High Powered DPV

[Jon Nellis]

Jon, do you disagree that power increases as cube of the velocity?

When your are talking about a eight fold increase in speed, definitely!


The small interval assumption used to derive the power vs speed equation is no longer valid, especially when the drive system would need to shrink to something that would fit on an arm. That is why all the real fluid dynamics equations are partial differential equations, as laminar flow transitions to turbulent and induced drag is overcome by parasitic drag among other things. And that does not even get into the issue of cavitation at the thruster and the power loss required to control it. That's is why I added the my fluid dynamics disclaimer to the simple drag force equation, since the drag equation assumes a constant drag coefficient, which is not likely with an eight fold increase in speed.

Clearly it has been a while since you cracked a fluid mechanics book.

Cheers,
Jon

 

[cool_hardware52]

When your are talking about a eight fold increase in speed, definitely!

We were. Do we really need diff eq's to tell the OP that 25 mph under water with an arm jet isn't with in the realm of reality?

Simple is often better understood.

Tobin

---------- Post added May 13th, 2013 at 02:36 PM ----------

The math is not quite that simple,

Clearly it has been a while since you cracked a fluid mechanics book.

Cheers,
Jon

Jon, Honestly I do wonder at times what motivates you.

I've attempted here, in simple terms, that many (most?) here could appreciate why 25 mph scooters just aren't going to happen. You seem bent on portraying me a unqualified to do so. Just seems odd.

Tobin

 

[Jon Nellis]

You seem bent on portraying me a unqualified...

Nope, I just don't like to see bad information perpetuated to an OP who clearly needs a better understanding of how complicated actual engineering is. All I said was "The math is not quite that simple.", because it isn't. You asking me to clarify my statement, just brought the rest of it upon yourself.


Cheers,
Jon

 

[ams511]

Seen this?

View attachment 154614

It was called an Armjet. I don't think it ever made it to production in any large numbers.

Developed by someone in California if I remember right. Might be the same people doing the Jetboots currently. Patriot 3 - Maritime Systems - Jetboots

According to this thread, it was a single jet boot with a battery.

 

[cool_hardware52]

Nope, I just don't like to see bad information perpetuated to an OP

Cheers,
Jon

More info is always welcome, but I fail to see the need for you to twice suggest I was unqualified.

Specifically what part of any of the information I provided was "bad", meaning factually incorrect?



Tobin

 

[Jon Nellis]

Specifically what part of any of the information I provided was "bad", meaning factually incorrect?

Your application of the cubic relation between velocity and power for almost an order of magnitude difference in velocity is incorrect, given the OP's parameters. I thought I made that clear enough in post #11.

Accept it and move on.

 

[cool_hardware52]

Your application of the cubic relation between velocity and power for almost an order of magnitude difference in velocity is incorrect, given the OP's parameters. I thought I made that clear enough in post #11.

Accept it and move on.

So tell me is the power required for an ~ 8x increase in velocity more or less than the cube of the ratio of velocities?

Tobin

 

[Jon Nellis]

So tell me is the power required for an ~ 8x increase in velocity more or less than the cube of the ratio of velocities?

That answer will cost you $120 an hour and you'll have to wait a couple months for my schedule to clear up.

 

[Mr.X]

Seems the OP took a powder.

Additionally, being towed underwater at high speed on scuba is not really pleasant. I was using a boat-towed sled for survey work and you have to tuck everything in just right - lest get something ripped off, or being yanked away. Hurts.

 

[cool_hardware52]

That answer will cost you $120 an hour and you'll have to wait a couple months for my schedule to clear up.

Typical. Exactly what I expected from you.

You are asking the readers here to believe that the OP, who is proposing a 25 mph, "lightweight", low cost DPV, needs diff eq's and $120 / hour "professional" services in order to learn that's not practical when a simple, free, "back of the napkin" quick estimate is more than enough.



Tobin