Thought this might be useful for some people in the future.

The original rubber on my speargun was weathered and needed to be replaced. Since my local dive shop stocked a different diameter rubber I had to figure out what length of new rubber to rig without having to use trial and error.

This is the formula I used with my case spelt out in full to make it easy to follow.

Length(new thinner rubber) = [radius(thin)^2/radius(thick)^2] x Length(old thicker rubber)

In my case when changing to a thinner rubber:

Length(new thinner rubber) = [(8x8)/(9x9)] x 470
= 64/81 x 470
= 371mm

This formula assumes the same elastic properties for both of the rubbers. This elastic properties probably won't be identical but they will be close enough.

Don't forget to make your length measurements from constrictor knot to constrictor knot. My old rubber measured 470 from knot to knot. Then when I cut my new rubber i cut it at 391mm to allow 10mm at each end for the new knots.

If you are adding a thicker rubber use the following formula instead

Length(new thicker rubber) = [radius(thick)^2/radius(thin)^2] x Length(old thinner rubber)

Ray

.....or you could just check out a website like Legendary that lists all rubber lengths/diameters for different gun lengths

_________________
bluedevilspear@hotmail.com

What tool do you use to get the radius of the rubber?
+ or - how many microns? or is that mm?
I love that you measured it to 391mm. Good humour!

Double A batteries are about 15mm in diameter so I was gonna estimate the dimensions by eye but thought that it wouldn't be accurate enough so I went to get the tape measure. But then I remembered what the dive shop owner sold me and used those numbers.