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Old 11-01-2020, 16:22   #1
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Explain to an idiot what spring lines are for

Don't understand the usefulness of spring lines when docked at a slip. I read many manuals and asked many sailors..... No cleat cut/definitive answer. I still don't understand. Am I missing something big or an I a total idiot?
Don't hold back your comments please.


My docking situation: I have two bow lines (secured on pilings) and two stern lines cleated on the pier.... (reverse situation if berthing the other way).

The boat can move a little fore and aft and side to side to adjust with a max three feet tide.

How in the world a spring line going to help with fore/aft movement of the boat which is already tied four way fore and aft?

Thanks
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Old 11-01-2020, 16:47   #2
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Re: Explain to an idiot what spring lines are for

You say you have bow lines and stern lines - are these breast lines (which would be roughly 90 degrees to the fore & aft axis of the boat & dock? If so, while they will tend to limit your fore & aft travel, they are at a significant disadvantage at dissipating the energy they absorb in stopping your vessel's movement - because of the angle. They will tend to get stretched quickly, and in a serious storm situation, could get overloaded. Spring lines, being much more in line with the axis, can absorb the energy and abuse much better.
If your bow & stern lines are actually leading well forward and aft, then they are doing the job of spring lines, and you have a much larger slip than would be usual for your boat size.
In that case, adding spring lines just reduces the load on those bow & stern lines.


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Old 11-01-2020, 16:55   #3
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Re: Explain to an idiot what spring lines are for

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Old 11-01-2020, 17:43   #4
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Re: Explain to an idiot what spring lines are for

My understanding, which could also be wrong since I'm not on docks often, is they prevent your boat moving fore and aft. The bow and stern lines keep it lined up with the dock. Spring lines keep it from moving along the dock.
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Old 11-01-2020, 18:13   #5
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Re: Explain to an idiot what spring lines are for

4 lines (1 on each corner) would need to extend out at approximately a 45 degree angle to kind-of hold a boat steady in windy conditions. In a lot of slips, that simply won't be an option. If the bow and stern lines come off at a 90 degree angle, there will be nothing to stop a boat from moving fore and aft until the boat moves far enough to change the angle significantly from 90 degrees.

The primary purpose of spring lines is to limit fore and aft movement of the boat.

If you use short breast lines on the bow and stern, they can work in modest conditions but are not well suited to tides or if there is storm. Because they are at a poor angle, the forces on the lines and cleats tend to be much higher. Even more important is they are short, they don't provide much shock absorbing effect. In calm conditions, you will feel the boat jerk now and then but in storm conditions, that jerk can easily pull a cleat out of the deck.

Ideal layout is to have 6 lines:
- 2 Bow lines, crossed (but not chafing on anything), to provide a good length to all for stretch. This keeps the bow from moving side to side.
- 2 stern lines...same thing but to keep the stern from moving side to side.
- 2 spring lines to keep the boat from moving forward or aft.

Obviously, you need to allow a little movement. This setup gives you a lot of flexibility and because the distance is significant, it can absorb a significant tide without the lines going slack or bar tight.

If you are at a floating dock, you can cheat a little as you don't need to account for tides. Likewise if the tidal range is small...but even there, it's not a bad thing as the unexpected can happen. On Lake Erie, we were in a marina one night with a bad storm. By morning the boat was 7-9ft down sitting on the bottom as the water was literally blown out of the marina. We had to adjust lines 4 separate times. Lots of boats with cleats pulled out as the staff couldn't keep up with adjusting the lines as most owners were gone.
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Old 11-01-2020, 18:15   #6
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Re: Explain to an idiot what spring lines are for

Quote:
Originally Posted by ferrailleur View Post
Don't understand the usefulness of spring lines when docked at a slip. I read many manuals and asked many sailors..... No cleat cut/definitive answer. I still don't understand. Am I missing something big or an I a total idiot?
Don't hold back your comments please.


My docking situation: I have two bow lines (secured on pilings) and two stern lines cleated on the pier.... (reverse situation if berthing the other way).

The boat can move a little fore and aft and side to side to adjust with a max three feet tide.

How in the world a spring line going to help with fore/aft movement of the boat which is already tied four way fore and aft?

Thanks
A sketch of your set-up would be nice..and would make for a more considered response.
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Old 11-01-2020, 18:17   #7
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Re: Explain to an idiot what spring lines are for

They also help prevent the boat from being pulled hard into the jetty under the effects of currents or the wind.
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Old 11-01-2020, 18:30   #8
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Re: Explain to an idiot what spring lines are for

They also act as backups in the event of a failure. If you’re only tied bow and stern and one let’s go you’re free to smash about the slip. With springs you’re still locked in place and hopefully someone would notice.
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Old 11-01-2020, 18:39   #9
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Re: Explain to an idiot what spring lines are for

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Originally Posted by ferrailleur View Post
... The boat can move a little fore and aft and side to side to adjust with a max three feet tide....

In fact, the max tide in Baltimore (storms) is about 8-10 feet. I've seen that much. If we count only strong south winds and strong north winds, 5 feet is possible and likely.


Some of this also depends on how a boat fits in the slip. The bigger the slip, the less the need.


But often fit is a minor factor. Springs reduce the horsing in the slip, making life on board in blustery conditions more pleasant. They also reduce chafe, for the same reason. They also reduce the peak loads by as much as 50%.


Many people never visit their boats when it is really blowing, because they don't go out on days like that. But that is when you most need to visit, at all states of tide, to see how she rides.


I watched this tie-up work through an 8-foot surge during Isabel. No problems. As you can see, the slip is a tight fit. A 4-point tie-up is impossible.
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Old 11-01-2020, 18:41   #10
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Re: Explain to an idiot what spring lines are for

I dont know anything about docking a sailboat and I put on spring lines. Seems incredibly self explanatory.
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Old 11-01-2020, 22:59   #11
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Re: Explain to an idiot what spring lines are for

Here's my quick test. Tie up the boat and pull on the bow line to see how far the boat travels in her slip. Does it run into anything? Then come around and pull on the stern line as far as the boat will travel. Then set up the spring lines to keep the boat from moving back and forth like this.
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Old 12-01-2020, 00:13   #12
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Re: Explain to an idiot what spring lines are for

Does it make any difference if spring lines lead from the bow & stern to a cleat amidships on the dock, or if they lead from bow & stern cleats on the dock to a cleat amidships on the boat?
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Old 12-01-2020, 00:57   #13
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Re: Explain to an idiot what spring lines are for

Springs are also used to pivot the bow or stern (as required) to move the bow or stern away from the dock under power without the boat moving forwards or backwards.
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Old 12-01-2020, 02:56   #14
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Re: Explain to an idiot what spring lines are for

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Originally Posted by Sonny36 View Post
Does it make any difference if spring lines lead from the bow & stern to a cleat amidships on the dock, or if they lead from bow & stern cleats on the dock to a cleat amidships on the boat?
Generally, no...as long as they are reasonably long. Ie: you don't want a 4ft spring line as it won't have much stretch before jerking the boat abruptly to a stop.
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Old 12-01-2020, 03:07   #15
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Re: Explain to an idiot what spring lines are for

In my ABC classI was taught the spring lines went from the bow of the boat to aft of the stern on the dock and the aft line of the boat goes forward of the bow on the dock and lines on the bow closest clew on the dock and the aft line to the closest dock clew. Remember KISS !not the band.
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