# How about an underwater helicopter? **URL:** \ **Category:** Factual Questions\ **Created:** [October 25, 2003, 1:22am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430 "2003-10-25T01:22:47Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Llama\_Llogophile](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@Llama\_Llogophile](https://boards.straightdope.com/u/Llama_Llogophile)\ **Post date:** [October 25, 2003, 1:22am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/1 "2003-10-25T01:22:47Z")
Would this work? I’m guessing that the water would create too much drag to use long blades like in a traditional helicopter. Would probably have to have small, rapidly spinning blades. And that would mean the blade assembly would have to be slung under the craft. Which basically describes a small submarine. But that’s not what I want. I want a real helicopter-LOOKING machine that works underwater. I saw a few things on Google, but they didn’t really look like helicopters. So is this possible? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 25, 2003, 1:28am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/2 "2003-10-25T01:28:18Z")
You described the problem yourself. There is no need for “helicopter-looking” blades. Instead, the setup that is currently used is like a ducted fan. Basically, you design a machine for the medium in which it is meant to function. “Ducted fans” for submersibles, long thin blades for aircraft. ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [October 25, 2003, 4:53am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/3 "2003-10-25T04:53:32Z")
First off, recall that the _rotary wings_ of a helicopter are just that: they are wings to lift the vehicle from the ground. Since water is so much denser than air, one does not need “wings” to lift one off the ocean/sea/lake bottom. Instead, one need only design a lighter-than-water vehicle that will float in the water, then concentrate on designing propulsion without worrying about lift. (That is the reason that no one has proposed an underwater “helicopter”, to date.) ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [October 25, 2003, 5:08am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/4 "2003-10-25T05:08:37Z")
Um, correct me if I’m wrong, but take a helicopter, turn it on its side, and shorten the blades, and you have a typical ship-driving screw. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 25, 2003, 5:34am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/5 "2003-10-25T05:34:44Z")
> [@](#): > > Um, correct me if I’m wrong, but take a helicopter, turn it on its side, and shorten the blades, and you have a typical ship-driving screw. Not quite. Helicopter rotors come in three flavours: fully articulated, semi-rigid, and rigid. A fully articulated rotor system has a flapping hinge, a feathering hinge, and a lead-lag hinge. Semi-rigid rotors do not have the lead-lag hinge (since they are two-bladed), and rigid rotors use the flexibility of the blades to handle the dynamic movements and only have a feathering hinge. Marine screws generally don’t have variable pitch (not to say there aren’t any that do, but I haven’t seen any myself), and they need to be more rigid than rotors. You _could_ shorten a fixed-pitch airplane propeller, but it wouldn’t be very efficient. Marine screws tend to have greater pitch, and they are wider than airplane propellers. Yes, you could bodge a helicopter rotor to propel a boat; but it wouldn’t work very well. Again, the design of the part needs to be based on the function and the medium. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [October 25, 2003, 6:44am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/6 "2003-10-25T06:44:25Z")
Just think of submarines as underwater hot-air balloons. 🙂 ---
**Author:** ![Doomtrain](https://avatars.discourse-cdn.com/v4/letter/d/0ea827/32.png) [@Doomtrain](https://boards.straightdope.com/u/Doomtrain)\ **Post date:** [October 25, 2003, 6:49am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/7 "2003-10-25T06:49:07Z")
Wouldn’t the (relatively) huge rotors require quite a bit of horsepower to turn against the resistance of the water? So wouldn’t you be better served attaching that massive engine to a boring ol’ prop? ---
**Author:** ![Kaptain\_Karnage](https://avatars.discourse-cdn.com/v4/letter/k/ebca7d/32.png) [@Kaptain\_Karnage](https://boards.straightdope.com/u/Kaptain_Karnage)\ **Post date:** [October 25, 2003, 7:04am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/8 "2003-10-25T07:04:21Z")
OK, as cool as an underwater helicopter would be the only reason to build one would be the comedy/novelty value. A helicoptors rotary blades pull it upwards with a change in the pitch turning that force into a upwards and forwards (or backwards and occasionally sideways) motion. Transfer that system into the water and you hit the largest problem. As it needs very little lift to raise a helicoptor like structure in the water, the blades would actually lift you to the surface before getting very far forward. Unless you adopted a climb and fall technique or found a way to counter-act the lift from the rotars it wouldn’t work and even if you did work round it you would find it very inefficient. The best suggestion i could think of would be to mount some decorative rotars onto and existing subs frame and see if it can produce the necessary visual effect. ---
**Author:** ![pilot141](https://avatars.discourse-cdn.com/v4/letter/p/e0b2c6/32.png) [@pilot141](https://boards.straightdope.com/u/pilot141)\ **Post date:** [October 25, 2003, 7:22am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/9 "2003-10-25T07:22:04Z")
Well, the subject has pretty much been covered. The purpose of rotor blades on a helicopter is to produce lift; underwater all you need is neutral buoyancy or something close to it. There is no need to direct all of your energy into rotating horizontal blades. If you want an underwater “helicopter”, build a hull that looks like a helicopter but has ballast tanks to control buoyancy. Then run the driveshaft out through the tailboom but make the tail rotor a traditional underwater screw (ie it faces aft instead of sideways). You may also have to put some rudders and elevators at the end of the tailboom, but you could make the screw one of those new fancy “ducted fans”, and no one would blink at the control surfaces attached to the “duct”. You rotor blades would be small, freely-rotating blades that incur the least amount of resistance (ie a very low AOA). As your “helicopter” moved forward in the water the resistance of the water would cause your rotor blades to rotate, thus providing the illusion of helicopter flight underwater. Or not, depending on how well you pull it off! ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [October 25, 2003, 10:40am UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/10 "2003-10-25T10:40:48Z")
> [@](#): > > \*Originally posted by friedo \* > \*\*Just think of submarines as underwater hot-air balloons. 🙂 \*\* No. Zeppelins. Bathyscaphs are underwater balloons. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [October 25, 2003, 12:02pm UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/11 "2003-10-25T12:02:45Z")
> [@](#): > > \*Originally posted by Johnny L.A. \* > Marine screws generally don’t have variable pitch (not to say there aren’t any that do, but I haven’t seen any myself) While they are definitely in the minority, they do exist. One advantage is that they allow “reverse” without changing the direction of rotation of the shaft. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [October 25, 2003, 12:38pm UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/12 "2003-10-25T12:38:26Z")
Inventing an underwater helicopter is about as useful or necessary as inventing a device designed to prevent parked cars from floating up in the air and drifting away. ---
**Author:** ![Scruloose](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@Scruloose](https://boards.straightdope.com/u/Scruloose)\ **Post date:** [October 25, 2003, 1:28pm UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/13 "2003-10-25T13:28:35Z")
> [@](#): > > \*Originally posted by sailor \* > \*\*Inventing an underwater helicopter is about as useful or necessary as inventing a device designed to prevent parked cars from floating up in the air and drifting away. \*\* They have! [I’ve seen 'em!](http://www.raleigh-nc.org/finance/Revenue/parking/pv_image/bootnr2.jpg) What? ---
**Author:** ![Fuji\_Kitakyusho](https://avatars.discourse-cdn.com/v4/letter/f/f0a364/32.png) [@Fuji\_Kitakyusho](https://boards.straightdope.com/u/Fuji_Kitakyusho)\ **Post date:** [October 25, 2003, 2:38pm UTC](https://boards.straightdope.com/t/how-about-an-underwater-helicopter/209430/14 "2003-10-25T14:38:47Z")
I used to work at a submarine engineering facility. The “ducted fans” that have been discussed here are actually Kort nozzles. If you look around the ring in cross section it resembles an aerofoil. What it does is act to channel the water passing through the prop with either an accelerating or decelerating effect, depending on design and intent. There are heavier than water submarines out there, that use foil surfaces to maintain lift when the vehicle is travelling forward (essentially flying underwater). Also, remotely operated vehicles (ROVs) are typically designed to be neutrally buoyant. In addition to the drive motors, they usually have a couple of props oriented vertically to move themselves up and down in the water column.