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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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4 posters

    Hemp skein?

    kiltedcelt
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    Hemp skein? Empty Hemp skein?

    Post by kiltedcelt Thu Apr 07, 2011 7:10 pm

    For my current Renaissance crossbow project, (a 1590 German bow), I'd like to get a more realistic looking skein. My last crossbow project used Dacron B50 to make the skein and it just doesn't look right. It's too skinny. So, I picked up some 48lb test Romanian hemp and was thinking of using this to make the skein. I suppose it would be something on the order of 15 strands or so, give or take. The prod is a Darkwood Armory prod that will probably span at about 160 - 170 lb. Do you think this will be strong enough? It'll certainly look period-accurate, but that doesn't do me any good if it's going to break.
    Basilisk120
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    Post by Basilisk120 Thu Apr 07, 2011 10:45 pm

    if it is 48lb string then 15 strands should be strong enough. I think the usual is a minimum of a 4x factor of safety so. (170lb * 4)/48 =14.2 strands so 15 should be fine. If your still worried about it then go with 20 strands or so. I guess it would also depends if 15 strands is thick enough or if you would want to go thicker to get a decently thick skein.



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    kiltedcelt
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    Post by kiltedcelt Sat Apr 09, 2011 10:18 pm

    Thanks for the calculation Basilisk. Very similar to the margin used when making up a string for a normal bow. I'll have to see what sort of thickness I get with 15 strands.
    mac
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    Post by mac Sun Apr 10, 2011 7:53 am

    Celt,

    Fifteen strands seems light to me, but I have never had good luck with natural fiber strings. They always wear out in a few dozen shots for me. I look forward to hearing how it goes.

    Mac

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    Post by kiltedcelt Sun Apr 10, 2011 5:59 pm

    Mac,
    I'd intended to start with 15 and see how it looked and decide from there whether I needed to go thicker. Granted, the bows I'm looking at all have prods that probably run in the neighborhood of 250 - 500 lb or more, thus they all appear to have skeins that would measure almost 3/8" or more in cross-section. I have no idea how many strands would be in one of those. I guess theoretically, as Basilisk had mentioned, as long as the breaking strain/test rating of the hemp adds up to the draw weight of the prod and then some then it will probably be strong enough. I'm thinking over-build slightly.
    mac
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    Post by mac Sun Apr 10, 2011 6:42 pm



    Celt,

    My experience with linen strings is that the strands which are on the tension side of the string where it comes over the "corners" of the nut fail one by one under the serving. I have tried to deal with this by making sure that the string skein had enough twist that individual strands would not be so heavily loaded when the string is bent. This helps, but I still don't get the sort of life span I would like to see.

    I have found that it helps to serve center of the string in the opposite direction from the twist of the skein. This keeps the serving from trying to untwist that portion of the skein that it overlaps.

    Mac
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    Hemp skein? Empty Re: Hemp skein?

    Post by Geezer Mon Apr 11, 2011 8:24 am

    Mac: Unfortunately, twisting the skein more than a turn or so, loads the outer fibers more than the inner ones, resulting in progressive failure from the outside in. And of course, once you've broken a single strand, you string is more or less doomed anyway.
    After more or less failing at making hemp or flax strings, I have come to the conclusion that it's 'stringer's arte" rather than Armbruster's arte-another whole trade to learn. So I have decided to stay with B-50 strings, whether single or doubled-eye. You young whipper-snappers can figure out the stringers' secrets. Geezer.
    mac
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    Post by mac Mon Apr 11, 2011 9:20 am

    Geezer,

    I see what you mean about the twist putting more tension on the outside of the skein.

    What I was trying to overcome is the different tensions experienced by the skein as it passes over the nut. Here the "belly" side of the skein is under significantly more tension than the "back" side. I was hoping that more twist would help by letting more strands share the load.

    I wonder if I got carried away with that idea....

    Mac
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    Post by Geezer Mon Apr 11, 2011 10:29 am

    Yeah, well it was that busybody Kurt Suleski who pointed out the problems with twisted strings. I never would have thought of it on my own. And of course the thicker the string is, the quicker the twist will kill the outer fibers. If you look at extant medieval strings, they're not twisted very much. Geezer
    mac
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    Post by mac Mon Apr 11, 2011 10:47 am

    Wouldn't it be just like a Roman siege engineer to point out a thing like that?! These hurling engine guys are so touchy about that twist thing! Just don't get 'em started on Payne-Galloway's skein twisting ratchets.

    It turns out that rope makers have fancy ways of avoiding that problem in heavy ropes. I read about it once, but decided that I did not need to understand it fully. Perhaps I should better find out more.

    Mac

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    Post by Geezer Mon Apr 11, 2011 1:27 pm

    There was a German author named Rohde, who collected information from one of the last guild-trained crossbow string makers, back in the 1920's (or thereabouts) His work is mentioned in Harmuth's "Die Armbrust" I have seen some quotes from this stringer, about how strings were fabricated... It was years ago... as I recall, the bit I read seemed very incomplete... but maybe a well prepared scholar could trace down articles by Rohde in German cultural history periodicals from the 20's and 30's.
    As for Picky Roman siege engineers... well they're a type, you know.... wonder if Kurt is listening? Guess we'll know if a big rock lands in the middle of the forum. Geezer
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    Post by mac Mon Apr 11, 2011 1:40 pm

    Geezer,

    I've got a photocopy of the article he wrote in Waffen und Kostumkunde. It's hard to find, because it was written during WWII, but there is a copy in the Kienbush library in the PMA. I also have a copy of a translation that appeared in the Journal of Archer Antiquaries. When you put the two together, it still doesn't make enough sense.

    If there is interest, I will try to post the both of them, and maybe it will make sense to someone.

    Mac
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    Post by mac Mon Apr 11, 2011 2:08 pm

    Harmuth only has one citation for Rohde, and I am pretty sure that it's the one I am thinking about.

    Rohde, Fritz, Über die Zusammensetzng der spätmittelalterlichen Armbrust, ZfHWK,NF Bd.7, 1940-42,S.56


    Richter sites this one as well

    Rohde, Fritz, Die Abzugsvorrichtung der frühen Armbrust und ihre Entwicklung, ZfHWK, Bd.IV,1932-34

    I found them both mentioned on this page.....

    http://www.thebeckoning.com/medieval/crossbow/xbow-resources.html

    ....which I had never seen before, and shall now go back and investigate.

    Mac

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