Need a tensile safety-break solution for a submersible slalom course.

swbca

I installed a course through the ice in January and will be adding the submersion feature in May. I have 700 feet of 3/32 cable that pulls the course down and is suspended across a valley with matching flotation to keep it from sagging or floating. This is a repeat of a submersible design I did on another lake many years ago that worked well for three decades until we moved to another lake.

In the event someone catches this cable with a boat anchor, I need a break-away feature to keep this from wrecking the course. I intend to put the "weak link" in the middle of the cable.

Normal tension for winding and holding the course down is around 300 lbs. I want the break away to happen around 400 lbs. I have been testing commercial cable ties with a tensile scale. 4 wraps with a 50 pound tie binding 2 stainless steel fittings breaks at 400 pounds. This is consistent with the fact that most commercial ties break at 2x their specified strength.

But I don't think cable ties will be reliable with frequent release and reload cycles that are within 75% of their break point. I think their break point will decline with constant reloading.

Any idea on another material to use as a break-away link that won't deteriorate with load cycles. For example a small high density poly rope that breaks at 400 pounds would probably be better, but I am just guessing.

BraceMaker

Why? What item that can catch your course with more than 300+ lbs of pull is going to be protected by a break away? At 300 lb someone ain't gonna be pulling up an anchor. They're going to cut it loose.

swbca

@BraceMaker if a 700 foot line has 300 pounds of tension, a momentary100 pound lateral conflict has the leverage to increase the tension by a more than 100 pounds.

BraceMaker

Gonna have to help me here. My question is more why or in what scenario does the mainline failing help out?

If a prop catches say a polypropylene and bungie bouy line would you rather it breaks the bouy off or breaks your safety and wraps the cable up.

swbca

@BraceMaker This illustration is from my original article in the Water Skier magazine with "how-to" instructions on the world's first submersible course. (Mike Suyderhoud apparently produced and patented the second submersible course)

On this illustration, if a boat's anchor line puts 100 pound of lateral or vertical pressure on the 700 foot cable, it increases the end-point tension on this cable by several hundred pounds. I want the cable to break at 400 pounds so it doesn't pull components in the slalom course out of position.

Someone is going to wonder Why this design ? I want a course that can be wound up or down from shore, 700 feet away from the course.

My friend Lance in Minnesota uses several hundred feet of plastic tubing to inflate 42 foot long bladders inside aluminum pipes for each set of buoys (straighter than PVC pipes). It was more expensive to implement and required a very long series of tweaks to make the deflation of the bladders reliable. His course has been reliable for many years and sinks to the bottom of the lake (45 feet down) when he lets the air out.

My course is low tech and was trouble free for 30 years the last time I built this design. On the 1975 original installation, the course went up and down with a button on the dashboard of our boat.

skimtb

My first thought would be to have something machined by a machine shop that would be similar to a “tensile test” specimen, incorporating holes in the ends to connect clips to your rope or cable.

Basically it would be custom machined such that the cross section of the middle would result in breaking at 400 lbs. The slight challenge will be that material properties can vary some so you would be 400 lbs +/- something....

Surface finish of the machined part may effect it too but that may be more important for fatigue vs and ultimate failure as you are looking for.

Kinda like the piece near end of this article.

https://en.m.wikipedia.org/wiki/Tensile_testing

mwfillmore

Not sure how to get a rope to break consistently... but would think that would be the most cost effective. Para cord? Maybe test with a piece of nylon? Something that won’t degrade in the water... else wise I’ve seen breakaway swivels that would work but I haven’t seen them in stainless ...
https://www.wctproducts.com/products/swivels-connectors/breakaway-swivel.php

Jody_Seal

Contact Ron Tannis of American skier boats. Ron invented the sled used in awsa towboat acceleration tests. The sled has a breakaway section in the tow rope that breaks at a specific tension.
Ron still has a shop in Louisiana.

swbca

@mwfillmore HDPE ski rope is good in water for a very long time, if not forever. (my previous slalom course was in the water for 30 years) I have some new bulk ski rope from Masterline. I think I counted 15 filament sets in the rope. A bundle of 4 sets broke a 420 pounds in a single test. With some testing while wet after many load/offload cycles I might be able to provide a sufficiently constant breaking point.

A better idea ?
Calibrating a tensile safety break based on a product breaking probably isn't the best concept for a cheap solution. Using a stainless cable clamp to splice cables end-to-end, a failure value could be calibrated by the torque value on the two nuts. This would be less likely to change in value over a long series of load and unload cycles because no materials are being stretched to their limit repeatedly.


Thanks for everyone's input.

swbca

@Jody_Seal I will contact Ron. Thanks

BraceMaker

@swbca gotcha I was envisioning a break away linearly along the course mainline.

Is there a reason not to engineer the break away such that it is above water on shore? If this is acceptable attach a pulley to the end of the line from the course. This pulley will have the winch cable go through it and back to a fuel line break away fitting. Those are 200 lb tension break aways.

The break aways have pipe thread fittings - but a pipe thread plug drill a hole in it and fish your cable through it and tie a knot. Then thread it into the break away.

Tension on the single line under to the course exceeds 400 lbs will cause tension in 1 leg of the system to hit 200 and POP. Part is reconnectable - disconnects repeatedly w/o failure and is twist to re-engage. Will also halve the amount of force on the winch to reach tension.

BraceMaker

7 LB reconnect force to reattach the unit... under 100 bucks and engineered w/ load rating. Probably would also survive underwater but the 200lb seems to be pretty standard on these units so you might need to do 2 of them in parallel on some sort of Y harness to do that.

swbca

@BraceMaker Interesting . . The pulley idea looks very interesting but creates a problem. I am concerned about an anchor line from a boat. When a boat finds it is hung up on something, they will probably maneuver the boat, or forcefully wind the anchor to get un-hung. If the release is on shore, that could end up pulling the 250 foot pipe protecting the cable out into the lake, and may not release the boat from the our cable. I expect that a break-away in the center that has no sizable hardware at the break point, would allow an anchor line or anchor to slip off the end of the cable.

But here's another design question and design seems to be your expertise

This is a second home, so we are frequently away. All critical functions at his home are monitored with programming conditions to send us an email alert identifying problems. I have been trying to think of the most simple method to detect when the course is "up" based on low cable tension near the winch. All I need is a contact closure or opening when the cable is at low tension.

We are under legal requirements to never have this course UP over-night or lake property owners would have cause to take action with the Sheriff. So I need to know if the course is UP due to some mishap.

I don't want to interfere with the cable winding in neat layers with something that inhibits the cable from moving laterally near the winch drum. So then I was thinking of some type of pressure or magnetic switch function that could detect when the winch mounting is stressed by high cable tension or not. If the winch-mount was spring-loaded and hinged to allow enough travel for a magnetic security contact, this would be the most compact method to input this change of state into our processor. I would just need to make it weatherproof.

Any other ideas ?

BraceMaker

Maybe some sort of cone ahead of the body of the release? Or the pulley could be a large open hook?

Maybe have the pulley on a board with a hook that is held up at an angle such that once the release tripped the board goes flat the arm of the hook then is beyond flat and releases a small eyelet?

BraceMaker

In this configuration whatever the hook/loop was could be very streamlined and could run down your pipe?

The unit I posted you'd have another option too - you could pressurize the unit on shore. I think they are rated to 50 PSI - Then a simple pressure sensing switch would read if the unit popped. The shorewards side of that unit could be threaded to some iron pipe which would be pressurized and have a switch.

Or when tensioned up the pivot in the drawing could hit an electric contact and open when pulled away. Connect that to one of the many available wireless units and away you go.

swbca

I appreciate you applying your skills and effort on my behalf but their is a logistical problem with the break-away near the winch.

If the break-away is in the middle of the cable, I can fix it in a half hour. I wind out an extra 50 feet of cable from the winch, Retrieve the 2 ends with my grappling hook, add temporary floats to the ends, pull the ends together and replace the link. If the cable has been pulled out of the pipe, my techniques for getting a cable back into the pipe don't work. (cant blow a pull string through a wet pipe floating in the lake). Water pressure would maybe work with fittings made to connect a water hose into the same end of pipe with the end of the cable and a piston fitted to the pipe interior to pull the cable.

I like the pressure sensor concept for monitoring . . I will keep working on it.

BraceMaker

You've got an interesting problem so its fun to think about. My personal dream is a fully water powered hydraulically raised ie. water pressure extends retractable cylinders to raise all the bouys those cylinders can leak all they want so long as I can pump enough water while in operating to overcome some retractive force be it from ballast or tension.

For your alert as I think about it the easy solution is to mount your winch on a plate that slides with a heavy spring maybe 50-100 lbs. When you crank the tension onto the line this plate would just slide forwards till it hit a stop and you could install a simple switch that would be closed by the plate. Figure out what you can install that will send you an offline alert maybe a smart thermostat? Ideally something like that which would work off low voltage. You'd run low voltage thermostat wire out to the switch which would open the common and the thermostat would go offline then that would give you an email.

swbca

@BraceMaker The Sensor After looking at your post, the winch will be mounted on a vertical plate with bolts and rubber bushings. It will be mounted so it doesn't move down but will compress the bushings at the bottom edge of the winch body when there is sufficient downward tension on the cable. A microswitch will be activated when the bottom edge of the winch presses in towards the mount plate, but with a physical stop so the switch doesn't get crushed.

The email alert Our local sheriff has relaxed enforcement of state DNR rules and there is no other jurisdiction. Without a permit the Sherriff allows submersible courses as long as they aren't UP from sunset to sunrise.

This is a second home so I need to know if the course comes up accidentally when we aren't around. We have a system that runs HVAC, Security, Lighting with unlimited auxiliary inputs and programmable conditions. Email alerts can be sent for any event. The microswitch on the winch has been assigned a sensor ID that is programmed to send us an email when the cable has low tension or normal course-down tension. I programmed this stuff remotely last night . . that was the easy part.

Thanks for your assistance.

buoyboy1

Two ideas: What about a small section of galvanized chain near the center with a snap clevis at each end so it is easily replaceable. You could do the tests yourself and see what load the chain will fail.

Not sure how deep your water is but what about a very long galvanized steel spring at each end of the course that has enough tension to keep the course tight but will allow for enough expansion that the course cable is able to reach the surface prior to the rest of the cable or springs to fail? You might need to replace the springs every couple of years unless you are able to find stainless steel ones.

swbca

@buoyboy1 This isn't a cable course. It has individual anchor lines that are pulled over at an angle to lower the course. The horizontal network of ropes connects at 44 feet above the bottom. Pulling the anchor lines 26 feet horizontally lowers the course 6 feet.

Below is my first 1975 installation on a different lake where the horizontal lines were all at the sub-buoys. I am going to try calibrating the safety-break on the cable that goes to the winch with torque values on a cable splice. I just got a 1/4" pound-inch torque wrench to test and set this up.

Deke

Don't laugh at me...

How about making up some sort of magnetic coupling?

BraceMaker

More slender 350 lbs - non-reusable 110bucks - smaller diameter. A couple of plastic V shaped bits to guide it through some sort of loop and you're in like flin - threaded so 2 wrenches and you can put together.

https://www.jmesales.com/opw-66v-series-1-in-breakaway-for-b20-w-350-lbs-pull-force/?utm_source=google&utm_medium=surfaces&utm_campaign=shopping_feed&gclid=CjwKCAjwu5CDBhB9EiwA0w6sLTiBxxtT4PV0qhOb59ckxFlK5chsXIYTT8uaEexVR9uHpNWWH5ohohoCB-wQAvD_BwE

swbca

@BraceMaker If the cable clamps don't calibrate with torque setting consistently, this looks like a good alternative.

swbca

@deke I did some testing with rare earth magnets for attaching buoys to sub buoys without going swimming. The concept is good but it would be hard to get to 400 pounds with a magnet assembly that is light and small. Plus it all has to be water sealed because they fail with corrosion. there may be some industrial magnet solution that would work. My knowledge is just with generic rare earth magnets.

Deke

@swbca got it about the magnets. Check out this line of breakaway connectors. There's a ton of options and pretty compact too.

swbca

@Deke The link you posted is dead. Interested in seeing what you found

Deke

@swbca Try this. There are wire and pin types. I just found this on a search, not by any experience.

http://www.breakawayconnector.com/products/breakaway-connectors.html
http://www.breakawayconnector.com/products/breakaway-swivels.html
http://www.breakawayconnector.com/products/index.html