norway spruce board bow

[joachimM]

I've never built a board bow before, because where I live is board desert and I have plenty of access to normal bow wood. About the only boards you can find around here are either fir (norway spruce), douglas fir (mostly only large beams) or meranti. But you can find straight ringed fir. So I stumbled upon an plank of 4.4 cm x 1.2 cm x 2.1 m and thought what the heck, let's just try this out.
Norway spruce is about the stiffest wood (and least elastic) for its mass (SG only 0.32!), only bested by sitka spruce and W Hemlock (which are a tiny bit denser). Not what you'd call ideal bow wood. 

Shortened it to 66", glued a 7" handle + fades section (a slat of the same plank), and cut it into a pyramid shape (4.4 cm at fades to 1 cm nocks), gently heat treated the belly and voila! Level thickness of 1.2 cm throughout the limbs, automatically tillered.
Draws smoothly to 28" at 35#, has 3/4" of set after shooting and since it has virtually no mass (335 grams) it shoots without any hand shock and very true to the target. Speed isn't terrific, about 145 fps from a 370 gr arrow, but it's a sweet shooter for sure.
The pics here just show the rough bow, just testing if these boards could make an OK bow. Seems they do.

[willie]

nice experiment

to what do you attribute the moderate arrow speed?

the top pic has a pinkish coloration that is more common with doug fir than spruce, here in the us, Is norway spruce reddish?

[turtle]

Did you add any reflex when you heat treated?

[aaron]

" Speed isn't terrific, about 145 fps from a 370 gr arrow"
I'd say that's pretty good speed. you're over 10gpp on arrow weight. I think an average wood bow typically shoots 100 fps + draw weight. So a 35lb bow shooting 135 is average. you are getting more than that at over 10gpp- good job!

[joachimM]

The pic is pinkish because of the incandescent light. The douglas we have is much darker still.
No induced reflex. I tried to give it 2", but it sprang back immediately. Weird. With a 500 gr arrow the expected speed would be 135 fps, not with a 370 gr arrow.

I might try to tweak it a bit more, or make another one from a 5.6 cm wide board, that should give a 45#.

[Del the cat]

Great experiment That speeds not bad.
Del

[k-hat]

That's cool, reminds me a little of the pine bow made in one of the TBB's.  I'm going to hypothesise that the arrow speed is due to the weight of the bow.  The bow being so light (11 oz.), means the outer 3rd limb mass (esp on a pyramid) is next to nothing.  That makes the  bow VERY sensitive to arrow mass.  The speed of the arrow depends more on the recovery speed of the limb mass + arrow mass than on arrow mass alone.   The smaller proportion of mass the arrow makes, the less the arrow will slow the bow down.  I bet if you used an ultralight arrow, you'd get closer to expected speeds.  Of course, I could be totally wrong here.  I wonder what Badgers mass calc would predict for your bow.

Thanks for sharing your experiment with us, I look forward to seeing the heavier bow.

[joachimM]

Badger's bow mass calculator indicates a bow mass of 439 g, about 100 g more than what it actually is. But I guess that's because I also have a very light handle.
The bow is not about to break and could probably be drawn to 31-32" but at the expense of a lot more set: even though it's so thin (12 mm), this is necessary because Norway spruce is so stiff (high modulus of elasticity for its light density) meaning it should be thinner than regular bow woods.

[make-n-break]

Cool! I've read that Douglas fir can make a bow, and we have these big straight/tight grained posts at the lumber yard here that I've been wanting to try. Your bow is inspiration to give it a shot. I wonder if firs/spruce could handle 45+- pounds or if it's better suited to lighter weights.

[PlanB]

That's excellent, Joachim! I'm sure your 5.6 cm version will give you the draw weight you want. I also think the speed is quite reasonable for the draw weight and arrow weight you mentioned

I've been planning for awhile to build an Eastern Hemlock bow and sawed some timber for it last fall. Unfortunately finding a long enough straight-grained piece, free of knots has been difficult, and the few I pulled out of the stack had started to check. I will saw some more shortly and see if i can do a better job selecting and drying.

The biggest problem I've found with EH trees is that they are extremely tolerant of low light. So they tend to keep their lower limbs alive without dropping them even in a dense canopy forest. This means big knots and many of them. I may try branch wood because EH tends to have very long lower branches that only fan out at the ends. Seems the branches can be clear wood while the trunk is knotty.

Anyway congratulations on a very nice and interesting bow!

[joachimM]

I found out that I was wrong with my arrow weight. I thought I chronographed a 370 grain arrow (35# bow), but it was in fact a 415 grain arrow. So at 12 grains per pound it did on average 145 fps. With the real 370 gr arrow it shot an average (four shots) of 154 fps, and with a 320 gr arrow I got an average of 175 fps (four shots). A lighter 243 gr arrow didn't shoot faster than the 320 gr arrow, so I guess that's about my dry fire speed.
I'm not displeased with that result 
For comparison, I also shot an elm flatbow with slight RD of same draw weight (35# at 28"), which shot close to exactly the same.
These aren't exactly flight bow speeds, but these aren't flight bows either

Joachim

[Springbuck]

Without a backing, even.  I am impressed!