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Can Lego Boats Float?

A lego man on a lego boat rowing on lego water.

Your child might like LEGOs and enjoy building structures and vehicles with them. The toy manufacturer previously released an erector set designed to make boats. Of course, the question arose, “Can Lego boats float?”

The answer is simple. Yes and no.

The Lego boats you see online may use the specialty building blocks that it crafted of ABS plastic for its boat kit. Those would float. On the other hand, you might have purchased a variety of kits by the manufacturer. In that case, these items would weigh different amounts.

Each model kit uses slightly different Lego pieces. Some of these bricks weigh enough to float while others simply sink. Part of this has to do with the design of the building blocks while another part concerns boat design.

Basic Physics in Action

If you have ever visited a shipyard, you can watch the boats sail in and out of the harbor. This amazing sight gets more amazing when you consider how much those steel or wood boats weigh.

The physics property that applies is displacement. If you craft a ship that weighs less than the amount of water it displaces, the boat will float.

You probably do not want to conduct physics math every time you build a building blocks boat. That’s understandable. You do  not need to do that. Just know that unless you use Legos specifically from the Lego boat kits they probably will not float.

3 LEGO Boat Kits that Float

Not every LEGO boat floats, which strikes me as odd. If your kids builds the boat, you know they’re going to want to float in the tub or the lake. I know my boys do. That’s why I set out to find boat kits that float.

The key is that the boat have an airtight hull.

Here are the 3 that do float. Note my son turned these kits into his own creations.

lego boat 1 may7

LEGO City Ocean Exploration Ship 60266, Toy Exploration Vessel, Mini Helicopter, Submarine, Shipwreck with Treasure, Lifeboat, Stingray, Shark, Plus 8 Minifigures, New 2020 (745 Pieces)

lego barge2 may7
lego coast guard may7

LEGO City Coast Guard Patrol 60014 (Discontinued by manufacturer)

Are there others?

Probably. I haven’t bought every LEGO boat kit.

But this build-it-yourself hull will NOT float

My son also likes to build out his own hulls but these won’t float. Here’s an example:

lego boat not float may7

What happens if displacement is off?

Your ship will sink. If your boat does not weigh the right amount – as in it weighs too much – it sinks. It will let water creep in typically from one side. The boat tips as water enters it, then it fills more quickly with water until it goes under completely.

Legos in Science Action

The proliferation of these erector sets and building blocks means you can easily use them in a classroom or at-home science experiment. You can use these Legos to teach physics and engineering and its sibling research area, materials science.

Material Design

These are various boats made of lego floating on water.

The design of Lego bricks makes them more likely to float than other building block types. Although the ABS plastic from which they are made weighs only a little more than the water they displace, put together, they can sometimes achieve the right density and surface tension to achieve buoyancy. Unlike other block types, these Lego bricks have an underside with hollow sections. This hollow area lets air bubbles accumulate which improves buoyancy. While solid ABS plastic sinks, the hollows that let air bubble accumulate enable the floatation.

Building block shape matters, too. A pieces that has a wide, think shape will float more easily than a rectangle or square solid piece would. Size and shape in engineering and the material from which the object is made determine whether it can float. Even pieces made of the same material from the same kit can have a different buoyancy since their shape and size differs.

With respect to materials, not all Legos get manufactured with ABS plastic. Some the company makes from foam. The foam bricks always float. These also work better for small children since they create less of a choking hazard.

Newer Lego kits will not use the same two materials as those that exist now. The Lego company made a consumer commitment to go green with its manufacturing and materials by 2030. As they do this, they discontinued some kits. They need more sustainable alternatives and that will take time to discover appropriate materials.

Water-tight Needs

This is a close look at various boats made of lego floating on water.

Another engineering tenet you can teach using Lego kits and boats is water-tight construction. When you design a water-going vessel, you must make it water-tight so it can achieve the proper buoyancy.

Besides needing water-tight design, you also need a design that keeps water from entering. This might seem silly on a Lego boat since you will not actually have any people inside it, but when you use it to teach design mechanics, you need to explain the needs of an actual boat. When you construct an actual boat, you need people and equipment inside it to steer it and navigate.

Your design must accommodate those individuals and equipment without making the boat sink. You can practice this using a few Weebles people for the ship’s personnel because as you probably already know, “Weebles wobble but they don’t fall down.” Weebles make a fine Lego ship personnel roster.

Basic tenets of boat engineering include requiring high sides and a good drainage system. If you do take on a tiny bit of water, you have to be able to rid the ship of it quickly. Any design that lets water in the boat with out providing a way to quickly drain it out will cause the ship to sink.

You also have to evenly distribute the boat’s weight, so it does not sink. The same is true of what you put inside the boat. You must distribute evenly the contents and individuals of the ship’s personnel.

Ship Size and Floating

This is a close look at a huge white luxury liner made of lego.

Ship size matters, too. You cannot put a 20-person crew into a canoe and expect it to float. When you place too much weight in a ship, you sink it. You can think of this in simple terms on land, too. Your bicycle has a weight rating. You can typically have one person of up to 250 or 300 pounds ride a mountain bike at 21-speeds.

You could double ride two people of 125 pounds. You would break the bike frame though if you attempted the ride with a person of 400 pounds. You can only put it into the boat or ship the number of people and size of people intended for it or you will cause it to sink or capsize.

That’s why large ships have many lifeboats. If the large ship takes on water or undergoes damage, the ship’s personnel or passengers offload into the lifeboats which can typically handle six to ten people. A sailboat, for example, would only have a lifeboat or raft suitable for one or two people though. If you exceed the capacity, you will sink the ship or raft.

Design Mechanics

This is a close look at lego Jack Sparrow, lego storm troopers and a lego lifeboat on a beach.

Certain design aspects can enhance the buoyancy and surface tension of your boat such as providing the Lego boat with long flat pieces that form side wings. Using a wider hull can help a Lego boat float. You can still find Lego kits online that offer sealed boat hulls as a starting point.

These let you build without concern about water penetration risks. Individual bricks in these kits are watertight but when you lash them together, you will inherently have some spaces between them. These can be placed so close together as to block most water, but you would still need to teach the concept of waterproof liners for an accurate engineering lesson.

Kit Combinations

Some Lego aficionados run into trouble when they mix and match from different kits. While all the parts in a Lego kit might float, including pieces of a Technic gearbox or DUPLO kit would not work. Made of different materials, these do not provide a watertight seal. Mixing and matching would sink your ship.

A close look at at a lego batman with his lego batboat on a beach.

Making Your Lego Boat Last

Perhaps you have achieved the ultimate design and you want it to last the test of time. You want your Lego kit boat to stay together and remain watertight so you can sail it with your own kid when you are 40. You can use a clear silicone sealant to fill the cracks between the blocks. Liquid latex works, too.

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