# What would boats look like on a planet with two moons?

For the purposes of this question, let us assume that one moon is similar in size to Earth's but slightly further away and the other is significantly smaller but approximately the same distance away from Earth as our own moon. Both are in a resonance of 2:3. My understanding is that this would create more intense tides that would evolve different seafaring technology than cultures on Earth but I'm at a loss at figuring out how this would be achieved in a realistic sense.

What I am specifically looking for are the unique challenges presented by more intense tides and the considerations that need to be taken into account when building vessels meant to traverse these waters. I would also appreciate consideration for the general shape, size, and materials used in their construction but these are by no means firm requirements.

• I can't see much change to the boats themselves, but getting small boats into/out of the water becomes much more interesting. You can't just pull it out of the water and leave it above the high tide line if the high tide line is hundreds of meters from the low tide line. Similarly, challenges with building ports and docks would be higher. Probably most cultures would prefer building around river mouths, and do all their launching/retrieval there. Commented Oct 9, 2021 at 5:16
• (1) If the two satellites are in a 2:3 resonance then the ratio between the radiuses of their orbits will be $\sqrt[3]{(3/2)^2} \approx 1.31$. The radius of the orbit of the satellite farther away is 30% larger than the radius of the orbit of the nearer. (3) Tidal force decays with the cube of the distance, so the tidal force produced by the satellite farther away will be 2.25 times smaller than the tidal force produced by our Moon. (4) Overall, spring tides will be no more than 50% larger. That's not a lot. (5) The question does not explain what those "unique challenges" are. Commented Oct 9, 2021 at 8:11
• At night, they would have two shadows. Commented Oct 9, 2021 at 23:07

No difference whatsoever

The ability to float in water is moon-independent. Waves on Earth are large enough to topple cruise and cargo ships today. I can't imagine any diffeence in how a boat would be built simply because two moons would occasionally create a larger tide. In fact, the only real difference would be in the tide tables, not in the design of the boats at all.

• Tides are effectively unnoticable once you're out at sea. What would need to change a bit would be the docks, which would have to float to stay at water level. This is done in many places today, e.g. resevoirs that are also used for recreational boating. Commented Oct 9, 2021 at 16:04
• @jamesqf That's a great point.
– JBH
Commented Oct 10, 2021 at 1:20
• @jamesqf: …or just build the docks on poles high enough that they'll stay above water even at maximum high tide (and preferably far enough out to sea that there's still water below them even at low tide). This is also something already done in many places on Earth today, and has been done for a long time. Commented Oct 11, 2021 at 1:18
• @Ilmari Karonen: If you have a large tidal difference, you need some sort of ramp (or stairs) to allow you to go between the end that's always on dry land, and the other end where there's deep enough water for ships at low tide. Commented Oct 11, 2021 at 4:11

Longer anchor chains needed, stronger engines advisable

Your ship would need longer anchor chains, because harbours will have higher quay walls and anchoring off shore to wait for entry will also demand longer chains. Stronger engines on the ship itself, or on tug boats, would be beneficiary for large cargo ships, when the unloading window becomes smaller (see below), unloading will be easier, when the ship can move into position faster. Also, larger engines could help the ship cope with the stronger sea currents, to keep a straight course. This may be a minor point.

Because of the larger tidal difference, your ship gets stuck easier, when approaching a harbour. Cargo transport would slow down: stacking your cargo up high must be avoided, because draft is to be kept minimal. Draft limits will affect loading capacity, you may need more ships. Tides slow down traffic, they do that now, on Earth.. Larger tidal difference could cause a long wait and a smaller window for loading the ship.

I'm afraid your little moon, traveling faster since it's closer in, is going to go a little crazy with the other Earthen moon out there. I feel it would not be stable.

But you want to know how a stronger tide would affect boat design

Many assumptions aside, if you look at extreme tides. We have them on Earth, depending on your geography. Your boats will have to be like our boats, ready to avoid tidal inlets during high/low tide. I don't anticipate any change other than timing. Just make sure to keep an eye on your littlest moon flinging about.

Happy worldbuilding!

I don't think more intense tides or tides with an "interesting pattern" will cause any additional problems while traversing the open sea.

Higher tides will mean less silty coastline, stronger current between inner seas and open ocean (think Gibraltar and Bosporus straits), higher tides in estuaries and it's likely estuaries well dominate the number of deltas. E.g. unlikely that the 20% of Bangladesh facing the Bay of Bengal would exist.

Sheltered ports may be at a premium (with larger commercial cities concentrating in their neighborhood) and docking/loading/unloadind the ships may be a tad different in all other ports.

The shallow water (like South China Sea or thr Great Barrier Reef) may present interesting challenges for navigation. Strong currents may discourage the use of canoes or rafts in such areas. But occasionally the tide may be low enough to expose the seabed and make from "navigating" between neighboring islands a matter of walking (or running, as it may be)

• The Bosporus connects the Black Sea (to the north) with the Sea of Marmara (to the south). Neither of them has noticeable tides. (In general, the Mediterranean and its annex seas are tideless, with the exception of a very few exceptional locations. When the Romans reached the shores of the Atlantic and saw real tides they were filled with awe.) Commented Oct 9, 2021 at 21:27
• @AlexP even so, if a current of mere 1.45m/s created troubles for the early navigators, stronger and more chaotic ones will definitely influence navigation. Commented Oct 10, 2021 at 1:38

Part One (of Four):

The Maximum Possible Vertical and Horizontal Tides on Habitable Planets.

There have been other questions and answers about worlds with high tides.

Using the search box in the upper black band for "tides", I found a number of questions:

https://worldbuilding.stackexchange.com/search?q=tides

For example:

Tides with 3 Largish Moons

In my answer to this question:

Would it be possible to have an earth-sized planet have much larger tides while remaining habitable?

I discussed how large tides could get on habitable planets.

It has been claimed that a habitable planet could have mid ocean tides as high as 20 feet. I don't know how correct that is, but if that is correct it is simple to calculate the maximum height of coastal tides by multiplying the higest tidal range on Earth by the relative height of mid ocean tides on the two planets.

So some spots on the shore of such a planet could have tides as high as 530 feet vertically. And if there are tidal flats in those locations the horizontal range of the tides would be determined how flat those tidal flats were. There are examples on Earth flat enough to extrapolate that the flattest tidal flats on another planet could have horizontal tidal ranges of tens of kilometers or miles. And possibly there oculd be even flatter tidal flats. That wuld be rather inconvenient for fishing boats in those locations.

And I guess it would be possible for someplanets with 20 foot mid ocean tides to have coasts the maximimum tidal range even more extreme, and to have even flatter tidal flats in those spots, and thus have horizontal tidal ranges of hundreds of kilometers or miles.

That would certainly cause problems in docking at ports at those extreme tidal locations, and so presumably there would only be ports in places with much lower tidal ranges.

Part Two:

Tidal Heights Won't Matter in the Open Ocean.

But for ships in the open ocean the height of the mid ocean tides would not matter. A mid ocean tide 1 kilometer high, for example, would be thousands of kilometers wide and would have a slope of less than 0.001, which would be much less steep than many wind driven wves which ocean going ships would encounter with no problem. And such a 1 kilometer high mid ocean tide would be many times higher than one that might erode away all of the continents and make the planet totally covered by ocean with no land.

So you can design a planet that has very high tides in some locations, causing problems along the shore line, and changing the designs of port and of ships, but you can't design a habitable planet that has mide ocean tides high enough to cause changes in ship design.

Part Three:

You May Need to Have Closer and/or More Massive Moon(s) Than in Your Question.

If you want to change the designs of ships and ports to cope with high tides in some coastal areas, you will probably need much more massive moon(s) and/or or much closer moon(s) than you asked in your question. I fyou are fine with not having to change ship design then you can go with the moons in your question, but if you want to have different ship and port designs due to much higher coastal tides, you will need to have more massive and/or nearer moon(s).

Part Four:

A Problem and a Solution.

But you have a problem with a habitable planet having very close and massive moons making very high tides, as I explained in my answer.

So you may be forced, if you care about plausibility, to make your habitable planet a very young palnet, only hudnreds of millions of years old, which has been terraformed to be habitable by a very advanced civilization.