I used to feel competent with basic Newtonian physics, but now I'm too rusty. Seems like such a simple equation.... 30,000lb boat moving at x knots, stopped by line that stretches x inches....

Anyone able to help with the calculations? Otherwise I'll just grossly overbuild everything like I always do.

E=Mc\2 ?

 

I hope not!!!

 

1. This is more about energy absorption than strength. While you could plan on easing the line and do this with steel cable, I think you want to build a passive system. I'm betting he slipped some line, though.

2. What is the maximum load you want on the cleats and line? I don't know your boat.

3. The length of the rope matters. Ropes will absorb X energy/ft.

4. Speed. Convert to feet/second.

4. The energy of the boat is about 1/2(M/32)V^2. Pounds and ft/s. At 5 ft/s this is about 12,000 ft-pounds. at 5 knots this is more like 35,000 ft-pounds.

5. The below is the maximum for GOOD nylon rope to survive a dozen times (empirical testing). You'll need larger. The risk of too large is that the forces go up fast. Knots and sharp turns reduce strength.

7/16"---300ft-pounds/ft. Peak load about 2000 pounds.
1/2"---400. Peak load about 2700 pounds.
3/4"---900. Peak load about 6000 pounds.

Assuming 30 feet of rope, 1/2" line will do it at 5ft-sec, but 3/4" rope will be needed at 5 knots. A conservative sizing would be 1" inch and peak loads would be over 8000 pounds! A big range of answers. If anything breaks someone gets hurt. I wouldn't dare this above 3 knots, and you shouldn't need to. In that case, ordinary dock lines are about right, if they are in new condition.

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Unless fusion or fission are involved, that is E=1/2 MV^2.

 

4. The energy of the boat is about 1/2(M/32)V^2. Pounds and ft/s. At 5 ft/s this is about 12,000 ft-pounds. at 5 knots this is more like 35,000 ft-pounds.

Looks like the pilings will be the limiting factor.

 

Med,

I think you might be over-thinking this. And I'm too lazy to do any math right now.

1st...I don't know why you need throttle at all when the wind is astern. I'd be in neutral with just enough way on to steer. ( approach a dock at the speed you're willing to hit it). Sounds like you do that already.

As for your neighbor: Suppose you missed his line handoff...or worse got your finger caught against the cleat. what does he do when you're not there?

Once you pick up your line, you can then put it in gear (idle) and it should tuck you into the finger dock and hold you there while you attach your other lines. Your normal docklines should be able to manage this nicely if they are properly sized to begin with. I'm sure they are ;-)

 

As for your neighbor: Suppose you missed his line handoff...or worse got your finger caught against the cleat. what does he do when you're not there?

That is the most frightening bit. For example...

Some marinas send line handlers that can manage any botched approach. I can think of one marina that has a nasty current across the docks that must really train the hands.

Some marinas send line handlers that you are better not using at all. I've had line handlers let lines run that I clearly and specifically asked that they simply secure so that I could power against them. No trouble, but confusing until I realize the problem.

I can't see trusting unknown help. In the PO's example, they may simply not understand that your intention is to apply 2-tons of force in a few seconds and try to hand-hold, believing that is better.

 

We have used this product and love it! It creates a huge easy to use loop.

dockingstick.com - Home (watch the video on cleat use)

For our 24,000 lbs sailboat the line and loop are composed of six feet of 3/4inch, 8 plait high quality nylon line. I would not want to go in any faster than 1kt though.

 

Interesting.... Most of the docking gadgets are total crap in my opinion. This one might be simple enough to actually work.... I have a lot more interest in single handed docking now that there's a kiddo aboard.

PDQ, the "magic line" as we call it, is about 15' long as to be long enough to reach the cleat easily but short enough to stop the boat before I reach the end of my slip. I was thinking of attaching it to a SS climbing bolt hanger and those are rated to 10K lbs. Surprisingly high loads you've calculated out there...

MedSailor

 

If the rope breaks while "full-throttling" in to the dock:

 

If you are concerned about rope strength, 3/4 inch would be fine. We uses 7/8th polly rope in the Panama canal and two lines held about 100 000 lbs of boats being pushed my a 3 or 4 knot current.

 

it seems to me the manuever is not only about absorbing motion (energy) into the rope (creating friction and heat) but about changing it's direction. By using the rope as a pivot, it not only absorbs a lot of the boats momentium, but because it is redirecting it, increases the friction on the hull from the water itself.

It's all very simple and elegant really.

you could probably do the same with a rope pretied off to bow and stern cleats and a waiting (high strength) hook attached to the piling in some fashion. As you sail by, clip in to the hook and let it do the work

 

The coolest bit of sailboat seamanship that I was ever witness to was when my next door slip neighbors in Seattle came into their slip with their Moody (45?) ketch with a solid 40kt tailwind.

They came roaring in at full throttle and one crewmember handed me a dockline with an eye in the end and asked me politely (but with some urgency) to please place the eye on the aftmost dock cleat. I complied.

Re: Full Throttle....My inclination in having to make such a landing would be full throttle in reverse on approach to try to check/minimize the forward speed. I suspect the neighbor was doing that and only shifted to forward after the line was secured to hold the boat to the pier.

Re: 40 kt. tailwind....what would have happened if someone fumbled the line....how many seconds to get it right on first try, then how many, if any, to recover the line and try again in 40 kts.? After the fumble, what happens and how expensive? And with the high wind, any wave action to bang the boat against the pier? Was the landing absolutely necessary or were there other, less risky alternatives?

 

No he was definitely in forward full throttle. The reason was that the wind wasn't exactly on his stern, it was slightly quartered. He was at full throttle so he could have maximum steerage.

What would have happened if I wasn't there? The wife, gets off and secures the cleat, that's how they've done it before in other hairy spots during their circumnavigation. They trusted me as a line handler, but I don't trust strangers to handle lines.

What happens if they fumble the cleat? THOUSANDS OF DOLLARS OF DAMAGE and possibly injuries. The reason I was impressed was that they were 100% committed. Really stupid/lucky or really good... still not sure but since they've been around the world and they did pull it off like it was no big deal, I'm leaning towards very good.

Me, I've done this kind of thing on a side tie, where I could afford to miss. I wouldn't have tried the stunt they pulled. I'd rather anchor on the nasty side of the breakwater and enjoy a crappy night's sleep.

The version of this trick that I do is much less desperate. Usually dead slow or up to 2-3 knots max. Still, I'd like to know the forces because I'm a geek like that.

MedSailor

 

People ferries and water taxis use a similar technique when they make their quick unload/reload stops.. Usually in fwd gear and the rudder turned away from the dock.. Thay basically motor against the spring line holding themselves on the dock.

 

I would be sure to use as stretchy a line as possible to absorb the impact. I would think either plait or three strand. I don't think this is a good use of double braided. You might want to put in some sort of rubber snubber on it to help make it a bit more comfortable.

 

Yup, the piling might be the limiting factor!

Since the snuber is going to add ~ 4-6 inches of stretch and the line is going to stetch 4-6 feet on this catch, the rubber doesn't affect the math. Rather like folks that put a bungie cord in a handline trolling set-up; the bungie stretches 18 inches, while 150 feet of fishing line stretches 15-30 feet!

If the line were smaller, I might even suggest climbing rope. BTW, climbing ropes are thin braid cover with 10-14 layed inner strands carrying the load; they are not double braid. One more bit of trivia; the inner strands are typically 50% RH and 50% LH to prevent rotation.

I would be comfortable with any nylon line of the correct size; the differences are not that great and the faster handling of braid might be more important.

 

The moving boat has "momentum." Momentum is simply mass times velocity, or kg times meters per second. It could be expressed as pounds per miles per hour also. Momentum of a 12000 kg boat moving at 2 meters per second (4mph) is roughly 24000kg or 52800 pounds of momentum. Any 1" nylon line would have broken in an attempt to stop this vessel, assuming the piling didn't break off first.

 

Med - Our ISPA instructors have to be able to do this single-handed.

The owner of Turicum (the Vic-Maui boat) does it slightly differently. He has a dock line with a hook that attaches to a specific spot on the toe rail.

I do not use full throttle.

 

I often have about a 10-15 kt cross wind (or maybe from just aft of 90 degrees) at my dock. I get the nose of the boat into the slip and then jump off and manhandle her into place. At worst, I can let the fenders do their thing (until I get the windward lines under control) as the wind pushes the boat sideways into the leeward finger. But, I would NEVER approach the dock at anything near full throttle. Even my little 6700 lb boat can have enough inertia to do a lot of damage.

 

Buy some 3/4" 3 strand and call it good.

 

Can we have a Google Maps link showing your slip and approach?

Or at least a diagram showing your slip and approach?

 

The coolest bit of sailboat seamanship that I was ever witness to was when my next door slip neighbors in Seattle came into their slip with their Moody (45?) ketch with a solid 40kt tailwind.

They came roaring in at full throttle and one crewmember handed me a dockline with an eye in the end and asked me politely (but with some urgency) to please place the eye on the aftmost dock cleat. I complied.

The line in question was run through a foot block at the quarter and then to the primary winch and back to the skipper at the helm. He hauled in the slack and the line stopped the boat. While still powering in forward (because the line was right at the picot point of the boat) it stayed stationary at the dock perfectly parallel to the finger pier.

I've used this technique quite a few times after witnessing this (though without as much throttle or tailwind!) and really like it. I'm planning on making a dedicated line that is the correct length for my dock, but when I got to sizing the line, I wondered, "how much force would be on this puppy?"

I used to feel competent with basic Newtonian physics, but now I'm too rusty. Seems like such a simple equation.... 30,000lb boat moving at x knots, stopped by line that stretches x inches....

Anyone able to help with the calculations? Otherwise I'll just grossly overbuild everything like I always do.

It is not a particularly difficult calculation. Google the "Work Energy Theorum" and go from there. The yacht has a kinetic energy equal to its mass x velocity. It's kinetic energy must be consumed/countered. The force necessary is calculated by taking the mass of the yacht multiplied by its acceleration. In this case, the yacht must go from its maneuvering speed as it enters the slip and the surge line is applied as a stopping force to zero speed as its bow reaches the inboard end of the slip and the surge line reaches its maximum extension. If the yacht's speed of approach is say, 5 knots, and its speed upon coming to rest is zero knots, the yacht's average speed over the distance of it's stop will have been 2-1/2 knots as it transitions from approach to landing. Assuming that the surge line extends half the length of the slip, from the end piling to amidships on the yacht, say 20 feet, and the line is nylon and hence cannot be stretched more than 20 percent of its length without rupture, or 4 feet (= .20 x 20), the yacht must transition from 8.43 ft per second (at 5 knots) to zero with an average speed of about 4.215 feet per second. With 4 feet of line "stretch" in the surge line, the yacht will traverse that stretch in, roughly, .95 seconds. The Delta V will be 8.43 feet/second with the Delta T being .95 seconds so the acceleration, which in this case has a negative sign (for slowing) is 8.88Ft/second squared. The required force to effectuate this Deceleration is the yacht's mass (= displacement/32.2 ft/sec^2) x -8.88 ft/sec^2. Let us assume that the yacht displaces say 21,000 lbs. It's mass is then 652.17 slugs and the force required to stop the yacht is about 5,793.30 lbs. The work energy is 5793.3 lbs applied over a distance of 4 feet or about 23,173.20 foot lbs. On the line side of the event, the issue becomes somewhat more complex because the amount of force needed to stretch the line 1% is decidedly different than the force needed to stretch the line 20% and it is the cumulative force moving over the length of elongation that must, in total, equal 23,170.20 ft lbs. Every increment of stretch can be described as a distance dx and the necessary energy to effectuate one such stretch or elongation as F*dx. To elongate the line the second increment, dx2, the force must be incrementally greater which may be, but is not necessarily, linearly related to the first incremental stretch unless the line is perfectly elastic, which of course it is not (tho' nylon in extremis is close). So, one is then confronted with a surge line that is applying a stopping force on the yacht that is not a steady 5,793.3 lbs but is increasing from, initially, zero, to some maximum, with the sum of (or the integral of) the forces applied over each incremental stretch of the surge line totaling 23,170.20 ft lbs from 0 feet to 4 feet. It will take a little pondering but it shouldn't be too difficult to come up with the size line needed from there. Note, however, as someone wisely pointed out previously, all of the kinetic energy of the yacht will be converted to potential energy in the line, which will surely take some time to burn off as heat, although it will, eventually.

FWIW...

 

It is not a particularly difficult calculation. Google the "Work Energy Theorum" and go from there. The yacht has a knitic energy equal to its mass x velocity. It's kinitic energy must be consumed/countered. The force necessary is calculated by taking the mass of the yacht multiplied by its acceleration. In this case, the yacht must go from its maneuvering speed as it enters the slip and the surge line is applied as a stopping force to zero speed as its bow reaches the inboard end of the slip and the surge line reaches its maximum extension. If the yacht's speed of approach is say, 5 knots, and its speed upon coming to rest is zero knots, the yacht's average speed over the distance of it's stop will have been 2-1/2 knots as it transitions from approach to landing. Assuming that the surge line extends half the length of the slip, from the end piling to amidships on the yacht, say 20 feet, and the line is nylon and hence cannot be stretched more than 20 percent of its length without rupture, or 4 feet (= .20 x 20), the yacht must transition from 8.43 ft per second (at 5 knots) to zero with an average speed of about 4.215 feet per second. With 4 feet of line "stretch" in the surge line, the yacht will traverse that stretch in, roughly, .95 seconds. The Delta V will be 8.43 feet/second with the Delta T being .95 seconds so the acceleration, which in this case has a negative sign (for slowing) is 8.88Ft/second squared. The required force to effectuate this Deceleration is the yacht's mass (= displacement/32.2 ft/sec^2) x -8.88 ft/sec^2. Let us assume that the yacht displaces say 21,000 lbs. It's mass is then 652.17 slugs and the force required to stop the yacht is about 5,793.30 lbs. The work energy is 5793.3 lbs applied over a distance of 4 feet or about 23,173.20 foot lbs. On the line side of the event, the issue becomes somewhat more complex because the amount of force needed to stretch the line 1% is decidedly different than the force needed to stretch the line 20% and it is the cumulative force moving over the length of elongation that must, in total, equal 23,170.20 ft lbs. Every increment of stretch can be described as a distance dx and the necessary energy to effectuate one such stretch or elongation as F*dx. To elongate the line the second increment, dx2, the force must be incrementally greater which may be, but is not necessarily, linearly related to the first incremental stretch unless the line is perfectly elastic, which of course it is not (tho' nylon in extremis is close). So, one is then confronted with a surge line that is applying a stopping force on the yacht that is not a steady 5,793.3 lbs but is increasing from, initially, zero, to some maximum, with the sum of (or the integral of) the forces applied over each incremental stretch of the surge line totaling 23,170.20 ft lbs from 0 feet to 4 feet. It will take a little pondering but it shouldn't be too difficult to come up with the size line needed from there. Note, however, as someone wisely pointed out previously, all of the knitic energy of the yacht will be converted to potential energy in the line, which will surely take some time to burn off as heat, although it will, eventually.

FWIW...

Good engineering my friend! I didn't account for the 0.95 seconds of deceleration, but then it was cocktail time also.
I redid a calculation using a boat mass of 11250 kilograms and deceleration time of 0.95 seconds and calculated about 28912 Newtons required or 6500 pounds approximately. Half inch nylon line might not stop the "Capt. Ron" docking style.
(I think I'll stick to my old fashioned methods.)

 

Per Faster,

People ferries and water taxis use a similar technique when they make their quick unload/reload stops.. Usually in fwd gear and the rudder turned away from the dock.. Thay basically motor against the spring line holding themselves on the dock.

Have Watched it many times.

My dock neighbor has perfected this technique as he has limited steerage.(he singlehands) I have used at it at the home dock on occasion. (while singlehanding) Once in a while i do come in hot due to weather conditions,BUT, my boat handles extremely well and i can stop her on a dime. I have confidence in my motor and the boat. I go around if i don't think i have it right.

 

I based my original post on drop testing and other dynamic tests of rope energy absorption. It will vary from one line to the other. However, This is a case where energy absorption is what matters, not strength, and the test methods are different. Unfortunately, energy absorption capacity drops off with use faster than strength (ropes stiffen), making the calculation more complex and in the end, empirical.

The other important factor that some understand but a few have missed (those reasoning everything will break) is the length of the line. For example, I'm betting I can stop a 175-pound swimmer with a head start with 15-pound fishing tackle; did it once at the local pool just for laughs. It's all about the math.

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Bottom line: a boat-length of standard dockline (3/4-inch for this size boat) will do nicely.

 

A spring line is a wonderfull tool but an old Maine salt advised me to never approach a dock at a higher speed than I would be pleased to ram the dock.

 

That point was brought up a couple pages ago. What about when the wind is high and blowing you off? Sometimes speed (water flowing over the rudder) is the only way to maintain any control.

Post on the same topic

MedSailor

 

OK.... I'm just not buying it anymore! Coming into dock at full throttle with a 40 knot tailwind. Good grief, what gives here???

 

What's not to buy about this? While I would never try to get into a dead end slip with a 40kt quartering tail-wind they chose to do so (and pulled it off).

How would you have approached getting into the slip under the same conditions?

Using a spring line to approach the dock with enough speed for steerage is a technique that I recommend everyone at least consider adding to their toolbox before poo-pooing it. You might need it some day, and it's a pretty standard technique.

T34 Changing approach angles is not always possible. A strong wind blowing from the beam is not likely going to allow you to approach the dock at <1kt (the speed you'd like to hit it at). Is there something else to your change of angle that you can describe for me? I'm having a hard time picturing it.

Medsailor

 

How would you have approached getting into the slip under the same conditions?
About the time my bow got close to the slip I would have been in full throttle... only reverse..... to stop the boat, knowing the 40K tailwind would be pushing me into the dock, then after securing a stern spring, used forward throttle to keep the vessel alongside the dock.

 

Knowing the energy absorption of the dockline might be a great idea, but whats the energy absorption of the cleat on the dock? Or the bolts holding the cleat, etc... Seems like using a better docking procedure might be a better idea since you would eliminate many of those variable.

 

Unless I really know a marina's equipment, I will only surge/snub from a piling. I have seen more than one bulkhead/dock cleat ripped out by a snubbing/surge line and with the line under much pressure, they are slung like a rock from a sling-shot.

 

40+' sailboat, full throttle in forward, 40 knot tailwind, one line coming tight on a cleat = something is going to give. Unless it's a perfectly straight, unobstructed path from the deck to the dock cleat there's going to be a chafe/break spot, and unless both ends are spliced eyes any knot is going to reduce the breaking strength of the line by about 50%. I suspect that the skipper was allowing the line to slip around the winch to avoid the shock load.