Hollow Back Bow no.4

[Jan de Bogenman]

Ash selfbow with a hollow back. It has been shot a few dozen times and I expect it to keep up!
The cavity is 11/32" and has been extended over the entire limb.

[Pat B]

Beautiful tiller and I love the back profile. Very nice work. 

[redhillwoods]

Fascinating.

[Jim Davis]

Weight?

[Jan de Bogenman]

Specifications:
Straight pyramid ash selfbow
length: 71" (1,8m)
Width: 2" (5cm)
Width nock: 0,4" (10mm)
Handle and fades: 8" (20 cm)
Handle cross section: 0,9"x1,3" (23x34mm)
Drawweight: 35lb@28"
Set after tillering: less than 1"

Weight 14 oz (398gram)
Speed 10gpp: approx 170fps (linen string 80 grain(5gram))

The aim of this experiment was to find out whether a hollow back can save mass and increase speed.
I think speed is pretty good, but mass is certainly saved.
As for speed, the picture shows the device I use. The finger blocking is on 28" from midd handle.

[DC]

Very nice, this should be telling us something but I'm not sure what it is

[joachimM]

I like your approach to bow making 

[Philipp A]

You have my undivided attention with this bow! The weight you have achieved for a 71"length bow is simply incredible! The feather light weight coupled with the speed makes this a truly an amazing bow. I have to try one myself, you certainly have set the bar very high!

By the way I also like the device you have set up to Chrony your bow. I would love to have you explain the details for it, I am always struggling with repeatability of my Chrony measurements and sometimes they are all over the place.

Again awesome work!!

[Philipp A]

Hi Jan,

Just as a follow up and I have mentioned that on your previous attempts of this design as well, this design has a good example in nature (nature usually gets its designs right) and that is the bird feather. It has a very similar design with two spars on the tension (back) side of the feather that do not join until close to the tip. It also features a slightly rounded belly. I have made a similar bow on a heavily crowned stave but did not nearly go to the extreme you have in your bow (mine had just a 1/4" slot fading towards the tips on each side). I wanted at the time to resemble a turkey feather. I guess the only difference between your design and the feather is that it has an elastic compound filling the spars so it is really a composite structure.

Here are a couple of turkey feather pics that show the resemblance.

[gfugal]

I'm curious if you were to flip a stave around and have the back be the belly. If you followed the growth ring it would be concave due to the curvature of the limb/trunk

[willie]

Very well done, Jan.
Hope you can find time to do a force vs. draw test.
 

[Badger]

  Beautiful job, Tim Baker has recently taken an interest in these style bows. I will see if I can get him to come over and take a look at it.

[willie]

Badger, are you willing to comment on the mass principle given the specs presented?

[Philipp A]

I would love to see Jan's design compared with the HLD design, with the same dimensions, same wood and shape and see what the cast will be and the corresponding draw weight. My intuition is that Jan's design will outshoot the HLD design. I believe the compression is more pronounced on the belly with Jan's design than in HLD and will result in faster cast.

I also think that this design should provide for a super smooth draw without stacking.

Jan: can you describe how it feels to draw the bow? Could you also measure the increase in draw weight over the length of your draw (every 2")?

[selfbow joe]

Very cool looking bow