocks are used. The use of a becket and its associated knot or splice would make it sensible to increase the drift to allow for extra distance between the blocks.

    The way to reduce that is to use a single part halyard with a self tailing winch, or to use my preferred form of halyard, two parts, with two attachment points at the masthead, again with a self tailing winch. The end of the halyard is made fast to an attachment point on the masthead that is above the sail (at 7 o'clock, if the sail is on the port side), and passes through a single block on the yard and a single block at the masthead (at 5'oclock), and then to the deck. If the yard block has a little rotational freedom (but not complete freedom), it is permissible to hoist almost to the "two-blocks" state, or at least to a drift of three times the length of one of the blocks, that drift being measured between the attachment points on yard and mast.

    But if possible, it's better to go for more than the absolute minimum drift that the blocks can tolerate. Another factor that might be considered is that a tricolour light really ought to be above the top of the fully hoisted sail.

    Last modified: 27 Mar 2019 09:16 | Anonymous member
  • 27 Mar 2019 04:54
    Reply # 7247056 on 7246671
    Graeme Kenyon wrote:

    I would be very interested (and grateful) to read some informed comments on this subject. Or a reference if this subject has already been covered in another thread.

    I Noticed this lack as well. I would be interested in what people with more experience have to say too.

    Having said that, what I have read indicates that the minimum height of the halyard anchor point above the yard anchor point is determined by the size of the block(s) and lines used to hoist the sail. There is first the size of the block anchoring on top and bottom. Then the size of the blocks themselves and finally room for the halyard line not foul. Then some extra to allow for moving the the sail forward and back. I recall seeing mentioned that someone had wished they had left extra to allow for that. Also someone who thought there was some binding when things were too tight.

    Then, as has also been mentioned, having the mast taller rather than just tall enough does allow for a higher aspect ratio sail to be used without remasting. In the few sail plan layouts I have tried, AR changes very quickly even with only inches of boom length or mast length.

    If the boat is to be trailed, trailing laws will determine maximum mast height as well. In such a case one may have to accept lower performance (or rather define performance in a different light).

    I have seen recommendations on on how many part block to use for size of sail but it seems most are three part unless they are really big and then four part. At the place where four part blocks are needed seems also to be the place where more sails of smaller area start to look good. So maybe start with the block and it's attachments and add length from there. Two 2x4s (or whatever they are in metric) with eyes in them to model the mast and yard. Hook up your block with some line for halyard and try it out. Doesn't cost much. Remember that the mast will stay still while the yard wants to be able to go left and right to at least 90degrees (for downwind). Also remember to hold the "yard" at the correct angle.

    One last thought. You mentioned the mast height compared to different sail shapes. While it is true that the high yard angle shows the mast only part way up the yard, a lower yard angle tends to have more sail area in the top panels and so for the same sail area there may be less difference in mast height than appearance shows. Do the math... for each change or idea you want to try.

    Last modified: 27 Mar 2019 04:58 | Anonymous member
  • 27 Mar 2019 04:35
    Reply # 7247038 on 7246671

    Let me put it another way.

    Once the sail area, plan form and aspect ratio have been decided, it is possible then to draw the sail with its CE where we want. We can then draw the mast in its correct position, and we can see where the mast crosses the yard.

    At that point in the process, however, minimum mast height is not yet obvious.

    The top of the mast will have to be higher than the point where the mast crosses the yard, in order to give sufficient drift to the halyard. My question is: how much higher? Perhaps my question should have been “how do you calculate the preferred minimum amount of drift” because that is, I suppose, what ultimately gives the minimum Mast height.

    I realise the answer to this question is different according to whether the planform is of a high peak or moderate peak type – which is partly the reason I am asking the question on this forum. For each planform there must be some principle on which minimum halyard drift (and therefore mimimum height of mast) is determined.

    In a way that David Th did not intend, it is indeed a question of “how long is a piece of string?”, but it is surely not arbitrary.

    It may be a dumb question, but how is minimum mast height determined once all the other important parameters are in place?

    (As a matter of fact, it is a real question for me. Like Scott, I can see the trade-off between mast height and sail area and, in general terms, where the various different planforms score. I am limited by mast height.)


    Last modified: 27 Mar 2019 05:01 | Anonymous member
<< First  < Prev   1   2   Next >  Last >> 
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software