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Leo Todeschini
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PostPosted: Wed 11 Mar, 2009 10:45 am    Post subject: 750lb windlass bow by Tods Stuff         Reply with quote

I have been wanting to make a windlass bow for a long time and finally I finished one I have been fiddling with for the last few months.

The bow is pretty standard stuff and has an ash stock, brass nut with hardened steel insert and bow irons. The prodd is calculated to be 850lb at 6" but for my own reasons I decided to set the distance at 5.5". for those that care it is 50 x 12mm at the centre and 700mm tip to tip. The string is linen.

What I was really interested in was the windlass and so that is what I got excited about. The crank end is copied largely from one in the Wallace and the hook end is an amalgamation. The wheels have the spokes brazed in. All in all I am pleased. Interestingly I have always read that that a double windlass produces about 45:1 ratio, but I thin this is well off. the pulleys allow for an 8:1 ratio and then the crank axel is 20mm diameter (ie 10mm from centre line to winding face) but the crank arm is 200mm long, so this give 20:1 so 20x8 is 160, so I think that the system gives about 160:1. Maths was never my strong point though.

I made 3 bolts with box wood flights and ash shafts, but they came out badly balanced and 2 of the 3 don't really fly at all, the third flies well enough to know the bow has some poke, but I will have to revisit the bolts in due course, but unfortunatley won't have time in the foreseeable future. I love the fact that my job is my hobby, but still work gets in the way of pleasure! It is hell to be so close to owning a windlass bow and yet not be able to find the half day to make some new bolts.



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Leo Todeschini
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PostPosted: Wed 11 Mar, 2009 10:53 am    Post subject:         Reply with quote

Sorry forgot to say that what I also found very interesting is the rate of shooting.

I have not yet had time to play properly and get to grips with it but what was immediately noticeable was that loading the bow actuallly takes very little time, maybe about 10 seconds, what takes the time is unwinding the windlass afterwards, because with those high ratios you cannot just pull the claw away from the crank, so you have to unwind the crank and then the cords go loose on the axle and then you have tease them straight again etc etc That is what takes the time, resetting the windlass.

Tod

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Sean Flynt
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PostPosted: Wed 11 Mar, 2009 1:00 pm    Post subject:         Reply with quote

Beautiful, as usual!

A technical question: What happens if the hooks slip off the sting near full draw? Does the friction make that impossible? If they slip off, would the tension on the windlass cords be sufficient to whip them back into the bowman's face or upper torso?

-Sean

"Everywhere I have searched for peace and nowhere found it, except in a corner with a book"- Thomas a Kempis (d. 1471)
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Leo Todeschini
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PostPosted: Wed 11 Mar, 2009 1:09 pm    Post subject:         Reply with quote

Thanks for the compliment Sean.

Quote:
Sean Flynt wrote
A technical question: What happens if the hooks slip off the sting near full draw? Does the friction make that impossible? If they slip off, would the tension on the windlass cords be sufficient to whip them back into the bowman's face or upper torso?


The hooks do look pretty precarious in that picture, but the pulleys are not under tension and so the they are resting the cross bar on the stock. When you start to wind and the tension comes on, the pulleys lift off the stock and the hooks engage much better.

I suspect that there would not be enough spring in the cords to do that, but even if there were, the bow actually leans away from you slightly as you outstretch your arms to wind the cranks, so they would go upward and in fact slightly away from your face.

Tod

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Jean Thibodeau




PostPosted: Wed 11 Mar, 2009 5:52 pm    Post subject:         Reply with quote

Very nice and interesting. Cool

With the time needed to unwind the crank and get it ready for use again being more time consuming than actually readying the crossbow for shooting I think pairing two crowbowmen with one shooting and the other doing loading one could maintain a very good rate of fire.

Oh, just my theory but I think one should use the heaviest possible bolts to get as much efficiency out of the power of a heavy crossbow on the theory that no matter how powerful the crossbow the maximum speed of the bolt can only be lower than the speed of the bow string if the crossbow was dry fired ( Although one wouldn't dry fire to avoid damaging the prod ).

Now using numbers just pulled out of the air just to illustrate my point the actual numbers could be very different:

1) Lets say that with no bolt the bow string would move at 200 ft/sec.
2) With a very light bolt 1x the bow string would accelerate the bolt to 195 ft/sec.
3) A bolt of 2x might still end up at 190 ft/sec and at close to twice the momentum.
4) The most efficient bolt would be the one giving the most momentum and this might be 8X the mass ( guess) even if the speed would be down to 150 ft/sec. this would be the optimum weight of bolt that would benefit the most from the stored power in the prod.

Anyway the whole point I'm trying to make is that I think these heavy crowbows only show their true power if they use very heavy bolts and that the use of light bolts makes less efficient use of the total stored energy.

You can easily give up your freedom. You have to fight hard to get it back!
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Leo Todeschini
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PostPosted: Thu 12 Mar, 2009 12:05 am    Post subject:         Reply with quote

In the 30 seconds I have before running the kids to school, Jean I suspect you are broadly right.

My impression of steel bowed crossbows is that they are not very quick, but they are good at shifting heavy weights compared to a modern composite. However I have had most experience with bows between 250-350lb and I can tell you for sure that a doubling of the bolt weight will provide a very steep decline in performance.

Tod

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Steven H




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PostPosted: Thu 12 Mar, 2009 6:00 am    Post subject:         Reply with quote

Very nifty! Thanks for showing off and sharing your experiences using it.

Cheers,
Steven

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Paul Kenworthy




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PostPosted: Thu 12 Mar, 2009 6:18 am    Post subject:         Reply with quote

Leo,

Tres cool.

I've been playing with a friend's crossbow that uses a cranequin and my experience is that the slowest part of loading is taking the cranequin on and off the bow. There are lots of motions:

1. Unhook cranequin from belt
2. Slide loop over stock of cross bow making sure to align the hooks with the string.
3. Unwind crank until hooks are past string
4. Wind crank until string engages tumbler
5. Unwind crank until hooks disengage string
6. Slide crank up and off stock
7. Hook cranequin onto belt

Do you have to take the windlass off the bow to shoot it?

Also, is there any way to figure out the initial velocity of the bolt? (muzzle velocity, if you will) Kinetic energy goes up with the square of the velocity, so velocity is what it's all about in kinetic weapons.

Best Regards,

Paul
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Sean Flynt
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PostPosted: Thu 12 Mar, 2009 6:45 am    Post subject:         Reply with quote

You know, of course, that you now need one of these....:

http://www.myArmoury.com/talk/viewtopic.php?t...ght=pavise

Big Grin

-Sean

"Everywhere I have searched for peace and nowhere found it, except in a corner with a book"- Thomas a Kempis (d. 1471)
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Leo Todeschini
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PostPosted: Thu 12 Mar, 2009 1:40 pm    Post subject:         Reply with quote

Quote:
Sean Flynt wrote
You know, of course, that you now need one of these....:


That has been on the 'to do' list for so long I must have been in nappies.............

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Glennan Carnie




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PostPosted: Thu 12 Mar, 2009 2:02 pm    Post subject:         Reply with quote

That's VERY nice, Tod.

Almost nice enough to tempt me away from my bow.

Almost. But not quite. (I REALLY like my bows!)



Looking forward to seeing it in the flesh on Saturday
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Gary Teuscher




PostPosted: Thu 12 Mar, 2009 9:43 pm    Post subject:         Reply with quote

Cool stuff Leo, you almost never see that weight of a crossbow in replicas.

I'd guess a 2500 grain or so might give optimum energy on impact, exit velocity maybe 175 fps or so.
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Ben C.





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PostPosted: Thu 12 Mar, 2009 11:17 pm    Post subject:         Reply with quote

beautiful work as always. I'd love to see a video of it in action.
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