Comments on: Shorefast Systems

By: Vyv Cox

Vyv Cox — Wed, 29 Sep 2010 17:23:39 +0000

In the Mediterranean it is very common to take lines ashore, or ‘shorefasts’ as you call them. Perhaps the primary reason for this is that anchorages are typically small and somewhat crowded, thus the use of shorelines allows many more boats to share the anchorage. A further reason is that anchorages in the eastern Mediterranean are often very steep-to, so that it would be necessary to free anchor in maybe 20 – 30 metres depth, whereas by taking lines ashore the boat can lie in much shallower water.

For many owners the material of choice for shorelines is polypropylene, which has the great advantage that it floats and will not snag underwater rocks when being rowed or swum ashore. Lengths of up to 100 metres are stowed on reels permanently mounted on the pushpit, if space allows. Tape reels are also seen frequently, especially on Turkish cruising boats, where steep-to anchoring is very common. Our narrow pushpit does not allow for a reel, so we stow our two 40-metre warps in homemade ‘flake bags’. Our warps are 12 mm braid-on-braid nylon, which is easily handled and has always been plenty strong enough.

By: Matt Marsh

Matt Marsh — Sat, 03 Jul 2010 20:32:28 +0000

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.

By: John Armitage

John Armitage — Thu, 01 Jul 2010 04:29:35 +0000

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.

By: Henrik Johnsen

Henrik Johnsen — Tue, 29 Jun 2010 19:29:49 +0000

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 of the forces applied to the boat during high winds.
Once again, thank you so much for sharing your experiences with us.

By: John

John — Tue, 29 Jun 2010 19:23:56 +0000

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.

By: Matt Marsh

Matt Marsh — Tue, 29 Jun 2010 01:25:51 +0000

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…. )