Lead placement

[Paul r s]

What makes something sink or float ? If it weighs less then the water is displaces it'll float.

So today was my 1st outing with a new toy (scooter) but I needs to get its weighting correct for neutral buoyancy and the leads placement for horizontal trim.
Played about before the dive with the weight, thought I'd corrected it to getting it as close to buoyant at 1ft per min.

Off we go and I discover it's now nose heavy so as soon as I release the trigger down the nose goes, made for a (slightly) less then enjoyable dive.

So I tried removing all the lead from inside the scooter and placement it externally where I can adjust its amount and location.

Now it's more buoyant then before and requiring more lead to sink.

Am I correct in thinking that when it was within the scooter it had more of a negative effect, as no more water was being displaced as before ?

[timthefish]

No expert but the space where the lead was is now air so will require more lead externally to sink. That's my guess

[nickb]

The location for the trim bags is fixed. One in the nose and one at the business end. The overall bouyancy is easy enough to sort out and then the trim is sorted by moving shot from the front bag to the rear or vice versa.

You say the nose was dipping but how was the buoyancy? For open water, the scoot needs to be ever so slightly positive. Once you've achieved that, then work on the fore-aft trim. It should be perfectly flat in the water or maybe just a tad bouyant at the nose.

[Janos]

Am I correct in thinking that when it was within the scooter it had more of a negative effect, as no more water was being displaced as before ?

Yes. The scooter needs enough lead so that it weighs the same as the water it displaces.
By putting the lead on the outside you've made the scooter displace more water, so you need to add a bit more lead.

Janos

PS - This is the main reason why aluminium cylinders are so floaty - you need thicker walls and so the cylinder displaces more water.

[MarkP]

Am I correct in thinking that when it was within the scooter it had more of a negative effect, as no more water was being displaced as before ?

Yes, although the difference with something as massive as lead is smaller than with most things. It will become less negative by the weight of the water it displaces once outside.

[nickb]

Paul, you'll find that just a few grammes of lead make all the difference here. So putting bags on the outside will not enable you to get it dead right.

Ignore that bloke on Facebook that's hanging double-enders off the tow-ring, that's OK for getting a ballpark, but nowhere near granular enough for getting it spot-on. Even a small double-ender weighs 40g or so.

My salt trim bags weigh 314g at the nose and 216g at the back. In fresh I'm using 116 front and 183 back, but I'm still working on this as I don't dive fresh very often.

These numbers won't work for you, as my battery is the 16Ah version so heavier than yours but they will reveal that the salt weighting is around 1.4% more than the fresh.

[Paul r s]

Looks like it'll be more trial and error next week......... If only next door would let me use there koi pond if I promise not to pull the trigger.


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[nickb]

Also, as you have two batteries, you'll need to make sure they are exactly the same weight or you'll need different trim bags for each.

[nigel hewitt]

Is one bit sealed and another not?
I don't know your scooter.

1Kg of Lead sealed within the airtight part of the scooter gives you 1Kg of down force.
1Kg of Lead on the outside in the water gives you 0.91Kg of down force.

I'm not sure it's enough to notice.
My scooter was very picky.

[nickb]

Is one bit sealed and another not?
I don't know your scooter.

1Kg of Lead sealed within the airtight part of the scooter gives you 1Kg of down force.
1Kg of Lead on the outside in the water gives you 0.91Kg of down force.

I'm not sure it's enough to notice.
My scooter was very picky.

It's a Sierra and the 30/40g or so difference by having the lead outside of the scoot would make a huge difference.