Thanks to all who replied.

Some very interesting comments.

So it is cavitation and air mixed with water that is causing the trails? Thanks, again, for educating me.

Jake

quote:

Originally posted by imadat:
Thanks to all who replied.

Some very interesting comments.

So it is cavitation and air mixed with water that is causing the trails? Thanks, again, for educating me.

Jake

Jut to throw a kink into it all, the subs and other ships have variable pitch props (meaning they can change the pitch of the prop on demand-unlike a fixed pitch prop like on a bass boat)

this is used to control speed & thrust direction while not changing engine revolution or direction of the driving machinery....

quote:

Originally posted by imadat:
Thanks to all who replied.

Some very interesting comments.

So it is cavitation and air mixed with water that is causing the trails? Thanks, again, for educating me.

Jake

Generally yes. It's the propeller sucking air and trying to compress it into the water as it propels the boat/ship forward. On the fast sportfish we have issues with cavitation and prop burn and it will actually burn the metal right off of the blades near the hub on about 1/3 of them to where if metal is not filled in every year or so, the propeller will start flinging blades off.

In regards to variable pitch props. Pitch is usually measured in inches and in a perfect world a propeller with 20" of pitch will propel the boat/ship forward 20" in one revolution. Variable pitch propellers you can adjust the pitch from a smaller number, lets say 5" of pitch....to say 25" of pitch. Less pitch would allow the boat/ship to move forward (or backwards) at a lot slower speed and have more torque, increasing pitch would increase boat speed assuming the engine is turning the same RPM's. Essentially you could run the engine at a much higher RPM and be in it's power band, yet move the ship forward at a slow speed. Think of it like staying in first gear in your car at 3000 rpms, versus 5th gear at 3000 rpms.

All ships and boats (as far as I know) have a single gear (transmission) unlike a car, so the speed at which the propeller itself is turning is strictly related to how fast the engine is turning (rpm's). So some ships (and that's a small percentage, most have a fixed pitch propeller that is tuned to the engine/ship so that the engine makes WOT rpms) have variable pitch props to allow for more acceleration or maneuverability in tight quarters or to allow slower speeds underway.

Other than knowing how they work, I don't have a lot of experience with using variable pitch propellers. I ran one 100ish' long range yacht that had one and it was a royal pain in the rear. You had to wait close to 20 seconds when docking and going from forward to reverse, each time you wanted to change direction of the propeller.

Regarding hull speed of displacement boats, the formula is k x square root of the waterline length. The K is usually about 1.34 to 1.5 for most boats, but a long and skinny boat might have a factor of 2.0.

if a 55' boat had a waterline length of 49', square root is 7, then hull speed is about 10.5. A ship with a 400' waterline length would have a hull speed of 1.5 x 20=30 knots. A skinny submarine the same length might have a hull speed of 2.0 x 20 = 40 knots.

quote:

Originally posted by MikeinNC:

quote:

As a side note, pretty sure the carrier can outrun them all to possibly include the subs' torpedoes.

I won't post what speed I have personally seen a carrier run operations, but suffice it to say that they are faster than any othe ship in that photo, and could certainly outrun a torpedo if put in a race, but the torpedo is akin to a motorcycle, smaller and faster and more agile than the carrier who can just run straight fast.

And the carrier can absorb more than one torpedo hit...

Oh yeah, baby. The Enterprise can easily outrun her escorts and regularly do so as needed.

Mike, either before or shortly after I got assigned to the Enterprise, I saw a picture of her with a rooster tail taller than the flight deck. I think she was also doing a turn in the process. Do you remember that? I've since searched periodically but have never seen a copy.

As to the reason, I can buy the explanation that the carrier is slowing down while the other escorts are just coming alongside for the pic.

I also believe it's because the propellers of the carrier are deeper in the water than the escorts.

quote:

Originally posted by jimmy123x:

quote:

Originally posted by imadat:
Thanks to all who replied.

Some very interesting comments.

So it is cavitation and air mixed with water that is causing the trails? Thanks, again, for educating me.

Jake

Generally yes. It's the propeller sucking air and trying to compress it into the water as it propels the boat/ship forward.

This is inaccurate. Cavitation doesn't occur because of air being drawn into the propeller, and it doesn't happen because of compression.

It occurs because of local areas of extremely low pressure, which cause the water to boil (turn into water vapor - a gas) and dissolved gasses in the water to come out of solution (yes, there is dissolved gas in water - e.g., the dissolved oxygen that fish breathe), causing bubbles to form, apparently from nowhere. A submarine propeller hundreds of feet under water can cavitate. Then as the pressure in and around the bubble rises back to ambient, the vapor turns back into water and the other gasses in the bubble go back into solution, causing the bubble to collapse and disappear. The bubble collapses very violently, and it is the collapse of cavitation bubbles that causes cavitation damage - the collapse produces shock waves strong enough to knock chips off of a metal propeller.

http://web.mit.edu/hml/ncfmf/16CAV.pdf

I will say that the WAKE, itself, is in part formed from air being drawn into the water, but that isn't cavitation.

Pretty good deep dive into wakes and bow waves here: http://www.steelnavy.com/WavePatterns.htm

Here is one of the Nimitz: