Engineering/handyman assistance needed

[SRanney]

[Let me preface this thread by saying this: I’m a pretty handy guy, but I know that there are others who are much more engineering minded than I. Recognizing this fact, I am aware that there are those on this board who are likely to be able to offer interesting insight into this project that I had not considered before.]

I am a fisheries science graduate student at Montana State University in Bozeman. I begin my field season on Ft. Peck Reservoir soon and recently decided that I’ll need to manufacture a “bongo net” and frame (conceptual figure below). Bongo nets/frames are relatively standard larval fisheries sampling equipment designed to be pushed by a boat at low speeds (< 1 mph). Standard they may be, frames for bongo nets are exceptionally expensive and my project’s budget will not support the purchase of one. I’ve decided that I’ll have to fashion one myself, but would like the thoughts and inputs of those who know more than I about engineering and handy work.

I should mention that the design in the image doesn't have to be exactly what I build. For example, the net end of the vertical frame does not have to be curved as it is in the image. I've provided only a conceptual design.



While the net I’m conceiving of does not look identical to the one in the figure, it is similar. I had originally considered using ¾” rigid conduit for both the horizontal and the vertical pieces but soon realized that the “adjustable height” part would be difficult to overcome. Granted, tack welding a small piece of 1” conduit (with set screw) onto the for end of the horizontal frame would allow for an adjustable vertical frame, but tack welds can fail, require specialized skills (which I do not have), and a completely self-repairable frame is ideal.

On my evening run, I realized that angle iron (the kind that is “perforated” every inch or so) may be perfect. The horizontal frame (including cross beams) could be easily attached to the bow—including an “extender” if necessary—and the vertical struts could be easily made to securely hold the net itself. Unfortunately though, I’m still up against the “adjustable height” problem, affixing the vertical piece to the horizontal frame, and (hopefully) being able to “hinge” the vertical piece so that it can fold back onto itself for ease of transport while on the water. (The hinge does not have to be an actual hinge; a bolt and cotter pin would be just fine.)

It would be nice if the hold apparatus were relatively durable (e.g., could be used for a few field seasons), but this is not necessary. It would also be nice if the entire thing could be disassembled for long-term storage. Permanently affixing the equipment to the boat is out of the question. Max depth of the net would likely be 10’ or maybe just a bit more. I will not have to change the depth of the net rapidly, but would have to do so while on the water. I will have a tool box on board, but using fasteners that could be manipulated by hand would be ideal.

If you have any questions, I’ll be happy to answer them. I would appreciate any thoughts you may have.

Thanks,

Steven Ranney
http://studentweb.montana.edu/steven.ranney" target="_blank" target="_blank" target="_blank

P.S. - If you'd like to communicate privately, feel free to send me a PM.

SR

[Matt G]

There is some stuff that electrical contractors use that may be handy. One name brand is "Unistrut".

It isn't "cheap", but it is fairly configurable and not too hard to find. Most of your local electrical wholesalers should have some comparable product in stock, along with brackets and fittings. It normally comes in a regular zinc-galvanized steel form, but is available in stainless steel and aluminum (for a definite price increase). It's been years since I sold this stuff, so I have no idea what the linear foot price estimate would be. It used to be between $1-$2 per LF and comes in 10' sections. It also normally comes with slotted holes drilled in the back for mounting.

It might be fitted to be "adjustable" but would require a bit of unbolting to do so.

Following your idea of using conduit, if you could bend it into a "U" the Unistrut product is designed to be used with adjustable pipe clamps. You could run two lengths of strut out to the front where the pipe clamps would hold the "U" of conduit. Those pipe clamps could be fastened with wing nuts for ease of adjustment. Furthermore, the strut would not only allow for vertical adjustment, but fore/aft adjustment as well.

[SRanney]

I should mention that the finished product does not have to be identical to the image provided. The image is only a guide rather than a blueprint. For example, the net end of the vertical structure does not have to be rounded. Ninety degree angles would be fine as well. Ultimately, I'm looking for design simplicity and ease of construction.

Thanks again for everyone's help.

SR

[rocksanddirt]

also, check out Grainger's website/catalog for various kinds of unistrut and attachment equipment.

http://www.grainger.com" target="_blank

[Donn]

Plastic plumbing is light, cheap and fairly strong. ABS (black) is a lot stronger than PVC (white), but PVC might do for parts of it. I'm thinking of a simple structure of essentially 3 parts - two "ladders" and a hoop. The hoop of course holds the net open. I haven't thought about how to make the hoop, because you evidently already have it? By "ladder" I only mean two parallel members connected by two or more cross braces.

I would connect these elements more or less as shown in your diagram. The vertical ladder's width matches the hoop, and is connected at the sides, and top, bottom or both to cross brace(s). I would make it taller than your diagram, so it stick up a ways, but suit yourself. The horizontal ladder width matches the vertical ladder, inside or outside your choice, and it's longer than shown in your diagram, because the vertical ladder is attached some ways back from the front. All these connections can be pins - just drill through both members and insert a bolt. Drill holes for every height you want.

Now string light cable from the front ends of the horizontal ladder, to the bottom ends of the vertical ladder. That will hold it in place when moving forward. It will fold up when not in use, to whatever extent allowed by placement of the various cross braces.

It will be light, and somewhat fragile - it might break into pieces the first time you use it, but at least it's cheap and easy to build, and might have a little less drag than some of the alternatives, due to the round cross section. ABS (or PVC) T fittings will be at least half the expense, but they're good for strength. The most obvious point of failure is the pivot, and it would very likely be worth the trouble to lash something to the bottom of the horizontal ladder to accept the pivot bolt holes, instead of drilling them through the pipe at this stress point. (And it will fold up better.)

[SRanney]

I am a fisheries science graduate student

...so you are working towards being a Master Baiter...??

Wow. I hadn't heard that one before, Bloke! To keep that joke from rearing it's head again, I've already got that title. Now I'm working towards a PhD.

Plastic could work, but the torque that will be applied at the net end would probably collapse the whole shebang.

I'll be aboard a 22' aluminum Wooldridge and yes, weight distribution is a concern. A friend suggested 2x6s reinforced with oversized washers at all key fulcrums and other points of contacts. That would certainly reduce the weight, but the frame likely wouldn't last as long as steel or perforated angle iron.

SR

[Donn]

I just recently bought some high pressure plastic pipe (for plumbing applications) that is charcoal gray (vs. white or manila in color). It costs more than white or manila pipe, but not tremendously more...Is that the stuff you're referring to?

No, I think that's just better PVC, but haven't done anything with it. ABS is large diameter waste pipe, softer and more flexible material than PVC which is too brittle. (Get the specific cement for this material, and move fast when cementing the pieces together.) I'm sure grey PVC is much better than white, if smaller diameter is desired, but I'd bet on ABS in its typical size.

My experience with these materials actually relates to the tuba - made a couple of bicycle trailers, first of PVC and then ABS.

[rocksanddirt]

The grey stuff is either schedule 80 pvc water pipe (higher quality than the white schedule 40), or it is schedule 40 pvc electrical conduit, which is unsuitable for the application.

Likely pvc is not a good material for this application.