Nose-Heavy Electric Boat (BG)

[Mars-Bound Hokie]

I'm designing an electric amphibious vehicle to send to Laythe whose design is based on my years-old Amphibian. The jet engines were replaced with Breaking Ground props, the passenger capacity was reduced to make room for scientific instruments and reaction wheels, the fuel tanks were (95%) emptied, and the pontoons were replaced with a lot of batteries. 

 

 

• Image of the (second-to-latest) prototype.
  • I named it "Saltrider" since the only way Laythe's waters can remain a liquid at below-freezing is if it has a high-enough salt concentration to make it have a significantly lower freezing point.
  • The latest one had wing connectors (and control surfaces) in the back holding up the engines, although it did not make a difference in the outcome of the water test.
• I'm going electric so I don't get stranded in the middle of the ocean forever when I run out of power.
• It has retractable landing gear on the side as an anti-crash measure in case it tips over.
• Solar panels not included, as I need to conduct a successful water test before deciding the placement.

 

Though the land tests looked promising - and I was probably going too fast (85 m/s at a 17-degree propeller blade angle) - the water test was not so much. It was nose-heavy when I got in, and when I tried turning right to I literally went belly-up. I brought back the engines to try and balance things out, but it didn't make a difference - structural pylons or wing connectors.

 

 

Could you please tell me what's causing the boat to get all nose-heavy and how I can fix this? Here's the craft file if you want to mess with it - https://mega.nz/file/zaAGyIpD#62WNRZzcqXpyO-ipP2ISX5W25pZinpWrzQUbe17sUyI

 

Thank you very much.

[Vanamonde]

Thrust too far above the center of mass? 

[18Watt]

A screenshot of the ship upright in the water, during daylight, would be helpful.

The higher you mount the engines, the greater the pitch moment will be, which will try to force your boat to pitch forward.  

Getting boats to float correctly in the water is not easy.  I have used had good luck with using a little fuel (or ore) as ballast, moving the mass forward or aft as needed to trim the balance.  However my best results are always when I manage to get the center of thrust (COT) as close to being inline with the center of mass (COM) as possible.  

The only real limitation you have is that the propeller blades can never touch the water- they'll be destroyed if that happens.  Otherwise, do everything you can to get the engines as close to the water as possible.  Using the shortest propeller blades helps greatly.  (You'll just need more of them..)

I have not tried using batteries as the hull.  In general it is helpful to have as few hull sections as possible.  The game looks at each battery as a separate hull section, which might give undesired results.

Another problem with those high-mounted engines is you are going to have additional forces when you try to turn the boat.  The boat will try to roll outboard of the turn, which will cause the front outboard pontoon to 'dig in' and produce more drag, which will progressively make things worse- eventually the boat will flip.  Again, mounting the engines as low as possible will reduce that as well.

Good luck!

[king of nowhere]

expanding what @Vanamonde said, your propellers are above the vehicle, so they not only push you forward, but they also give you an angular momentum. to counteract that, you can move them down - contrary to what @18Watt said, I used a lot of plane/boats with propellers touching the water, and nothing bad ever happened, though you do not get thrust from using propellers underwater. alternatively, you ccan put more floaters in front, to counteract the nose-sinking tendency

[Mars-Bound Hokie]

22 hours ago, 18Watt said:

The higher you mount the engines, the greater the pitch moment will be, which will try to force your boat to pitch forward.  

With my old motors, the problem with that was the propeller blade length. My solution: use a smaller motor with smaller blades.

• And I won't have to sacrifice thrust, either. The latest Mk. I prototype was reduced to 15% power (60 kN) per rotor, while the EM-32S standard rotor has a thrust of 70 kN at 100%.

 

22 hours ago, 18Watt said:

Another problem with those high-mounted engines is you are going to have additional forces when you try to turn the boat.  The boat will try to roll outboard of the turn, which will cause the front outboard pontoon to 'dig in' and produce more drag, which will progressively make things worse- eventually the boat will flip.  Again, mounting the engines as low as possible will reduce that as well.

Which is why I not only did gradual turns in my test run, but I switched the front nose cone with Type Bs. I also added rear horizontal stabilizers to the Mk. II along with horizontals, although I don't plan on flying this thing on its own. When it's on Laythe, it's not going for a speed record or turn-heavy action; just an exploration cruise around the water.

 

• Here's the latest variant.
  • I added retractable solar panels to decrease recharge time for when this thing is stranded.
  • As I said before, after the water test was complete, I added solar panels on the exterior.
• The center hull and floats were based on my Amphibian, which worked so well in the past for what it was designed for.
• When the front nose was digging into the water a bit, I replaced it with a Type B.
• The nav lights are there to make it easier to see in the dark, although it's probably smarter not to go swimming at night due to not only lowered visibility but the lack of solar power to reduce (if not cancel out) the rate at which power is drained.

 

• Though the boat is still a bit nose-heavy, it was nowhere near as bad as Mk. I's was.
• For best speed on the water, set blade angle to 15 degrees (AND NO MORE).
  • To my surprise, I went faster on props than the Amphibian did on Panthers.
• You can still go at max speed at just 2/3 throttle.

 

• I made it to the abandoned airfield right as the sun was rising.
• After that test run was over, I decided to put gear on the front and rear tips so that the wheels can have a better grip on the terrain should it get too steep.
• That was when I increased the blade angle and pushed forward at maximum throttle.

 

• Air time.

 

So, anyways, thank you for all your help. Now that the preliminary test runs are done, all I need to do is copy this into my sandbox save, slap it on a rocket to Laythe, send it there, see if it survives re-entry, do a test sail, and then I'm all set for my career save.

 

If anybody has any questions/concerns/comments, please let me know. Again, thank you. 

[18Watt]

1 hour ago, Mars-Bound Hokie said:

My solution: use a smaller motor with smaller blades.

This also worked wonders for me.  I highly recommend this approach.

1 hour ago, Mars-Bound Hokie said:

but I switched the front nose cone with Type Bs.

I never saw an advantage to the slanted nose-cones.  However, that doesn't mean one doesn't exist!  I tried them and did not see any improvement, but your boat is much different than mine.  So if you are seeing an improvement with the slanted nose cones, stick, with them..

1 hour ago, Mars-Bound Hokie said:

I also added rear horizontal stabilizers

I've used aero control surfaces for trim also.  They (sometimes) work extremely well.  One problem is controlling them.  For trimming at certain speeds, you probably don't want them tied to any primary controls.  I've had luck pairing trim controls with action groups (to move them up/down).  Certain speed / thrust combinations may require different amounts of up/down force.  Being able to move the trim surfaces independently of other control groups was handy, in my experience.

1 hour ago, Mars-Bound Hokie said:

When the front nose was digging into the water a bit, I replaced it with a Type B.

Again, I never had luck with that, but it 'looks' like it should work better.  I am NOT an expert here, just passing on what worked for me.  If your experimentation finds solutions, you should use them!

1 hour ago, Mars-Bound Hokie said:

The nav lights are there to make it easier to see in the dark, although it's probably smarter not to go swimming at night

First, awesome idea!  Second, yeah, high-speed runs at night are risky.  But you can always hire more Kerbals, right?

1 hour ago, Mars-Bound Hokie said:

To my surprise, I went faster on props than the Amphibian did on Panthers.

Yup.  I wish the BG props were available in KSP2..

Keep at it!  Enjoying watching your boat/rover/speed machine evolve!