GPS surface float

This project comes about as a result of a small problem encountered while scallop diving. My regular dive buddy and I often go for scallops in an area with a rather unimpressive 20 feet of water at high tide. The bottom is fairly flat in this area so to get to the deep water the scallops like you’re faced with either a surface swim of approximately half a kilometer or swimming out on bottom. The latter option is relaxing and enjoyable but leaves you with very little air by the time you find the scallops and their significant risk of winding up somewhere other than where you intended, navigation underwater is not an exact science at least not for me! The surface swim is not an ideal option either, firstly I’m no athlete so a half kilometer swim in scuba gear leaves me more in the mood to call the Coast Guard for a pickup than to dive for scallops. Furthermore, the navigation is only simplified slightly by the surface swim. Certainly you can see the surrounding landmarks but being at the surface of the water gives a fairly poor overall perspective, you can certainly tell your out in the middle of the bay, but where exactly? How close are you to the spot you picked out on the chart before you left?

So it occurred to me it wouldn’t be too difficult to construct towable surface float equipped with a GPS. Although this wouldn’t solve all the problems it would give you a substantive empirical record of where you have been. You can compare your impression of where you were with where you actually were, right after the dive. Hopefully that develops a more accurate sense of place out there in the blue void. Further, your keeping a record of where you’ve been and perhaps more importantly where you having been and together with notes on catch sizes could develop into a map of where the little beggars are!

And thus concludes the back story on why the following curious gadget was brought into existence.

It occurs to me just now that althought it has served me very well my workbench is not a very good backdrop for photographs.

The core of the concept is to confine an inner tube between two plates creating a flat “cargo deck” on top. The Innertube size is that used for wheelbarrow tires and plates are constructed from 3/8 plywood, quarter-inch aluminum would’ve been preferred but I had the plywood and it’s easier to work with. Unfortunately it necessitates a coat of paint and the truth is I hate to paint stuff, between waiting for it to dry and waiting for me to get around put another coat on it took two weeks to paint the damn thing! But I guess it don’t look too bad

Most of the cutting and drilling is done with the two pieces clamped together so the holes line up and it looks slightly more “slick and professional”.

We have a pattern of four holes to hold the two plates together, a perimeter of 1 inch holes which allow for drainage in the center of the device, and a hole right in the center to mount the ballast weight and tow eye. The oblong hole allows access to the inner tubes Schrader valve.

The ballast is a 5 pound lead dive weight with a hole drilled in the center for mounting(careful drilling lead, rumor is the stuff’s toxic). All bolts I’m using are 5/16. The eyebolt serves double duty both mounting the ballast weight and serving to affix the tow line. Of course this is for use in seawater so all the hardware is stainless steel.

The ballast weight of course goes in the bottom side of the bottom plate. On the top side of the top plate we mount the waterproof electronics bay. This is a lock’n’lock container by star Fritz which I acquired from the local Walmart. I first encountered these containers while Geocaching and noted that they were the only cash containers that seem to be consistently dry inside.

I drilled appropriate sized holes in the corners through both the container and the plywood then taking the container away I cover the holes in the plywood with a blob of 3M’s 5200 marine adhesive sealant(this is notoriously sticky stuff you can even apply it underwater). The container goes back down secured with stainless steel wood screws, threads coated with more 5200 for good measure. The only trouble with 5200 is the stuff gets everywhere, once it’s applied to a project it’s best just to walk away till it sets.

The planned occupant of this electronics bay is my Garmin etrek legend but I don’t want it just beating around in their, it needs to be secured somehow. The etrek series uses car mounts which replaced the rear battery cover which seems to me a clever idea and I took my cue from that. I acquired a couple of replacement battery covers from since the back of the GPS is nothing close to flat I fixed the battery cover to a piece of strap aluminum with another nice blob of 5200.

Of course the access hole was cut before hand, this lets you get at the bolt that holds the battery cover on. Actually I guess it’s not a bolt, it’s to Dzus(zeus) clip.

Let the 5200 dry and cover the bottom of the Velcro and now you got a GPS with a nice flat bottom that stays where you stick it. For an application like this I like  the soft fuzzy side of the Velcro on my device. Inevitably the GPS will be held in hand with the dry box adapter still mounted so I give it the comfy side.

Of course the bottom of the electronics bay needs to be covered with the other side of the Velcro. It’s a loose fit but this is the smallest container the GPS fits in and this allows room for additional electronics in the future Perhaps a radio, or a data logger, or some small computing device. The Velcro keeps everything secure so no need for a tight fit.

Time for final assembly then. Bolts go through the bottom of the bottom plate, innertube on top. Spacers are cut from half-inch pex pipe, in this picture the innertube is just full. All the parts are sized so that when fully inflated the innertube just touches the spacers all around. By fully inflated I mean just at the point when it starts to bulge out one place. With the tube fully inflated you kind of need a press to put it together I thought it would be easier just to let the air out. The bottom plate sitting on a stack of books to make room for the eyebolt, if you ever get a chance to get your hands on a set of vintage handyman encyclopedias I highly recommend them, not only are they full of knowledge both useful and humorous there all the same size which makes them very handy for impromptu jigs such as this.

Tighten up the nuts, top off the tube with air, and there you have it. Unfortunately it’s getting quite late in the season as I finish this project, fall rain and wind stir up sediments and visibility becomes atrocious. So it’s not clear that this device will get its field trial this year however when it does I shall append a complete report.

1 thought on “GPS surface float”

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