Air flotation boat lift

Moderator: GreenLake

Air flotation boat lift

Postby hectoretc » Fri Mar 30, 2012 8:50 am

Greetings all,
Pulling a quote from the floating dock topic from this winter where I asked what the displacement of a 55 gallon drum would be.

ctenidae wrote:8*55= 440, so I'd say, not accounting for the weight of the drum itself, you get 440 pounds of flotation per drum, give or take.


So now I need some additional math/hydro-dynamics help please.
Maybe this is a no-brainer but I just don't know enough about this to figure it out myself.

Without getting into complicated specifics, from the above posting I am lead to understand that a 55 gallon drum full of air, will displace approx. 400+ lbs. of water, which I assume transforms to the ability to lift 400+ pounds of weight, out of the water.
Assuming that's true (or true enough);
- If I have four 55 gal drums submerged under a platform, when the barrels are blown dry, that should give me over 1600 lbs of lift, correct?
- If I assume I have 1000 lbs on the four barrels, assuming an even load, that means each barrel will need to displace 250 lbs. to float.
- My intent is to have a moderate sized hole on the bottom edge of each barrel, and simply bubble air in through the hole, displacing the water and forcing it out the bottom through that same hole.
- If it matters, my lake is at about 1000 ft above sea level.
- When the platform (with the 1000 lbs on it) is lifted from the water, what will be the air pressure inside the barrels (relative to ambient). Is it just 250 lbs divided by the number of cubic inches? That can't be right because air pressure is not measured volume... so what is it measured by?
- If I run a 1" hose between the barrels (at the top edge) for equalizing pressure, (and also to facilitate a vent valve for submerging) will the pressure in the hose be greater than than in the barrels? In other words if I used 2" connecting hose, would the pressure with-in that connecting hose be 1/2 of what it is in a 1" connecting hose?

What is confusing me (I think) is that pressure is measured in psi (pounds per square inch) but the drum is a 3 dimensional object, so in that context pressure should be measured in pounds per cubic inch. Since it is clearly measured in pounds per square inch (PSI), I have to assume that PSI reflects the outward pressure (per square inch) on the inside surface of the displacement container. Therefore it would be rational that a larger hose would have more surface area, and therefore less pressure per square inch. But maybe the pressure is so low it's not relevant, or maybe none of this is applicable...

Anyone who's not gotten lost in this and can offer an answer or suggestion as to how to figure this out, I'd be appreciative.

Thanks - Scott
DS #6127 - Breakin' Wind - From the land of 10,000 lakes, which spend 80% of the year frozen it seems...
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Postby GreenLake » Fri Mar 30, 2012 12:39 pm

Scott, pressure is indeed the force per unit area. If you connect several chambers, then the pressure should be the same, everywhere.

If you had a piston in the end of your hose, the force on a larger piston would be greater, that's the only difference between a large and a small hose.

Now, the same pressure acts on the much larger area of the water level inside a (partially) filled drum. You take that pressure and multiply it by the area to get the total force that's pushing on that water-level. That's then the buoyancy at that particular pressure.

For your question, you work the thing backward. Take your lift (1000 lbs) and divide it by the combined area of the water level in all the barrels. That will give you the pressure.

As long as the structure floats (and is supporting 1000lbs) the pressure will stay the same, it doesn't change whether the barrel is immersed a bit more or a bit less.

There will be more air in the barrels when it floats higher, but the pressure would be the same - except for the small contribution due to the extra weight from the part of the barrel wall that's now no longer immersed.

Also (practically) constant would be the difference in height between the water level inside the barrel and that outside the barrel.

In designing this, make sure that your pump will be able to move air at sufficient volume at this pressure. (If you bubble air from below the bottom of a drum, your pump would need to provide even higher pressure to initially force the air that low into the water column).

If each drum is submerged at 400lbs of ballast, but you need 250 lbs to float the structure, then you have less than half the displacement (by volume) to use for changes in how high your structure floats in the water (unless your idea is to submerge it fully to the bottom).

If you connect the barrels with a hose, nothing prevents all the air from accumulating in only one barrel (with the effect that your platform submerges on the three other corners).

Those would be some thoughts to your problem.
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Postby jdoorly » Sat Mar 31, 2012 7:13 pm

Hi Scott, if I had to float a dock and I knew I had 4 drums with 1600 pounds of up, but only needed 1000 or so I would:

1. strap the drums to the dock frame leaving the filler bungs, the only open opening in the drum in the up position, i.e. farthest away from the water.

2. don't connect tubing drum to drum as there is some question about de-stabilizing the dock by doing this.

3. put the dock in the water and pump about 25 gallons into each drum for 800 pounds plus frame weight total.

4. evaluate the desired height and stability of the dock. Add more water a gallon at a time to each drum to increase stability and sink the dock closer to the water. I would aim for keeping the wood frame an inch or 2 above water.

5. secure the bung holes when satisfied .
DS2 #6408 "Desperado"
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floating boat lift

Postby hectoretc » Sun Apr 01, 2012 12:00 am

Hi Guys,

Re-reading my original post, I guess I wasn't very clear what I was trying to accomplish nor had I provided any information on how I would address the unbalanced drum syndrome when drums are connected together.
The idea is for a floating boat lift. Somebody mentioned it in the the floating dock discussion, and it kinda stuck with me this winter. (I tried not to think about it... I really did)

I knocked out a quick drawing in SketchUp for a reference. This is not to scale and is not anywhere near complete, but hopefully it will get the idea across.

1247

The verticals are a 8'w x 3'd x 12'h pallet stand that I picked up this winter. My intent is to cut it in half about 6' up so I have two 8'w x 3'd x 6'h sections. The thing is connected together by the steel rack bars included in the purchase, or other materials. The vertical tube slides are 2-3' tall sections of 5-6" PVC pipe (have to see what fits snug but still slides up/down" connected by aluminum (probably) horizontal pieces attached firmly to the slides. There are four 55 gal plastic drums attached on the outside to the slides. Maybe I'll put a boat trailer on top of the moving cross members, or build a wood/carpet bed for the boat to sit on.

The plan being, when the drums are vented and filled with water the slides, trailer and barrels sink to the bottom of the frame and I float the boat in/out as desired (through the 8'w opening). By filling the barrels with air, they raise up the bed which raises up the boat. That's pretty much it.

I'm thinking the slides will more or less keep the platform level, so connecting the barrels together by 1-2" PVC tubing (at the top) will provide a natural balancing of air/water. Attached to that tubing will be a vent valve to let the air out of all four barrels at once eliminating the need to balance the release of air from four barrels as the boat is lowered into the water. I guess using the larger tubing will allow the air to vent faster (letting water in through holes across the bottom edge of the drum) much like s submarine ballast tank. (always open on the bottom with vents on the top)

My thinking about feeding the air into the bottoms of the barrels (bubbling in in from one of a dozen or so holes cut into the bottom edge of each barrel, is that with the connecting tubes (and stabilizing frame), I only need to feed one drum with air and it will equalize itself between the four barrels. This as opposed to using the vent pipe to feed air in at the top, to force the air out of the bottom. My expectation is that it will take the same air pressure to do both, and by feeding the air in at the bottom, it seems a more passive method rather than having an active valve at the top that I have to monitor and turn on/off with the blower.
Depending on air volume it may take 30 minutes (or more) to float the boat, but I don't care. I turn on the blower, leave and come back later to turn it off (when the barrels fill with air the excess just bubbles out the bottoms, or I put it on a timer. I've stuck the business end of an electric leaf blower under water before and it's not had any trouble blowing bubbles so that could possibly be the air feed for the tanks. I can't say I've put it 3-4 feet underwater so if I need something with more pressure, that's fine. Again I don't need any massive volume to do this... when it the tanks fill up, they fill up... I guess I'd want it to finish is 1/2 hour or so but even that is sort of an arbitrary number.

So my question about the pressure in the connecting pipes was more one of wondering how to connect the lines to the drums. I didn't think it would be much pressure, but since I can't think of how to calculate it, there's know way I could know without trying it, or asking someone smart.

GL - I think you told me how to calculate the pressure, but it's not clicking yet. I'll re-read it a couple more times and maybe I'll get it. (thanks)

Anyway, that's the plan (as it is today)...

I guess as long as I'm asking pressure questions, is there a formula for water pressure at a given depth of fresh water? (3-4 ft) Maybe GL's answer will tell me that too, when/if I figure how to apply it to my case. That would help me to understand the air pressure needed to feed air at the bottom of the drums.

Thanks,
DS #6127 - Breakin' Wind - From the land of 10,000 lakes, which spend 80% of the year frozen it seems...
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Postby Moose » Mon Apr 02, 2012 11:27 am

Hi hectoretc,
I gave your system some thought over the weekend. Generally I think you have a good concept. However I think there are some oversights and design flaws that you have over-looked. Before focusing on the details we should look at the system in a broader light. Below are my recommendations.
You should consider using six drums for increased buoyancy, safety, and stability. Roughly figuring 8.34 lb/gal H20 and 0.01 lb./gal 02 and a 10 lb. drum you’re looking at 448 lb. buoyancy per drum (not factoring in the cradle). If you use four drums and one were to burst or spring a leak you go from having 1792 lb. buoyancy to 1334 lb. This is not really enough to float the boat and the cradle. With 6 you would still have 2240 lb. buoyancy and the boat would high and dry.
Very roughly speaking 1792 lb. buoyancy loaded with 1000 lb. ((this is an ideal, really you will have more weight) consider 750 lb. boat, 175 lb. man, 180 lb. cradle, possible rain water, etc.) will only float the barrels 56% out of the water. In your current design the cradle is set up such that the centerline of the barrels is the centerline of the cross-beams. I recommend you make the cross-beams such that they extend out past the vertical guide rails and over the top of the barrels. This will gain you some clearance between the bat and water. This will also simplify mounting the barrels and reduce strain in the mounting hardware by allowing them to “couple” directly. The buoyancy force will be transferred from the barrel right into the beam.
The only disadvantage to this is raising the center of gravity of the platform/boat combo but this would be helped by using 6 barrels and using long guides over the vertical guide rails.

If nothing else, eliminate the two separate floating pads. Link them together to create one integrated platform. Consider that if half the boat weight is resting on the front pad (-500 lb.) and one of the barrels fails you now only have +448 pounds supporting the front half of the boat. At this time the front pad will begin to sink slightly. Allowing the boat to tilt which will transfer more and more of the boats weight to the sinking front pad. This might create a resultant force that would “push” the boat off the cradle.
Two separate pads also assume the CG of the boat is right between the pads. Likely it will not be. So by creating a single rigid pad you eliminate the possibility of pitching the boat by distributing the load equally to the barrels. I’d also recommend that you add a cross-beam between all of the vertical guides.

I can put together some sketches if it will help.

I hope you find some of these points valid and helpful.

Thanks,
Moose
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Postby hectoretc » Mon Apr 02, 2012 9:05 pm

Hi Moose,
Thanks a lot for the thoughts and suggestions. Some of them I had been thinking about but others I had not, so I appreciate the inputs.

I agree with adding the two additional drums for 6 total. One of the secondary intentions of this lift was to be able to raise any of our hull boats (not pontoon) for maintenance, bilge draining etc, and so the extra lift is a very smart move (thanks). I had already planned to do as you suggested by extending the platform cross-members out to sit on top of the drums. Lift by pressing up has less stress than with a side mounted float.
Also I had planned all along to make the raising platform a single piece rather than the two in the drawing. I just forgot to include the connecting bars between the two in my quick drawing.

I plan to link the drums (all 6) at their tops with PVC plastic pipe so that a single venting valve will allow the air out of all 6 at once. Further, air filling will be more uniform. This means if one drum should leak, the whole platform will sink rather than tipping at a weak corner, but I would prefer that. The worst that will happen is the platform sinks and the boat floats in the center of the lift assembly (I will include attachment lines to insure it remains centered.

Thanks again - Scott
DS #6127 - Breakin' Wind - From the land of 10,000 lakes, which spend 80% of the year frozen it seems...
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Postby GreenLake » Mon Apr 02, 2012 9:54 pm

I would look at your planned connection to the uprights. Currently these are tubes. What will happen if for any reason the platform does not float level? They might twist and get stuck. Rings?

Also, contrary to your belief, if you connect the air pockets, the structure will not self-level. Rather the opposite.

Disconnected. If you were to stand in one corner of an empty platform, that drum would sink a bit deeper. As air is locked in, pressure increases in that drum, preventing the water level from rising.

Eventually, by being pushed lower, that drum will generate more lift than the others, limiting further tipping of the platform.

Connected, the pressure in all drums would instantly be equal. As more water flows in the drum below your corner, the air pushed out of it runs through the tubes and pushes more water out of the other barrels.

Your drum generates less lift, despite sinking, the others more: tipping is increased.

That PVC pipe is not a good idea.....
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Postby hectoretc » Tue Apr 03, 2012 7:07 am

Thanks GL,
I see your point, but why would several drums connected together behave differently than one long container such as a pontoon? Unquestionably, I am out of my knowledge zone here, but with the drums inter-connected wouldn't the pressure in all drums should be the same? Or maybe that's your point.

I'm going to have to do some more research and try to understand the dynamics of this. I know there are air floatation boat lifts on the market, and to me (externally) they look like big plastic pontoons. Unless there is some form of exotic active leveling air distribution system (which there doesn't appear to be) I'm not seeing why the drums would behave differently.

An alternate design I was playing with would have the boat platform longer (the two vertical sections further apart) with the bow end of the platform attached on a horizontal pivot to one of the frames and the air tanks at the stern end of the unit only.
In this instance I would drive the boat onto the platform, and then only the back portion raises using more of a lever action to pick up the boat. The down side to this design would be changes in water level would be more impacting since the lift range is more finite. But that is probably less of an issue on a lake. Maybe I need to go back and look at this design some more.

Thanks again for the thougths. More work to do...
DS #6127 - Breakin' Wind - From the land of 10,000 lakes, which spend 80% of the year frozen it seems...
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Postby hectoretc » Tue Apr 03, 2012 7:13 am

jdoorly wrote:Hi Scott, if I had to float a dock and I knew I had 4 drums with 1600 pounds of up, but only needed 1000 or so I would:

1. strap the drums to the dock frame leaving the filler bungs, the only open opening in the drum in the up position, i.e. farthest away from the water.

2. don't connect tubing drum to drum as there is some question about de-stabilizing the dock by doing this.

3. put the dock in the water and pump about 25 gallons into each drum for 800 pounds plus frame weight total.

4. evaluate the desired height and stability of the dock. Add more water a gallon at a time to each drum to increase stability and sink the dock closer to the water. I would aim for keeping the wood frame an inch or 2 above water.

5. secure the bung holes when satisfied .


I am at this time, constructing the floating dock that I'd addressed in the other post, and this is an excelent idea for adjusting the float height of the decking. I'll use this plan for the dock floats. Thanks!
DS #6127 - Breakin' Wind - From the land of 10,000 lakes, which spend 80% of the year frozen it seems...
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Postby GreenLake » Tue Apr 03, 2012 2:15 pm

hectoretc wrote:..why would several drums connected together behave differently than one long container such as a pontoon? Unquestionably, I am out of my knowledge zone here, but with the drums inter-connected wouldn't the pressure in all drums should be the same? Or maybe that's your point.


Yes, that was my point.

And depending on the other details of your design, that equal pressure isn't the advantage you think it is. Let's look at some more examples:

A long pontoon (sealed) does have the same pressure everywhere.

If it is empty, then, when you stand on one end, that end dips in, displaces more water, generates more lift and, if you are not too heavy for it altogether, will stop further tipping at a new equilibrium.

If you fill this long pontoon with water to the half way mark, and then stand on the same end, the water will remain level, even as the pontoon inclines. If it's a long pontoon, just a few degrees of tilt will mean that the water level at the lower end reaches the "roof".

So, you would be standing on top of a fully water-filled portion of the pontoon, and you would sink (because that portion can't have any buoyancy).

If you had a series of baffles that prevented the water from moving between compartments, then the system would act more like an empty pontoon again. A series of barrels, with ONLY the air pockets connected and the water trapped would act like such a pontoon with baffles. (This system provides no benefits over a series of unconnected barrels, except to add additional points of failure - the hoses).

However, a series of barrels with the air connected AND each barrel open at the bottom (for water to get in and out) would act like the partially filled pontoon with water free to move. That was your original configuration as I understood it, and that is totally not stable.

The details matter, but in general be leery of "free water surfaces" in your designs, whether in flotation elements or tanks on board. Free surfaces means the water can shift - and it will.

hectoretc wrote:I'm going to have to do some more research and try to understand the dynamics of this. I know there are air floatation boat lifts on the market, and to me (externally) they look like big plastic pontoons. Unless there is some form of exotic active leveling air distribution system (which there doesn't appear to be) I'm not seeing why the drums would behave differently.


I've seen those. I don't know how they are designed, but I doubt they have a free water surface.

You could make bathtub experiments :)
  • Take a tupperware container, fill it half, and close the lid. Push on one corner. See what happens.
  • Leave it open and upside down and trap air inside. Push on one corner. See what happens.
  • Put some weight (that can't shift around) in a closed container so it floats half submerged. Push on one corner. See what happens.

hectoretc wrote:An alternate design I was playing with would have the boat platform longer (the two vertical sections further apart) with the bow end of the platform attached on a horizontal pivot to one of the frames and the air tanks at the stern end of the unit only.
In this instance I would drive the boat onto the platform, and then only the back portion raises using more of a lever action to pick up the boat. The down side to this design would be changes in water level would be more impacting since the lift range is more finite. But that is probably less of an issue on a lake. Maybe I need to go back and look at this design some more.


That would make things more stable. But instead of a fixed end, you could have the same design with one end floating (empty, sealed chamber). Then the whole setup tracks the lake level...
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Postby hectoretc » Tue Apr 03, 2012 8:33 pm

GreenLake wrote: That would make things more stable. But instead of a fixed end, you could have the same design with one end floating (empty, sealed chamber). Then the whole setup tracks the lake level...


Ah... best of both worlds. Thanks!
And I finally do get the theory on the other. Took a little while but if finally sunk in (pun intended).

Thanks again!
DS #6127 - Breakin' Wind - From the land of 10,000 lakes, which spend 80% of the year frozen it seems...
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Postby hectoretc » Mon May 21, 2012 5:57 pm

GreenLake wrote:That would make things more stable. But instead of a fixed end, you could have the same design with one end floating (empty, sealed chamber). Then the whole setup tracks the lake level...


Greetings all,
Spring is moving along nicely,
1292
Breakin Wind is at the dock, although not quite ready for open water and in serious need of a bath... I'm finishing up the floating dock (2nd dock) hopefully over Memorial Day weekend, weather permitting.

Once the floating dock is finished, along with getting the DSII sailing ready, I'll also start working on the floating boat lift for her. I have the sinkable floats and the outside frames ready and now am thinking about the actual up/down lift structure. I have a quite junky boat trailer from which I'll remove the axle and hitch, weld a bar laterally across the front as a pivot point and then install the submersible floats at the stern end.
The plan would be to lower that end into the water (so the trailer is tilted down), float the boat in up to the winch roller (no winch needed), clip the boweye to a stationary hook, and then raise the back of the trailer to lift her out of the water.
Since there is no roll on/off need, just an up down lift I am of the mind that bunks are the easiest and probably least costly way to go, also being most forgiving in actual centered positioning, but structurally, can I lift the boat with wide side bunks only, or does there need to be keel support as well?
Anyone have any thoughts or experience with that? Thanks - Scott
DS #6127 - Breakin' Wind - From the land of 10,000 lakes, which spend 80% of the year frozen it seems...
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Postby GreenLake » Mon May 21, 2012 7:58 pm

My take:

If your trailer frame drops far enough that you can float the boat all the way in, bunks are fine.

You will want to support the CB by a bunk under the keel. And you do need to support the front part of the boat, and that is best done under the keel (as it's sort of the "strong point" for the boat from about the CB case forward).

For support, a simple three-bunk setup whould work, but, depending on whether you need guidance when floating your boat, you may want to fashion a short V bunk forward of center that can help guide the boat. Location would be as far back as you can go while the hull is still very V shaped at that point.

I've had wind, waves and current push the boat sideways and off my trailer, so unless your dock is really protected, or you have the extra hands to manually hold the boat until it's raised, I would put some guides in.
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Postby Alan » Mon May 21, 2012 9:32 pm

I'm with GreenLake on the guides. My boat drifts maddeningly to starboard when it's being trailered, no matter what. Even if I stand in the water and push it into alignment, it still ends up with the centerboard on the starboard edge of the rollers (four of them).
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Postby hectoretc » Tue May 22, 2012 6:43 am

Thanks Guys,

I agree about the positioning guides.

1294

Unforunately, the area of our property does happen to be in a very protected cove which equates to a very lily pad friendly environment (thousands of them) but not-withstanding, the frame of the lift will have corner posts which I'll pad. The bow should be end up pretty well centered due to the hook in the boweye, and once the boat is in the approx location I can use swing out positioners on the aft posts to more or less lock it into the center of the lift over the bunks before raising.

Thanks again
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