Considering Jet Drive for Micro Sub

I am looking for informed opinion on the merits of a jet drive for a tiny,

micro submarine I want to build. It will be a toy, for fun, no commercial

gain. It is small. To acheive 15 knots it needs 500 Newtons of thrust.

I have an 8 kw electric motor with enough battery power to run on and off

the throttle for one hour. My motor and prop designs are in the

neighbourhood of 90%. The only jet drives I am familiar with are Hamilton

42's that were powered by two, 16 cylinder Detroit Diesels putting out 1450

ponys each. It would do 42 knots but at 160 gallons/hour. At 30 knots it

would suck back 90 gallons. 20 meter mono hull, 54 passenger, aluminum.

It was a pleasure to drive. I digress, but that's what I'm familiar with.

I've pretty well ruled out a jet drive for my sub but I thought I'd through it

out there for comment.

Thanking you in advance,

Tom

 

Making a torpedo?

 

????

No, a micro sub, but you can call it what ever you like.

Why do you ask?

 

Well, a torpedo is the only "micro sub" that needs to to go 15 knots (and about the only shape that will also).

Just wondering which agency to forward this to.

 

Paranoid

You sound like a paranoid American.

You can forward it to whatever agency you like.

Anyone else out there have an answer to my question without trying to

make something out of nothing?

Tom

 

The physics of the situation is:

Thrust from the impeller (or Propeller) is a function of velocity squared.

Power is a function of velocity cubed.

A small diameter requires high velocity to produce the thrust. A larger diameter requires less velocity. So the larger diameter will be more efficient up to the point where the drag on the blades offsets any advantage increase in diameter.

If you look at what the human powered subs use you can bet that big diameter , low velocity ratios work the most efficiently. I doubt that even a ducted prop would have any merit at 15kts, 500N thrust unless prop protection was a priority.

Sublime won the 10th ISR. Here is some video of it being tested:
http://www.tampabay.com/video/?bctid=26532235001&p=2652468001
The prop has a very aggressive pitch. That is related to the losses in the movement from legs driving it rather than any inherent hydrodynamics. An electrically driven prop would have more normal pitch but looks to be well designed for high efficiency - as does the hull.


BTW - John was just having dig. Although given the sensitivity it might not be all that wise.

Rick W

 

I'm only half joking, really.

Anything running around submerged at 15 knts with a strong enough structure to can carry at least 8,000 W-hrs worth of batteries (160 lbs x 1.2 cf minimum even if using Ag-Zn, ) and a 8 kW DC motor (say 120 lbs for a brushed DC motor) is going to hurt when it hits something. Think of driving a quad in traffic at 20 mph with a bag on your head, or if unmanned, a quad driving around with some guidance system that cannot sense what is in front of it. 15 knts is way too fast for most water visibility and if you want to do it acousticly then "mirco" is not an operative description. Remember, a sub is always the burdened vessel and always at fault for a good reason.

Additionally, 15 knts is deadly fast in a wet or dry sub. A 18 ft/sec excursion rate will cause an embolism in a wet sub and a designing for plane jam on a 1-atm hull would mean at least a 300' rated hull. Right now HPV wetsubs are pushing the limits of safety with respect to vertical speed accidents.

 

Interesting point on speed verses safety on sub. I ran into this problem being towed behind boat while snorkeling with board.

 

Embolism is not a problem while snorkling a towed sled as the SCF volume of air in the lungs is constant at the 1-atm volume. All that happens is the lungs squeeze to pressure and you really cannot be down long enough to worry about N2 bloodstream loading. This is why free divers never bend, but have other problems related to squeeze.

When breathing off a SCUBA rig or re-breather, the SCF volume in the lungs could be several times the 1-atm volume. If you take a breath at 33', you have twice the volume of air in your lungs and becuase the partial pressure is twice as high, gases (mostly N2) are forced into the bloodstream and tissues. Rapidly coming up from that depth to 1-atm causes the air to expand rapidly in you lungs as well as in your blood and tissues. If the physical damage to your lungs doesn't get you, the gas bubbles in your blood will. The speed of swimmer tugs are actually limited by this.