Where to step the mast on a convershion monohull.

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  • 26 Aug 2022 07:27
    Reply # 12896759 on 12885037

    Today I received my copy of " Practical Junk rig" and " Building badger".

    Tomorrow I get to start reading both. any other books that can be recommended?

    Thank you for all the help


  • 21 Aug 2022 09:43
    Reply # 12890410 on 12889687
     There are some good pictures of tabernacle builds both in the forums here and in the JRA magazines.


    The tabernacles that Pete Hill builds are brilliant imho. Certainly the way I'd go!

    Last modified: 21 Aug 2022 09:43 | Anonymous member
  • 20 Aug 2022 02:03
    Reply # 12889687 on 12888699
    Anonymous wrote:

    I am hoping to step the mast far enough forward to put it in the locker between the chain locker and the forward bunk.Even at the foot of the bunk would be cool.I need the tabernacle to get under bridges. and work at the top of the mast without climbing.

    That sounds like a plan then. It would be interesting to watch you put it together, post pictures of your progress as you go. There are some good pictures of tabernacle builds both in the forums here and in the JRA magazines.
  • 19 Aug 2022 07:01
    Reply # 12888699 on 12885037

    I am looking for a Chrysler 26 that I can gut completely.  I am hoping to step the mast far enough forward to put it in the locker between the chain locker and the forward bunk.Even at the foot of the bunk would be cool.I need the tabernacle to get under bridges. and work at the top of the mast without climbing. I understand the lower half of the mast will need to go down to the keel. I don't care if the boat I find has soft decks as I also know I will need to reinforce the deck to support the mast and windless anyway. I am a towering 5ft 3 in tall. The Chrysler 26 has a cabin height of over 6 ft . So I can sacrifice some head room for supports and insulation. I think the fun-est part will be removing the insert from the hull and keeping shape until I get the new bulkheads and ribs in her.

  • 17 Aug 2022 23:11
    Reply # 12887178 on 12886291
    Anonymous wrote:

    OOP!   Sorry!  Tabernacle not pulpit. I am learning. LOL

    Tabernacles are great for sure but even then you need something to extend the tabernacle to the keel. So there is not much one can do to get around having something go through the cabin. If the tabernacle is steel (or maybe aluminum) it could be made in a wishbone shape if it would otherwise be right in the middle of a passageway, thus straddling the passageway. Should you like to be really outlandish, the mast should be able to be mounted off centre (to avoid the above passageway). Most people avoid this because it troubles their sense of right. It is also harder to affix the mast step. I have heard (maybe read) of some steel boats where the mast is bolted (lamp post style) to the deck with a flange. However, in such a case the deck still needs the structural strength to deal with it and that probably means an interior structure of some sort. Basically, a free standing mast goes all the way to the keel in some manner. Actually, a deck stepped mast requires support all the way to the keel as well but as the force is mostly straight down, the support(s) can be off to the side so long as there is a strong plate to bridge the mast step to the supports, like the walls on either side of a passageway for example. If you look at deck stepped mast boat layouts for very long you will be surprised at how many have a wall for a head right under the mast step, the passageway to the V berth very often being offset. What this means for someone going from deck stepped to keel stepped, is that there is often a wall of some sort centre line where a mast might be added without blocking too much of the interior. Once you get into that V berth area though, you pretty much have a mast in the middle of the bed, unless you can get to the aft of the chain locker. For any given sail area (and balance percentage), a lower AR seems to want the mast more forward and a higher AR wants the mast more aft. There are a lot of variables that allow moving the mast around.
    Last modified: 17 Aug 2022 23:40 | Anonymous member
  • 17 Aug 2022 09:31
    Reply # 12886362 on 12885037

    Nope.  Mast step & partners!!  Have fun.

  • 17 Aug 2022 07:08
    Reply # 12886291 on 12885037

    OOP!   Sorry!  Tabernacle not pulpit. I am learning. LOL

  • 17 Aug 2022 07:01
    Reply # 12886290 on 12885037

    Thank you. You have brought clarity to some questions.

    I can that there are several things that all have there place in consideration.

    From interior space to placement of the mast in connection with the forces applied by the varying sail shape. I can also see a benefit to a forward angled mast to keep your battens from flogging the mast in ruff waters.

    Lots of wants that too will require consideration. Having the mast on a pulpit and mounted as far forward as possible  would solve several problems.

  • 17 Aug 2022 05:56
    Reply # 12886236 on 12885037
    Anonymous member (Administrator)

    That’s good, do all the reading you can: Arne’s notes and Hasler McLeod. Then you will see that placement of the mast is not a simple question which is decided in isolation. Its not a difficult question, though. I’ve only ever done one conversion so not an expert, but I do understand the slightly over-whelming prospect of having to think it all through for the first time. Be reassured, you’ll get plenty of good expert help once you get started on the design.

    If that sail plan I posted matches the one on your boat – and if you are happy with the helm balance of the vessel as it is, then some of the work in calculating mast placement has already been done for you, and you need not worry too much about calculating the so-called centre of lateral resistance of the hull. You can simply calculate the centre of area of the existing bermudan rig and use that as a guide to the placement of your proposed junk sail. So, you first step might be to superimpose a proposed junk sail onto your drawing, with its centre of area where you deem it should be.

    (In the case of a split junk rig, vis-a-vis the original bermudan rig, the two centres of area can actually coincide. For other types of junk rig, get advice from those who have designed them, as the best placement of centre of area may not quite coincide with the centre of area of the original Bermudan sail plan, though probably not too far away).

    Now, your proposed junk sail will have a percentage of balance, which determines where the mast will go. (The “balance” of the sail being the proportion of the sail outline which lies ahead of the mast centre line). If you have a fixed figure in mind for balance (it might be anywhere between, say, 5% and 35%) then your mast position is determined.

    However, if you are open as to the geometric shape of the sail and how much balance you are happy to have, then you will have a certain amount of choice as to where the mast can go. The position of the centre of area of the sail can’t be changed much, but the position of the mast can vary. In simple terms, if your proposed junk sail is one with a low amount of balance (and the concomitant high yard angle) then the mast will be at the forward end of the range of possible positions. If the sail you propose is the type with a high amount of balance (and correspondingly a low yard angle), then the mast will necessarily go towards the aft end of the range.

    You can reverse this reasoning to some extent: if the internal accommodation arrangements and/or structural considerations make it desirable to place the mast in a particular position (within the range of possible positions) – then you don’t move the centre of area of the sail, but you can re-consider the shape of the sail, ie the different amount of balance which this mast position makes necessary. That in turn decides on the type of junk sail which fits your choice of mast position– in simple terms, the mast position can determine the shape of the sail: a high yard-angle/low balance sail, or a low yard-angle/high balance sail, or something in between.

    In practice, the boat, the type/geometric shape of junk sail and the placement of the mast will all be considered together to find the most harmonious solution.

    A further variation on mast position is also to consider the rake of the mast. A forward rake means the bottom part of the mast can be placed aft a little, if other requirements dictate. Or, to reverse the reasoning, a little bit of tweaking of the rake might allow movement of the centre of area of the sail if that is found to be necessary later. Some people prefer a certain amount of forward rake. Aft rake is not considered so desirable on a free-standing rig.

    There are people reading this thread who are considerably more qualified to comment further, and my guess is, as David Th has suggested, if you have a go, and put forward a scale drawing of your proposed sail plan superimposed on the underwater hull drawing (or better still, superimposed on the hull drawing with the original Bermudan rig) you will get plenty of help and guidance from those people, some of whom have designed many junk sail conversions. (Also needed to be considered: span clearances, sheeting arrangements, structure, accommodation requirements – they will all fall into place).


    Last modified: 17 Aug 2022 06:11 | Anonymous member (Administrator)
  • 17 Aug 2022 04:35
    Reply # 12886214 on 12885037

    Thank you that was just what I was looking for. Answered all my questions and more.

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