Amen! (NM)
Let's just agree that we are talking about dry boat - on top of water, and there are no breaking waves involved.
The OP was not dealing with any sea states, so this observation is valid.
In WW, submerged boat, or SK, similar, breakers or anything involving sudden release of energy, most of the bets are off. Make sure you have, at least some, idea what you are facing. Being jerked by a dumper while towing out is NOT fun.
Just some final remarks
I thought I was done, but it’s raining and that changes my schedule at the moment, so I’ll write a little more.
“You have spoken of the forced required to achieve an acceleration of a mass (weight) . I have spoken of the forces attained at an abrupt decceleration of a mass , such as an abrupt stop of the towed vessel could create.” There is no difference between the two situation. In physics, there is no such thing as deceleration, only acceleration. Deceleration is a lay term that describes the direction of acceleration. The force applied to the object is the same in either case. Even driving a car at constant speed around a curve requires an input of force and is defined as acceleration and the force is defined the same was as for all forms of acceleration.
“In the story where the Warden held the rope in his hand , my guess is did so for safty purposes knowing that if he had attached the rope to boat directly it was possible for the rope to break or cause a capsize or injury should you all being towed have gotten snagged on something . The highest possible loading that could of been placed on the rope in his hand was only whatever lbs. force he could hold on to , not the weight of his gross boat load as might have been applied in the snag scenario.” He held the rope in case our boat got out of control. Here again, if you had ever towed a canoe behind a motor boat, you would know that the real danger is the boat taking off at a cockeyed direction and suddenly flipping and swamping. It’s not the rope you are worrying about in that situation, as the rope has plenty of strength to destroy the canoe in that scenario. just as sometimes happens when freeing a pinned boat in swift current but typing the rope to a convenient place instead of a location that causes the boat to lift up before being pulled in. We actually told the warden this quick-release option was necessary because keeping the canoe pointed the right way at that speed was a pretty sketchy and nerve-racking affair. Had the boat turned sideways and and flipped but not been allowed to release, it would have been seriously damaged. The correct way to tow a canoe at high speed is with a harness (which puts the anchor point right on the keel line, a few feet behind the bow), just like what whitewater paddlers used in the old days for lining through rapids. That would have been perfectly safe, and no quick-release would have been needed.
“In the story of you and your Dad the lb. force applied was most likely a uniform graduation of lbs. force and not a shock load . If your Dad would have allowed a running slack and then accelerated until the anchor rode stretched tight , it may have snapped the line as it realized the full weight of the pulling vessel combined with it’s forward momentum . My guess is he did not want to break the line or have something else break instead and therefore applied a gradual uniform stress to the anchor rode as opposed to jerking it out.” When you read it again you will see that there were some pretty strong jerks during those crazy corrections to the steering that were needed. You are right that a really strong jerk on the line would have broken something on the boat. The rope was wrapped over the gunwale a few feet behind the bow, putting a sideways force on that gunwale, and that’s what was creaking. But how could you duplicate such forces when towing one kayak with another? That was a much stronger force than any towing situation, even one motorboat towing another. With two kayaks, even if the towed boat stopped dead, the pulling boat is so light, the speed so small, the rope has a bit if give, and it would be no big deal. No worse than bumping into a log when paddling alone.
“I agree that you or I could not resist a 300 lb. sudden force applied to a rope in our hand , perhaps a gradual force applied to 300 lbs. is possible though certainly not req. to accelerate a kayak on the water … but I do believe that a towing boat could resist a 300 lb. (or greater) force if the same rope attached directly from kayak to kayak in motion such as being towed , were to experience the towed vessel suddenly and abruptly being halted such as in a snag or hang scenario as I suggested prior.” You just need to do this yourself a few times to see what (to you) will be an amazingly small amount of tension in the rope, even during the “worst” jerks. Maybe reply to the other posters in this thread who are saying the rope tension will never exceed 20 pounds and see if they can explain it better than I can. Those are people who have towed boats too, it seems.
suriam , I agree as well …
… no contest as to the amount of energy (lbs. force ??) required to pull or get a boat into motion on the water (overcome it’s inertia on water) , kayak or 4 ton vessel … never was , nor needed to be , it’s always been more than obvious to me that the required force is very minimal , dah . (here’s a capitol L to wear on the foreheads of those who think it nessasary to argue the point w/me)
I offered examples of the breaking point of 1/4" poly rope (lbs. weight ??) . I also suggested the 1/4" poly line could break under a shock load situation if connected directly from boat to boat .
As a side note I haven’t noticed anyone else expressing any understanding of how a line’s breaking point is deternined . Seems one might want to know that for various reasons .
I offered a scenario of the towed vessel becoming stopped abruptly at a speed of 3 mph. , and suggested that the tow boat’s weight (plus the potential energy of that weight at 3 mph velocity) , could be transfered to the tow line and result in exceeding the 1/4" poly ropes breaking point at a knot or attach point .
I think it’s a reasonable thought the possibilty exist that the towed vessel could become snagged (or severly interfered with) abruptly … but it seems a number of responders can only grasp the minimal force of what it takes to overcome the inertia of the towed vessel to get it moving on the water , and are not capable of (or ingnorantly refusing to) acknowledge that such a snag could happen and the resultant shock forces may be capable of breaking the line .
Because I do not feel capable of formulating the proper physics equation to express the amount of energy that could be transfered to the tow line in that snag scenario situation , I had ask you if you could do it … I still would like to know how much energy (preferably in relationship to the way lbs. are used to determine a ropes service ability) , could be transfered to the 1/4" poly line
Safety
From the safety side of things you want to ocnsider the worst case scenario which is that some powerboater thinks it would be cool to zip in between two kayaks and doesn’t see the tow line.
If I’m in either kayak and see this about to play out, I want that quick release or a knot that can be released. Second, I want to know that the rope is weak enough that it will break well before it jerks my kayak apart or does bodily damage if it is on a tow belt.
If you have static line rated at 4000 lbs tied into your boat the jerk is going to be bad and the rope will not fail before parts of you or your boat start coming apart. A stretchly, weak line diminishes the jerk and breaks more quickly.
And imagine if you have that line get wrapped around your wrist when the jerk occurs. Would you rather have a weak 3/8 inch rope or smaller diameter and much stronger paracord? The small, strong rope creates a lof of pressure on a small area and tends to slice into flechy things when enough force is appled.
Do all the math you like, but use medium to large, weak rope that floats and have a quick release on each end – even if it’s just a simple knot that unties quickly.
I seriously doubt there is any commercial rope 1/4 inch or larger that is too weak for towing.
I use a heavy chain
Damn the practicality. I know it won’t break!
if a boat w/a gross weight of 250 lbs.
… is traveling at 3 mph. pulling a vessel behind it , and the vessel behind gets stopped abruptly … how much energy will be transfered to the tow line at the moment the line becomes fully stressed , or shock loaded (in lbs. please) ??
I think the tow line will realize quite a bit more stress in the shock load situation than what’s required to simply get the towed vessel moving on the water (gradually overcome it’s inertia ??) .
If I understood you correctly gbg , you think the two forces would be equal .
I’ve towed small vessels as well as larger vessels before gbg , and I’ve even seen a good quality tow line break once in the tow operation . The towed vessel simply bounced on a wave , the line became momentarily slacked just a little , and at the point of retightening , snap it went . The towed vessel only weighed about a 1000 lbs. , the towing vessel weighed about 3000 lbs. , the line’s breaking point was rated at more than weight of the two boats combined .
got it
Sounds like a fun set of calculations, I might do it if I have time.
Too much misunderstanding gojng on here
No, at no point have I said that a quick snap applied by the rope is the same as gradually changing the speed of a boat by pulling on the rope. Please read more carefully and stop saying that. All I have said is that you won't experience such a violent, quick snap as that when towing someone. You need to get real. Next time you are out, tie 40 or 50 feet of rope to your canoe and tie the other end to the another canoe and paddle away slowly (remember, when towing another boat you'll be doing good if you can reach 3 mph. More likely you'll go 2 mph). I predict that you won't observe the kind of force you imagine. Try it at 5 mph if you want and you still won't see anything remotely violent. Try it when tied to a tree on shore and paddle 3 mph and still no big deal, but try it at 5 mph and maybe then you'll break something, but most likely it'll be the thwart on your canoe. It sure won't be the rope. Let us know what you find.
I say you should try this yourself, because I already know that you CAN get a running start against the tow line while the other boat is stationary. It happens a lot, and nothing violent happens as a result. There are too many options for "give" in the system (a couple inches of stretch in the rope, boat that pivots slightly from an off-center pull, boat rolls slightly due to the attachment point being higher than the center of gravity, and only ONE boat needs to act as a cushion to affect both). And don't come back and say it makes a difference which boat initiates the shock.
Oh, yeah, and probably the biggest cushioning effect is due to the fact that the heaviest thing in the boat is a live person, not a solid, inanimate object that is structurally connected to the boat. If the boat instantly stops, the person's upper body will simply rock forward a few inches, and that's all it takes to eliminate nearly all of the shock that would be due to his proportion of the total weight. After all, if a person were to jump off a two-foot-high step they'd land at a higher speed than a boat will travel, and if they landed with locked knees and a straight back, they'd do serious harm to their body. Instead, just a few inches of "give" in the knees is all it takes to make stopping at the bottom of that two-foot drop require a negligible force and no severe strain to the body results. The cargo in the boat isn't structurally part of the boat either. Even if tied in, it would shift a tiny bit if the boat somehow stopped instantaneously. Would you expect the ropes that lash your gear packs to the floor to break too?
If you don't try this for yourself, take it up with any of the other people here who are saying that 20 pounds of tension is all you will ever experience, and mostly much less. Maybe they will have another way of explaining it.
good luck
If you think the engineers are bad, just wait until the attorneys find this thread.
that’s an interesting point gbg …
… the part about the occupant(s) and gear shifting forwards .
That does seem as if some of the weight in motion might not be fully applied into the shock loading on the tow line . Seems occupant(s)/gear deceleration rate would be taking place at a bit slower rate than the fixed tow boat .
Sounds interesting to me . Yet I’m not certain at the moment the actual shock load is imparted that the full weight of boat/occupant(s)/gear aren’t still fully realized on the tow line . I can see the forward momentum of occupant(s)/gear is a resultant reaction of a body in motion and all that , I’m just not sure how it all plays out in the time span of the instant .
Besides , I’m still not sure how much shock load in lbs. force could be generated to the tow line in the scenario I offered of a 250 lb. boat moving at 3 mph. being stopped abruptly .
If some of the 250 lb. weight can be displaced prior to the moment of shock loading , that should make a significant difference . Also as you mentioned if the boat rolls slightly (but what if it rolls the other way in favor of the shock loading ??) , and some of the other things you mentioned … all possibly contributing to a form of cushioning effect (energy absorption) . I think they are valid points worth considering . How they would play out in the generated force equation I don’t know , but it seems they could reduce it to some degree , “if” they took place .
I really think the main factor here in the abrupt stop scenario I suggested is the weight of the towing vessel (magnified by it’s velocity) , always have .
Flags
Of course, to avoid the power boat situation described above, you would want to fly your “vessel in tow” flags and lights.
This is the longest thread for what seemed to me a very simple question that I have seen. It’s not even really debatable, like the rudder/skeg or British/North American fiascos.
yeah , wish I knew how to do the calcs
...... thanks if you can find the time or motovation to show me how it's done , if not don't sweat it no big deal really just curious .
Out of context with OP's situation but perhaps not out of context with using a 1/4" poly rope for towing a very light boat (a kayak , couple in a canoe , etc.) ... same scenario of course it's understood the towed vessel get's snagged abruptly to a stop , the line is attached directly boat to boat (let's just say it's a direct knot attach) , so those things are constants I beleive ... but what happens if the towing vessel's weight is altered ??
Maybe the speed could be altered some too , perhaps 2 mph instead of 3 ... or perhaps 4 , or all of them resulting in different values .
Altering tow vessels weight and speed become variables I believe .
Maybe this 1/4" poly rope is being used by a couple guys in a Jon boat , weight aprox. 1000-1200 lb.(good samaritans) , maybe it's a Tanker and the Dingy gets wrapped on a channel bouy or Moby Dick comes up and swallows it , lol ...
OP don't need to fear in the circumstances he explained , I think he's pretty much always know that .
I realize these things are not in context w/the OP's situation , and the Tanker / Moby Dick thing is rediculous . Mostly just curious about the 1/4" poly rope and what "minimum" weight towing vessel it would take to reach a 310-387 lb. stress point range .
1st thing that comes to my mind is a 310 lb. tow vessel should have no problem breaking the rope in the snag/shock loading situation ??
not out there
Similar concept was used by Mythbusters for their crash scenarios. Basically, they used sacrificial rope segment to make sure that the towing vehicles did not get damaged when the tow-ins smashed against obstacles.
I didn’t think of that at first.
I’m pretty sure that movable contents of the boat is a factor. I was trying to think of a good reason I’ve never felt the slightest jolt when towing another boat, and that may be part of it. I’m still confident I could tow a boat every day for the rest of my life and never feel a huge shock or break the connection to one of the boats (because the more I think of it, and whitewater rescue instances I’ve been part of, the more I realize that thwarts or carry handles will break long before the rope does).
I understand what you are saying though, even if I don’t think it will happen due to a rope between to tiny boats. Once in my guide-boat, I drifted very slowly into a bridge pier while trying to explain to a newbie how to do eddy turns. I bet the boat was only going 1.5 mph, but it was a direct hit on the stem and it fractured the hull at the point of impact. I’ve had lots of other impacts that were much more severe which caused no damage at all, but they were in a manner that allowed the boat to shift ever so slightly at that moment of impact, which must have made all the difference.
I vote for trilliumlake’s recommendation
Go ahead and go pop for a good tow line system. After all, you’re towing your son, not a log. At least buy some floating line.
Nope
You are correct that F = CdA(1/2)rhou^2
However the A in this equation is the frontal area. I have never seen a 6’ X 2’ kayak, perhaps 0.6 ft x 0.2 ft.
Further, so far, the stretch in the line has been neglected. One needs to integrate the tension of the line over the distance which the line stretches, then equate that to the kinetic energy of the running start. Then solve for force. That is the only way to do it.
Guideboatguy is right.
I use 1/4" floating NRS line.
chuckle
OP- has your question been answered? LOL
reading is a lost skill
I believe I used words "worst case scenario" a couple of times.
Oh, since you are such an expert, might run a quick search on WW pins and look for pictures. Then we can discuss how unrealistic my number were.
Reading?
The OP was regarding towing a child.
The tangent went off to the maximum force in the line.
The sub-tangent involved the drag force.
I missed (and still miss) the connection between the shock loading of a tow line and a WW boat pinned in a strainer.
I am certainly not an expert in anything, but I have a fundamental understanding of dynamics. The max acceleration experienced by a rock climber after a fall is a classic problem - very similar to the max shock load in the tow line. Hence, the integral approach to the problem.
No personal offense intended.
The max force in the line is a function of acceleration (d2x/dx2), not the jerk (d3x/dx3) or the drag. Please correct me if I’m wrong.
D
this works
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this works even better
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