Triumph of the obvious: Bollards: A victory for ParaTow!

March 3, 2009

So.  I’m super-duper into this ParaTow wind-based ship-propulsion scheme, right?  Right.

And a remaining mystery, to me anyway, was the question of how the hell to hook it to a ship in a safe and inexpensive way.  I was assuming that some kind of harness contraption would have to be bolted/welded/strapped to the boat to allow the ParaTow to attach to the ship in one place.

And that’s probably true… if I wanted to hook to the ship in one place.


Take a look at this.  There are these thingies on ships called bollards.  They’re called bollards whether they’re part of a ship or part of a dock.  They’re the hard points where you tie on ropes or chains.

Bollard (I think on a ship, but I can't tell for sure)

Bollard (I think on a ship, but I can't tell for sure)

Bollards on a dock.

Bollards on a dock.

Okay.  So.  I finally decided to ask just how much load these things were built to take, and then compare that to how much tension a ParaTow will be exherting on a ship to pull it around at full speed.

I couldn’t figure that out, actually, but I did figure out that the “bollard pull” of normal harbor tugboats (how hard they can yank on a ship’s bollard through a long rope) is around 50 tons.  Okay.

Further, the bollard pull of an oceangoing tugboat (basically, a tow truck for ships that break down at sea) is around 100 tons.  Okay-okay.

So great.  We know that ships’ bollards, and therefore the structure underneath supporting them, are built to take somewhere between 50 tons and 100 tons of force safely.  Okay.

So how many tons must a ParaTow exhert in order to pull the ship at full speed?  If power = force x speed, then force = power/speed, and the million-dollar linux utility “units” will spell it out for us.  For reference, a common Panamax cargo ship goes about 20 knots and has an engine no bigger than 50,000 horsepower, so:

You have: 50000 hp / 20 knot You want: tonf * 407.33261 / 0.0024549962

Aha, so about 400 tons of thrust.

Well then, if a ship’s bollard is easily good for 50 tons of oomph, then we’d (theoretically) only need to hook to eight of them (four on the left side plus four on the right side) in order to safely yank this thing along at full speed.

The good news is that according to the pictures I’ve found in Ships Monthly magazine, they all have at least ten bollards up front.

So what do you know, there it is!  I’m imagining some kind of super-tugboat platform with not just one winch and tow line, but ten or so.  Their relative lengths are tuned to the geometry of the given customer vessel, and it goes through some procedure to let the ship haul up each of them and loop it over the bollard in question.

So wow.  The real world is never as simple as the sixth-grade math in a casual blog post, but the basic message is still loud and clear: When working together, the bow bollards on a ship are together rated for the same range of cumulative line tension as a ParaTow needs to route into the ship’s structure.

So done intelligently, it’s looking possible to demonstrate the ParaTow concept at full scale without having to add new structure to the ship (what I think most of you have suspected all along, but I was trying to make complicated).  Wow!  Let’s hear it for logic!

13 Responses to “Triumph of the obvious: Bollards: A victory for ParaTow!”

  1. Matt Brubeck Says:

    Is it safe to assume that a hundred-ton tugboat can deliver all that force through just one bollard? Might it need to be connected to more than one?

    Is it necessarily true that eight bollards can take eight times the load of one? If they’re all attached to the same structure, couldn’t some underlying component fail under a much smaller load than that?

  2. craigrmeyer Says:

    Oh! I sure have had the understanding that all tugboats use just a single connection, Matt. Just one. If I’m wrong I’m wrong, but I’ve never had a clue that it’s not ALWAYS just a single connection.

    This has everything to do with stiffness. If the multiple parallel connections are “stretchy” enough then the load SHOULD share pretty well. That’s the idea anyway. Indeed, though, if they were relatively stiff then most of the load would end up going through just one or the other, which ever was slightly tighter than the others. Which Would Be Bad.

  3. Peter Boothe Says:

    I think I’ve seen a tugboat using 2 bollards, but I may also be misremembering.

  4. Frank Says:

    If a simple stretchy hookup doesn’t work you could always use a series of pulleys to more or less force the load to be evenly distributed.

  5. Dan Says:

    @Frank: Or a series of tubes!

    Seriously, though, the second point is an issue. Imagine (a silly simple case) where all the bollards were tied to one metal beam, and that beam would separate from the rest of the ship with a >50 ton force. Each bollard could hold 50 tons, but you hook in to two of them, and you can still only exert 50 tons, even if the load was evenly distributed.

  6. Matt Brubeck Says:

    Even if the load is shared perfectly, what if all eight bollards are anchored to the same part of the frame, and that part can only take (say) 200 tons force? Then maybe any one bollard could withstand 100 tons, and any two bollards 200 tons, but loading all eight bollards with 800 tons would cause the frame to fail.

  7. craigrmeyer Says:

    I’m going to have to ask an expert, Matt. I can’t give a convincing answer to your ever-so-astute question without some help.

    Here’s what little I can say with confidence:

    1: Bollards are hardly ever alone, but almost always manufactured and installed in pairs. So if the left bollard is good for 50 tons then the right one MAN be able to take 50 MORE tons, or maybe not. I just don’t know yet.

    2: A ship’s bollard pairs are mounted quite separately from each other, at least 20 or 30 feet apart from each other. To me this suggests that their loads are going into different interior sub-frames, but the heck do I know?

    3: 50 tons of bollard pull sounds like a lot, but that’s just the hanging weight of a single loaded container.

    So it’s not like the a above PROVES much, but just supports my optimism.

    My next job to check this against the judgment I a real-deal Marine architect.

  8. craigrmeyer Says:

    (I mean “…MAY be able to…”. iPhone.)

  9. craigrmeyer Says:

    Okay, I’ll elaborate on this a little later, but it IS more complicated that the above. It is.

    I found that in all my pictures of tugboats bulling ships around, it’s always through a port at the very bow or stern. So it’s only at the bow or stern that tugboats hook to ships.

    There are many OTHER ballards on the sides, but they look a little different and are apparently only for mooring, and all logical bets are off as to their structural capabilities.

    So foowee. I reckon that now my job is to package this mystery up as best I can and actually find someone with professional experience with tugboats. I’ll keep you posted.

  10. ekpaulson Says:

    While I believe it is somewhat useful to speculate on how your Paratow could attach to ships, I think you are putting the cart before the horse. I think this particular problem will be easy/obvious to solve if or when you reach the point that it needs to be solved. No sense spending all your time solving problems you don’t have to face yet.

    Personally I think you should be dreaming about a canoe-scale Paratow, with possible applications to rich-dude sailboats first. Cargo ships should come later.

    It is true that we *need* (in a global sense) the “cargo ship” application much more than the “rich dude” application, but you’ve got to target the low hanging fruit first. Like Tesla Motors has been trying to do.

  11. craigrmeyer Says:

    You’re right about the cart before the horse, Eric, but I just can’t help it. I need to have SOME kind of picture of what this thing can ultimately be. I can’t move without it.

    In conversation with my handsome colleague Mike Shope, we got into the idea of a tiny-scale non-electric manually-controlled baby-ParaTow that converts a canoe or kayak into a sailboat, but a sailboat that:
    + can go through very shallow water (by just hauling the fish up out of the water),
    + can be hauled up onto a beach just like any other kayak or canoe
    + can be operated by just one person sitting in one place.

    It could be a real product all on its own, and would bring experience and credibility with every unit sold, used and *seen*.

    In fact, I’m right now in the throes of coming up with just such a proof-of-concept contraption for pulling a kayak across Lake Washington and back. It won’t be a perfect product, but it WILL show the basic ideas in operation. Nothing garners interest or credibility like a scale model doing its thing. Yup.

  12. Nathan Says:

    Hey Craig, have you heard of the Maltese Falcon …which has computerized, automatic, airfoil-like sails that allow one guy to run the whole thing.

  13. craigrmeyer Says:

    Word up, I read the BOOK about it. It’s called “Mine’s Bigger,” and is plenty interesting. Homeboy blew around $100,000,000 on that boat, and as far as I can tell he’s perfectly happy with it, too.

    The ‘Falcon is a great data point. It shows that people will put up HUGE money to develop, pretty much from scratch, a novel sailing system so that they can have the first one.

    If I could take a crazy course in understanding and exploiting the psychology of the super-rich, man, I’d take it ten times.

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