Question: We’re about to build a shoreline system. We have two reels from Easyroll that can take about 150 m of 12 mm line each. I’ve seen the system used by Polaris, and it looks very solid, but my concern is, obviously, the cost of using Dyneema [Spectra].
As far as I can see the only alternative to Dyneema will be polyamide [Nylon] lines, less cost, but also by far less strength. 12 mm gives about 3,4 metric tones breaking load, versus 10 mm Dyneema with 9 tons.
- How strong (breaking load) should a shoreline be to be sufficient to suit our 20 ton boat?
- What kind of, and how much shore line do you guys carry?
- Are they often used (normal anchorage situations during cruising season)?
- If you were about to make a system, like we are, which system would you choose, money taken into consideration?
Answer: Let’s start with our shorefast usage and then move on to our system on Morgan’s Cloud.
Despite the amount of information about shorefasts on this site—put “shorefast” in the Google Search box (above right) and you will see what I mean—we don’t use shorefasts ourselves very often at all, perhaps once or twice a season. Our reasons are:
- We have an oversized 55 kg (120 lb) SPADE anchor that we have huge confidence in that sets in almost any bottom type, so we don’t feel the need for shorefasts to replace or backup the anchor.
- We don’t like to anchor really close to the shore since such positioning would give us very little time and space to manoeuvre if anything went wrong. In fact, if we are expecting a really nasty blow, we often look for a larger anchorage than many people might choose, say half to a quarter mile across, and anchor well off the shore—it’s not the sea that kills sailors (and boats) but the hard bits around the edges.
- Shorefasts prevent the boat from swinging to face the wind, which puts huge strains on the shorefasts themselves and any anchor that’s down.
Having said that, there are times when only a shorefast(s) will do: Usually when the only available anchorage, like the one above in East Greenland, is too small to safely anchor in, or we are trying to get into very shallow water in the high latitudes to get away from ice. Incidentally, shorefasts can be a real pain if there is any ice drifting around since it tends to get hung up on the lines, producing truly frightening loads.
Our shorefast system consists of one 300 foot (91 meter) length of 1/2 inch (about 12 mm) Spectra single braid and a further 900 feet of 7/8 inch (22 mm) Nylon single braid in three lengths that double as secondary anchor rode(s) and for streaming a Galerider drogue.
All the lines have thimbles spliced into both ends so that they can be joined using shackles—knots would reduce the strength of the rope too much and will slip in Spectra—or be shackled to the two 50 foot (15 meter) 7/16 inch (11 mm) stainless steel wires with soft eyes spliced into each end that we use to go around boulders on the shore.
All of the lines are kept flaked, never coiled since that will cause tangles, in bags. The bags work fine, although line reels would be more convenient. However, we don’t like the clutter of reels, and since we don’t use shorefasts that often, bags are a good compromise.
In all our years of cruising we have always been able to get secure with this system, although there was one time when every dock line on the boat got co-opted as well; however, that was when we were still anchoring with a CQR, which was, as usual in the high latitudes in our experience, refusing to set properly.
Shorefasts are not just for remote high latitude cruising grounds. Here we are using one to stop the boat from swinging into shallow water or the green ketch at Great Wass Island, Maine
We plan to replace one of the Nylon lines with a second Spectra line simply because my aging back is starting to object to dragging over a hundred pounds of wet Nylon rope around. The Spectra will, as you point out, cost a bundle, but less than a back operation!
By the way, you can reduce the cost dramatically by sourcing Spectra or Dyneema from a commercial supply house rather than a yacht shop. We got ours from Machovec and they even custom dyed the rope high visibility red and spliced it for us.
As to a recommendation on the appropriate strength for shorefast lines, it’s hard to be precise. But given that the loads can be huge when the boat is caught on the beam by storm force winds—as was graphically brought home to us on Polaris last winter when we had winds gusting to well in excess of 50 knots on the beam—and the penalty for a shorefast failure close to land may be loss of the boat, we have sized our entire system for our 52,000 pound (23 metric ton) boat for a minimum break-load of 17,000 pounds (8 metric tons), including the shackles.
Your boat is a bit lighter than ours, but unless you can come up with some really good engineering to support it, I would not want a shorefast system that was much lighter, in relation to the boat’s weight and windage, than ours.
If you have a shorefast system that works well for you, or any other thoughts on the subject, please leave a comment. We would be particularly interested in hearing from any reader with an engineering background that could make a stab at calculating the loads on a shorefast with the boat caught abeam in storm-force winds.
{ 5 comments… read them below or add one }
A great post as usual, John… some very helpful insights.
)
As a very rough guess, a ‘typical’ 17 m monohull sailboat beam-on would present something like 30 square metres of hull, and perhaps 6 square metres of deckhouse to the wind.
Let’s say the air density is around 1.3 kg/m3, the beam-on hull has Cd of 0.8 or so, the superstructure closer to 0.5; this gives us about 11 kN of wind drag on the hull and deckhouse alone in a 50-knot wind.
What’ll the rigging add? Something like 120 m of wire (say it’s half-inch) for another 0.3 kN, more if the wire’s heavier; a 30 cm mast section 15 m high for another 0.9 kN (again, more if larger/taller)….. by the time you add all this to the hull, you’re probably around 12-14 kN for static wind drag alone (or close to 2500 lb).
As for waves- well, let’s guess a 10-degree slope on the wave face, make a whole bunch of extremely unrealistic assumptions, and get something like 17% of the boat’s weight (you mentioned a 23 ton boat, so perhaps four tons- say 40 kN- due to the boat being bashed around by waves).
Based on this very hasty, very rough back-of-the-napkin guess, I’d say it’s not unreasonable to expect repeated loads of 50 kN or so (about five tons) on the shorefasts for a boat this size, with peak loads potentially much higher. Considering that the real killer is going to be the shock loading as the boat is kicked back and forth (not calculated here), the system John describes above would seem to be about right for a boat the size of MC, and I doubt I’d be comfortable with much less if I had to face these conditions.
(Thankfully, my boat is far, far too small to ever get in such a situation….
Hi Matt,
Thanks very much for running the numbers with what is obviously a lot of knowledge. It’s really great to have some solid engineering to back up our gut choices.
To our other readers,
Matt designs boats and has a web site that is well worth a look. Just click on his name above his comment. He has a particularly insightful post on situation awareness here:
John
Thank you for the detailed and highly useful information. Your website is very helpful for less experienced sailors, like ourselves. The comment regarding the physics, submitted by Matt, I also found very interesting, and for sure made me aware the forces applied to the boat during high winds.
Once again, thank you so much for sharing your experiences with us.
When I cruised Norway I used shorelines only as a last resort, partly for the reasons discussed above, but also because single-handed I found it so difficult to extricate myself from the tangle if the weather turned difficult. Very time consuming and tiring. In an emergency weather deterioration, I felt shorelines greatly limited my options. The Norwegians thought I was crazy; without a history of using modern anchors, they were reluctant to trust an anchor and always opted for shore lines.
Thanks for the kind words, John, although I’m not sure I’d put too much faith in a back-of-the-napkin calculation with so many assumptions, as I gave above. (There’s a big difference between knowledge from books and calculations, and knowledge from actually being out there like you are!)
A proper answer to the question of “how much load must the shorefast take” should take into account the motion of the boat and the stretch of the lines, rather than the static approach (boat sits on wave face, gravity pulls down, line pulls sideways, buoyancy pushes normal to wave face) I used above. The choice of rope type (spectra, poly, nylon, etc.) will make a big difference in how the secured boat reacts to waves (stretchy vs. high-modulus) and will thus change the loads on the lines.
It’s not a trivial problem, and I agree with John’s recommendation to err on the beefy side.