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Nov 01, 2004, 04:22 AM
Thread OP

Robbe U47 with Engel tank - CG issue?


I've retro fitted my Robbe U47 with a 750 ml Engel tank and have got to the
stage of having a dry boat that sinks and rises in stationary tests. The boat
also has the bulkheads in the saddle tanks ground out to accomodate inflatable airbags. It has an Accurate Armour tower with all the internal
excess resin milled out to reduce top hamper.

This has been about three years work to date....

My problem currently is that the boat goes down stern first quickly
once below the surface. It requires some stern weight to sit level on the

Being the only model submariner around here there's nobody to compare notes
with. So I'm asking if anyone else with this kind of setup has any suggestions
on how to trim it to work. It seems to me that its really about the centre of
gravity shifting aft.

Comments anyone?



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Nov 01, 2004, 07:52 PM
Registered User
zorrow's Avatar
Perhaps Try A Small Piece Of Closed Cell Foam Taped To The Rear Of Boat And Try It Again. You Will Have To Experiment With Size And Placement Then Can Add To The Interior Once You Find The "sweet Spot"
Nov 02, 2004, 05:15 AM
Thread OP
...well, maybe...

the problem is that the stern is too high in the water when surfaced, then
sinks so fast once submerged.

Nov 02, 2004, 04:05 PM
Crazy bubble-head!
Nemo's Avatar
You need some foam in the area ABOVE the waterline in your stern. Then it's bouyant properties only come into effect when submerging.

The problem with a single Engle piston tank is that you CG wil shift as the tank is filled because it fills from front to back (or back to front) and not bottom to top like most ballast tanks. This shifts your CG forward and back dependant upon the position of your plunger.

It sounds to me like your tank fills from the back forwards. The only way I can really see getting around this is to add another ballast tank that works opposite to this one so that they fill opposite each other and maintain your trim.

DOes your model even out after her tanks are fully "vented", or is she still stern heavy?
Nov 03, 2004, 11:07 AM
Man from Atlantis
Sub culture's Avatar
You can also make a moving weight i.e. the battery inside the pressure hull to counter the piston tank induced shift of C.G.

This is also handy for depth control at low speed (creeping) when hydrovanes aren't very effective.

Nov 03, 2004, 01:59 PM
Thread OP
She's stern heavy with the tank fully flooded.
I'll try the foam this makes a lot sense.

Basically I put weight in the stern to counter the forward position of the batteries.

Moving weights internally not really possible...its a real squeeze in there!

I'll post a couple of pics to illustrate. Thanks for your help

Nov 03, 2004, 04:34 PM
Crazy bubble-head!
Nemo's Avatar
Try the foam out. If you're trimmed at the surface and not submerged, you need to adjust floatation that's above the waterline.

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