Everything an amateur bowyer does to turn a log into a bow throughout the year.
Making bows, longbows and primitive bows with all the tips, tricks and problems.
Tuesday, 1 May 2012
Comparing Two Longbows
I've got two bows finished, 55# and 60#.
Well is the 60# any faster? With some trepidation I shot it through the chronometer and averaged the 6 results getting a respectable 173.7 feet per second (fps) average.
I then strung the 55#, would it be just as fast? Again 6 shots and average, 168.9 fps.
I'm happy with those results and it's a bit of a relief to find the 60# is faster (I shot the same 3 arrows with each bow, 5/16" shaft 100gn point). I took a pic once I'd finished as I happened to get a nice group with the last 3 arrows from the 55#, you can see the reading on the chrono'.
I take the arrow speed divided by the draw weight as a sort of "efficiency" indicator. The 55# bow gives 3.1 whereas the 60# gives 2.9 implying the 60# bow is somehow less "efficient". I think the reason for this is light arrow favouring the lighter bow. If I repeated the test using my 11/32" arrows with the 100 grain points I expect the 55# bow would loose more speed than the 60 and the "efficiency" figures would even out or swing the other way. The arrows used for the test feel fine with the 55#, but the 60# definitely feels happier with the slightly heavier arrows.
Out of interest my best speed/draw weight figures tend to be the primitive style flat bows with 'Twister' my Yew primitive (45#) giving a figure of 3.7 Of course the longbow is designed for delivering energy rather than velocity as is better matched to a heavier arrow.
Figures need treating with caution of course and reducing the weight of the arrow will always give increased speed, but it's about matching the arrow to the bow rather than pure speed, unless you are into flight shooting where you are after maximum range with little interest in smoothness or longevity of the bow.
Explain more? Not sure what to explain really, I put "efficiency" in quotes as it's not really a measure of efficiency as it doesn't take into account the arrow weight, but it gives some sort of comparison.
These 2 posts explain more about velocity/energy/ speed and power. http://bowyersdiary.blogspot.co.uk/2011/11/mass-velocity-and-energy.html http://bowyersdiary.blogspot.co.uk/2011/10/graingram-scales.html
The chronometer has two photo cells which detect as the arrow passes over it. I've stuck thin pieces of cane in to show the active area which I have to shoot over. (The chronometer comes with metal rods which get severely bent when hit by an arrow so the thin cane is better!) I have two100w incandescent lights which hang down above it to give suitable illumination (fluorescent lamps flicker and can confuse it) Sometimes it's tricky to get reliable readings as the arrows can be flexing at short range as they leave the bow, I generally shoot at about 5 paces and take several reading to make sure I'm getting reproducible results. A chronometer is the only way to really tell how a bow is performing, as they can feel quite different and with the 2 bows I tested here it would be hard to tell which was shooting faster with out the chronometer.