very basic; mooring to a harbour wall question.

[laika]

On this, would 10m for mooring warps, 12m for springs sound about right?

Stretchy ropes and elementary trig.

Any questions, see me after prep and before cocoa.

Oh dear. I confess that the only times I've tied against a wall wth a big tidal range it's been with the benefit of bars and loops. Plus Fareham's finest 6th form college where I last did a maths exam more than 30 years ago didn't issue "prep" or cocoa so I'm probably going to show myself up as a dunce on a public forum but...

The problem with Gwylan's calculations would seem to be that at the top of the tide the drop you've allowed for in the vertical plane translates to an ability to wander away from the wall in the horizontal plane. Obviously this can be addressed by use of weights as others have suggested but I strongly suspect that birdseye has some rather good experience here and a slightly more complex bit of maths might take account of the stretchy properties of the 3 strand nylon we've bought because it's stretchy.

As I (with an inexperienced-in-this-department eye) see it you're not looking to rig a 3-4-5 triangle. The longer the line the better because:
- the amount a line stretches is proportional to its length
- pythagoras tells us that the hypotenuse of Gwylan's triangle (our mooring line) needs to increase by proportionally the same amount (if the line is perpendicular to the side of the boat: i.e. the triangle is not a triangle but a straight line) or less (if it is secured laterally fore/aft) and that that proportion reduces the longer the base of the triangle (i.e. the further fore/aft away form the boat you're secured).

So as we increase the initial line length (i.e. base of the triangle or distance fore and aft from the boat that we've secured), the *proportion* of its length it needs to increase to accommodate a given drop decreases.

Ideally we'd like to make the amount the line needs to increase in length to accommodate our drop less than the amount the line will comfortably stretch. That way we're snug against the wall at the top of the tide and snug (but with tighter lines) at the bottom.

If the amount our line can comfortably stretch is s (e.g. 15% would mean s=0.15, for 20% s=0.20), the depth we need to accommodate is d, then presumably the length of line we need to achieve our goal in an idealised world (where the cleat of our 2 dimensional hypothetical boat is flush against the top of the wall) is:
l = sqrt((d^2)/(s^2 + 2*s))
Which for an 8m drop and rope which stretches 15% stretch would be:
l = sqrt((8^2)/(0.15^2 + 0.3))
l = sqrt(64/0.3255)
l = sqrt(196.62)
l = a bit over 14m

Obviously that's not real world and your lines need to be longer to be tied on somewhere and your boat probably isn't 2-dimensional and I have no idea how much 3 strand nylon can comfortably stretch (ideas?) and it probably gets less stretchy the more you do this kind of thing....but is the theory along the right lines?

 

[steve yates]

I seem to remember being in a harbour with a significant tidal range where I saw rings in the wall just about at water level low tide. The idea was that you loop a line through this ring first chance you get (don't tie it off there unless you plan on leaving at low water) and the two ends of the line are then made fast to a bollard immediately above it at the top of the wall. You do this twice, one for the bow, one for the stern. You then pass your respective mooring lines around these vertical lines and make fast. As the vessel rises and falls on the tide your mooring lines simply slide up and down your vertical ones.

Now that makes an awful lot of sense!

 

[Triassic]

......
l = sqrt((d^2)/(s^2 + 2*s))
Which for an 8m drop and rope which stretches 15% stretch would be:
l = sqrt((8^2)/(0.15^2 + 0.3))
l = sqrt(64/0.3255)
l = sqrt(196.62)
l = a bit over 14m

Yup, you've convinced me. I'm going to anchor and take the dinghy ashore!

 

[C08]

Apart from everything else that has been said about walls and large tidal ranges if you leave the boat when you get back it is likely to be either pulling your cleats off or yards away from the wall when dried out. Best to stay on the boat and adjust lines as you drop.

 

[Vega1447]

I've been in Portpatrick a couple of times, once on a friend's Albin Vega 27.

We (Vega crew) attached our longest lines as springs (i.e. took a line from the bow to a bollard (?) at the top of the wall aft of our stern and a line from the stern to a bollard or whatever at the top of the wall.

I think we also attached a line at the beam (no cleat at beam on a Vega so took a line round the base of the shrouds) and tied it to top of ladder (we were at or near LW).

Worked fine...

 

[alahol2]

I've only dried alongside a wall a few times but I set up a masthead halliard tightly to a cleat at the base of the mast. A snatch block is set to run up and down the halliard and the rope attached to the snatch block goes ashore to a bollard/ring. The stern ropes and springs can be set more or less as loosely as necessary to accommodate the rise/fall of tide.
The object of the snatch block line is to prevent the boat falling out from the wall but it also serves the purpose of holding the boat in position next to the wall. Maximum rise and fall I've had to cope with is about 2 metres (Newport, IOW).

 

[LadyInBed]

If the amount our line can comfortably stretch is s (e.g. 15% would mean s=0.15, for 20% s=0.20), the depth we need to accommodate is d, then presumably the length of line we need to achieve our goal in an idealised world (where the cleat of our 2 dimensional hypothetical boat is flush against the top of the wall) is:
l = sqrt((d^2)/(s^2 + 2*s))
Which for an 8m drop and rope which stretches 15% stretch would be:
l = sqrt((8^2)/(0.15^2 + 0.3))
l = sqrt(64/0.3255)
l = sqrt(196.62)
l = a bit over 14m

Obviously that's not real world and your lines need to be longer to be tied on somewhere and your boat probably isn't 2-dimensional and I have no idea how much 3 strand nylon can comfortably stretch (ideas?) and it probably gets less stretchy the more you do this kind of thing....but is the theory along the right lines?

Ignoring stretch, so you definitely won't be left hanging
This table will give an approximation - but I have been known to be wrong!

 

[laika]

Ignoring stretch, so you definitely won't be left hanging
This table will give an approximation - but I have been known to be wrong!

Ah but the stretch was the point. This table is what Gwylan's calculations were about. Allow the lengths in your table and at the top of the tide you may drift out as far as you've allowed for the drop (which may of course then be mitigated by weights, lines on the shrouds etc.)

What I was suggesting (possibly incorrectly and on the back of a virtual fag packet: I invite comment) was calculating the length of line (when fully extended but not stretched) which would stretch to the required length within comfortable tolerances given permissible percentage stretch and depth of fall to allow for. Thus the boat would be held in both at the bottom of the tide and the top (although it might be pushed out by wind or tide).

 

[LadyInBed]

Ah but the stretch was the point.

Yes, accepted.
I put the table up as much to show that if you can get lines well spread, the percentage change in length from High to Low is hardly more than the acceptable stretch of the line, so only a slightly slack line at High will be ok at Low.
The secret is not to use short scope lines.

 

[lpdsn]

I'm confused. If the tide has ebbed and the boat is 3 or 4 metres below where it was when you left it, how is grabbing a loose spring and hauling on it going to help you?

Richard

You're right, you are confused.

Leave loose lines when expecting the tide to drop - in meantime haul in on loose spring when wanting to go ashore.

With rising tide, the lines will become loose, so then you grab a loose spring and haul in when wanting to get back on the boat.

 

[[3889]]

Like TSB240, Port St Mary has the largest tidal range I know in a non-drying harbour without facilities for your lines to ride up and down the harbour wall (chains etc.). Max range is 6.5m. Do what Gwylan says and maybe add a breast line to haul the boat to the ladder when coming and going. Best avoid wearing your best blazer when negotiating ladders there and have a head for heights. A line shoreside is useful for lowering provisions to deck. PSM also has buoys if a dinghy trip ashore seems preferable to the above, definitely recommended in northerlies if you don't want to grind against the wall all night. And finally, the SHM beacon is nowhere near the end of the reef which extends parallel to the wall.

 

[garvellachs]

We've moored on the wall at PStM in the IOM several times. There's an inner harbour which dries for the brave and those with two keels; we prefer to float - with a fin keel - on the outer wall. Long lines are needed: typically I'll dig out two old halyards which will be circa 30m long and use those up to bollards on the quay. If the inside boat, we'll use those long lines and leave them quite slack; the boat will need to be fendered at the stern (big ball) and the pulpit (fender tied to the railing) to avoid scaping on the wall. If rafted, the long lines are needed for shore lines to avoid straining the inside boat's cleats. Boats seem to moor on long slack lines more comfortably than you imagine - they just seem to settle in a roughly constant place.

Beware of tying to ladders to try and keep the boat tight in - we got caught doing that in Douglas on the wall in the inner harbour. Tide was dropping and the line went tight. We were in the saloon and only realised because the boat started to heel. Panic and a knife: I thought quickly where to cut the line - it was bar tight - and I just needed to touch it with the knife and it snapped violently (you need to keep clear of the loose end). Maybe a shackle on a stay would fare better than a cleat (we use that trick when drying out) - rigging is immensely strong and will happily heel the boat a bit in the event of a mistake?

 

[RichardS]

You're right, you are confused.

Leave loose lines when expecting the tide to drop - in meantime haul in on loose spring when wanting to go ashore.

With rising tide, the lines will become loose, so then you grab a loose spring and haul in when wanting to get back on the boat.

LOL

Read post #29 again and maybe have a re-think about who is confused. :encouragement:

Richard

 

[jimbaerselman]

Ideally we'd like to make the amount the line needs to increase in length to accommodate our drop less than the amount the line will comfortably stretch. That way we're snug against the wall at the top of the tide and snug (but with tighter lines) at the bottom.

If the amount our line can comfortably stretch is s (e.g. 15% would mean s=0.15, for 20% s=0.20), the depth we need to accommodate is d, then presumably the length of line we need to achieve our goal in an idealised world (where the cleat of our 2 dimensional hypothetical boat is flush against the top of the wall) is:
l = sqrt((d^2)/(s^2 + 2*s))
Which for an 8m drop and rope which stretches 15% stretch would be:
l = sqrt((8^2)/(0.15^2 + 0.3))
l = sqrt(64/0.3255)
l = sqrt(196.62)
l = a bit over 14m

Obviously that's not real world and your lines need to be longer to be tied on somewhere and your boat probably isn't 2-dimensional and I have no idea how much 3 strand nylon can comfortably stretch (ideas?) and it probably gets less stretchy the more you do this kind of thing....but is the theory along the right lines?

Yes.

Expressed more simply, 3 strand Nylon can give you from 5% to 10% stretch without permanent distortion. So:

6% is the increase in the hypotenuse length when the rope drops from level to 20 degrees (cos 20 = .94); for 25 degrees it's 10% (cos 25 = 0.9). I know, these should be reciprocals, but this is dirty maths, so give or take an inch . . .

Tan 20 degrees is 0.36, so the rope length needed for a given range, assuming 6% stretch, is Range/0.36
Tan 25 degrees is 0.46, for 10% stretch use Range/0.46

8m range, take your pick from 22m ropes to 17m ropes. Depends what note you want to hear when you twang the lines at low water. But if you're going for 10%, here's hoping your nylon lines haven't previously been over-stretched!

When I was regularly using alongside Peter Port berths for nice big old gaffers in the outer harbour, we used stretchy 25m lines. Worked a treat.

 

[[3889]]

Re tying to ladders, the object is not to keep the boat tight in but to be able to haul the boat alongside o sufficient line length is required. Rigging is strong but not designed to resist lateral strain, which will cause problems at deck seals which can be a PITA to resolve.

 

[garvellachs]

Re tying to ladders, the object is not to keep the boat tight in but to be able to haul the boat alongside o sufficient line length is required. Rigging is strong but not designed to resist lateral strain, which will cause problems at deck seals which can be a PITA to resolve.

Good points.

 

[lpdsn]

LOL

Read post #29 again and maybe have a re-think about who is confused. :encouragement:

Richard

Nope, still the same. I would avoid tying to a ladder even just to haul a boat in - it's an unreasonable strain on the ladders and there'd be no cause for complaint if a HM got grumpy about it.

As I said, you can achieve the same by hauling on a loose spring to bring the boat into the wall whether getting on board or stepping ashore. If you ever come sailing in the Irish Sea and are still confused I can give you a demo.

Or anyone else can as it's the way most boats do things. I'd tie up to a wall several times a season and I only remember one small boat at Portpatrick tied to the ladders - that had to cut themselves free when the tide went out unexpectedly whilst they were in the pub.

If the OP does his Great Harbour Walls of the Irish Sea tour he'll also find that not all walls have ladders. The visitors berth at Donaghadee and the berths inside the breakwater at Peel both have stone steps, for example.

 

[RichardS]

Nope, still the same. I would avoid tying to a ladder even just to haul a boat in - it's an unreasonable strain on the ladders and there'd be no cause for complaint if a HM got grumpy about it.

As I said, you can achieve the same by hauling on a loose spring to bring the boat into the wall whether getting on board or stepping ashore. If you ever come sailing in the Irish Sea and are still confused I can give you a demo.

Or anyone else can as it's the way most boats do things. I'd tie up to a wall several times a season and I only remember one small boat at Portpatrick tied to the ladders - that had to cut themselves free when the tide went out unexpectedly whilst they were in the pub.

If the OP does his Great Harbour Walls of the Irish Sea tour he'll also find that not all walls have ladders. The visitors berth at Donaghadee and the berths inside the breakwater at Peel both have stone steps, for example.

But I keep asking you to re-read post #29 because I'm sure that the only reason PRR mentioned ladders is because the post before his mentioned the boat falling 4m as the tide goes out. The ladder is therefore presumably needed to climb down back onto the boat from the harbour wall.

What continues to confuse me is how that relates to any of your posts?

Richard

 

[[3889]]

Nope, still the same. I would avoid tying to a ladder even just to haul a boat in - it's an unreasonable strain on the ladders and there'd be no cause for complaint if a HM got grumpy about it. I can haul my boat to a ladder without any real effort but I can't lift my own weight. The ladder can support it easily.

As I said, you can achieve the same by hauling on a loose spring to bring the boat into the wall whether getting on board or stepping ashore. If you ever come sailing in the Irish Sea and are still confused I can give you a demo.

Or anyone else can as it's the way most boats do things. I'd tie up to a wall several times a season and I only remember one small boat at Portpatrick tied to the ladders - that had to cut themselves free when the tide went out unexpectedly whilst they were in the pub. What caused the tide to go out unexpectedly - or do you mean it was only unexpected by them?

If the OP does his Great Harbour Walls of the Irish Sea tour he'll also find that not all walls have ladders. The visitors berth at Donaghadee and the berths inside the breakwater at Peel both have stone steps, for example. With rings for lines set into them.

 

[fisherman]

If you moor up with sufficient slack for the fall, you can hang weights on the warps. However, only part of the weight acts on the boat as the triangulation varies with height and part of the weight is taken at the quay end. Obviously the ideal is to have the warp taut at low water to keep the boat snug, but at HW there will be slack. Can't quite work out the dynamics, but what if you have two slack warps and a third, lightweight line fore or aft running through a snatch block hanging on the quay wall, with a weight on. This will exert the same effort on the boat at all times. But at different angles. Now my brain hurts!