Little L.A big WTC - Ballasting

[david h]

Hello all,

I have a small L.A that I have just completed. Sits nicely in the water however the WTC only just fits inside. The Dia of the WTC is about 90. The L.A is about 100mm. It's totally snug.

I'm having difficulty getting the boat to go from Surface trim to anything more than decks awash. Could this be due to the fact that there is not much free flooding area. The free flooding area is purely at the stern or Bow there's no room for water in the middle (Except the B tank!)

I would love some comment on this?

regards

David Hughes.

[thierryc]

It sounds like your ballast

[color=#0000FF]It sounds like your ballast tank is too small.
You might have to work the other way around]

[anonymous]

Hi David,

I agree,

The less

Hi David,

I agree,

The less volume you have above your water line, the less amount of lifting (then volume) of the ballast tank you need.

Between the WTC and the hull you have only 10mm, so a lot of your wtc is above the water line.

[petn7]

exactly how big is this

exactly how big is this sub your talking about?

[JWLaRue]

Another thought....

How is the WTC

Another thought....

How is the WTC mounted within the hull? (vertically, that is)

Ideally, you would like the top of the WTC to be no higher than the surfaced waterline of the sub. Any part of the WTC that is above the waterline adds parasitic weight to what must be 'lifted' by the ballast tank and it reduces the effectiveness of the ballast tank by whatever volume of the tank is above the waterline.

So......depending on your WTC location (height), you may be able to get more lift by moving the WTC lower......

-hope this helps,

Jeff

[anonymous]

1. Your ballast tank volume

1. Your ballast tank volume is too small.

2. At surfaced trim, a portion of your WTC is surfaced too, making the volume of the ballast tank required to compensate greater still.

Suggestions.

Fortunately the LA 688's have a minimal reserve buoyancy to begin with which we here have to play with. 1. Make a smaller diameter WTC, around 60 to 65 mm diameter (instead of 90mm?) , what ever will allow the WTC to be completely submerged even when surfaced. Make the ballast tank size so if slightly more buoyancy is needed to get your 688 correctly trimmed on the surface, add foam as long as what ever it is stays below the waterline while surfaced.

The volume of the ballast tank should only need to be large enough to compensate for the volume of the model structure itself above the surfaced trim water line that is displaced by water. No other variables. The important word here is 'displaced by'. If your above water surface trim structure is thin and light, say fiberglass.... the volume would be correspondingly less. If the same here have thick resin parts, say the sail or the towed array fairing, the volume of the ballast tank will need to be slightly larger to accommodate. The goal is to compensate only for the surface trimmed structure displacement, no more, no less.

My two cents. Hope it helps.

Steve Reichmuth