Bulletproofing your dock

I have seen several different ways of securing floating docks. Below, mine is explained. I modified what I could to strengthen its weaknesses. It has survived 72 mph winds recently where other docks failed. I have seen some interesting ways of securing a dock. Telephone poles seem really good but lake management here won't permit new poles because of creosote. One dock I saw had 4 galvanized 6" steel poles on each corner with larger pipe sleeves so the dock could move up and down. Another had a galvanized 5 inch steel pole about 75 feet long running diagonally from a hinge mount on shore to a hinge mount on the corner of the dock. It and the walkway securely position the dock and it could also move up and down without adjustments. Big triangle with no cables. A dock on my street has 4 winches, one on each corner. Two run to shore anchors and two to deep water anchors positioned a good distance outward from the dock. The walkway has wheels on it and allows chasing the water. To move the dock you let out on one side and take up on the other. It works well.

To the right is an illustration (not 100% to scale but you get the idea) of my dock setup. When I bought my house in 2014, the dock was already wadded up in a ball on the shore. I found pictures on Google Earth Pro that show my property once had a 150 foot walkway. It is now 100 feet long from multiple failures previous owners experienced. I now have to chase the water when they drop the level of the lake every winter to be able to use it year round. Seems every time it broke, they cut off the bad part.

The realtor said a storm a few weeks before broke the dock. Neighbors said it had been that way for 3 years. Regardless, I had to fix it. The walkway joint at the dock had failed due to torsion. It had been twisted apart, breaking welds and twisting the walkway back onto itself. I fabricated a 20 foot section of walkway matching the existing walkway to replace the bad part and welded it in.

Look at the illustration to the right. With a southwest wind the dock is pushed to the left, tension is put on the right cable and anchor. This pulls on the right corner of the dock, creating torsion, trying to rip the walkway loose. It also pushes the walkway towards the shore, cramming it against the shore mount.

The triangles are the key to a strong dock. I used cables to strengthen both joints and support the center of the walkway. The cables bracing the joints are tensioned with zero free play. They are tight and essentially now a part of the structure.

Think of the middle joint in the walkway as your elbow. Keep your arm straight and it will support more than you can lift. Let it bend a little and it fails. Same here with walkways with a joint in the middle. Keep it straight and you keep it strong.

I added a second set of cables running from the middle walkway joint to the anchor poles. They keep the "elbow" straight. Each cable has just enough slack in it to let it touch the water, matching the slack in the winch cables from the corners of the dock. When the dock moves in the wind, both cables become tight at the same moment.

Where the walkway joins the dock is a weak spot. With wind and current pushing the dock to the left and pulling on the anchor, there is torsion on the dock joint. Cables making triangles here brace that joint.

The winches are mounted on the corners of the dock and have the cable run down to the dock floor where a pulley turns the cable towards shore. I have had the pulley mount welds fail, the winch frame distort, and the cable on the winch drum stripped. I upgraded to 3500 lb hand winches and have not had a problem since.

I also pulled the flooring up and welded in cross braces to the corners of the dock frame using 2 inch square tubing. This makes the structure much stronger. The frame has to withstand the tension from the anchor cables.

You can see the direction of the forces this dock experiences with a southwest wind. Dock springs do one thing. They catch the momentum of the dock as it is pushed by the wind. They prevent anything resembling shock load. All the hardware is loaded over time instead of all at once. That has to be a good thing.

See the "What is Shock Loading" page on this site. I do my best to explain shock and impact loading on our docks.

Materials

What should you use? In the land of floating docks secured by cables to the shore, bigger is always better.

Cables.
Cables are really just wire rope. They have industry standards of strength ratings. These ratings are the minimum breaking strength limits for the cables. Working load limits are about 20% of breaking limits. With the forces nature puts on our docks, we are really asking a lot from our cables. Bigger than 1/2" is hard to work with, won't like the diameter of the winch drums we want to use on our docks and costs more.

1/4" = 5480 pounds
5/16" = 8520 pounds
3/8" = 12,200 pounds
7/16" = 16,540 pounds
1/2" = 21,400 pounds

I have 3/8" cables on my winch lines and 5/16" cables on the walkway support lines.

Cable is about $1.50 a foot. I have seen guys doubling their cables, running them from the winch to the anchor pole, through a pulley, and back to the dock. This is also a very good idea and halves the load on the cable. You only need a few layers of cable on your drum to allow adjustment for water level fluctuations. It is a cost effective way to double your cable's strength.

Stainless Steel vs Galvanized steel cable
Stainless steel is not affected by corrosion created by the marine environment we need to use them in. Stainless costs substantially more than galvanized. I first used galvanized steel cable to support the center walkway joint. It drooped into the lake most of its life, which was short. After just a year, it had rusted into a solid line. The individual strands of the cable had rusted together and would break if you bent the cable at all. Galvanized steel cables will not live very long on a dock.

Cable clamps (also called cable clips and wire rope grips)

Did you know there is a right way and a wrong way to install cable clamps? A cable clamp is a saddle, a u-bolt and nuts. The saddle is placed on the live side of the load. The u-bolt is on the dead side of the cable. There is a saying, "never saddle a dead horse". There is a torque specification for cable clamp nuts.
5/16" = 35 ft. lbs.

3/8" = 45 ft. lbs.

7/16" = 65 ft. lbs.

1/2" = 65 ft. lbs.

Cable clamps should be placed at least 6 cable widths apart, a few inches. After the cable is tightened, it is recommended that the nuts be torqued again as the wire rope will have stretched slightly and become smaller in diameter. Use 3 cable clamps at the thimble cable termination. When properly applied, they have a termination strength of about 80% of the cable. They are a weak link in our dock anchor systems. Always use thimbles. Thimbles protect the wire rope from abrasion and distribute load on the cable.

Rope

One recommendation I have heard is to use marine HDPE rope to secure your dock. Rope has the ability to stretch 10% before failing and absorb the load created by the momentum of your dock like the Dock Springs do. But rope has a few disadvantages. It is relatively fragile. It can be cut and abraded. It will degrade from exposure to UV light and weaken. 10% stretch introduces several feet into the motion of your dock. It catches and keeps hooks.

Shackles

Anchor shackles also have industry ratings.

The most common sizes used are 1/2" and 3/4". One manufacturer stated, "any variation such as angular lifts, shock loads, and modifications to the shackle will result in a substantially reduced working load limit". See... shock load is bad. I include a pair of 3/4" anchor shackles with each set of Dock Springs I deliver.

Here are the ratings for working load limit / breaking strength limit.

1/4" = 1000 lbs./5000/lbs.

5/16" = 1500 lbs. / 7500 lbs.

3/8" = 2000 lbs. / 10,000 lbs.

1/2" = 4000 lbs. / 20,000 lbs.

5/8" = 6500 lbs. / 32.500 lbs.

3/4 = 8750 lbs. / 47,500 lbs.

7/8" = 13,000 lbs. / 65,000 lbs.

1" = 17,000 lbs. / 85,000 lbs.

Winches
Winches are rated for the weight they will lift. Its a working load rating. I found only one company that publishes a breaking limit rating, Fulton, and it is only twice the working load. I started with 2500 pound hand winches on my dock with both failed. Once because of a shock load administered by my neighbor's dock and another from a failed weld that changed the angle of pull to one it was not designed for. Don't cheap out if you are replacing your winch. The bigger the better here. 3500 lb. hand winches have a minimum breaking strength of 7000 lbs. You need that level of strength and the 3500 lb. size is not that much more money. I have searched for better solutions instead of using the "trailer hand winches". Marine grade dock winches are a thing but start at 10 times the cost of a trailer hand winch.

Shore Anchors
Go big. If you are repairing a failed anchor post, don't skimp. Money spent on concrete today will prevent you from having to spend money on dock and walkway repairs tomorrow. See the below reference to the 3200 lb. anchor pulled out of the ground and through the sea wall. It weighs as much as a mid sized car. Wasn't big enough for the dock it was securing and it was not installed in solid ground. That dock has Dock Springs. The springs did their job, absorbed and prevented shock loading but the overall load from the storm overwhelmed the capacity of the anchor to secure it. You have to have good anchors located in solid ground. And, the closer to the ground you attach your anchor cable, the less leverage the load has over your anchor. See my design below for an upgraded shore anchor.

Deep Water Anchors

If you have a dock over deep water or in a location where you don't plan on moving it, go for them. They are an effective way of keeping your dock where you want it. They don't prevent shock loading. The cables going to them have to be constantly monitored because as the water level changes, the slack in their cables change. I have held my breath long enough to go down 15 feet and reattach the cable to one for a neighbor. Its cable attachment point had failed. When one of his shore anchors failed, his deep water anchors were drug across the lakebed but did limit the dock's movement.

Mounting stuff
Our docks are mostly made from thin wall metal tubing. When we attach anything, like a winch or eyelet for a cable, always add a backer plate. A big washer would be a minimum but a steel plate with the appropriate sized hole for the bolt would be better. I have used a 6 inch long piece of 2"x2"x 3/16" angle behind the tubing. It is much stronger than the tubing it is backing up. Drill your mounting hole in the backer then use it as a guide when drilling the hole through the tubing. Use either nylock nuts or a lock washer so they won't loosen on their own. Grade 8 hardware bolts is preferred and available pretty reasonably at Tractor Supply. Remember... overkill is underrated. Go big. Go stronger. Upgrade where ever possible every time you have to work on it.

Failures
My dock has been on the beach a few times. Some from my stuff failing and some from someone else's stuff failing and taking mine out. I spend a fair amount of my free time on a boat fishing. I have covered a chunk of the 550 miles of shoreline here and observed countless broken docks. I always look to see if I can identify what caused them to fail.

This dock had a walkway joint failure. The anchors were fine. Nothing supported the walkway to keep it straight. It now has a second set of walkway cables like mine.

Neighbors upgraded their dock with this two story fun palace. Its really a huge kite shaped like a dock. Almost instantly, the north anchor pulled out of the ground, breaking the walkway joint at the dock and letting it hit my dock.

The addition 35,000 pounds overwhelmed the left winch, breaking the pawl and stripping the cable off the drum. My walkway now has a little banana to it.

I helped fix by upgrading their north anchor with 1440 pounds of concrete. It wasn't enough. See below.

On my dock, I had cables at the dock walkway joint that were attached about 10 feet from the corner. This held up awesome and saved my walkway from being twisted off but the walkway bent just beyond where these cables attached. I upgraded and move them outward on the dock to the pulley mounts and an equal distance out the walkway to form a much larger triangle. Had I put this in place to start with, my walkway would still be perfectly fine.

This time my right winch failed. It was winter and the water was down. I had chased the water and had the dock pushed 100 feet out and the cables lengthened. The pulley mount on the lower right corner broke off due to some pretty crappy welding. Without the pulley to direct the tension down from the winch to a point just above the water, the cable now ran straight from the winch to the anchor. The direct tension distorted the winch body, making the drum no longer align with the pawl that held it stopped, and let all the cable off. I made new pulley mounts and welded them on. Did the one on the left because of crappy welds and having to fix your dock sucks.

When the lake floods and high winds hit, bad things happen. Here the lake had come up 19 feet from winter pool and we had 65 mph winds. Here the dock/kite pulled its 3200 pound south anchor out of the ground and through the retaining wall. The ground was saturated and the anchor had been created in backfill and gravel. On that day and at that time, it was the weak spot.

I helped my new neighbor pour 14,000 pounds of concrete into a new anchor hole in solid ground.

The 1440 pounds of concrete we added the first time it pulled out were not enough. High waters again had saturated the soil and high winds pulled it out. I used my tractor here to pull the anchor out of the lake and stuck it in the hole, then as an anchor until we could fix it.

Because overkill is underrated, there is now 27,000 pounds of concrete holding this anchor post in place.

Just in case you thought I was kidding about the volume of concrete, here you go.

This is how you one and done a repair.

Did you notice the arrow on the illustration above showing that there is force applied cramming your dock against the shore when the wind blows? This is what it looks like.

This is a 1440 pound block of concrete with 2 inch rebar rods driven 5 feet into the lakebed. It wasn't enough. Rolled it right out of the ground.

To fix, I upgraded with additional 2000 pounds of concrete and used my anchor pole upgrade design (see below). This time poured into four 10 inch by 48 inch holes dug on the corners, reinforced with a rebar cage around the existing mount.

Upgrading an anchor mount.
When your stuff starts wiggling, you need to do something about it. I used a 3' x 3'x 12" form around my north anchor when it started to move. I had let my neighbor hook his monster dock/kite to it while lining up the fix for his anchor getting pulled out. It was too much for it.

I used an auger to pop an 8 inch hole 3 feet deep at each corner. I then constructed a cage around the existing anchor using 1/2" rebar. Each hole got 3 pieces of rebar to reinforce the "legs" of my mount. 18 bags of concrete later I have a very solid mount with reinforced legs that reach down under the mount and help lock it in place. This anchor fix has been tested with 72 mph wind without issues.

The finished product looks sharp.

Most recent attempt to make a bulletproof anchor. Neighbor had his pole break loose. It was held driven in ground 12 inches and had 4 inches of concrete at the top, tied to the newly poured ramp. It lasted until the first storm. It is now 4 feet deep with 4 legs surrounding it with 70 feet of rebar and 42 50 pound bags of concrete. Took a couple hours to do. We learned that 50 pound bags are not meaningfully more expensive than buying 80 pound bags. Plus, they are way easier on my back.

My mount design... The above anchor mount design is something I came up with that just makes sense. If one anchor pole in the ground is good, then 5 has to be much better. In theory, it locks the anchor into the soil with 258% more surface area contact with the soil than a single pole mount surrounded with the same 3x3x1 cube on top around the pole.

The typical mount is a single pole stuck in a hole about 3 feet deep with concrete filling the void around the pole. Might weigh a couple hundred pounds. If you add the 3'x3'x1' form around it, you increase its weight to 1509.79 lbs.

Here is the math... Concrete weighs 143.38 pounds per cu ft. The 3'x3'x1' cube is .33 cu yds. or 9 cu ft. The hole is 10"x36" which is .06 cu yds. or 1.62 cu ft. Add that up and its 10.62 cu ft. x 143.38 lbs. per cu ft. = 1509.79 lbs. of mixed concrete.

Now lets add 4 more holes, spaced out as wide as we can get them under the box. Each hole adds 232.28 lbs. so 929.10 more pounds for a total of 2438.89 lbs. for the mount plus the weight of the pole. Its heavy. Its solid. It also added 31.42 sq. ft. of surface area to engage with the soil. It is an economical way to get the most of out the space under the mount.

Concrete is expensive. Lets say you dug a square-ish 3 foot hole under the 3x3x1 form and filled it with concrete. It would now occupy 1.33 cu yds. or 35.91 cu ft. and weigh 5655 pounds but have a surface area of 57 sq. ft. engaged with soil. The 5 hole model has 57.28 sq. ft. engaged with soil and I did not have to dig a huge hole, haul off a bunch of dirt, or buy 59 80 lb. bags of concrete.

Each leg has 3 four foot pieces of rebar making it a structural part of the mount, impossible to break off. Each leg is tied to an internal double cage around the pole. A powered auger makes short work of the holes. They rent by the day and Harbor Freight has one that isn't too expensive. An 8" auger makes about a 10" hole, with it wallowing around and using post hole diggers to clean the bottom of the hole. Using post hole diggers instead of an auger, a 3 foot deep hole is not unobtainable. Rebar is at Lowes for pretty cheap as well.

Size matters? This is a 3200 pound anchor that was pulled out of the ground and through the sea wall. It had been poured in backfill near the sea wall, not solid undisturbed soil. Water had gotten high and saturated the soil. Then we had a thunderstorm with gusts in the 60's. This dock had anchor springs I made. The winch, cable and all hardware attaching it was fine. The soil here was the weak link.

Like I said earlier on this site, mother nature has the power to stomp your stuff like a bug. Dock Springs eliminate shock loading. Your anchors have to be done right. Locating a mount in backfill was not a good idea.