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Author Topic: Need some engineering advice - little DIY project in steel/plastic  (Read 1512 times)


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Hello all.

This is totally unrelated to boats, but seeing as there are a lot of clever engineery types on here, I wonder if I can pose a bit of a design question to you and get some feedback.

OK background first:

I go fishing, and from the day I started, have made a lot of my own tackle. Rods, floats, weights, lures, flies etc etc etc. This trend continues to this day, and I only buy what I can't make, such as reels, hooks, line etc.

However, I have a requirement for a "widget" which I can't buy commercially and my own home made version is not satisfactory. Therefore, I'm wondering if someone could take a look at my current design and suggest a better way of making it.

Aim: one of the main ways of catching fish is to attach a lead weight to the line in a variety of different ways. This sinks the line, hook and bait to the bottom. When the fish takes the bait, the line goes tight.

There are a close to unlimited number of mechanisms that can be used as an indicator to show when this is happening, however, when fishing is fast flowing water, most of them cease to function effectively.

One widget that remains functional is called a bobbin. You put the rod in a rest and draw out some of the line between the reel and the first rod ring. You then clip on the bobbin about half way between the rod and the ground. Any movement of the line is signalled by a movement up or down.

A picture of a bobbin in action:

close up of a bobbin design

One end screws into the rod rest, the other end, with the weight on it, attaches to the line.

Problem: I use a nowadays very uncommon method of fishing ( Called bowed back-legering ) which means that I can fish right in the very centre of the roughest water in a river, such as a weir pool in full flood, and at the same time be very effective at catching fish and use delicate tackle.

This method uses bobbins as well, but the bobbin needs to be very rapidly adjustable from between 1/4 ounce to up to 12 ounces. By this I mean that, depending on where my cast lands, I might need the bobbin to weight 3 ounces, and the next cast, 5 minutes later, I might need it to weigh 10. This rapid changing will happen every cast for 5 or maybe even 10 hours.

There is no commercially available bobbin design that allows this sort of easy or rapid adjustment. Most allow a few grams or maybe an ounce, and that's it. There are also no commercial bobbins that weight more than 4 ounces.

I have designed made my own solution by taking a 1/2 ounce bolt and cutting a slot in the end of the threading. To this, I epoxy in a hair clip of my wifes with a more severe bend in it so it clips to the line. To alter the weight I add or remove washers as required. Add a full thread of washers and I have about 8 ounces, remove 1/2 and I have about 4 ounces etc. the other end is tied t a string, which is tied to the rod rest.

The issue is that when you hands are freezing cold, soaking wet, covered in ground bait, corm syrup and (if you have been sucsessful) fish slime, possibly in gloves as well, this is close to impossible. Also washers get lost very easily. While this design is fine for the middle of summer with warm and dextious fingers, when I actually need it, i.e in the winter when the river is flooding, it's freezing and chucking it down with rain, it's not actually very usable.

I would be incredibly grateful if an engineering type person could look at the problem with "Outsiders eyes" and see if they can see a better way of having a large amount of adjustable weight on this bobbin.

The requirements are:

1) must have a clip at the top to attach to line
2) Must be alterable in 30 seconds or less to any weight between 1/2 ounce and 12 oz
3) Must be operable with cold, wet fingers or while wearing gloves
4) the mechanism must be more difficult to loose than washers ( i.e easy to find in a bag, easy to find if dropped on the bank. Ideally, it should not roll to far if dropped either )
5) Must not be fragile: It will get wet, dropped in river, covered in mud and sand and frozen on a regular basis
6) must have something to tie a cord or string to so I can attach it to the rod rest
7) must be practical to transport - for example no computer controlled winches setting a fixed pressure
8) Must be in a material I can work at home - mild steel, brass, copper, perhaps aluminium, or plastic.
9) Must fit in a box or bag with all assorted bits, variable weights etc, to allow it to be shoved in a carry bag along with rest of my kit.

I'll give you some examples of idea's that I have thought of:
1) Balloon on a line clip that allows water to be injected or removed to get to the right weight - can't see how this would actually work though

2) instead of using a big weight on a string, use a motorcycle chain with just a clip on it. Come up with some way to rapidly add or remove links to add/remove weight. Issue- can't see how this would be done, also issue about it being easy to do with cold fingers..

I know that this is fairly tricky request, and having spent 3 months trying out all sorts of different idea's like the above, I think I am probably going to get an answer of "Your about as good as your going to get" and I'll just have to deal with it.

But if any one fancies a problem to play with in their heads, please have a ponder and let me have any idea's, no matter how odd sounding. I will be eternally greatful.

Many thanks indeed.


Ian Robins

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Re: Need some engineering advice - little DIY project in steel/plastic
« Reply #1 on: December 28, 2007, 09:48:15 pm »

Hi steve,
I am also into fishing and I machine plastic for a living so I also qualify as an engineer.
Have you tried a arm swinger, as depending where the weight is, from the pivot the weight changes (hope it makes sense to you)
ie 4oz at pivot load 4oz  or 4oz at 3" distance equals 4 x 3 oz = 12oz I hope.
I can check this out but

load = distance from pivot x Weight

some law from my school days in physics

in other terms  look at it as if it was a see saw

Hope this helps if not pm me and I will try to find arm swinger for you



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Re: Need some engineering advice - little DIY project in steel/plastic
« Reply #2 on: December 28, 2007, 10:40:14 pm »

Hi guys....have not been fishing for some years now , but , thinking sideways , would a swim feeder or something similar with no holes in , and lead shot not do the trick....easy to adjust the weight ( add or remove shot ) has a loop at one end which can be modified as necessary , cheap , easy to transport ..  might be a good idea...then again it might not......anywhoo....good luck



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Re: Need some engineering advice - little DIY project in steel/plastic
« Reply #3 on: December 28, 2007, 11:49:20 pm »

I get the gist of what you are asking but not sure I am going to be able to translate this into a finished article as A) I have never fished in my life and B) I'm not 100% sure what this device is doing.

However I can understand your drift so all I am going to do is offer suggestions for you to develop into a mechanism.

My first thought rotates around the use of water, as you suggested.  What you need however is something quickly and easily adjustable with cold hands and that's where I think you have missed it a bit.  Take one large medical syringe, I have one up to about 60 cc for bilge pumping on the model boats, and fit a length of silicone rubber to the nozzle.  Manufacture an arrangement whereby this can be attached to your line as required however the operation of the plunger needs to remain unaffected.

All you do to increase the weight is to stick the silicone rubber into the river and draw the plunger, sucking water into the syringe.  To remove the water simply compress the syringe and eject the water through the tube.  You don't need a valve as the water will remain in the syringe under the vacuum effect until you decide to eject it.  You can easily calibrate the syringe in weight of water and mark up the scale at home so adjusting the weight higher or lower would be a matter of a couple of seconds pushing the plunger one way or another.  Couldn't be much quicker or easier.
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Re: Need some engineering advice - little DIY project in steel/plastic
« Reply #4 on: December 29, 2007, 01:35:38 am »

Immediate thought


Only the first one needs a hole for attaching. Or you can use a thin sheet steel plate attached to rod and then put magnets on this.

A steel plate on the rod rest for attaching those not in use.
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