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    Maximum Power for a Wooden Prod

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    Maximum Power for a Wooden Prod

    Post by Warbowman on Fri Oct 11, 2013 1:18 am

    I was wondering what would be the practical maximum power for a wooden-prodded crossbow?  I have only ever seen evidence of wooden crossbows being hand-spanned, and they were used as military weapons early in the medieval era - largely being replaced by composite and steel in the 13th and 14th centuries.  Then the power of military crossbows started to increase considerably.  I think wooden bows were still used for hunting - but then you don't need any more power for hunting that would require mechanical spanning aids.  (I think my crossbow history is right here - someone correct me if I am wrong).

    The practical maximum for hand spanning is about 200lb and this was probably true for tough, hardened medieval warriors as well.  (Of course there will be some tough buggers who could span more, but on average this would probably be about the top end.)  I know a bowyer - and a very good bowyer at that who has made a yew prod of about 500lb.  He tillered it using a car jack, but to my knowledge he hasn't made it into a crossbow yet.  I would have thought that if you tried to increase the power of a wooden prod too much, you would just increase the mass and lose too much draw length and you would not get anything much back for it in terms of power and velocity.  There would also be the maximum speed that the material could recoil which could not be exceeded by increasing draw weight beyond a certain level.

    I would be interested to hear about experiences anyone might have on this.  What is the most powerful crossbow known to have been made from wood?  Is there any evidence of mechanically spanned wooden crossbows from the past?
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    Re: Maximum Power for a Wooden Prod

    Post by hullutiedemies on Fri Oct 11, 2013 1:59 am

    Typical wooden spring can take ca 1 Joule load per 5 grams of wood.
    In other words about one liter of bending wood is needed for every 100J of stored energy. So a powerful wooden bow needs to be quite large.

    There is no theoretical limit to draw weight. But a small wooden bow with very heavy weight will have very fat limbs and very short powerstroke, and it may be so stiff that it will be impossible to string.

    Wooden prod for a kiloJoule class siege crossbow would have to be size of a telephone pole.
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    Re: Maximum Power for a Wooden Prod

    Post by Geezer on Fri Oct 11, 2013 7:54 am

    Nerd:  Thanks for the post concerning limits of wooden bows.  You succinctly state something I've had trouble expressing. 
    One might also mention that really long, massive bows run into problems of scale... large timbers can have weak spots that don't show up in smaller ones... so it gets progressively harder to find suitable wood.  There are very good reasons why Leonardo's famous giant crossbow is a very bad idea.  That's why bowyers of all sorts eventually go to alternative spring-materials, in search of more compact and reliable materials for stronger bows. 
    Well spoken, Nerd!  Geezer
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    Re: Maximum Power for a Wooden Prod

    Post by Geezer on Fri Oct 11, 2013 8:03 am

    Geezer with a post-script on wooden prods.  I have seen photos of very large wooden siege bows... prods on the order of 2 meters in length... (that's an estimate) So some bows had very long prods indeed.
    And yes, all bows eventually run into diminishing returns as size and power increase.  A 2000 lb. bow is not necessarily twice as powerful as a 1000 lb. bow, and it certainly won't move any faster.  Once you approach the maximum return-velocity of a spring, you can't make it faster by making it stiffer.  You CAN increase the weight of the projectile.  That will give you more foot/pounds or jouls of energy on target.  If energy = 1/2 mass X velocity squared, and you can't increase the velocity, you HAVE to increase the projectile mass to get more punch.  Also, heavier projectiles of similar shape will have relatively less cross-section than lighter ones, so less wind-resistance.  That means higher velocity at longer ranges and more punch. What does this mean for a hunter?  Not much, unless you're going after something very hard and thick, like elephant or rhino.  But for armored human targets, you may need lots of force to penetrate armor, as well as a really Hard and Sharp point on a very resilient shaft.
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    Re: Maximum Power for a Wooden Prod

    Post by Todd the archer on Fri Oct 11, 2013 12:30 pm

    Another limiting factor of wood prods is time. Time spent in a cocked position puts a lot of stress on the fibers which eventually give way, maybe not failure but diminished performance. In combat a crossbow probably is not at full draw long before being shot. This is not the case in a typical hunting scenario where it may sit for hours before before being used. So for a wood prod to be practical it is best to be under stressed. In other words made much longer and drawn shorter than it's maximum potential.

    Believe this is also a short coming of rubber powered weapons as well.

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    Re: Maximum Power for a Wooden Prod

    Post by Warbowman on Mon Oct 14, 2013 4:08 am

    Very interesting discussion guys.  As I have mentioned, I make longbows including some powerful 'warbows' with draw weights over 100lb at 32".  I usually use laminated woods but have made some self - and would like to get into this side of it more.  I usually use experience plus some intuitive judgement to assess how much power I can get from any particular stave of wood.  The equation of 5g of wood per joule sounds useful though.  I suppose with very large siege crossbows, it would not matter how long they were as you weren't lugging them around much - and I suppose they must have had long power strokes as well.  As an interesting aside, on King Henry VIII's flagship the Mary Rose (that sank in the Solent in 1545) many very well preserved yew longbows have been salvaged with the ship.  These have been extensively analysed and draw weights were found to be impressive - ranging up to about 180lb at 30" maximum.  Although such draw weights have been found to be possible (by a FEW archers), it has been speculated that the biggest bows may have been made to be somehow clamped to pegs in the ship's gunwhale and then drawn and released with two hands.  These larger bows also appear to have a squared off section in the centre that would not be very comfortable to hold but would help with locating in some kind of mounting.  However, I don't know how an archer could release a heavy bow in this way without falling back on his arse afterwards!  This view is not generally in favour at present.

    The wooden warbows/ heavy longbows we make are all long, some very long at nearly 7'.  They also look very stout and intimidating! They work very well with their long power stroke of 30"-32".  I suppose one of the advantages among others of the crossbow is its compactness and long wooden prods would not be good for this.  I have noticed myself, also when putting a wooden bow on the tiller how the draw weight can drop off the longer it is held drawn and how most shed a pound or few between the first draw and the second after stringing.
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    Re: Maximum Power for a Wooden Prod

    Post by Taxus on Mon Oct 14, 2013 7:58 am

    There is a very large siege crossbow depicted in Walter de milemetes de secreta secretorum from 1326 that looks to have a wooden bow,as it has the knobbly back similar to longbows of yew. If the bow is in proportion to the men in the picture, it is about 1.8 meters long, and must have a very large draw weight.
    One is draw by a wooden screw, another a wooden windlass.
    Jean Liebel estimated these siege crossbows with windlasses to draw around 1300kg, though this could be for composite prods.

    There are several other references to crossbows of this size with bows of yew in French cities such as Rouen from the early 15th century.

    As for smaller bows,one was found in the moat at Berkamstead Castle in England.
    It is made of yew,about 4 feet long and estimated at 150lb draw weight, possibly from the siege of 1216.
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    Re: Maximum Power for a Wooden Prod

    Post by Warbowman on Tue Oct 15, 2013 5:24 am

    Taxus - that is really interesting.  I've tried to find some images of what you describe on-line without any luck.  I suppose it was not very hard to produce a really big crossbow for people used to making powerful yew longbows.  You could bulk it up and get hundreds of pounds of draw weight to propel an awesome arrow - possibly with better 'anti-material' (as they call it these days) capabilities.  It's just if you got too long with the prod it would expend too much of it's energy moving its own mass.  When the composite and steel prods came along it would have just allowed these big siege bows to be made smaller and more compact.

    I know of the Berkhamstead Castle bow and in fact looked at it when designing my first crossbow (illustrated elswhere on the forum under that title).

    BTW - I notice you are a fellow Englander...do you shoot longbows as well?  ...Your avatar looks just like one of my warbow arows!  jocolor

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    Re: Maximum Power for a Wooden Prod

    Post by MoeM on Wed Oct 16, 2013 5:32 am

    Here are two pics of wooden prod- or better to say split log Shocked  
    Btw. I`m new here and looking forward for good time with you fellas bounce 
    wooden prod
    I really wonder about the drawweight but I`d say much more than 200#...
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    Re: Maximum Power for a Wooden Prod

    Post by Warbowman on Wed Oct 16, 2013 12:28 pm

    Wow...that looks a brute!  It's got lugs for attachment of cocking device too on the tiller.  In that position would it be for a goats-foot or a cranequin? Shame about the woodworm...looks like they've had a field-day!

    It looks like it has no track either - bolt support at fore end, interesting advanced features with a wooden prod.

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    Re: Maximum Power for a Wooden Prod

    Post by MoeM on Wed Oct 16, 2013 2:26 pm

    I tend to say the bolt is for a cranequin; lever bearings for goatfoots are closer to the nut as far as I know.
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    Re: Maximum Power for a Wooden Prod

    Post by Taxus on Wed Oct 16, 2013 3:37 pm

    There is a picture of the siege bow I mentioned here: http://nativeurals.narod.ru/westcrossbows.htm in the XIV B section, middle picture.
    Hard to say whether wooden or composite is intended though.

    Another interesting wall crossbow here that does have a yew bow:
    http://www.tgorod.ru/contentimage/crossbow/swiss_rampart.jpg

    As for longbows I'm looking to get hold of one, probably as Ash self bow.
    I've been shooting an old 'barebones' fiberglass recurve bow for a while (one without any sight/stabilisers etc) and want to try something a bit more like the traditional longbow our countrymen were known for.

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    Re: Maximum Power for a Wooden Prod

    Post by MoeM on Wed Oct 16, 2013 3:52 pm

    Damn this reflex is really sick...

    (from http://nativeurals.narod.ru/westcrossbows.htm)
    Hard to imagine that the bow doesn`t twist in the briddle while stringing/after shot...
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    Re: Maximum Power for a Wooden Prod

    Post by actionbow on Wed Oct 16, 2013 10:33 pm

    Yeah, I was wondering about that myself. I wonder if the prod flipped when the string broke? It seems impossible to even string much less span. Is that all wood or wood/horn? Also, being rounded on the back and flat on the belly seems like an odd profile for a wood bow.
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    Re: Maximum Power for a Wooden Prod

    Post by Warbowman on Thu Oct 17, 2013 1:21 am

    Taxus - thanks for posting images.  Really interesting, and almost without doubt a yew prod, though massive - probably made from half a tree trunk!  The weapon looks like a descendant of the roman arcuballista.  The other bow is interesting to see - it also has a simple push-pin notch lock, like a Skane bow...I suppose even until quite late, such crossbows were relatively simple and inexpensive to make.

    MoeM - I think your bow there is almost certainly composite with that shape. On the end of the left limb it looks like there is some sort of covering that has flaked off there.
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    Re: Maximum Power for a Wooden Prod

    Post by bpnelson on Thu Nov 28, 2013 11:24 am

    I have seen some old photos of what appear to be very, very large (wooden) prods. Perhaps in the 800-1000 pounds range. Of course these limbs would move very slow, but in warfare the real purpose of higher draw weights was not greater speeds, but greater bolt/arrow mass while keeping speed the same. Remember, momentum is all-important, energy not so much. Also, if more of the momentum is due to mass, the bolt will penetrate farther. 

    If I have two situations: 700 grain bolt at 280 fps and then 1400 grain bolt at 140. Well now, the 700 grain bolt will have twice the kinetic energy. And the momentums are the same, we want the 700 grain bolt, right? Nope, not if we just want raw, armour punching, penetration.

    The problem is that RESISTANCE is based on velocity squared. So, while the momentum is the same, the 700 grain bolt will meet 4 TIMES the resistance during air travel, and also when penetrating something like flesh. The 700 grain bolt will bleed momentum much faster than the 1400 grain bolt. So if I want as much penetration as possible, always want more momentum, and if momentum is constant, I want more mass than velocity. 

    So while the wood prods may be slow, it's really just about launching a heavier missile.

    Also, I think 300 pounds is a more realistic draw weight for a strong man. I've managed to get used to 240 very easily, and I am by no means that strong.

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    Re: Maximum Power for a Wooden Prod

    Post by jeep on Fri Nov 29, 2013 2:50 am

    Here a picture of a  large balista with a wooden prod. It part of the crossbow collection of the war middle age museum of Castelneau castle in Dordogne France. Prod power unknown, prod material unknown.

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    Re: Maximum Power for a Wooden Prod

    Post by jeep on Fri Nov 29, 2013 2:52 am

    .

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    Re: Maximum Power for a Wooden Prod

    Post by jeep on Sun Dec 01, 2013 3:47 am

    This Balista is a reproduction. There is an original model  in Germany in the Stadtischen museum , Quedlinburg.
    Referring to there info, the bow is composite (horn,tendon,wood,leather). Lenght 1,78m, wid of the bow 1,62m.
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    Re: Maximum Power for a Wooden Prod

    Post by Warbowman on Tue Dec 03, 2013 3:25 am

    Really interesting stuff.  I suppose wood is pretty easily accessible and wooden bows relatively simple to make and costs are low.  If there is no limitation on size you can build a wooden crossbow as powerful as you like...though you will eventually run into length and mass problems at some point, but then that is true of any bow.  Then, the clever Romans invented torsion-artillery to overcome that problem.  I would think that wooden crossbows have always been around even when steel and composite were the state-of-the-art, for use in situations where out and out power and range may not have been paramount...such as defending castles against attackers i.e. 'rampart bows'.

    BPNelson...interesting points re. mass vs. velocity.  This is something I am well familiar with being experienced with longbows of all types ranging from target bows to heavy warbows. I have seen good 60lb recreational longbows fling a little 5/16" arrow as far as a warbow chucks it's 1/2 inchers.  (And you couldn't use the small arrows in the warbow as they are too light for it and it would probably blow up!).  With a well designed longbow you reach max. speed potential at about 70-75lb and any increase in draw weight simply allows you to use heavier arrows while maintaining range which was of course vitally important in the old days.  As you rightly say, when it comes to impact and penetration it is mass that matters.  The little 5/16" target arrows may reach 12 score...but they land soft as a feather!  Turkish flight bows that could reach prodigious ranges were generally in the range of 60lb - 90lb draw weight...and used tiny arrows.  Turkish warbows however were usually over 100lb draw, less extreme in profile and used much heavier arrows to more moderate distances.

    I suppose your point about draw weights of crossbows depends on how you do it.  I am new to crossbows and have made one about 150lb.  I draw it by hand with the 'butt' against my waist/hip-bone.  I don't find it taxes me really.  Whilst also not the strongest guy around, I am also no weakling and have shot with longbows in the 120lb-130lb range.  I can see that if you use a foot-stirrup and draw more as a dead-lift you could pull considerably more...in the 300lb+ range possibly. There are also contemporary descriptions of crusaders lying on their backs with feet braced against the prod and pulling on the string with both hands.  That seems to imply large and powerful weapons (- probably in the 300lb+ bracket?).  However, there is the point of efficiency - if it half kills you, you would be better off using a goat's-foot and being in a fit condition to shoot straight as soon as you place the bolt.  I thought that aids such as gaffa, whippe and belt started to be used at about 200-300lb up to about 450lb and beyond when you got into cranequin and windlass territory.  Your regular 16th century steel 'field' crossbow, spanned with a goat's foot drew about 300-400lb?  I know it also gets more complicated than that with 'one foot' and 'two foot' bows being mentioned in the past - presumably referring to the number of feet required on the foot stirrup.  There is also the matter of spanning and locking the string straight.  As I am sure you know, if you are off-centre the bow won't shoot straight - and you could risk the prod.  It would be much harder to ensure you were properly square if you were sweating and puffing at full strength.  BTW...how do you hand-span - is it with a foot-stirrup?

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