Propelloring question

[TheBronzeAge]

Actually, two propellering questions:

1) Why does every single bow I make start to propeller?  I have this idea that maybe the grain isn't quite straight in the boards I got and it's twisting to follow the true grain.  Would this make sense?

2) More importantly, do I need to fix it?  What are the issues with propellering, other than looks?  Does it pose a breakage risk?  Is there some degree of twist that's acceptable?

Thanks.

[Pat B]

Generally selfbows can handle some propellar twist without any problems.
 If the prop twist develops while you are tillering it is probably you removing more wood from one side of each limb than the other. It is a common problem especially if you are using a vice on a work table to tiller. You tend to favor one side over the other when using this setup and when you flip the bow you get the opposite side of the other limb so it prop twists when under stress.

[TheBronzeAge]

Any general guidelines on how much twist is too much?  One limb is out at something like 45 degrees near the tip.  That just seems extreme.  The other one is more like 20.

I'd been wondering if I'd shaved it down unevenly, but I've done two things that make me wonder if there isn't another issue.  First, I rearranged my set up so I'm working on this bow from both sides - it sticks out into the middle of my work space.  Second, I've tried taking wood off unevenly to correct the propeller and that's not worked really well.  I'd imagine that if unevenness is behind the twist that using the drawknife at a deliberate angle should have rapid effect.

What's your preferred method of holding a bow in place to avoid these issues?

[Pat B]

I use my hand(thumb and fingers) as a thickness guage as I work. You can feel the slightest changes in thickness more so than see them.
  Think about it...if the wood on one side of the limb is thicker it will take more energy to bend it so the limb will twist to the weaker side(bends easier) when drawn.
 I use a bench vice for the rough work like removing bark and sapwood. I also do some shaping with the vise. For tillering I use a shaving horse(bow bench) so I'm working right over the limb I'm scraping.
  Have you looked at the limbs at full draw or only at brace? With a wood bow, only full draw will show you good tiller...twisting included. You can tiller twist out of a limb by removing wood from the belly of the stiff side of the limb as the draw increases.

[Art B]

Are you using bias ring cut boards for your bows?

[George Tsoukalas]

Choose straight grained board tip to tip. More likely uneven wood removal is the culprit and we tend to favor the same side in each limb. Visually inspect and use your hands as Pat said. Info on boards on my site.  Jawge
   http://georgeandjoni.home.comcast.net/~georgeandjoni/

[4est trekker]

Here's a thread that deals with tiller-induced twist.  Perhaps you'll find it helpful as well.  

 http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=125;t=006654

[TheBronzeAge]

I'd have to check on the cut of the boards.  This has happened on three different pieces of wood, and one of the bows now belongs to a friend (after I fixed the twist).

Thanks for the advice, though.  I'll try to make some changes next time I have a chance to work on the bow.

[TheBronzeAge]

In case anyone is wondering, the bow in question died a violent death.  I wanted it to draw 28".  At 26" it was still propellering a bit and decided to end its own life by snapping 2/3 of the way up the more propellered limb.

A post-mortem revealed that the wood fibers had torn straight out from the back, leaving ridges and valleys running down the arm, and that the "strong" side of the bow arm was half the width of the "weak" side.  So I really have no idea what was going on and have recently acquired a different batch of wood to work with.