Critique please - RC Groups
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Apr 03, 2014, 10:04 PM
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DanL's Avatar

Critique please

Converted a Trumpeter Seawolf way back in 2006 with a Dave Welch WTC (air pump fills external bladder to surface, valve lets air back into WTC to dive.)
My first and only sub build - great fun. But just got it out of storage after many years. Started to read up on the changes in the hobby, and am thinking of rebuilding the WTC using the approach in the attached sketch.
A small positive displacement, reversible water pump fills two small diameter bladders (bike innertube sections sealed at ends) to dive. Reversed pump ejects water from bladders to surface the sub. Two bladders are used for fore/aft balance. Using bike tubes because they collapse flat and are thick-walled enough to hold shape pretty well for repeatable fill needed to maintain desired trim.
May add bladder compression servo, controlled by the old attitude controller, to trim the attitude. Servo arm would "squeeze" water between tubes to shift water ballast weight. Need to test that approach....

Fore, center and aft chambers all vented together with small openings at top of divider plugs to allow for maximum volume for internal air compression as the bags get pumped full.
Any thoughts on pros/cons, suggestions (including "toss the plan" is OK) would be much appreciated.
Last edited by DanL; Apr 03, 2014 at 10:24 PM.
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Apr 03, 2014, 10:52 PM
Registered User
1. You have to have the room to put the water bladder inside the wtc.
2. The water pump & motor will be larger than the vacuum pump currently used needing more room inside.
3. Having two bladders in the past seem to have issues. Yes, a leveling servo will work but it seem to me if I remember right that the servo was always behind the needed balancing and the boat would porpoise continually. Using a weight that side back and forth was also tried. Had the same porpoising effect.
4. I too have the older system that you described. In fact I just refurbished it for a local run next weekend.
If you can find a small enough water pump/motor combination, I think you could try going with the single water bladder centrally located.

Keep us informed on your progress.
Apr 04, 2014, 02:05 AM
Registered User

Rva1945 RC conversion of the Trumpeter Seawolf.
Here he used a reversible windsheild wiper pump and a vented tank.

Read about it here:
Apr 04, 2014, 07:49 AM
Registered User
DanL's Avatar
Redboat -
Thank you. Really a great reference. The ballast system looks like a winner.

Love to see the Dave M comments again.

Apr 04, 2014, 08:36 AM
Shanghai'd Expat
herrmill's Avatar
Dan, if you want to see David's comments post over on the S-D forum. Am sure he'll be more than happy to chime in on your project.

I had the pleasure of finally meeting him early in Feb. He's one amazing person in many ways. Has a cool cat, too. ;-)

Apr 04, 2014, 01:52 PM
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DanL's Avatar
Hey Chuck!
Thanks - will go to other forum.
It's the "Forever Winter" this year in Minnesota. Still 2-3ft of ice on the local lakes, so I'm going after some old models - the sub, a half-finished tug and upgrading cannons on Syren.
Seawolf is in good shape, but I think I'll strip parts and rebuild her with a whole new system if this proposed ballast system proves better than the old.
Will see what Dave M thinks - if it's worth his attention
Apr 04, 2014, 06:19 PM
AKA Mr Pita
you may need to fasten your end caps some how. when the bladder expands with water it is going to compress the air in the wtc.
Apr 04, 2014, 08:40 PM
Registered User
Nice idea about compressing the water bag with a servo for attitude control. Usually ballast shifting involves using a servo to move a sliding weight ( lead or battery pack). You should test it out and show us the result.
Apr 05, 2014, 12:21 AM
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DanL's Avatar
Spent the afternoon going over and adjusting old Seawolf. Even after about 8 years unused, everything worked just fine in dry run and in the tub. All I did was put new NiMH's in the sub and in the Polk Tracker 3. The stern was a very slight bit heavy - added a very small foam block and attitude control is great.
I was expecting to have problems, and so just re-build it with the new WTC proposed above. But it works so well, I want to keep it as it is.

Sooo...I think I NEED to build a second sub and try the proposed ballast system.
I want to keep the model simple - don't want to spend time superdetailing the hull - so a modern sub is probably going to be the choice. I'll build the WTC and mechanicals so I can experiment along the way.
Attached is a pic of Seawolf and an updated diagram of the ballast system concept.

Any modern sub hull in the 3ft long range recommended? Would prefer moderate cost (ABS?) vs high cost (fiberglass?).
Apr 05, 2014, 01:21 AM
Registered User
Can you explain the purpose of the 2 vent valves?

I see one possible problem with your balance servo. Since you're using a compressible bag (innertube) there's no guarantee that not all of the water will be pushed to the opposite end. The bag could expand under the compression plate on the same side.

The bottom drawing is what you want. The top is what could happen.
Last edited by redboat219; Apr 05, 2014 at 01:41 AM.
Apr 05, 2014, 09:39 AM
Registered User
redboat219 assessment of the air bladder is correct. Using two compression shoes would allow the bladder to bulge at the ends and in between the shoes.

A solution would be to

1. use a single long compression shoe so there is no where for the bladder to bulge past the shoe.

2. invert the system so the shoe is under the bladder, pushing up. There would be no high spots in the bladder as it would all be against the top of the cylinder. By inverting the system, two smaller shoes may do just fine.

3. with one long shoe there would be no where for the water pressure to effect the air in the bladder pushing the air to one end during dives and accents.

4. having the air bladder at the top of the cylinder would keep the center of gravity as high as possible in the cylinder. May not be an issue but a positive.
Issues -

1. moving the air back and forth would move the positive center of gravity fore and aft. But this is the point of the system to keep the boat level.
Apr 05, 2014, 10:21 AM
Registered User
Why not just get a commercially available automatic pitch controller?
Apr 05, 2014, 10:36 AM
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To make the air bladder work I would think a APC would be necessary. Just hooked up to the bladder servo instead of the rear planes.
Apr 05, 2014, 11:11 AM
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DanL's Avatar
Originally Posted by redboat219
Can you explain the purpose of the 2 vent valves?

I see one possible problem with your balance servo. Since you're using a compressible bag (innertube) there's no guarantee that not all of the water will be pushed to the opposite end. The bag could expand under the compression plate on the same side.

The bottom drawing is what you want. The top is what could happen.

Cliffhanger -
To make the air bladder work I would think a APC would be necessary. Just hooked up to the bladder servo instead of the rear planes.
Redboat, Cliffhanger,
Thanks for your interest in this and the feedback.
Reason for two is simply a one-way pressure relief valve. Since the pump is positive displacement, if it runs too long it can blow out the ballast system. Once the pump fills the ballast system, any excess water blows out the relief valve.
The second valve is a failsafe valve. If the sub loses the Tx signal, the failsafe circuit blows the ballast after about 10 seconds of lost signal. In the current Seawolf setup, the failsafe circuit starts the air pump and blows up the bags. In the proposed system, the failsafe would open the valve and WTC internal air pressure would force water out of the ballast bags, surfacing the sub.

More detail on the balancing-servo approach....
On the Seawolf - my first and only sub from way back in 2006 - I had problems with the ballast bag. It was a wide, soft blood pressure cuff bag. It shifted, changed shape, etc, just as you show in your sketch. The sub would never stay level and would dive/surface at different angles every time. So I made new airbags from two narrow bicycle tire tubes. They are laid side-by-side logitudinally. The air line feeds both tubes. The tubes are held in place by rubber bands. When empty, the tubes are totally flat. The walls are stiff enough that, along with the rubberbands to hold them in place, they fill and empty very repeatably.
The bags are easy to make at exactly the right length to give the desired ballast capacity -no excess volume, etc to cause variability. The number of rubber bands and their position can be adjusted if necessary to give exactly the right trim.
Seawolf is perfectly level on surface and under. I can hold it at periscope depth, etc etc. I'm surprised it works so well. Seeing this again is what changed my mind about modifying it. But I still want to do the new system too (on a bigger hull maybe). Note that the current Seawolf does have APC, but on the rear dive planes only.
So, if this works so well with air, I think the same bag design would work really well with the proposed water system. Again, rubber bands (or zip-ties...whatever) could be used to hold the bags in place if needed to get primary buoyancy/level adjustment, and the balance servo(s) would be the level trimmers. I do see a problem when looking at the existing bags...they are too long for one servo to work. Maybe no problem, just use one servo at fore and one at aft end of bags. And the water bags are likely to be smaller/shorter.
See the pics of current Seawolf bags and proposed water bags with servos roughly sketched in . The compression arms shown would likely be longer to cover more bag length - sketch just shows general idea. Servos would have low load because they would be on same channel, working opposite directions. APC would add some auto-level in addition to manual leveling capability.
So waterbags would have three controls - 1. surface/dive depth by the amount of water in/out by the pump, 2. manual pitch control by fore/aft compression servos, and 3. auto pitch control by the APC. Would have to play around with servo speed/travel and APC setting to tune control and hopefully eliminate porpoising.
Let me know if you think it will solve the problem of the sloshing bag that you described.
Last edited by DanL; Apr 05, 2014 at 11:34 AM.
Apr 06, 2014, 09:38 PM
AKA Mr Pita
The mobius skipjack comes in about 42" loa and just over 5" wide. You can find them in the $100 range

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