Moebius Skipjack and proposed ballast system

[redboat219]

I did test the system without a check valve. Leakage flow thru the pump was drop-by-drop. So maybe it would be good to eliminate the check valve and count on the leakage as the slow failsafe.
Dan

That's with the bags at ambient pressure. But once inside the WTC air compressed by the expanding bag will push the water out faster.

Btw, how about adding a small thumb screw at the bottom of the pinch tube opposite the servo roller. This way you could throttle the amount of "pinch"?

[Slats]

There's blood in the water guys!

"Dave Welch and bike inner tubes" - all in the same sentence! Phew! This should be good! Where's my popcorn?

I must have missed 60 minutes ......



Dan - have done something similar (not with a sealed bag on the inside). I found using a centrifugal pump and a check vale to hold the water in under pressure in a sealed tank worked well. Its the only system that I have ever achieved neutral buoyancy in. Left the boat parked 3m down in a 5 m deep dive pool. Had lunch came back and she hadn't moved. The safety margin was the centrifugal pump. Should it keep running but won't force water in typically when the compressed air to water ratio is 40/60.

I no longer use that system. The gear failure rate was high. The pumps didn't like the work, eventually the tanks need replacement too and any servo activation (in my case the pinch valve), expect high amp draw and eventually destruction of the servo. It was a system of high energy demand (both pressure) and power required. A minimum 12v was need to utilise the pumps I had to get a decent qty of water in the tank and the tank had to be of course baffled.

The reason why I like the SAS SD so much is cost effectiveness, simplicity and reliability. Its low energy system - one servo movement (not under any great load) to open a free flood valve. One low pressure blower air pump, that can run successfully on 6 volts. Passive use of rising and falling water for valves, and simple use of air outside and borrowing air inside the system. In such low energy requirement systems you have a small footprint requirement in your wet hull sub for the WTC. Other high energy / pressure dependent wet hulled subs loose a lot of their wet area weight and associated maneuvering advantage accommodating ballast utility.

Please keep a good account of your expenditure from get go until you can say that you have the system dialed in. I have been down many roads of experimental ballast systems -all cost money regardless of where parts come from. Be good to see how your system stacks up cost wise against off the shelf products.

Good luck

John

[Subculture]

A few points to note. If you eliminate the check valve, you will almost certainly find the boat will not keep trim. If you want to see how 'tight' the pump is on the bench, fill the bags with water, and placed a couple of bags of sugar on top of the bags, which will simulate the kind of pressure they will be under when inside the cylinder. If you get anything more than drop or two coming out of the hose, then you need the pinch valve.

The system John describes is the sealed tank system commercialized by Sheerline models. This operates at much higher pressures due to the air remaining inside the tank, typically 40psi or thereabouts. The tank needs to be of sturdy construction to handle the pressure, although normal plastic pipe is more than upto the job. Although not a user of this system myself, I haven't heard of Sheerline boats requiring regular changes of pumps. A few people use the system with geared pumps, which permits bi-directional pumping. I agree with John that the pressures the sealed tank operates at result in a system which is thirstier than others, but it's fairly simple and works well if the parts are engineered for the duty required of them.

I am rather bamboozled by the explanation of wet hull boats with small wtc's being more agile. They will certainly be lighter to lug about to the pond, but once in their element and full of water you will still have the inertia of the water inside the free flood areas to alter course, and unless you build your hulls like a tea strainer that isn't likely to alter direction or velocity without putting up a fight. I would say that distribution of mass e.g. heavier components close to the boats c.g can certainly have a marked impact on the boats handling, but you can get that with most systems, it just depends on the way you lay things out.

[DanLew]

"Data trumps conjecture." Some Roman scientist, i think.

I sealed the 3-bag setup inside a WTC with an internal volume of 1773cc (ID=2.5"(6.35cm), L=22"(56cm)).
The pump was external, feeding water to the ballast bags thru an endcap fitting.

Pump rate filling bags at ambient P, OUTSIDE of WTC: 415g/40sec
Pump current at 6.3V = 650mA

Pump rate filling bags INSIDE sealed WTC: 390 to 410g/40sec (only about a 4% reduction)
Pump current at 6.3V = 730 to 790mA (about a 17% increase. Current increases to the higher value as bags reach maximum fill.)

To determine the WTC internal air pressure increase as the ballast bags fill with water, it's easily calculated using Boyle's Law: P1V1=P2V2, or restated, P2/P1=V1/V2.
The approximate WTC air Volume with empty bags = 1773cc. The internal pressure is 1Atm (atmosphere).
The increase in bag Volume is assumed equal to the water pumped in, in this case a nominal 400g (or 400cc).

So the internal air volume of the WTC after the 400cc of water fills the bags is: 1773-400= 1373cc.

Then using Boyles Law: New Increased P/1Atm = 1773cc/1373cc = 1.29Atm. 1Atm = 14.7psi, so the increased P = 18.9psi, or an increase of only 4.2psi.

Contrary to some expectations, that's not a lot of pressure increase, and it would be much less in a 3.5"ID WTC (more like only 1.9psi increase). I am not taking into account the battery, motor and other component volumes, but you get the idea.

So...this system is still theoretically a GO. There is no huge internal P increase, there is no huge energy demand, and ballast stability seems excellent.

Additional observations: The bags fill and empty very evenly and stay nicely in place inside the WTC. A backflow check valve is needed to prevent bleed-back thru the pump. The servo to pinch the tube will have very low load if a roller is used to ride the tube (I've done this in other apps. - no issues). The pump load of 700mA is really trivial given only a 40second on-time during dives/ascents, and given the 5, 7 or higher Amp-hour battery packs likely to be used.
As usual, all the above is qualified by my motto: "Often wrong, but never unsure!"

Cheers.

[Slats]

I am rather bamboozled by the explanation of wet hull boats with small wtc's being more agile. They will certainly be lighter to lug about to the pond, but once in their element and full of water you will still have the inertia of the water inside the free flood areas to alter course, and unless you build your hulls like a tea strainer that isn't likely to alter direction or velocity without putting up a fight. I would say that distribution of mass e.g. heavier components close to the boats c.g can certainly have a marked impact on the boats handling, but you can get that with most systems, it just depends on the way you lay things out.

Actually Andy its an easy explanation. It has little to do with the weight of the equipment required for operation. It has far more to do with the layout and overcoming buoyancy.

A dry hulled sub has by its very nature far more buoyancy than a wet one. The dry air space is distributed along the subs length, save for perhaps stern control areas, and the ballast tank unit.

This high level of buoyancy (higher than a wet hulled sub employing a WTC) requires far more ballast weight to counteract the superior buoyancy that the dry hull has over wet. This weight is a lot heavier than a wet hulled sub's essential ballast. You need to offset all the bouyancy above the WL in a dry hulled sub - a lot of wet hulled WTCs are designed to be below the WL to begin with. Then there is the bouyancy along the whole sub in the dry hulled too.

My recent Permit class build - more photos coming next weekend - has virtually no additional ballast foam added. The entire 3.5" diameter WTC (SD) sits below the WL.

The mass mometum in the water of the dry hulled sub is therefore dealing with much more weight than just the weight of the hull filled with water. Lead weighs how many more times that of water?

Then there is the issue of where you cram all the required offsetting ballast weight in a dry hulled sub. The ballast tank should be at the CP / COR/ COG. Yes you could cram the bulk of the weight around this location but the mass will still be a lot higher, and you might find that you need weight forward or aft too, depending on the equipment weight distribution.

Real world examples - my RCABS experiment boat. This wet hulled boat required so much dry air space to supply the inflation bag, that the WTC was twice the length of that of the SD I ended up fitting to do the same job. The ballast weight to over come the dry space was 3 times that of the the setup with the SD. The boat in question - an X fin Collins with the RCABS large WTC set up had twice the turning circle as the SD smaller WTC setup that I finally put into the boat.

Also all dry Engel boats I run with of similar size to my boats turn poorly.

Dry boats don't just weigh more out of the water, they weigh more in the water too and by a significant margin.

Key issue is this:
-In a wet hulled boat you can take full advantage of ballast weight primarily being used to simply keel the boat (make sure it has a degree of stability).

-In a dry hulled boat the primary purpose of ballast is to offset excessive bouyancy so as to keel the boat.

J

[DanLew]

Why struggle...

The boat in question - an X fin Collins with the RCABS large WTC set up had twice the turning circle as the SD smaller WTC setup. Also all dry Engel boats I run with of similar size to my boats turn poorly.

J

Why struggle with turn radius? An easily made, EZ-on/EZ-off "fore-rudder" could be added, no?
See sketch. Opposing control horns would give desired aft and fore-rudder rotation. When displayed for scale accuracy/appearance the assembly would be removed. But for in-water performance, the fore rudder shaft could be simple a threaded rod screwed into a nut embedded invisibly in the hull. Since rotation would only be maybe 30 deg max, the threaded mount would be no issue for rotation. The aft-rudder control horn and the mid-hull bottom rod-guide would likely be the only two non-prototype hardware left attached to the sub when the fore-rudder is removed. On larger models, the control rod etc could be internal.

Seems it would work.

[redboat219]

Sort of a bow plane rotated 90 degrees. Take note that bowplanes have the effect of changing the boat depth not pitching the bow up or down. So, wouldn't your fore rudder just move your boat sideways?

[Slats]

Why struggle with turn radius? An easily made, EZ-on/EZ-off "fore-rudder" could be added, no?
See sketch. Opposing control horns would give desired aft and fore-rudder rotation. When displayed for scale accuracy/appearance the assembly would be removed. But for in-water performance, the fore rudder shaft could be simple a threaded rod screwed into a nut embedded invisibly in the hull. Since rotation would only be maybe 30 deg max, the threaded mount would be no issue for rotation. The aft-rudder control horn and the mid-hull bottom rod-guide would likely be the only two non-prototype hardware left attached to the sub when the fore-rudder is removed. On larger models, the control rod etc could be internal.

Seems it would work.

Hi Dan
we'll actually I don't struggle with turn radius issues.
Read the post above again - the boat that used RCABS - had RCABS replaced by a far more efficient system.

The boats I build are set up by first overlaying a grid over the boat's plan views, finding the exact COR / CP point and plonking the COG and centre of ballast tank on that point.

I have over 40 builds under my belt in 2 decades in the hobby. I really don't like un-scale visible features in any boat. Sorry but YUK! I've seen plenty of Engel subs running with over-scale control surfaces in attempts to improve maneuvering.

Build it right the first time.

Whilst some people think that I am the marketing arm of Merriman and Casewell, (I'm not), the praise I have for them comes from experience with years of my own experimentation, my own ballast systems, purchase of other systems, and driving RC subs pretty much every other w/end over warmer months.

David will tell you I certainly don't give him a free pass with his technology, I ride this guy with questions, my own testing, and real world experience. In fact I'm a PITA at times to Merriman and Caswell. Bottom line is his stuff works. The sticker price might seem high, but the longevity of the system and its reliability is beyond question.

I certainly don't want to put you off experimenting - its a path I have taken, it can be fun and fruitful, but if your doing it to avoid the sticker cost of proven technology it can be a false economy.

Best

J

[redboat219]

I really don't like un-scale visible features in any boat. Sorry but YUK! I've seen plenty of Engel subs running with over-scale control surfaces in attempts to improve maneuvering.

Build it right the first time.

Actually there's a patent for an anti-roll keel sail.

http://www.google.com/patents/US5058521

Dan could always say his model was based on this.

[DanLew]

Hi Dan
we'll actually I don't struggle with turn radius issues.
But you said "Also all dry Engel boats I run with of similar size to my boats turn poorly. " No argument, I misinterpreted. Just tossed out an idea that you don't need or like. That's OK.

I really don't like un-scale visible features in any boat. Sorry but YUK! I've seen plenty of Engel subs running with over-scale control surfaces in attempts to improve maneuvering.
So, wouldn't some hardware, nearly invisible under the hull, be better than enlarged control surfaces (for some other folks, anyway...)

Build it right the first time.
Many "right" ways to approach any hobby. My approach is to build to see what works. Rebuild as necessary...

... Merriman and Caswell. Bottom line is his stuff works. The sticker price might seem high, but the longevity of the system and its reliability is beyond question.
No question they are the gurus. Prices seems very reasonable to me if you want to simply buy and build a kit.

I certainly don't want to put you off experimenting - its a path I have taken, it can be fun and fruitful, but if your doing it to avoid the sticker cost of proven technology it can be a false economy.
Not cost avoidance. I reckon one of my other boats, the brig Syren, a 7ft long fully RC square-rigger with 18 firing black powder carronades, cost at least triple the most expensive sub kit I've seen. And that was the basic kit before all the customization. My priimary interest in subs at this point is simply exploring this ballast approach for fun and learning, and then maybe popping it into a Skipjack.

J

One of my Syren threads: http://www.rcgroups.com/forums/showthread.php?t=1136541
John, I do appreciate your input. I am learning a lot from all the feedback in this thread.
Best,
Dan

[Subculture]

The mass momentum in the water of the dry hulled sub is therefore dealing with much more weight than just the weight of the hull filled with water. Lead weighs how many more times that of water?

Lead weighs about eleven times that of water. I did say that weight distribution will have an effect on the boats handling. A good example, the Sheerline Trafalgar class once had a chunky sealed lead acid battery up front in the wet space, and was known for being a bit of a camel on casters handling wise. Chris Cloke breathed on the design, lightening the upper hull, and replacing the lead acid pack with a smaller set of NimH's which ran underneath the boat. The result was a transformed boat, with much better handling owing to a redistribution of weight.

Also all dry Engel boats I run with of similar size to my boats turn poorly.

Not owned an engel boat. I have had a bit of stick time on three of their kits- Lafayette, 212 and Typhoon and they all were very nimble save for the Typhoon, which is a big old barge.

Dry boats don't just weigh more out of the water, they weigh more in the water too and by a significant margin.

Assuming we're talking about a fully dry hull here (unusual as most boats have at least some free flood areas), weigh it, then take the wet hull, tape up all the flood slots and fill it up with water, how much will it weigh?

[Slats]

The mass momentum in the water of the dry hulled sub is therefore dealing with much more weight than just the weight of the hull filled with water. Lead weighs how many more times that of water?

Lead weighs about eleven times that of water. I did say that weight distribution will have an effect on the boats handling. A good example, the Sheerline Trafalgar class once had a chunky sealed lead acid battery up front in the wet space, and was known for being a bit of a camel on casters handling wise. Chris Cloke breathed on the design, lightening the upper hull, and replacing the lead acid pack with a smaller set of NimH's which ran underneath the boat. The result was a transformed boat, with much better handling owing to a redistribution of weight.

Also all dry Engel boats I run with of similar size to my boats turn poorly.

Not owned an engel boat. I have had a bit of stick time on three of their kits- Lafayette, 212 and Typhoon and they all were very nimble save for the Typhoon, which is a big old barge.

Dry boats don't just weigh more out of the water, they weigh more in the water too and by a significant margin.

Assuming we're talking about a fully dry hull here (unusual as most boats have at least some free flood areas), weigh it, then take the wet hull, tape up all the flood slots and fill it up with water, how much will it weigh?

The weight difference in the water Andy of dry vs wet is huge. Pulled 2 subs off the bottom of the pool at a run - an Engel Layfatte (dry enough to call dry) and a 1/72 LA (wet hulled). The LA is bigger boat. The LA was retrieved with relatively little effort - one man mask and flippers. The Layfatte was a 2 person op. The extra lead plus all that extra material is the obvious difference. Engel boats by the way overscale the control surfaces for good reason.

My point is Andy the more you need to complicate you wet hulled ballast system (RCABS is a great example due to the massive air supply required), inevitable the bigger dry space people load up. The bigger the dry space the more you head towards a dry boat and its issues. Little improvements in dropping weight make big differences in tightening up turning radius. I had one of David's twin motors single shaft setups. I removed one motor because it was overkill. The boat turning radius with that small drop in weight of a 540 type motor was noticeable.

I do like these threads Andy, but do get around to buying a SD if you haven't yet. Seriously, you and I could flog a thousand of these threads to death, but experience a SD and you'll see what I'm banging on about. Serious top bit of kit.

Best

John

[Subculture]

Were the boats that required rescuing flooded? Otherwise the Lafayette should have been easy to raise up, being either neutrally or slightly negatively buoyant, with only the hulls envelope providing drag as it's pushed upwards.

I do have one of Dave's WTC's an early WTC 3.0 in one of his Skipjack hulls. Works well enough, have no issue's with it, although I'd say the cylinder fits the hull very snugly indeed, so although it will displace less than dry hull Skipjack would, not by much. For most of my projects, ready made WTC's just aren't suitable, you need something bespoke, plus I like to tinker. With me it's all about the journey.

[Slats]

Were the boats that required rescuing flooded? Otherwise the Lafayette should have been easy to raise up, being either neutrally or slightly negatively buoyant, with only the hulls envelope providing drag as it's pushed upwards.

I do have one of Dave's WTC's an early WTC 3.0 in one of his Skipjack hulls. Works well enough, have no issue's with it, although I'd say the cylinder fits the hull very snugly indeed, so although it will displace less than dry hull Skipjack would, not by much. For most of my projects, ready made WTC's just aren't suitable, you need something bespoke, plus I like to tinker. With me it's all about the journey.

Gwad I like this thread - sorry Dan for walking it off topic.
Andy we need to go down the pub and continue this over Lager's - my treat!

Andy both boats weren't flooded. -Power issues. Both were 2-3m down in a pool. No doubt any slight positive displacement was eroded at depth.

The ballast lead inside is the difference Andy between the two approaches weight wise.

I agree that a wet hulled boat primarily filled with water whilst submerged will be weighty, but also that there is an optimal size WTC. A lot of experimenter boats I have built and seen first hand have required WTC / dry spaces that ingress above the WL. These still work Andy, but handling can be improved when ballast doesn't have to be added purely to deal with excess bouyancy.

BTW - all of my SDs are not off the shelf jobs. I have been a complete PITA to David over custom sized fitting SDs, and my early purchases involve buying SD kits so I can customise and optimise. So yes agree 100% my SDs are Bespoke. Think of David as being Mercedes and me being AMG. (yes I have an ego!) But seriously anyone can do it - they are so simple to put together. Lost count to how many SDs I'm up to. And whilst they are designed to go from one boat to the other, like you I prefer each boat to have its own purpose built SD - it ads cost to the hobby but its worth it.

Cheers

J

[DanLew]

Gwad I like this thread - sorry Dan for walking it off topic.
Andy we need to go down the pub and continue this over Lager's - my treat!
J

Andy and Johnny, off on a Walkabout....

Here's the latest on the bike innertube ballast system. The mockup to test pumping, fill repeatability. ballast capacity, etc has worked well enough so far to look ahead.
Sketch shows components.
ADF2 feeds back Failsafe to pump motor to empty ballast water.
ADF2 feeds APC signal to dive planes and to servo actuating the ballast bag fore/aft compression. (Manual Tx pitch control, too)
A single servo controls water in/water out pumping and closes off the water line by compression when the pump is off.
A strainer on the water feed keeps particulates out of gear pump (always some thing in the water, except for pools maybe)
Pressure relief valve - simple spring loaded valve - blows off any excess water pressure to prevent blowout
Moisture sensor in ballast chamber senses start of any leakage and activates pump to blow the ballast
Air vents to fore and aft chamber - very small openings to minimize any ballast tube water leakage in ballast chamber getting to other chambers
Three bags act as lateral baffling
Longitudinal baffling from bike innertubes that are molded flat and have stiff walls
Bag compression shifts water weight slightly fore/aft. Need to design details of mechanism.
Manual ballast air vent line allows all air to be pulled from tubes before water fill.
Next: order polycarbonate 3.5"OD, 1/8" wall, ADF2. Have pretty much everything else for the WTC.
Feedback, advice appreciated.
Best.