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Fire Boat

A simple fire boat. Features a working water cannon on a moving boom, double propellers and steering with tunnels instead of rudders.


Completion date: 19/07/2015
Power: electric (Power Functions)
Dimensions: length 80 studs / width 23 studs / height 26 studs
Weight: 1.29 kg
Suspension: none
Propulsion: 2 x PF M motor
Motors: 5 x PF M motor, 2 x PF L motor


This boat was one of the rather crude models built to test/demonstrate some concepts. In this case, there were two concepts: a rudderless steering and a working water pump.

I have come across idea of steering with tunnel in a German WW2 mini submarine Seehund. The idea is to steer using a fixed propeller without a rudder. Instead, the tunnel around the propeller is turned left and right to direct the thrust. It seemed like a simple design to recreate at small scale with Lego, using 3 studs big propellers and ready-made Lego tunnels.

As for the pomp, I was trying to build a functional Lego water pump for ages. I have tested a number of designs, from a peristaltic pump that proved that Lego tubing is too thick-walled, to a pressure pump in which air is being pumped into an airtight reservoir, pushing water out of it. The pressure pump proved to work, but the output pressure was below my expectations, and refilling it was quite tedious. In the end, I decided to use a regular medical syringe as the watertight container that would be operated by a Lego mechanism.

My pump design was based on the gear rack from the Lego 42043 set, which was used to hold the syringe while allowing to move its plunger. I have initially used a 20 ml syringe, which worked fine, but the large amount of water required plenty of torque to move the plunger, making the pump slow and not very effective. I wanted the pump to provide high exit pressure, ensuring good range of the water cannon it was powering, and I was also concerned about the possible damage from spilling 20 ml of water. I have then tested a much smaller 5 ml syringe, and it worked perfectly. It had low capacity, but thanks to that it required little torque to operate and could be operated fast – which meant both high exit pressure and quick refilling.

Since it seemed very inconvenient to manually refill the syringe every time, I have connected it to a Lego pneumatic valve with two tubes coming out of it. One tube went to the water cannon, the other went overboard and into the water, acting as a water intake. Switching the valve allowed to connect syringe to one or the other tube. Thus it worked in two cycles: when the plunger was pulled, the syringe would pull water from the intake tube, and when the plunger was pushed, the syringe would push the water into the water cannon tube. It took me a while to synchronize the valve with the plunger mechanism – it proved crucial to switch the valve as quickly as possible, because it would cut the syringe off the tubes when in neutral position, thus blocking the whole mechanism. In the end I have used the smoothest-working valve I had, and it still jammed from time to time. In these events, all water coming out from the syringe would be stopped at the valve, exerting a lot of pressure on it. I have later found that this made the valve leak, but very little – literally a few drops of water for some 30 minutes of operating. Given the circumstances, I would say the valve proved more reliable than anticipated. The syringe worked great too, it was perfectly watertight even when water was held in it for extended periods of time, it fit the Lego pneumatic tubes, and it wouldn’t allow the plunger to be pulled out of it when the pump mechanism moved it too far. All in all, the pump was pretty efficient and very easy to operate – it would unload and the refill in quick, continuous cycles. Ideally, the mechanism should be automated so the cycles are changed without input from the remote, but I didn’t want to make the already large mechanism even more complex. The valve function could be perfected too – ideally, it should switch before the plunger changes direction. In my mechanism, there was a little delay between the change in plunger’s movement and switching of the valve, resulting in some water being returned to the intake tube.

The entire boat was built around 74 studs long hull, which ensured good stability and load capacity. Most of the space was taken by the pump mechanism and electric components, including 3 PF IR receivers and two motors operating a crane with the water cannon on the bow. The crane could rotate and elevate, but in practice, when aiming at the fire, it proved much easier to just move the entire boat by hand. It also proved that tealights are nearly impossible to extinguish, as they can stay lit even with water spilled on top of them. The only way to effectively put them out was to hit the flame directly.

The boat was propelled by two 3 studs big propellers driven by two separate PF M motors. One of the motors was powered through a Power Functions switch that could be shifted remotely by a third PF M motor. This allowed to revert one of the motors, making the propellers work in opposite directions. I was hoping this would help to maneuver, but it did very little in practice. Yet another PF M motor was rotating the tunnels on both propellers. This proved to work, although I think it was less effective than traditional rudders. All in all, it was clear that the propulsion system was underpowered for a vessel of such size and weight – this could be helped by running the propellers at higher speeds or using larger propellers.

As usual, I have taken some safety measures before putting so many motors on a boat. All electric elements were mounted above the deck, not directly on it, in case of a spill. Also, all PF plugs were covered from the top, to protect them from any water drops that could occur with the water cannon shooting water at high pressure – some drops could bounce off the walls of the bathtub. I have also made sure that the water cannon’s nozzle stayed off the deck at all times, which proved a good move, as the water pressure would drop when the syringe was nearly empty. This meant that the last of the water coming out from the cannon was expelled at minimal pressure, basically dropping straight down from the nozzle.

The boat was great to play with, even if not exactly pretty to look at. I only wished it was more agile and the water cannon didn’t need such a careful aiming to be effective.


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Categories: Watercraft Tags: , ,
  1. Sariel
    July 26th, 2015 at 12:04 | #1

    Sure, I think the only problem would be connecting their exit tubes together. You know, we want to avoid situation where they pump water into each other.

  2. July 25th, 2015 at 15:56 | #2

    Could you use two syringes in tandem ?
    Eg as one is emptying the second is filling, then vice versa… For a constant stream of water?


  3. Ev3fan
    July 23rd, 2015 at 14:17 | #3

    Hmm ok no problem. 🙂

  4. Sariel
    July 22nd, 2015 at 23:31 | #4

    It is already disassembled.

    Sony Smartband works with all Android devices. And no, it’s absolutely not suited for controlling SBrick.

  5. Alex
    July 22nd, 2015 at 14:55 | #5

    Hey Sariel do you have a sony smart band talk as I noticed it on your hand and does it work with LG g3
    and by any chance can you control your sbrick through it.

  6. Ev3fan
    July 22nd, 2015 at 14:13 | #6

    I guess it is already disassembled, isn’t it? Because I think my idea is worth trying :). Or have I missed something important what makes this impossible in the first place 😉 ?

  7. Sariel
    July 20th, 2015 at 14:43 | #7

    No, I did not.

  8. Ev3fan
    July 20th, 2015 at 14:41 | #8

    Must be HUGE fun to play with 🙂
    And for the water cannon – did you consider controlling elevation and slewing with the speed control remote (maybe mechanically limited to the first few steps) ?

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