Help needed with sea ship propulsion.

[davaeron666]

I just spent a few hours building a sea worthy battleship and I was experimenting with different type of propulsion. I was wondering what you guys found what was the most efficient? Propeller under water or above....jet engine ...? Any tips would be appreciated.

https://steamcommunity.com/sharedfiles/filedetails/?id=1478415734
https://steamcommunity.com/sharedfiles/filedetails/?id=1478415662

[d.c.1939]

If you are looking build heavy sea craft, you should get the Maritime mod. It comes with all the parts you need to make a good boat/sub

[RoofCat]

1. propulsions is not the problem, drag is. It's much higher in water.

2. buoyancy can't be cheated. Your mass requires the same amount of water pushed out. Amount of that water basically defines your drag too. Thin or fat pontons will work pretty much the same. Ponton will work the same as the main body (just add more mass for construction).

3. you can use hydrofoil to get out of water. It slightly optimizes the drag, but still it is bound to mass you have to lift, so not much
https://steamcommunity.com/sharedfiles/filedetails/?id=1394380070
4. 80m/s is actually 288km/h. Not that bad for a boat in real life. It's also very good in KSP. My top speed is above 100m/s for a boat, but it's just about power and keeping it under control. Most boats are stuck at 30..60m/s. Which is very boring travel speed in KSP (and in real life in fact). Boat tours look good only with drinks and half naked people of your choice. So be ready to go slow.

5. KSP propulsion works everywhere. You can have engine above water like on hovercraft or below - it will still work. Scott Manley once created electro-mechanical paddling boat. I guess not just him. The speed was ridiculous though.

6. there are 2 kinds of drag - frontal and surface. Optimizing one you will increase other, because you still have to submerge exactly same volume due to same mass.

7. best results can be achieved making water plane - transfering most of the lift required (~drag) to air wings instead of water. Air has less drag for the same lifting work so you can go faster and lift most of the vessel out of the water reducing drag a lot. The problem here - it is very hard to keep those "floating". As your speed grows, so does your lifting force, you lift a bit more and your water drag is reduced further (considerably) and you can go even much faster... and it's more like one moment you float and the next one you fly already. It's is rather easy to make plane fly above water with some tiny wing in water. But that is not really hydrofoil, you just glide on your air wings while "scratching" water. Also at high speeds it is hard to keep it in water due to KSP SAS flying straight line and Kerbin being a sphere.

8. Top speeds of realistic hydrofoil assisted hovering are close to 100m/s just like for the most powerful boats. Slightly less actually because of the "wing" problem. It's like some kind of sound barrier for boats you are stuck with. Too much lift from air, too high drag in water above that. Once hydrofoil reaches surface, it turns into air wings. Tiny jump on wave and you are flying on those wings intended for water! Even faster, just not in water anymore. Because your drag is so much less in air. Real life racing boats have the same issue. At some point lifting the body above water at pitch you turn into deadly flipping "plane". Because at that speed flat boat body alone has enough lift. It's easier to fly at those speeds than glide the water. So you get kicked out of water. Sooner with hydrofoil than without.

9. don't build wide ships in KSP. Trimarans and stuff. Keep pontons as close as possible. Or even better - avoid. Drag grows faster/stronger than buoyancy kicks in on the side that submerges more. Even worse at high speeds. Again, drag grows as speed². Buoyancy doesn't. As a result, you will have trouble keeping your boat straight at high speeds due to leverage growing with the distance from CoM on those designs and random bounces.



Short - use engine with best Isp. Goliath? Reverse can be useful!
Or some electric solution. Electric won't work during night though. Also due to high water drag you will waste a lot more fuel compared to flying the same distance. A lot. The higher your speed, the more. Add mining ability reaching shores to be able to go around the globe. Also buy a lot of chips and drinks, as it may take a whole day even with time warp. Also high speed boats don't like time warp at all.

3700km at 290km/h? Good luck.

[davaeron666]

Amazing RoofCat ! A big thank you. Nicely put. I have an idea in mind i'll post a scrennshot of the final product.

[Toastie Buns]

Originally posted by RoofCat:

1. propulsions is not the problem

2. He asked what engines to use
3. Jet engines are the answer lol

[XLjedi]

I have a lot of fun with this one... submarine bay, loading ramp, flight deck.

https://kerbalx.com/XLjedi/HCS-16-Hurricane

The goliath engines start submerged, then skim above the water when on plane. The associated video shows how it works. I prefer large wide catamaran / trimaran designs for hydrofoils. Mine work like giant sleds powersliding in turns at safe maneuvering speeds (80-100m/s).

I like your cool ship design BTW!

[van Leeuwenhoek's mustache]

@XLjedi

That's an awesome design.

[Lystent]

I do wonder if jet engines output more thrust when deep underwater instead of being just above sea level; they are the only form of propulsion that I tried that takes the deep sea drag in stride...

[XLjedi]

Originally posted by Gramer Natzi:

@XLjedi

That's an awesome design.

Thanks!

[RoofCat]

Originally posted by Toastie Buns:

2. He asked what engines to use
3. Jet engines are the answer lol

well, every engine does. He ain't stupid. As for additional concepts, you may want to add maneuver engines like on big ships. Thrust reverse works nicely too. I used action groups for maneuver engine toggle.
https://steamcommunity.com/sharedfiles/filedetails/?id=1480079974
https://steamcommunity.com/sharedfiles/filedetails/?id=1480080902
https://steamcommunity.com/sharedfiles/filedetails/?id=1480081138
one thing I realized is Goliath engines don't like even touching the water with 105m/s. They just vanish. So either steer very carefully at top speed, or make hydrofoil wings slightly lower or engines higher. It is recommended to keep engines close to CoM though as otherwise they will pressure your boat to dive or surface depending on where they are. Which may be profitable at some point. Or not. As for slowing down, turn off engines to slow down softly. Or just redesign. This was made for quick test mostly. Also it doesn't make boom so probably simple struts would help too. May be similar to wings flying of on heavy planes during sharper turns.

Another thing I noticed MK1 Crew cabins don't allow to EVA through painted end hatches anymore. Or they didn't like my particular design. Had to install command pod exit for leisure deck which worked just fine.