Forward mast rake and friction

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  • 22 Nov 2023 15:31
    Reply # 13282544 on 13282091
    Anonymous member (Administrator)

    A closer look at the CG offset angle

    Now it dawns to me that this shown CG offset angle is the one which decides the friction between a sail’s parrels and the mast.

    The three factor contributing are...

    • ·         the sail’s mast balance,
    • ·         its aspect ratio,
    • ·         and finally, the mast rake.

    I got a hint about this when I rigged my Ingeborg in 2016. Since she initially had some weather helm on the first trips, I moved the sail forward about 15cm so the mast balance increased from 12 to about 15%. I then noticed a reduction in friction, with easier hoisting and lowering of the sail.

    Now I took the liberty and had a closer look at the rig on Frederik Elslo’s JR, designed by Paul. When checking the CG offset angle, I found this to be as low as 15° (against Ingeborg’s 21° with her present rig). Thanks to the Hogfish sail’s high AR and mast balance, this resulting low CG offset angle should ensure easy enough hoisting and lowering of that sail
    Good!

    Conlusion: I think I will keep an eye on this new-found CG offset angle on future designs, and maybe try to keep it below 21°...

    Arne

    PS: One may wonder how the Hong Kong seamen handled their mainsails. I guess there was some brute force involved...

    (Arne's sketches, section 7)

  • 22 Nov 2023 10:20
    Reply # 13282464 on 13282091
    Anonymous member (Administrator)

    Sometimes, when I am pondering over details in the JR, I look up the material I have about the real Chinese junks (mostly Worchester), or I carefully study the photos taken by the Dane, Karsten Petersen in the seventies. On these 2-3-masted Hong Kong schooners, the big driving mainsail in the middle appears to be set on plumb masts or with the mast raking maximum 1-2° forward. The much smaller foresails are rigged on masts with quite a lot rake. The relatively tiny mizzen masts sometimes rake a little aft. I tend to think that these professional sailors knew what they were doing.

    I remember Graham Cox mentioned friction problems when he set the black, cambered sail on the forward-raking mast on his steel-boat, Arion, and Pete Hill also mentions it in the last magazine (#93).

    Of course, friction when hoisting and lowering a junk sail, can have several causes, so I will not try to shout out “thou shalt not...”. It just appears to me that raking the masts forward, is generally not necessary. With the new freedom to vary the mast balance of my sails (gained from Paul’s experience with his La Chica), I see no big reasons for not doing it the easy way, with plumb masts.

    However, this is not a big deal, anyway, so personal taste will probably have the last word. I find that raking masts look odd, and will only use them when strictly necessary. Besides, I am not 60 anymore, so prefer climbing plumb (and stationary) masts, these days  -  and only if I have to...
    Live and let live...

    Cheers,
    Arne


    PS: I certainly am not going to send this to the Facebook. Everything seem to be kind of blown up (Edit: better, exaggerated?) there, not as badly as on the now dead Yahoo JR group, but still...


    A schooner in Hong Kong, 1974. Photo: Karsten Petersen

    (See section 8 from my album photos)

    Last modified: 22 Nov 2023 22:43 | Anonymous member (Administrator)
  • 22 Nov 2023 00:52
    Reply # 13282393 on 13282091
    Anonymous wrote:

    Why I avoid forward mast rake where I can

    All fore-and-aft sails have their centre of gravity, CG, moved out to the side (or aft) with respect to the mast top, they being gaff, lugs or junk sails.
    For a junksail this means that the sail constantly must be held from falling forward when raising and lowering it. This means that lines like batten parrels or standing luff parrels will have to rub on the mast as the sail goes up and down, and this adds friction. The angle between a line from the sail’s CG to the mast top could be called the CG offset line, and its angle to a vertical line would then be the CG offset angle. This angle is critical to how much friction there is between the sail and the mast, both as the sail goes up, and when it is coming down. The lower the aspect ratio of the sail, and the lower the sail’s mast balance is, the higher goes this CG offset angle, and the friction with it.

    On the sail I use on my Ingeborg, this offset angle is about 21°. On the diagram below, I have swung the lower end of the mast 4° aft around the present mast top (and the sail with it). This increases that CG offset angle to about 25°.

    The sail of my Ingeborg has tolerably low friction in it when hoisting and lowering it. I can mostly crash-dump it without any need for hauling it down. In a pinch, I give the shown THP a light tug, and down comes the rest.

    However, if I had rigged the sail with the shown 4° mast rake, that THP certainly would have been needed more often, and hoisting the sail would have been harder. Therefore, minimising friction is my main reason for sticking with a plumb mast.

    Even so, sometimes there are good reasons for raking the mast forward. On a foremast on a schooner, for instance, or if the deck layout or interior dictates the mast rake. Annie Hill has another argument for it. She sails up and down the east coast of NZ (North Island). There she often experiences a combination of light winds and old swell rolling in from the Pacific. She finds the mast’s forward rake helpful in taming the sail in these conditions.

    Well and fine, I respect such good reasons. However, I will always aim for a plumb mast as long as I see no real need for rigging with forward (or aft) mast rake.

    Cheers, Arne

    (..PS: And don’t come and tell me that a forward-raking mast looks smarter or saltier...)

    (Full size diagram at Arne's sketches Section 7...)


    Well Arne, I beg to differ. I always put forward rake in my designs as I've mentioned to you elsewhere. I find it helps to stabilise the sail when off the wind and of course it can be used to get the CE where you need it when it's no possible to get it there by other means. 

    I've used everything from 0° forward rake to 10° of it and I've experienced no problems with any of my sails coming down.

    Last modified: 22 Nov 2023 07:34 | Anonymous member
  • 21 Nov 2023 17:05
    Reply # 13282171 on 13282145
    Anonymous member (Administrator)
    Anonymous wrote:

    Always good to listen to your experience.

    in the first sentence, do you mean ‘aft’ rather than ‘to the side’.

    Ah, I see. Aft is probably best although that depends if the sail is squared out or sheeted in. Now I have corrected the term "fore and aft" to "fore-and-aft" sail...

    Arne
    Last modified: 21 Nov 2023 19:32 | Anonymous member (Administrator)
  • 21 Nov 2023 16:27
    Reply # 13282145 on 13282091

    Always good to listen to your experience.

    in the first sentence, do you mean ‘aft’ rather than ‘to the side’.

  • 21 Nov 2023 14:37
    Message # 13282091
    Anonymous member (Administrator)

    Why I avoid forward mast rake where I can

    All fore-and-aft sails have their centre of gravity, CG, moved out to the side (or aft) with respect to the mast top, they being gaff, lugs or junk sails.
    For a junksail this means that the sail constantly must be held from falling forward when raising and lowering it. This means that lines like batten parrels or standing luff parrels will have to rub on the mast as the sail goes up and down, and this adds friction. The angle between a line from the sail’s CG to the mast top could be called the CG offset line, and its angle to a vertical line would then be the CG offset angle. This angle is critical to how much friction there is between the sail and the mast, both as the sail goes up, and when it is coming down. The lower the aspect ratio of the sail, and the lower the sail’s mast balance is, the higher goes this CG offset angle, and the friction with it.

    On the sail I use on my Ingeborg, this offset angle is about 21°. On the diagram below, I have swung the lower end of the mast 4° aft around the present mast top (and the sail with it). This increases that CG offset angle to about 25°.

    The sail of my Ingeborg has tolerably low friction in it when hoisting and lowering it. I can mostly crash-dump it without any need for hauling it down. In a pinch, I give the shown THP a light tug, and down comes the rest.

    However, if I had rigged the sail with the shown 4° mast rake, that THP certainly would have been needed more often, and hoisting the sail would have been harder. Therefore, minimising friction is my main reason for sticking with a plumb mast.

    Even so, sometimes there are good reasons for raking the mast forward. On a foremast on a schooner, for instance, or if the deck layout or interior dictates the mast rake. Annie Hill has another argument for it. She sails up and down the east coast of NZ (North Island). There she often experiences a combination of light winds and old swell rolling in from the Pacific. She finds the mast’s forward rake helpful in taming the sail in these conditions.

    Well and fine, I respect such good reasons. However, I will always aim for a plumb mast as long as I see no real need for rigging with forward (or aft) mast rake.

    Cheers, Arne

    (..PS: And don’t come and tell me that a forward-raking mast looks smarter or saltier...)

    (Full size diagram at Arne's sketches Section 7...)

    Last modified: 21 Nov 2023 19:28 | Anonymous member (Administrator)
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