Hey guys,
Besides being completely confused about how much kilos 13 lbs is I gotta tell you that in my opinion there is no strict formula you can apply, even though I've seen java and flash applets on different sites calculating the "perfect" ballast for you. It depends on too much things and not only the weight (muscules or.. well... fat?), the suit, the depth, the type of hunt, the weather, and whether you had beans for lunch!
It all depends on the individual preference methinks.
For shallow dives you can put more weight for the reasons I already shared, as long as you can get rid of all the weight with one move. I had an argument before with a Spanish guy I think - he was strongly desagreeing with the ankle weights, while I was saying that 0.5 -1 kilo you can't get rid of immediately but after 2 kicks with the heel is not of too much difference for a shallow hunt. He was saying that he never had a problem or felt any need in using the ankle weights or had a way to avoid the problem. That obviously works for him, but for some reason, without the ankle weights even at depths of ~8 meters I can't keep my legs on the bottom! This is not a problem for most types of hunt besides, of course the most applicable technique for our sea - the ambush. You can hear many solutions but you have to find the perfect one for yourselves without compromizing your safty or accountinf for any risk!
Octopus gave probably the best formula I've heard so far about spearfishing (and deserves a piece of karma for it!
): If you can pass by a school of small fish without scaring them - that's a good descent. If you pass too fast - you'd scare everything around - not only the small fish. If you start waving fins and hands to slow down you get the same result. Same sutff happens when you end up on the bottom with a big bang and in a cloud of mud. That's why you need less lead for the deep dives.
And in the shallows you'd scare everyone off, including the mid sized sea eagles
if you start plucking sea weed and pushing rocks around in attempt to stay on the bottom...
How it works for me for the average depths: You only need the weight to pass the point (depth) of the neutral buoyancy of your body with less effort possible. If you have stiffer fins, that's one of the things they help for, the other is to give you a better start from the bottom to the surface. Other than that I don't think there is any need for speed in spearfishing, which logically leads to the conclusion that you need the stiffest fins only when diving extremely deep. But that's my opinion - somebody may say they need to cover greater area. My answer would be that if you know the behaviour of the fish and the area, and you can link that to the current weather, you don't need to cover big areas. But anyway...
After you pass the neutral point you need to watch for the speed. If you descend too fast, then you have no option but to take off some weight and compensate it with efforts on the surface - otherwise you reduce your chances to get fish... The slow descend allows actually saves you much more bottom time than you think: you can check the bottom and guess the most probable route of the fish and you can pick the best ambush spot; you can look and remember where the caves are so you don't waste time in searching; and you can spot the grouper catching sun at the front porch of his cave.
So you must test it for yourself. It usually steals one day off of every season, but there's nothing you can do...
Cheers!
IB