Implications of propeller twist

[UserNameTaken]

Can somebody explain what the implications of extreme propeller twist are? I've noticed several people making bows out of some pretty twisty wood, and I would like to know what that does to an arrow when it's released. Is it purely a cosmetic issue?

[JW_Halverson]

I think there is some differences in definition of propellor twist.  The two most common are as follows:

1)  limbs that twist/rotate along the longitudinal axis as the bow is drawn.  A result of one side of the limb having more resistance than the other side, result of bad tiller.

II)  A limb profile when seen from the front or back forms a gentle S shape.  A result of the grain of the wood, unrelated to tiller.

If you have number 1, bad tiller results in reduced cast in the arrow in all cases and often handshock or a bow that is inconsistent in where it throws an arrow despite all efforts to group a quiver of arrows.  If you have Roman numeral II, it's just coooool!

[UserNameTaken]

I'm referring to a split log that looks like a double helix; not a tillering error.

JW, it's not going to cause the bow to turn in my hand or anything? I think I remember reading something--probably in the TTBB--about avoiding trees with bark that spirals up the tree.

[JW_Halverson]

I'll give you this....it's a lot easier tillering a set of limbs that are not trying to illustrate Watson and Crick's awesome discovery of the double helix!  It can be done however.

I prefer to use a heat gun on the stave when it has been reduced to about 1/2 inch thick and the limbs have been brought to close to their final shape.  There is a whale of a lot of twist that can be taken out of the limbs at that point if you are patient.  Heat 8-10 inches of limb at a time and untwist.  Let that limb rest, flip it around and heat another short section and take a little more out.  Let the whole thing rest overnite and repeat until you got the whole thing looking right. 

I have fixed staves with well over 90 degrees of twist this way.  If you chose a design with longer and narrower limbs you will have better luck, in my opinion. 

Good luck!

[UserNameTaken]

JW, but you're saying that I could just leave the propeller twist alone if I wanted to, and it wouldn't interfere with the functionality of the bow? I'm fine with a twisty bow, but would also like it to be functional. I have no interest in making a bow that doesn't shoot right.

I know that propeller twist can be removed, but that's starting to sound an awful lot like work. = ) I would love to just leave it twisted, if that's a possibility.

[JW_Halverson]

It can be done, yes.  In my opinion you really gotta know your chops to tiller a twisted bow.  If the twist is less than 20 degrees, I would tiller it considering my own experience and skills.  The tough part will be knowing if your tillering is causing the limbs to twist more or even untwist...either case is bad tillering and could shorten the life of the bow and/or reduce the efficiency and case of the arrow. 

Search a few of the posts on here for pics of people taking prop twist out of bowstaves.  Some of the best help will be leaving the limbs a couple inches long and using a pipe wrench on the limb while you are heating.  Hanging a three or four lb weight from the end of the pipe wrench creates steady torque.  Then as you heat, moving the heat gun back and forth slowly to avoid scorching, you will see the wrench turn when the wood softens to the right point.  Torque it a little more by hand and hold it or brace it untilit cools for an hour or so.  Trust me, it's easier than tillering a corkscrew!

[Del the cat]

You can make a bow with twisted limbs, upto about 45 degrees is do-able. If you tiller it with extreme care it will draw straight and shoot perfectly.
I'm not saying it's easy, just possible. It's probably easier to take some of the twist out with heat.
The last bow I made 'Twister' is one of the sweetest shooting, best gouping and fastest (for it's draw weight) bows I've made. Here is a pic of it in the early stages, with thin bits of cane taped on to show the twist which is mostly in one limb, but is tillered so that the twist is shared by both limbs on the final bow.

I've seen bows with more twist, but I don't know how they shot.
Here's the link to the finished bow http://www.primitivearcher.com/smf/index.php/topic,28324.msg376265.html#msg376265
Del

[UserNameTaken]

Do you have to modify how you tiller when a limb is twisted, or is the challenge to not deviate from normal tiller, because of the twist.

What I'm asking is: if I took a straight stave, tillered it correctly, and then twisted it, would that still be correct tiller?

[JW_Halverson]

The challenge is to not overtiller one edge of a limb over the other and complicate the twist. 

Having never induced twist INTO a limb after tillering, I cannot say how that would change how the bow draws or shoots.  Makes me wonder, though. 

Try taping straws on the limbs like Del did in the above photo and post them here.  I'd like to see how much twist this Chubby Checker stave is doing!

[UserNameTaken]

Okay, so you're not changing how you tiller. The twist just makes it difficult to keep thickness even. Got it. This is one limb. The other limb is about the same.

http://i1133.photobucket.com/albums/m599/biggorillasuit/IMG_0283.jpg

[johnston]

Really think it would be easier to just untwist this one after looking at the pic.

What kind of bow you making?

Lane

[UserNameTaken]

Lane, there's no plan. Maybe I'll try a twisty pyramid. That would be cool. It's unlikely that I'm going to straighten it though. I don't really enjoy steaming wood. I can do it, but I don't enjoy it. I really wish my ocean spray stockpile could somehow dry faster, so I could take a break from the vine maple. It always needs straightening.