Trim weights with a more negatively buoyant tank

[RIHappyDiver]

I just purchased a Faber M100DVB tank which is rated -6.69# buoyancy empty and -14.11 full. I’ve been diving Faber LP85’s rated at 0 empty and -6.7 full with 2# in each back trim pocket and 18-20# ditchable in the front. Don’t know if it’s important, but I usually dive 7 mm. Obviously, I will need to cut back on lead when I start diving the 100, but do I remove the trim weights completely since I have the decrease in buoyancy on my back? I know this is probably one of those “ymmv” situations that diving the setup will tell me more, just thought I’d try to get some input from people who have experience with this situation ahead of time.

Thanks,

Erik

 

[Beau640]

The whole point of trim weights is to use a low amount of weight in a specific area to make you "trim out" - be horizontal when neutrally buoyant. Every tank/setup is going to be different in regards to how much and where you need those trim weights. If this tank is a different length/width/etc it will trim out differently than your last one.

The best thing you can do is to just start from square one again. Dive the new tank without trim weights, and then add trim weights as needed depending on how this new tank dives. As a starting point, given this tank will be 6lb more negative when empty compared to your last tank, ditch your 4lbs of trim, ditch 6lbs in the front, and see how it dives (this is assuming your trim weight was actually trim weight and not weight needed to get your neutrally buoyant on an empty tank). The important thing to know is how much weight you truly need to be neutrally buoyant. Trim weight should only be minimal amounts put in specific areas to help you stay horizontal.

 

[stuartv]

I would start by taking 8# off my front and then test and tweak from there.

 

[Bob DBF]

When I change tanks, I just adjust the weight for the buoyancy difference on my belt and adjust the trin weights if necessary. Most of the time the trim is the same, but that would depend on the tank and how one wears it on their rig.

I use a notebook to keep track of different configurations I use so I don't have to reinvent the wheel each time.


Bob

 

[claymore]

Having to much weight on your back can cause stability problems because there is to much weight above your center of gravity causing you to want to turn turtle. I would start by loosing the rear trim weights and what ever else weight it takes to equal the buoyancy change is between the two tanks. Go from there to figure out what weight needs to be where. It is kind of trial and error for a while to find the right amount of weight and where it needs to be.

 

[The Chairman]

I get nervous with anything over 10 pounds of ditchable weight. I've done it, but I don't like it. I prefer 4 pounds ditchable or less. I don't want to be a cork.

 

[RIHappyDiver]

Gentlemen, thank you for all the advise, it’s given me some good ideas on where to start the process.
@Bob DBF, I like the notebook idea. I keep that info in my log, but it’s difficult to find it in there sometimes (it’s the same log I started with in 1979. A 15 year period of not diving has left a lot of it unfilled).
@The Chairman, I have a couple of pockets on my waist belt that I could move a fair amount of weight to, which would get my ditchable down to a much smaller amount. Sounds like a good thing.
Now, to go diving and experimenting.

Erik

 

[Colliam7]

I just purchased a Faber M100DVB tank which is rated -6.69# buoyancy empty and -14.11 full. I’ve been diving Faber LP85’s rated at 0 empty and -6.7 full with 2# in each back trim pocket and 18-20# ditchable in the front. Don’t know if it’s important, but I usually dive 7 mm. Obviously, I will need to cut back on lead when I start diving the 100, but do I remove the trim weights completely since I have the decrease in buoyancy on my back?

My general bias is to move as much weight as practical, from the waist (either on a belt or in weight-integrated pockets) to a position higher on the torso (with trim weight pockets and/or a negative steel cylinder and/or a steel backplate). That way, my center of lift - the lungs - and my center of weight are more directly aligned. Now, I dive a 5mm, not a 7mm, so the floatiness of the legs with a 7mm might affect that approach just a wee bit. Nonetheless, if you are wearing 18-20 lbs on your waist, my suggestion would be to keep the 4lbs in the trim pockets, and reduce the weight on your waist by the difference in empty cylinder buoyancy as a starting point.