Wednesday, December 25, 2013

Got Rope?

I've had this 30-year old anchor rode hanging in a tree for 15 years, serving a second life as a Tarzan swing. I've got a Kevlar genoa sheet with a shattered core, no longer trust worthy for anything critical.

The rope ladder was a simple project. I've wanted something that was compact, couldn't harm gelcoat, and could be used climbing in or out of the tender. This took about 20 minutes and about 30 feet of line (2x the length + 3' for each rung). A fun night-before Christmas project, complete across my lap, with a glass of hot tea at my elbow, while watching The Polar Express with the family.

The door mat required 100 feet of 1/2 inch line and about 2 1/2 hours to complete. The knot in the middle is rather a lump underfoot, but it looks very traditional. The wrap and sewing took the real time. I soaked it with the borax/washing soda/baking soda anti-mildew blend when finished. My mom thought it a charming gift; since we had enjoyed it on the boat and in the backyard for many years in previous incarnations, it has history with her children and grandchildren.

And 2 lumps of old rope were thus consumed. Good.

Via Ferrata Tethers--Some Good Ideas, But Not for Sailing

Via ferrata trails, literally iron ways in Italian, are climbing trails equipped with rungs and safety wires that allow fit but unskilled personnel to move quickly and safely over difficult terrain. Though they date back into the 19th century, they are best know from their use during World War I, moving troops through the mountains.  Since then they have expanded to thousands of tourist routes in Europe and the United States. Since tether technology is the focus of this post, I'll leave history research to the reader; the internet is great for that.

The climber is protected falling all the way by a pair of tethers that follow a safety wire to the side of the rungs. Periodically, the wire is fixed to the face, and this is the challenge; these points are generally  10-20 feet apart, allowing for  serious falls before the tether takes, and requiring the tether to absorb a great amount of energy, more than a dynamic rope alone can handle. Dynamic ropes are rated for a fall factor of 1.8, whereas a via ferrata fall can be fall factor 5 or more.

The solution is a more extensive energy absorption system, including either friction through an aluminum plate or a tear-type energy absorber, like a Screamer.

Why wouldn't these make useful sailing tethers?
  • The allowable impact force is much greater, since the load is taken on a seat harness rather than a chest harness; the human tail can take a lot more force than the ribs. Sailors need a softer catch.
  • The falls are really much greater. A sailor will never see an impact beyond fall factor 1; the ISO drop testing required by the ISAF rules is based on a FF1 with a 100Kg mass.
  • The end clips on via ferrata tethers are applicable and gaining popularity. The West Marine tethers are now Kong Tango via ferrata biners (pictured on tethers to right), and I like them very much. Fast on and off, one-handed even when cold and wet, locking, light, and fit over most handrails too. Because they are aluminum, they do require periodic lube and rinse. Mine have held up very well, like new after 2 seasons.
  • Harness attachment. Sailors like a quick release on the tether, something that can be removed under load. Climbers opt for a locking biner, since no possibility of accidental release is acceptable. Most often the tether is cow-hitched to the harness.
  • Length. Sailors like dual lengths (2-3 feet and 4-8 feet) while climbers are restricted to 2 matching arm's length tethers; they never want the clip to move out of reach (if it snags below you this is a real drag).
Certainly there are some ideas here, some worth adapting, but the application is different.


Note on recalls. Because of the extreme nature of the falls and untrained nature of many users, there is some history of failures, most often fatal. In general, the causes have been:
  • Fall too great. Anything of 10 feet is trouble and over 15 feet is beyond the equipment rating. The via  ferrata trails generally recognize this, placing tie points ever 10-15 feet, but risk is often part of  climbing; if it looks too scary, go down.
  • Worn friction rope. Some designs rely on a rope slipping through a plate with many holes. If the rope wears through use--tour groups use them daily--the rope slides more easily and insufficient energy is absorbed. This type has gone out of production because they are too difficult to inspect.
  • Poor or worn stitching, or any other construction fault, though this type of failure can result from too long a fall.
ALL via ferrata falls are bad, far more dangerous than falls on a climbing rope. They feel safer but are not. The impact forces are high and there is much to hit on the way down.

Monday, December 23, 2013


When we need something, we run to the corner store store, or worse, the mall. When we some thing for the boat, we run to the chandlery, West Marine most probably. What we need is on the shelf. Seldom do we give a thought to making simple tools, though isn't that capability what made us human? Back in the day, for a workman to make his own personal tools was central to the craft. What sailmaker bought his fids from a store? Seam rubbers and wooden fids were crafted as needed, to the individual taste and hand of the worker. Some made fids with a handle or a turk's head worked in a turned grove; this probably worked best for large course hemp lines, but on a small boat with synthetic line, needs have changed. Even my wooden fids are smooth, so that they can pass through one line, towing another. The point is they were made by me to work best for me.

My finishing bench is of my own creation, fits nicely next to my favorite chair, always holding what I need near at hand (I have another sewing kit on the boat). It is a bit smaller than old-time benches, better suiting the size of my projects.

My wooden fids were turned from scrap hardwood dowel in a drill press (I have a lathe, but the drill press is handier for this) and have been worn smooth. There is a hole in the butt to accept a towed line, when that is required.They took perhaps 15 minutes to make, and I enjoyed the process.

My hollow fids are discarded knitting needles. Ask your wife or daughter--I'm they have a few mismatched needles about--or go to a thrift store. Cut to length, smooth the outside edge with a file or sandpaper, and clean the inside edge with a hand held drill bit. I seriously doubt the commercial hollow fids were purpose built, as the volumes are far too low--they are re-puposed knitting needle blanks, I'm sure of it.

Wednesday, December 18, 2013

Tangier Island Seeking More Cats--Who Would have Thought

When we first visited Tangier, we were greeted at the Park's Marina by about 50 cats, all fed by the owner. At some point, someone had a problem with this state of affairs, and the town’s leaders brought in a SWAT team of veterinarians to spay and neuter their increasingly out-of-control cat militia. Unfortunately, they hadn't done all of the math.

Now the otters have the upper hand, stealing soft crabs so conveniently placed in shedding tanks, available nightly under convinient work lighting.

Some of the waterman have gone back to keeping cats on their shedding platforms. Though a cat is no match for an otter, a group of cats is sufficient deterrent.

I suspect we'll see more cats during our next visit.

Read the full text here.


Sunday, December 15, 2013

Heater Efficiency

Marine space heaters are not typically rated for efficiency by any approved method, and Internet folks are forever commenting on what is efficient or not efficient, without any engineering basis or understanding of the math. But it isn't complicated and engineers have long known you can calculate everything you know from the stack conditions. After all, what goes up the stack didn't go into the room.

I described my heater installation and early experiences, but I only estimated the efficiency without rigorous measurement. Time to fix that.

Stack Temperature. Obviously, cooler is better. Since the stack temperature on my Dickson P9000 is 185F on a cold day (40F), that gives me a starting point efficiency of 88.5%. However, since the Dickson unit preheats the combustion air with flue gas, this gives a 4% increase to 92.5%. So little, you say? In effect, I'm simply moving the exhaust temperature 200F. It is worth doing simply to keep the hull fitting cooler and to reduce burn potential on-deck. More importantly, while dropping the temperature of the exhaust 200F is not a big deal in terms of efficiency, it means the P9000 exhaust can't damage a nylon rope. That is a big deal.

Excess Oxygen. However, the easiest way to get a cool stack is plenty of excess oxygen. A few percent is desirable, but the rest is energy wasted heating air. I get a reading of 5% on the Dickson heater, and that costs me 2% on heater efficiency; down to 90.5%.

Ambient Temperature. It is also necessary to do a correction for ambient temperature, compared to the base temperature used int he first graph (70F). Colder inlet give the impression of higher efficiency. By observing performance at 40F vs. 70F I gave myself some false credit. Subtract 1%. Down to 89.5%.

Overall Efficiency. Just start at the top and add them up. 88.5% + 4%(preheat) - 2% (excess O2) - 1% (testing temp correction) = 89.5% thermal efficiency.

AFUE (Annual Fuel Utilization Efficiency) Rating. This rating, found on every home furnace, is a bit different, as it includes losses up the stack when the unit is not firing; for example, fire places can be great heat wasters since warm air goes up the stack even when not in use. However, all of this is irrelevant to a simple marine heater such as the Dickon P9000 (it runs steadily without cycling--it can be turned down), I have not considered it. However, for the purposes of comparison, the AFUE is generally about 15% lower than the thermal efficiency. However, since the Dickson is sealed (all combustion air comes from outside) only a minimal 5% penalty is applied. Electric heat is assigned no penalty, so I am being conservative. 84.5% estimated AFUE.

Bottom Line. So how does this "stack-up?"  This little marine furnace rates as mid-efficiency. Not too bad for something the size of a toaster and much better than more primitive predecessors.