Heavy hitting crossbow

First topic message reminder :

Hello From SW Florida USA, been reading this forum for a few days now an love it.

The reason I came here is I'm trying to plan a crossbow build that will give me the best possible hard target penetration.

Currently I'm using a 150lb barnett (fiberglass prod no cams) and a 60lb bear (modern split limb compound) with modern "big game" fiberglass bowfishing arrows and the power/penetration is completely unsuitable, my arrows are bouncing off.

What I'm hunting is alligators (from a boat with license/tags) and they have extremely heavy scales over most of their back. Now the proper way to shoot one is to get a broadside shot and hit them low in the neck/side where the skin is soft but this isn't always possible and many times the animal dives before I can get a shot like that.

So I want to build or modify a crossbow that will penetrate about half an inch of solid bone scale on the head or back of an alligator. The normal shot is 3-8 meters so it doesn't need to shoot very flat just penetrate extremely well.

I'm open to about any design from the oldest to newest even a torsion or rubber powered bow and I don't mind spending some time or money to build what I need. Draw weight can be about anything and the bow can be about any size or weight however it must be managable enough to fire it handheld, mounting it on the boat like a whaling harpoon is not legal.

I have a basic shop with a lathe, mill, grinders, welders and normal woodworking tools saws/drills/routers.

Thanks Ken I just picked up two six footers for testing, only $14.88. As soon as I asked about thicker tension rods the guy asked if I was building a crossbow, I said yes and he got all excited and asked me about 10 times to bring it in and show him when finished. Nice to see not everyone freaks out a the mere mention of building a ""weapon"".

I've also been playing with the $5/50  hack saw blades from harbor freight, they are surprisingly good springs if you weld a few together to stiffen them up. I broke a couple and welded a stairstepped prod out of 4 blades total and it's perhaps 20lbs with a 4" powerstroke. Obviously not a useful prod but I really do think you could build a serious crossbow out of these blades if you had enough of them and a good understanding how to lay them out and weld them...HF is out at the moment though.

Yeah, I need to take my finished bow over to show him.  If you build a "multiple laminate" prod from metal strips you don't want to weld the ends together.  The laminations need to slide against each other.  The multiple lams is what 'stiffens them up'.  If you search here under Chinese or Ming you'll find some information on multiple lamination Chinese crossbows from 1500+ years ago using bamboo laminations.  The principles of construction still apply today regardless of whether you use steel, fiberglass or bamboo

These hacksaw blades just aren't thick enough (.024") to make useful springs if you simply stack them, they must be welded together. Two blades welded together are much, much stiffer than two blades stacked and this stiffness is required with these flimsy blades otherwise they just aren't strong enough to get out of their own way.

Now with the .250 fiberglass it works brilliantly simply stacked up and and if I can figure out a way to get 300lbs from a single stack I think four stacks compounded down to 700lb X 20" would be perfect.

I'm having much less luck with the 5160 spring steel, nobody locally seems interested in ordering me a 20' stick so I'll have go abroad and pay UPS shipping if I use steel.

I think if you use a 28", 24", 20" you'll exceed 300 Lbs.  I've got about 250# @ 13" using a 28" and 20".  I used heat-shrink tuning to hold the pieces together, sawed pin-nocks in the 28" piece, and reinforced them with a bit of epoxy putty.

Is there a reason that your longest piece is 28" rather than 32, 36 or 40"? Also how do you decide on 28, 24 and 20 rather than say 28, 20 and 12?

Also is the heat shrink meant to keep the limbs in contact with each other during full draw or do they develope a gap?

I'm not doubting your methods just trying to understand them better, I already built a simple test bow with your 28/20 setup and I'm amazed at how much weapon you can build for $6-7 in materials.

Truth be told I was following another member here who used the 28" max length.  No reason you couldn't use 30" but too much longer would IMHO start to get cumbersomely wide.  Using 4" different lengths gives you 2" on either side and gives the prod a more C shape.  Staggering the length otherwise can give you a more efficient shape, perhaps.  

I was actually trying to reduce the draw weight.  So I trimmed the 24" lamination back to 20", and removed the third lamination entirely, so that I could cock the bow without giving myself a hernia or a ruptured shoulder.  Initially I was a bit leery, thinking that the tension of the string on the single lam might cause it to kink and snap at the point where the two lams meet.  No such problems.

The heat shrink just helps the lams stay in alignment, not necessarily contact with each other.  They will develop gaps here and there, and that's OK.  The lams do need to slide on each other, not be rigidly fixed.  At least that's the way the Chinese did it, and the concept worked for them... If the lams are rigidly fixed you get too strong of pull too quickly.  By sliding, the draw is smoother and easier.

The Chinese developed this technology more than 800 years ago. All we're doing is updating the material from bamboo slats to fiberglass bar.  

Here's a link to an ATARN article about a Chinese 'build along' for the bamboo version of this kind of prod.  Stephen Selby's comments and the illustrations help you understand the relative dimensions
http://www.atarn.net/images/ming_xbow/ming_xbow.htm


It is rather interesting just how much prod you can create for a few bucks worth of tension bar.  Traditionalists can even cover the fiberglass to make it look more authentic.

Geezer here concerning the 28 inch prod length.  In fact, 28 inches is a fair length for medieval prods... I've seen them as short as 21 inches in steel and as long as 36 inches.  The average seems to run around 30 inches, but in fact the reason we have 28 inch steel prods from Alchem, Slobows, and Darkwood, is because sheet Aluminum comes in 48 X 144 inch sheets.  That cuts down conveniently to 24 X 28.8 inch plates, which are reasonably easy to handle, and avoid wastage.  So back in 1982 or so, when I started making my own aluminum prods, I used a pattern from the old Jayhawk co. and THEY made their prods 28 inches wide, mostly to get the most prods out of a sheet.  So with careful work, you could get 12 or 13   28.8 X 1.75 inch prods out of a 24 X 28.8 slab of aluminum alloy.  In fact, steel comes in sheets the same size, so economy of materials still rules the business.  There's no reason you can't make prods longer or shorter, but if one is actually in the Business of making prods, 28 inches comes out great for metal prods, and indeed that's a reasonable length to make them.  30 inches might be more common on full-size medieval bows... I dunno, having never made an exhaustive study.  But in case anybody wondered, the standard 28 inch prod is an artifact caused by the sizes of industrial sheets of metal.  Funny ol' world, ain't it?  Geezer.

Thanx for that bit of history Geezer.  I appreciate that.  I was just going along with the crowd, as it were.

Kenh:  Still 28 inches is a perfectly reasonable length to make prods for 'field' bows.  Siege bows would have longer ones, horse-bows often get down to the 24 inch range, and some of the little Padre Island type Spanish bows are much smaller.... the original Padre Island prods I examined at the museum in Corpus Christi Texas, measured @ 21 inches.  They might have been a quarter-inch longer at one time, but not much... the rolled ends were still intact, if rather eroded.  So yeah, 28 inches is fine for the sort of bow one might carry in the field, to hunt or discourage poachers.  I just thought folks would be interested to know that the precise 28 inch size was dictated by modern economics... then again, Medieval prods may have been dictated by forge-size or limitations of hand-forging large prods up out of fairly small bits of iron/steel.  Like I said, it's a funny old world.  Geezer.