Dock Building/Engineering questions

[Sublime]

I am attempting to build a floating dock. It's at a rather high traffic area so it gets pretty rough. On top of that it needs to be removed every winter and is in 15' of water.

Doh.

I knocked up two 10' x 4' frames with the decking being made out of cedar and the frame outta PT. I would have preferred the whole thing to be made out of cedar but it was 19.76 for a 2' x 6' x 10' well out of the budget. See ghetto fab pic below.

Prior to this we got hold of eight 55 gallon drums. To find out roughly how much they would support I rigged up a empty milk gallon tied it to a bucket ghetto I know but filled it with weights until it lost buoyancy and sunk. Then I took it out and weighed it and roughly got 23 lbs give or take a few. So by that 55 gallons of air should be able to hold up 1265 lbs. So even if we say conservatively it can hold 750 lbs before sinking. So between the 2 of them they can hold 1500lbs. The dock maybe weights 300 lbs so two 55 gallon drums should hold it up no problem right?

I secured the barrels to the dock by the means of 2 eye hooks and 5/8 braided nylon rope. I went for the rope because some strap designs could only be fastened cheesy by screws/nails. Where I felt the rope is much more secure and solid.

I was thinking about adding 2 additional barrels to the dock. Screwing in the poles then the dock to the poles. The one we use look like this{ see bottom } but they have a large set screw in the side to connect to the pole. I would then set the dock at a higher height then sink the barrels thus raising the dock slightly above the water line.

Make sense? Or would gravity win over and the dock would just slide back down? Am I better off just screwing in the poles and not putting in a set screw so the dock can free float along with the waves?

I am in desperate need of ideas but the only thing is that my project is lone wolf. I am rather stocked in tools: all the saws table chop circular sawzall etc pretty much the only thing i am missing is a set of nail guns.

[runethechamp]

I design ships for a living and know all there is to know about bouyancy and waves and shit . I'll give you a reply later tonight. Quick answer: you want that dock to be able to move up and down.

[Mulletizer]

Sounds like a fun project. I sort of follow most of what you are saying.

I have done some of this stuff so might be able to help. First I have a couple of questions: will the dock be perpendicular to the shore or will it be parallel? Will the water level rise/fall beyond the turbulence caused by wakes?

Seems to me that the critical thing is how you attach it to the shore - you don't want the thing to break off. If it is in an area without significant seasonal change in depth you can anchor it to the shore. Say, to some old tyres filled with concrete and buried or to eyes drilled into concrete if there's already concrete there. I would go with some type of rope lashing - dynema or spectra - or preferably chain, with a very solid anchor on both the dock and the land.

If the dock will be parallel to the shore this will be fine. If it is going to be perpendicular you'll need to anchor the end further away from the land too. Basically, if the dock is pointing outshore and you don't anchor the offshore end the stress caused by wind/current will most likely break the inshore moorings/structure of the dock. Bye dock, bye boat.

A typical way of anchoring to avoid this is to a pole driven into the river/lake/sea bed at each outshore corner and to the land at each inshore corner. Tricky in deep water but possible. If you can't do this you could try using guys to the shore from the outshore corners but this would make it impossible to come alongside the dock - you could only come to the end.

If the water depth changes or it gets quite rough things get a whole load more complicated. Anchoring it to the land or poles will mean it gets pulled under when the water rises and left hanging when it falls. So you need an anchor it can more up and down on with the water's change in height. Follow? I've done this by driving poles into the bed and anchoring the dock to the poles with rings that clamp around the pole in a O shape. These rings allow enough space for the dock to be free to rise or fall with changes in the water depth. I've also seen this done with a rail sunk vertically into the bed with special wheeled clamps that make this movement smoother. You then have a short ramp down or up to get onto the dock.

Another thing: to work out the number of barrels you need to use Galileo's shit. Forgetting it's own mass, a sealed, empty 55 gallon drum will displace 55 gallons of water - or float the weight of 55 gallons of water before being submerged (I don't know what a gallon of water weighs - we have metric system here but do the maths). So you can work out how many barrels you need. Remeber AKPM's sig? - it's like that.

But it ain't quite this simple. You don't want your barrels to be submerged totally or floating high - you want them to float at a depth that gives optimum stability (prolly somewhere between the two). That'll be a point where the raft isn't too high so top-heaqvy, like standing up in a canoe or so low that you get wet feet. So you have to take into account that height you want the raft to sit at and then decide how many barrels to use to get the right volume of barrel below the water to give this height. The calculations get more complicated, I'll let you figure it out.

Still with me? Nice one if you are. I can't believe I just wrote so much... I can't sleep. Hmmmm.

[schuss]

do what we did, get a bunch of wood, a thirty rack and a chopsaw and have fun. Oh, we also had a construction engineer, but he was drunk.

[Squatch]

i'm pretty sure a gallon of water weighs about 8 pounds. I don't know how you did your milk carton test, but i feel like 23 pounds is way too high.

[Sublime]

The dock is going to be perpendicualar to land which makes things even harder.


I bought some square stock steel to make a bracket where one was going to be attached to the dock and the other to a steel seawall via some lag bolts and some carrge bots and self tappers. But since you mentioned water levels which do go up and down 3-4'' or 7cm on an average. But if the water level rose plus combined with high traffic that seems like a combo for some shit to break. So i'd imagine getting the dock as close as possible to the seawall would be better then attaching it to the seawall.


Well if you use displacement a gallon of water weighs 8.33lbs or 3.7so..

A 55 gallon drum should be able to hold up 458lbs or 207 kilos. So two drums can hold up 916 lbs 414kgs. The docks dry weight is around 300-350 or 136 or 158 kg

More if too sleepy to type

[Snow Dog]

A 55 gallon drum should be able to hold up 458lbs or 207 kilos. So two drums can hold up 916 lbs 414kgs. The docks dry weight is around 300-350 or 136 or 158 kg

You forgot to include the drunk fat people using the dock.

[runethechamp]

Mulletizer knows a lot more than me about docks, and all his advice looks good. I'm thinking in your case it migh be possible to use some ancor chain to restrain the outer end of the dock. Don't they make some things with a spring in the middle so you have some room for expansion with higher/lower water levels?

As far as your barrels go, I'm pretty sure you're going to run into stability trouble with only two barrels. You want the footprint of the barrels to be as wide as possible for max stability, and the length of one barrel just looks too narrow. 4 should cut it, but if you have the separate weights of the barrels and the dock structure, I can do a quick stability calc for you.

And all these bouyancy calcs are about a million times easier in the metric system.

[Sublime]

Rune the barrels measure 30cm diameter by 137h. The weight of the drum is 10kg and the weight of the dock dry is roughly 136kg.

I was thinking last night about suspending some sort of weight around the barrels. To put some resistance on them so they don't rise/fall as fast. The dock is going to be on pole that is going to help with the stability. If 2 can hold 414kg then 4 should be able to hold up 828kg. If that's the case I would be worried slightly that the dock would be sitting too high in the water and bounce like a fat girl on a trampoline.

[Mulletizer]

I was thinking last night about suspending some sort of weight around the barrels. To put some resistance on them so they don't rise/fall as fast.

You could part fill them with water. A lot easier and the same effect, I think.

[runethechamp]

Rune the barrels measure 30cm diameter by 137h. The weight of the drum is 10kg and the weight of the dock dry is roughly 136kg.

I was thinking last night about suspending some sort of weight around the barrels. To put some resistance on them so they don't rise/fall as fast. The dock is going to be on pole that is going to help with the stability. If 2 can hold 414kg then 4 should be able to hold up 828kg. If that's the case I would be worried slightly that the dock would be sitting too high in the water and bounce like a fat girl on a trampoline.

Those measurements do not equal 55 gallons.

And to lower the speed of the up/down movement you need damping, not extra weight.

[shmerham]

May sound like a silly idea from Bill Cosby, but I think you want to put gelatin in the barrels

[Ski Monkey]

And to lower the speed of the up/down movement you need damping, not extra weight.

Perhaps something like a disc, hanging from a rope in the water, hanging from the end of the dock?

I'm a bit confused, is this going to be a floating dock like a swim platform in the lake or is it going to be a standard dock that starts on shore and goes into the 15' deep water? I'm assuming the later.

[Camel Toad]

we had a bfa (big f*cking anchor) at the end of our dock.

rough dimensions. 30' by 10' all wood, floated with foam blocks. hinged boardwalk to accomodate the high traffic waves and regular water level fluctuations. the shoreward 1/3 had cables that went to eye bolts drilled into the rock (keeps the dock perpendicular). bfa at the end to help keep the dock straight, the anchor is attached with very large link chain this dampens the up and down motion of the dock and allows for water level adjustments without doing anything.

we also built a livebox in the middle of the dock for storing our catch until we sobered up enough to clean em.

in short, be sure all of your hardware is bomber, overkill even. once you factor in the stresses of a boat tied up during a storm, the occassional poor docking job (read: collision), etc you want all of your attachment points to still be holding.