SJR forward & aft?

  • 11 Oct 2021 18:49
    Reply # 11333688 on 11311647
    Anonymous wrote:

    In another thread (comparing rig choices) David noted that "the [split junk] type is not appropriate for a two masted rig", that it "will certainly not be worthwhile on the after sail of either ketch or schooner" and that he "can't see any aerodynamic benefit in making the after sail a split junk, as it is operating in the dirty wind of the foresail".

    This is only true when sailing to wind. However, even at 45 degrees, the aft sail is not directly shadowed by the fore. I don't have enough experience to say really but the reason anyone uses fore and aft sails is because they do work. the wind direction is almost never directly fore and aft with running before the wind being the big exception and a time when sail shape is less of an issue.

    In the same thread Barry wrote that he "cannot see a reason to put a split junk sail on any mast except the front one".

    I understand the dirty wind concept. However, if one accepts an efficiency advantage of a SJR, wouldn't that efficiency advantage still hold for the aft sail, albeit to a lesser degree?

    Or if the point is that the efficiency advantage becomes negligible, what's the great harm in doing it anyway?

    Sorry if it seems I am labouring the point. Happy to be a pioneer, but also keen to avoid a big, ignorant mistake.

    There are very few vessels with SJR that even have two sails (I only know of the one actually). So the field is wide open. The SJR is (as these things go) a brand new idea that has just started to reach maturity in single masted vessels. Those with double masted vessels are crossing oceans (ie have a bigger boat) and so want to be more conservative in their rig. The cost of changing things if the rig don't work is double and maybe more if it is mid ocean where how it doesn't work becomes apparent.

    So start off with points of sail where an SJR doesn't really matter. A flat JR has been proven to work just fine anywhere from 90 to 180 degrees to the wind, being competitive with any other kind of rig. So while at 90 degrees dirty air is a non-issue, an SJR is also a non-issue. Unless it has some other value. The balance for example may allow a "nicer" mast placement or make handling easier. The rigging in an SJR is under less stress because it hangs straight without extra lines to hold it there. (so less complex rigging?) if the SJR is easier to build or rig or use, it may still be worth while even if it doesn't do any better than flat. On downwind the SJR is known to pull to the upwind side so maybe two SJR wing on wing would cancel that out.... except the fore jiblets would certainly be blanketed in that case.

    So this leaves upwind (forward of 90degrees). I would note there are a number of schooner and ketch rigged JR that do use cambered panels fore and aft and I have heard/read of no issues with these. People do tend to use less camber on the aft sail than the fore though. I am not sure that this is through experimentation or carried over from pointy rigs. So far as double SJR are concerned, all we have are guesses... maybe quite reasonable guesses based on long experience... but who would have guessed that the SJR would work so well before it was tried?

    Edit: I forgot to add that it might be a good idea to try to make sure that whatever your aft mast placement is, it is still possible to change to a flat sail and have good overall balance. This may mean that the aft sail if flat needs to be lower over all area for example or that the aft mast should be a little taller while using the SJR. I expect moving the mast(s) would not be something one wants to while experimenting though this is one of the advantages of a two mast rig, the balance can be adjusted with sail size alone.

    Last modified: 11 Oct 2021 18:57 | Anonymous member
  • 11 Oct 2021 11:35
    Reply # 11311647 on 11138799

    In another thread (comparing rig choices) David noted that "the [split junk] type is not appropriate for a two masted rig", that it "will certainly not be worthwhile on the after sail of either ketch or schooner" and that he "can't see any aerodynamic benefit in making the after sail a split junk, as it is operating in the dirty wind of the foresail".

    In the same thread Barry wrote that he "cannot see a reason to put a split junk sail on any mast except the front one".

    I understand the dirty wind concept. However, if one accepts an efficiency advantage of a SJR, wouldn't that efficiency advantage still hold for the aft sail, albeit to a lesser degree?

    Or if the point is that the efficiency advantage becomes negligible, what's the great harm in doing it anyway?

    Sorry if it seems I am labouring the point. Happy to be a pioneer, but also keen to avoid a big, ignorant mistake.

  • 04 Oct 2021 20:17
    Reply # 11140009 on 11138799

    Hmm, interesting. Thank you for the webpage links.

  • 04 Oct 2021 19:15
    Reply # 11139802 on 11138799

    I should  have added that the SJR main with flat mizzen seems to have worked out just fine. See this post

  • 04 Oct 2021 18:49
    Reply # 11139732 on 11138799
    Anonymous wrote:

    Hello all

    I’m considering the split junk rig, and specifically its appropriateness for both sails of a two masted monohull (schooner or ketch).

    Having not come across this arrangement previously, I am wondering whether there is precedent for it, and if there is any compelling technical reason why it might be a stupid/unworkable idea.

    I personally can't really comment but you are not the first person to consider this arrangement. There are two posts on this page.

    These show a design that started out as two SJRs. In the end they went with a forward SJR and a flat mizzen see: rethinking the rig

    So in the end, the mizzen rig choice was not based on windward performance but practicality while anchored. Just my two cents on it.

    So for and aft SJR is still an unknown, you can be the first to try it  :)  Let us know how it goes... this might be a good time for poly tarp or tyvek prototype sails.


  • 04 Oct 2021 11:35
    Message # 11138799

    Hello all

    I’m considering the split junk rig, and specifically its appropriateness for both sails of a two masted monohull (schooner or ketch).

    Having not come across this arrangement previously, I am wondering whether there is precedent for it, and if there is any compelling technical reason why it might be a stupid/unworkable idea.

    I am fascinated by, but have an immature understanding of the junk rig, so would appreciate your thoughts on this.





    Last modified: 04 Oct 2021 18:06 | Anonymous member
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