What's up dock?

[kokko]

My ds ended up under the dock in the sommer of '10. How that happened I could explain over a beer.
Since then I have used a couple of rigid bumpers to keep it From happening again. Last summer I also experimented with home made boat whips, which I made from a section of 1/2" schedule 40 inside 3/4" schedule 40.
It seemed to work very well and kept my boat off the dock

[K.C. Walker]

Another possibility that might be easier and less expensive than building a floating dock is to haunt craigslist for an old pontoon boat. My wife's cousin got one for free. He uses his as a dock extension/barge. The aluminum framework and pontoons lasts longer than the plywood deck, upholstery, and motor.

[hectoretc]

Another possibility that might be easier and less expensive than building a floating dock is to haunt craigslist for an old pontoon boat. My wife's cousin got one for free. He uses his as a dock extension/barge. The aluminum framework and pontoons lasts longer than the plywood deck, upholstery, and motor.

That's another good idea... except you get kind of limited in length based on price. I really need about 32 ft. of dock, so that would have to be a big pontoon (or two) and I doubt it would be cost effect at that size. But it costs nothing what-so-ever to shop...

[GreenLake]

My ds ended up under the dock in the sommer of '10. How that happened I could explain over a beer.
Since then I have used a couple of rigid bumpers to keep it From happening again. Last summer I also experimented with home made boat whips, which I made from a section of 1/2" schedule 40 inside 3/4" schedule 40.
It seemed to work very well and kept my boat off the dock

Here's one of the more challenging docks I've visited.

The wind wasn't very strong and the water looks peaceful in the picture, but it was the wakes that were the problem. If the stern had gone under the dock, the wakes would have slammed it upwards. A really oversize fender might have worked, but what I had was too small to stay put.

[kokko]

Ok I will explain how it happened, but you guys owe me a beer

My father-in-law thought my boat was too close to shore and might bump the bottom with a passing wave. He took the boat further out he dock into deeper water. He did not properly retire the bow line to the dock. The bow line slipped allowing the bow to swing out and the stern to swing under the dock

He has never realized that this ws the result of his well-intentioned actions, and just as well. In the interest of family harmony, and that he gives me dock space

[hectoretc]

Ok I will explain how it happened, but you guys owe me a beer

I'll pay up in January when we all get together for the Minnesota DaySailor winterfest...

They use barrels for my dock on white bear lake. You may give the boatworks a call.

My father-in-law...family harmony, and that he gives me dock space

Does your father in law own Boatworks? or am I reading way too much into these disconnected sentences? (you could quickly become my new best friend... )

[kokko]

Sorry about the confusion. No my father in law has no connection to the boatworks. They have a summer place on teal lake near Hayward, wi and he let's me keep my ds1 there.
I also have mt Catalina on wbl, but I pay like everyone else. The boatworks does have docks floated on barrels so you can see how they are put together.

[hectoretc]

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

As for stability- just remember, they roll. That's about all I know.

Oh, double check your drum size- a standard barrel is 42 gallons. a "55 gallon drum" isn't always a 55 gallon drum.

Looking again at the drums, one if them is a slightly different color but the same size, and is marked with graduated levels topping out at 55 gallons so I'm assuming thats what they all are.

So given that, would 100 lbs of weight added, suggest the drum would be approx 25% submerged, and at 200 lbs, approx 50% submerged, or is this not a linear equation (being that the drum is round and therefore displacement occurs at some geometric rate? I don't know that it matters (but I suppose it could), for now more of a curiousity on how things work.

I always say, you're never to old to learn hydrodynamics... or if I haven't actually said that, I've probably thought it a couple times...

[GreenLake]

If a drum were a cylinder, and you were to immerse it upright, the result would be linear. If you immerse it sideways, you get half immersion at half load, but the rest of the curve is not linear. As, initially, the curved volume displaces less at same depth, the immersion at 1/4 load is a bit more than 1/4 depth. The difference is probably not something you'd need to know with precision...

A barrel is curved in two dimensions at the same time, which would make the solution even more complex (and immersion depth at 1/4 a bit deeper).

A real-life drum may have a bit of a taper but not as severe as a barrel. But still, not something you'd usually worry about in such a design.

[hectoretc]

Just posting a conceptual design for comment.

The building blocks for my conceptual dock will be 4 x 8 sections made from 2 x 8 x 8' treated boards (edge pieces), 2 x 4 x 8' treated boards (barrel positioning and support), a sheet of 5/8" 4 x 8 green treated plywood (not shown) for deck surface (painted with non-slip paint) and various hardware pieces as needed for assembly and of course one or two 55 gal plastic drums depending on the position in the overall assembly.

If it's not obvious, two 2 x 8 x 8's form the long edges, the remaining 2 x 8 is cut into two 46 1/2" end pieces (forming a 48" width section when attached between the edge pieces). I'm expecting to use full width inside corner braces for the 2 x 8 corners to insure connection loads to adjacent sections are distributed.
Two of the 8 ft. 2 x 4's are cut to four 46 1/2" pieces and placed to hold the barrels in position exactly so the barrel will touch the decking bottom flush with the remaining surfaces. I intend to use upside down joist hangers for the 2x4's as the barrel pressure will work to push them up and out. The last 2 x 4 is cut to provide a center support for the plywood and help support the inside cross piece 2x4's. There is probably enough 2x4 material left after that cut to insert extra similar braces between the outside 2x4 barrel brace and the 2x8 end cap if needed.
Rather than using decking boards ($$$), I'm going with 4 x 8 5/8" plywood which is green treated and painted with non-slip paint as the dock surface.

This "dock section" will hold one or two barrels depending on floatation needed, and can be bolted together end to end or side to side (or both) to create whatever shape and size of dock needed. I paid $5 each for my barrels so my quick calculations show that each section will cost about $55-60 to build. One positive of this plan is that you can build small to start and add on as budget allows if you can't foot the whole thing at once.


One of the challenges of a narrow floating dock design is the inherent left-right instability problem when you have a high center of gravity, and a narrow footprint. The stability in these cases is commonly addressed (with floating docks) by building them double wide to create a wider footprint. I didn't want an 8' wide dock (pier) so I'm planning to address the stability by adding fingers off to the sides as shown. They will work like outriggers, each part helping to stabilize it's connected neighbors. The fingers can literally be placed anywhere to create berth spaces as wide and long (in 8' increments) as desired.

This image is only showing one barrel per segment in the assembled dock, which I think will be sufficient for the finger sections and possibly a few of the core pieces, but I accept I am likely to need double barrels for many if not all of the central dock sections, certainly anything on an edge.

For my implementation, loose corner augers will provide some stability, but in my case I have a boat lift for the pontoon and speed boat and one will go on each side. Barring some amazing winds, I think they should keep the dock from floating away.

So what am I missing... what have I not considered about this plan?
Opinions appreciated - Thanks, Scott

[GreenLake]

Scott,

looking at your design. It seems it could be re-analyzed as a series of L-shaped units, or some T shaped ones (depending on where you place your outriggers).

I would pay special attention as to how long a section you'd want to bolt together rigidly. A two piece L or three-piece T might pose no particular problems, but for longer stretches I'd think you might want to consider hinges of some sort.

The way I imagine testing your design: if you were to support a section on its end points only, you should be able to put the design load anywhere, without the rigid connections coming apart.

I think that the building blocks you designed might work in L2 or T3 configuration.

What do you think?

[hectoretc]

What do you think?

I don't think I understand...(completely)
I understand the reference to L2 - two pieces make an L and T3 - three pieces make a T.

Let me establish a frame of reference for myself so I can better ask questions. Lets say the long core, spine or main section of the dock is on the X axis, and the laterals, fingers or outriggers are on the Y axis. I guess waves would be the Z axis and maybe that's what you were referring to with your comments on rigidity.



My thinking (just to start somewhere) is that the main dock being only 4 ft wide, with one or more people on it will want to easily tip in the X axis (splash). My thinking was to add the Y axis pieces (attached rigidly) to add width to that plane stabilizing the X axis. To do that, I think I'd have to rigidly attach those pieces to the main dock. I think that still works because of your "L" and "T" model. So then what you are saying (I think) is that I should consider flex joints along the X axis where-ever possible to allow for that Z axis movement.

(Pardon the Graffiti style painting)
And now looking at the picture I can clearly see your L's (3) and the T (1). That last L on the end is complicated by the wider piece I wanted to add as a fishing platform, but if that gets too wierd I'll do it another way. Or put it on the other dock.

I can see (again if I'm understanding) that wave action and people walking on the dock will want to flex those joints (GREEN), but less clear is how to accommodate that. I don't see where I can hinge them that would allow for Z+ and Z- flex. The only way to get a full flex is to have a space between the sections big enough to allow whatever my worst case expectation for flex might be. I'd almost have to do something with a very big (inch or so) hinge at the top - between the sections to allow flex both ways while still offering some protection from pinched toes.

Not sure where to go with that, but I'll stop here just in case I've been analyzing the wrong problem.

[GreenLake]

In my shorthand notation the T you drew is a T2 (made from 2 pieces). I think you can combine one of the L2 and your T2 piece into a single T3 (two x and 1 y oriented piece) using one fewer building block.

I think a single outrigger like that can stabilize two dock elements. The total span is inside a 16' x 12' rectangle, which I would expect is about the limit that can be made rigid.

For hinges I would use linked eye-bolts. (Or a U-bolt through an eye bolt). The idea is that the hing should be sloppy to allow for some canting, as well as a bit of misalignment in your y direction. A little bit of play goes a long way to reduce the stress on these hinges while still constraining the sections enough to keep them together.

[hectoretc]

For hinges I would use linked eye-bolts. (Or a U-bolt through an eye bolt). The idea is that the hing should be sloppy to allow for some canting, as well as a bit of misalignment in your y direction. A little bit of play goes a long way to reduce the stress on these hinges while still constraining the sections enough to keep them together.

Thanks GL - Did a little reworking of the design to allow for an eye-bolt to eye-bolt or (as you suggested) more likely an eye-bolt to u-bolt hinge. That would by the nature of the bolts, put enough space between the dock sections so up and down pivots would not be a problem. One of the concerns though of this extended hinge with appropriate gap is pinching little toes (or my big ones) if a foot (or I suppose hand) ends up in the wrong place during the wrong wave.

I wonder of that danger/risk can be mitigated by canting the end caps of the dock sections, and placing the hinge higher, as close to the deck level as possible.



By placing the pivot point high, (even higher than in this sketchup drawing) there will be no pinch point where the dock sections come together and flex. I have some leftover rubber roof (large sheet of heavy rubber) that could be cut into small covers for the flex hinges (attached between to dock frame and top piece) to keep fingers and toes out entirely.



An added benefit to this modification is that now the end cap board becomes one of the barrel supports as well. Having a space between sections would allow me to use an outside corner bracket as well at add additional strength as this key stress point.

I welcome any additional thoughts or concerns from anyone, regarding anything you see that I've not yet considered.

Thanks - Scott

[GreenLake]

Looks like overengineering to me. I'm sure I've seen docks with just such hinges that simply leave a reasonable gap between straight sides. If you need a connected surface to support carts with small wheel diameters, you could add a metal cover, "fixed" to one side and free to slide on the other. That's how larger gaps tend to be bridged on "real" docks. Again. probably overkill unless you have a cart with wheels that just won't fit.