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Paul r s
08-02-2015, 05:13 PM
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
08-02-2015, 05:16 PM
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
08-02-2015, 05:30 PM
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
08-02-2015, 05:31 PM
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
08-02-2015, 05:31 PM
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
08-02-2015, 06:00 PM
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
08-02-2015, 06:06 PM
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
08-02-2015, 06:07 PM
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
08-02-2015, 06:17 PM
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
08-02-2015, 06:25 PM
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.

Barrygoss
08-02-2015, 07:03 PM
Lol. I can remember an evening of hanging a cuda 650 off the side of weight watcher, repeatedly adding a bit/taking a bit off either nose and tail to get perfect trim.
Only way to do it right.
Luckily Bob Cooper did my Gavin's and I have a drilled length of stainless to go from fresh to salt. (Plus each battery is marked as to its weight and where the weight is ;) )

Did you get a pass for Sunday? ;)

B

Steve Clark
08-02-2015, 07:08 PM
I'm not familiar with Sierra's. I have a Suex that just works out of the box.

For scooter weighting, there are three things to consider, in order of importance :

1. Overall buoyancy. This is a balance of volume vs. weight. The volume of the scooter is pretty much fixed (unlike lots of diving things). Some scooters with face sealing o-rings will compress slightly at depth, reducing volume and making them less buoyant. You've correctly identified that by putting the lead on the outside, you increase the volume as well as the weight. It's better to try and sort it with the lead inside, but this is obviously laborious. You'll also need to do the same exercise to balance it in the sea. Salt water is heavier and provides more lift for a given volume. You can expect to add a fair bit of extra lead for sea weighting (~3% of the scooter weight). My Suex has a big lump of stainless you add for salt.

If you want a very rough idea how much lead to add, just float your scooter in a lake and see how much plastic is above the surface. You need to add about 10% of this in lead volume inside to make it neutral.

2. Fore/aft trim. Once the overall weighting is sorted, you need to move it forward and backwards (or vary the amounts in fixed containers) to set the trim. I'm no expert on this, but flat is nice.

3. Torque-control weighting. This is the complicated one. When a scooter is working, the prop blades generate both lift and drag. The lift provides thrust, the drag imposes a torque on the scooter body and causes it to rotate (similar to why helicopters have tail rotors). Most scooters have an offset battery configuration that works like a pendulum to counteract this. Well trimmed scooters will rotate about 90deg until they balance and sit nice so you can drive them with one finger. If you are having a problem with this, you need to change where the lead is relative to the axis of the scooter. Simplest to think of the front of the scooter as a clock-face and move the lead around so it's over a different number, until it balances properly.

I had no idea about this stuff until I actually had a scooter. If scootering is new to you, and I don't mean to be patronising if it's not, it can be hard to figure out what's a scooter problem and what might be something else (tow-cord length, personal trim, arm-bend angle, grip on the handle etc.). I had a perfect trimmed scooter on day 1 and it took a number of dives to get comfortable. Now it runs and turns just using my index finger on the trigger. The first few dives I was wrestling with it and got out knackered. I've not changed the weighting at all.

Steve

Paul r s
08-02-2015, 07:08 PM
My pass has been sorted for weeks. Waiting on you and your dodgy finger.


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nickb
08-02-2015, 07:29 PM
I'm not familiar with Sierra's. I have a Suex that just works out of the box.The Suex is a bit more modern and has a great weighting solution. The Sierra is a bit more 'industrial' with pouches fore & aft that contain lead shot that needs to be tailored for each scooter/battery combination. The pouches have velcro backs that stick to velcro patches stuck inside the scooter body at the bottom (when the handle is at about the 1 o'clock position).

I understand the new Dive Xtra scooter has a similar system to the Suex.

dimitri
08-02-2015, 10:27 PM
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 ?

<hypothesis>
Can it be that while the scooter was nose-heavy, this dipping was counterbalancing the positive buoyancy so when the trim was corrected the positive buoyancy became evident?
</hypothesis>