Unbeaching a Boat — With Physics!

Did I observe an ingenious ad hoc pulley operation, an accidental rope configuration that helped get a boat unstuck, or neither? [NUC #23]

This weekend, I went out on a boat for a kids birthday party.¹ Overall, it was a fun event, but as the tide went out where we were anchored, a nearby boat was left beached.

Not here. But near here. Image

By the time we noticed it, another boat was already hooked up to the beached craft with a rope, ready to pull it out by a cleat attached to the bow. Our small party stayed clear, the rescue boat pulled at a slight angle, and — to our surprise — the stuck was successfully pulled out to sea.

Incidentally, the formerly-stuck boat had carelessly left itself anchored in the water offshore… Or was this careless? It may have actually been ingenious.

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I didn’t take pictures, but to put things visually, here’s the rough setup:

I didn’t do the math exactly, but you get the picture

If the rescue boat (BoatR) was directly tied to the stuck boat (BoatS) and it pulled straight out, the total pulling force on BoatS would be equal to whatever force BoatR directly generated. However, if BoatR was instead tied to the anchor line looped through the cleat, the total force on BoatS could potentially be much greater. In fact, this pullout force could be nearly double what BoatR was outputting if the angle between the anchor and the boat line was near zero.

This is, in fact, a simple pulley system, as shown below👇

The cleat acts as the pulley. F3 is the anchor line. F1 is BoatR,

Looking a this another way, and simplified, BoatS would be represented by F2, while the anchor would be represented by the fixed F3, and BoatR by F1. F2 = F3 + F1, and since F3 = F1, F2 = 2 x F1.

Caveats

Of course, there is no free lunch. In order to increase force, the distance BoatS travels is cut ~in half as compared to BoatR when the force is doubled. Since work = force x distance, this preserves a sort of balance in nature, and no fissure in time-space is created. But distance isn’t really important here, since it should only take a few feet to get BoatS free.

More practically, there’s also the chance that the anchor would come loose, the cleat breaks, or the rope snaps. Also, the hull can certainly get scraped on the bottom, even if the boat is freed. so the real solution here is to not get your boat stuck in the first place. Also, it appeared that BoatR pulled at an angle, which may have helped torque and rotate the boat just a little bit to get it in motion.

It’s also very possible that BoatR just pulled it out, and that the anchor was not implemented as an ad hoc pulley. Either way, BoatS was freed, and everyone was presumably happy.

Don’t get stuck

What’s the lesson here? Well, first, don’t get stuck.² Secondly, pulleys are awesome. This rabbit hole goes much deeper if you care to examine this further. Third, welcome to Florida. Perhaps this is a well-known technique among boaters, or perhaps there’s some reason its frowned upon. Either way, this is the first time I’ve seen it.

Let me know your thoughts in the comments!

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Addendum/Footnotes:

1. As one does in Florida

2. Not that our party was entirely immune to this. As we returned to our pontoon boat under threat of an impending rain, the offshore anchor had released, and the shore anchor/current was torquing this boat in such a way that it was itself lightly beached. ~Five of us pushed the boat free. I ran and grabbed the anchor, dipped it in the water to get the bulk of the sand off, and scurried up the bow ladder as the boat was backing out. A little adventure/team building activity to finish our trip/birthday celebration.

That’s going to be a bad day. And perhaps night.

Minutes later, we encountered another, much larger, boat in the water, but fairly near the shore. As we continued on our way, I noticed that way more of the hull was visible than it should have been, with unnatural gaps between the water and the sides. It appears they were badly beached. There was no way we were pulling them out.

Likely they were going to be stuck until the tide came in many hours later. We did observe a Sea Tow boat take off in that direction on our way in. Maybe they have some way to deal with that, but it seems unlikely.