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How to make a geocache for extreme conditions


Do you get tired of finding poor quality geocaches with containers that don't keep out the moisture?  Often these are sheltered locations as well.

You end up with stinking, sodden logbook and contents. YUCK!!  Not a pleasant experience.

How to make a floating geocache buoy

Floating Geocache Buoy

To to see if it's possible to create a waterproof geocache that stays dry in the most extreme environments, I created a Floating Geocache Buoy that is placed in open water. It  is constantly being pounded by wind, waves and every passing storm.

The cache is called "Buoy Oh Boy" and the GC code is: GC33XG0

OK, to you Americans, that name doesn't sound very good (Boo-ee Oh Boy).  However here in Australia, it's pronounced "Boy Oh Boy"!

The video below shows how the  geocache was created and placed.

Also because of all the quality geocaches that have been shown on Progeocaching I was inspired to create a high quality one of my own. Hopefully it will give you some inspiration as well.

How to make a geocache buoy

So how do you make a geocache buoy?  In essence it's a waterproof container inside a waterproof container.

The outside container is made from a 5 inch plastic drainpipe cut to 450 mm (~18 inches) in length.  It is sealed at one end with a plug and a screw top at the other end. The inside container is a 4 inch plastic drainpipe that is sealed in the same manner. The overall length is about 500 mm (~20 inches)


To float vertically a buoy needs to be weighted in the bottom.  To determine how much weight was needed, I kept adding sand (in ziplock bags) until the buoy floated at the right height in a water tank.  It took about 2.5 kg (5.5 lb) of sand to achieve that.  I then took a short length of 5 inch drainpipe, sealed it at the bottom with sticky tape and measured out 2.5 kg of Quickset concrete.  This was mixed with water in the tube and allowed to set overnight.

Surprisingly, after setting hard, it still weighed 2.5 kg.

Because concrete expands slightly, I needed to use an angle grinder to take off some of the concrete from the circumference of the block so that it fitted inside the geocache.

Cavity Filler

Once that was in place, the cavity above the block was filled with expanding foam (Space Invader), up to the bottom of the inside container.   This is so that even if water does manage to get in, the buoy will still float.

Some expanding foam was also put inside the screw-top lid to give flotation, just in case it's dropped into the water. The thread of the lid was masked with sticky tape and given enough room around the foam to be able screw the lid on.

Attaching points

The buoy needs to be anchored, so it needs an attachment point.  How do you attach a rope to the geocache without breaking the waterproof integrity of the container?

To do this, a stainless steel screw clamp was attached to the bottom of the buoy.  A couple of brackets from a bike rack were used as the attachment points.  A screw was inserted into each one, then the brackets were slid under the clamp, and the clamp was tightened.

2 Nylon cable ties were used on each bracket to attach a loop of chain.

2 carabineers (for redundancy) were used to attach the rope to the chain.  This is to allow finders to remove the buoy from the rope and bring it into their boat so that they don't drag the anchor. A slice of pool noodle was attached to the carabineers as flotation when they are removed from the cache.

The anchor

An 11kg (24 lb) concrete garden block was used as the anchor.  A hole was drilled through the middle and a long bolt inserted.  An old pulley from a sailing dinghy was attached to the bolt and secured with a couple of nuts that were locked together.  This setup allows the pulley to swivel around the bolt as the buoy drifts with wind and tide.

Waterproof Geocache Logbook

OK, so if it all goes to water (ha ha), then the last line of defence is to have a waterproof logbook.  I found one at the local Office Depot.  The pages are made from some sort of polymer and shouldn't mind getting wet.   It was cut down to size to fit inside the inner container.  I inserted 3 different writing implements - a normal ballpoint pen, a retractable ballpoint pen and a mechanical pencil. The purpose is to see which one survives the best in such a salty environment.

Placing the buoy geocache

Finding a secluded location

Because it will be in plain site, the most important thing is to find a location that doesn't get a lot of passing traffic.  We found a spot in the backwaters of a bay near some old oyster leases.  Not many boats come here because there are so many submerged posts from the oyster farm.  Even if you know the GPS co-ordinates, it's not easy spot until you are about 30 metres (100 ft) away.

Tide watching

The other benefit of this location is that the water is fairly shallow, so it doesn't need a lot of rope.  However the anchor block was still lowered over the side very carefully in case it disappeared into the depths and took the buoy with it.  It turned out that there was plenty of rope - even taking into account the highest tide of the year.

In this location, maximum tide height variation during the year is about 1.5 metres (5 ft).  Average variation is about 1 metre (3ft).  So it was important to check the tide tables for maximum height throughout the year to make sure there was enough rope, otherwise there was a risk that the buoy would be completely submerged at some times.

GPS co-ordinates were marked at the anchor point. This is because the buoy will move around with wind and currents. It was difficult to keep the boat located in one spot long enough to average the co-ordinates.  However they don't need to be that accurate because the cache is easy to spot when you get near GZ.

Anchor drag

Cache finders need to be careful not to drag the anchor.  There is enough weight in the block to hold the buoy, but any sort of boat will pull the block along the bottom. Talk about a bad case of cache creep!

This is why it is recommended that people remove the cache from the rope when they sign the log. There is an instruction in the cache description for this purpose. If they do drag the anchor they will need to bring it back to the original co-ordinates.

I'll check back in a few weeks or months to see how it's faring in the environment.  Hopefully it's still there!

The cache page for "Buoy Oh Boy" is here.

My thanks to Usat31 for helping me to place it.  It was quite windy and choppy on the water that day which made it difficult to place it by myself.

Check out the video.  If you aren't interested in the construction details, skip to 10:45 on the video to see where and how we placed it.


Comments (6) Trackbacks (0)
  1. Thank you for sharing how to make this. I wonder if you would be interested in making another and selling it to me? My husband is an avid crabber here in savannah, ga but his crab traps have been repeatedly stolen. I would love to put a gps transponder into a waterproof buoy to attach to his trap, so that we can find them again when they wander off. If I could get you the dimensions of what needs to go inside, is there any chance you would make me one? It would be his Xmas present, but it does not need to be here for Xmas, I would give him an IOU. I wish I had the skills to just follow your directions, but I fear I am not up for it. Thanks for considering it.

  2. Oh, and come to think of it, design would be a little different because no anchor is needed, we would attach it to the trap.

  3. I have to say I find it amusing that you talk so much about “How to geocache with quality” on your site, but when this cache when missing, you simply ignored it, made no attempt to replace or archive and just waited for the reviewer to have to do it for you. Doesn’t being a “quality geocacher” include cache maintenance?

    • Hi Paul, yes you are right. However in this instance I decided to leave it up for as long as possible because I have various links pointing to the cache from YouTube and my site. If the cache was archived I was concerned that those links would disappear. I’m not going to put it out again because there is some nefarious cache thief who has been operating in the area, and has removed other caches in the bay.

  4. Great idea and video. Appreciate your taking the time to document the process.

    I might suggest the use of an anchor knot instead of a bowline as my experience is that a bowline may come loose with the wave action.

    • Sorry, for the late response, I missed your comment.
      Yes you’re right. However a bigger problem than wave action is muggle action! The thing disappeared a few weeks after I put it out, and it was in a fairly secluded area.

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