Coasting to a stop

[swdw]

One of the things bugging people in SH4 is the "eternal" coast once you hit one knot.

May have found a fix for this, but would like a little more info. I have the Tambor taking 8-10 minutes by the game clock to go from 5 knots to 0 knots with no reverse bell applied. Will be putting the file out to the RFB testers to make sure the change doesn't screw something else up.

Any one got an idea what would be accurate? Any diesel boat sailors that can throw some numbers at me?

[Roger Dodger]

Originally Posted by swdw

One of the things bugging people in SH4 is the "eternal" coast once you hit one knot.

May have found a fix for this, but would like a little more info. I have the Tambor taking 8-10 minutes by the game clock to go from 5 knots to 0 knots with no reverse bell applied. Will be putting the file out to the RFB testers to make sure the change doesn't screw something else up.

Any one got an idea what would be accurate? Any diesel boat sailors that can throw some numbers at me?

What would be accurate would be 2000 tons of steel ploughing through the water at 5 knots. If you just ring up all stop, you are just putting the engines in neutral - you have not turned off the Kinetic Energy of the boat. I would figure that it would take at least one mile to 'coast' to a complete stop.

If you need to stop faster than that, I would suggest you ring up 'All Back'. The faster you need to stop, the faster you will need to reverse. When the Speed-O-Meter reaches '0', THEN hit All Stop.

Newton's 'First Law of the Conservation of Energy' states: "A body in motion tends to stay in motion in a straight line until acted upon by an external force" (like friction or gravity). There are formulae to express this phenonoma, but one of our wiser folks on the forum will have to give it to you.

[swdw]

Originally Posted by Roger Dodger

What would be accurate would be 2000 tons of steel ploughing through the water at 5 knots. If you just ring up all stop, you are just putting the engines in neutral - you have not turned off the Kinetic Energy of the boat.

No kidding? Actually RD, there's a reason I'm testing it this way

I would figure that it would take at least one mile to 'coast' to a complete stop.

Based on? And how do you measure one mile of travel in the game on the ocean? Time is an easier measurement to use.

If you need to stop faster than that, I would suggest you ring up 'All Back'. The faster you need to stop, the faster you will need to reverse. When the Speed-O-Meter reaches '0', THEN hit All Stop.

Newton's 'First Law of the Conservation of Energy' states: "A body in motion tends to stay in motion in a straight line until acted upon by an external force" (like friction or gravity). There are formulae to express this phenonoma, but one of our wiser folks on the forum will have to give it to you.

You misunderstood the reason for this post. I understand how all the physics work. I can even write out and explain the formulas you mentioned as they are often a part of trying to figure out problems on my job. I actually have college level classes in both thermodynamics and fluid dynamics (no degree though- that's another story).

This is not a post on how to stop a boat quickly, but what would make a realistic adjustment.

There's a lot of data that I don't have access to that would be required to even remotely calculate a decent number. parameters like total surface area in contact with the water, drag coeeficients of the hull, props, rudders, planes, shafts, reynolds numbers for some of these features just to mention a few. Then there's both parasitc drag and dynamic drag that need to be calculated in.

Personally, it'd be easier with some feedback from someone who's familiar with the time it took a real boat to come to a stop with no reverse bell being applied

Currently in the game, I've had a boat coast at one knot for as long as 30 minutes by the game clock. This is not even cloase to realistic by any means. At one knot a boat will come to a stop and drift with the current much sooner than that.

[pythos]

Actually taking 30 minutes to coast to a stop sounds quite resonable. This is what makes large boats such a trick to handle. Remember it takes nearly 4 miles for a mile long train to come to a full stop, and that is using breaks. A boat is 2000 or more tons of metal and machinery, and men, that is a whole lot for drag of the water to stop on its own.

A boat is much like a tail dragger aircraft. It is not finished moving until it is tied down.

From my understanding about subs is that they don't want to stop once moving. Remember in hunt for Red october during the Crazy Ivan scene, Jonesy had to explain to the trainee that the trick of it was that a boat like this doesn't exactly stop all by itself, so if the Ruskie slows down, the pursurer would crash right into the Boomer's stern.

That last bit of kenitic energy can and should take an eternity to bleed off, which is modeled in this game quite well.

[Old Dog]

Hi swsd

"Based on? And how do you measure one mile of travel in the game on the ocean? Time is an easier measurement to use."

Granted that knowns are easier to deal with than unknowns, but let's let's deal with some "knowns" here. Let's see...one knot is...Oh yeah ! One nautical mile....per hour. Ten minutes of coasting after the "speedometer" reads one knot equals about one sixth of one nautical mile or +/- 1000 feet.
http://en.wikipedia.org/wiki/Mile

I'm personally surprised at that distance being so short. I would have thought that the designers of subs would have done a better job of reducing the coeficient of friction in their design. Actually, looking at a WW II sub, with deck guns, periscopes, radars, etc festooning them, "they look about as aerodynamicly clean as a bag of walnuts."
(I remember reading that in a magazine piece describing Formula I racing cars back in the sixties, and I had not found a place to use it until now.)

Old Dog

[GeoffBelding]

Hi - disregarding kinetic enery for the moment...

When I am at my desired intercept point for a convoy, and wish to stay quiet, I kill the forward speed by alternating full rudder, first one way, and the the other, quickly, so that my heading in not materially altered.

Hope that helps....

Cheers

Geoff

[AVGWarhawk]

I'm good with the sub coasting. It is the DD that stop on a dime and reverse like a car does. This needs work.

[swdw]

Originally Posted by pythos

Actually taking 30 minutes to coast to a stop sounds quite resonable. This is what makes large boats such a trick to handle. Remember it takes nearly 4 miles for a mile long train to come to a full stop, and that is using breaks.

Well yes and no. The train example is NOT from one mile an hour, but from a normal operating speeed. Go back and I think you'll find that the four miles stoping distance is for a train that outweighs the sub by a huge amount and is moving faster than 25 mph- so not a good analogy. Looking at this conservatively, a typical boxcar's max weight including the weight of the car is 236,000 lbs or 115 tons. Ue an aqvergae weight 80 tons to be conservative, and a typical train consisting of 65 cars, you are pulling 5200 tons of train not counting the locomotives at more than 25 times the speed I'm looking at.

I'm personally surprised at that distance being so short. I would have thought that the designers of subs would have done a better job of reducing the coeficient of friction in their design. Actually, looking at a WW II sub, with deck guns, periscopes, radars, etc festooning them, "they look about as aerodynamicly clean as a bag of walnuts."
(I remember reading that in a magazine piece describing Formula I racing cars back in the sixties, and I had not found a place to use it until now.)

Here's the thing, the nuke boats I was on did not take 30 minutes to drift to a stop from a speed as slow as one knot, and they have less drag than a fleet boat. The thing is, the hull isn't the only thing causing drag. The sationary props are a huge drag on the boat, then you also have the drag from the rudder, dive planes, shaft supports, and all hull penetrations.

Oh well, I'll continue to do some more research on this and see what comes up.

[pythos]

Have you ever watched a train come to a stop? I have and this auto train going no more than 5 MPH took for ever and a day to stop. The breaks were screeching for all they were worth and the sound went on for what seemed 10 minutes. Yes the weight of the train is many times more than that of a sub, but the train has both dynamic and pneumatic breaks stopping it, and yet it took all this time to stop. A surfaced sub has little drag and no breaks, and so it is going to take a very long time for it to stop. 30 minutes at least, if not more. One under water has much more drag induced by the tower, guns, railings, and so forth, and there for should stop faster.

[Peto]

Well--admittedly, I'm a big fan of anaolgies. But trying to compare something that uses wheel on land with a very small grade (ie--little natural drag) and something that uses water for it's travelling medium (automatic drag) does strike me as the proverbial Apples & Oranges scenario.

I looked into this a little today and couldn't find any decent info. Maybe there's info for similar tonnage vessels that could be found on some commercial boat site. :hmm:

[swdw]

Granted that knowns are easier to deal with than unknowns, but let's let's deal with some "knowns" here. Let's see...one knot is...Oh yeah ! One nautical mile....per hour. Ten minutes of coasting after the "speedometer" reads one knot equals about one sixth of one nautical mile or +/- 1000 feet.

Not really, because the boat is decelerating during this time, the distance will be shorter. If the deceleration is a linear rate, distance would be easier to calculate, but the deceleration observed from higher speeds in the game shows deceleration is not linear. This is how it should be as the drag forces change in a non linear manner as the boat slows down. Besides, break this down even further and you are talking an object at one knot slowing to a stop with a forward velocity of only 1.7 ft per second.

Originally Posted by pythos

Have you ever watched a train come to a stop? I have and this auto train going no more than 5 MPH took for ever and a day to stop.

Yes, we have coal drags consisting of 100+ cars running through the town all the time. One of them with 110 loaded hoppers came to a stop from ~10 mph in less than a mile after it struck a car that was stalled at the crossing (their speed limit is actually higher through town, but there are 3 street crossings, so they go slower). The car was knocked off the track, so the car was not increasing the drag.

You are talking about a controlled brake, and this situation was an emergency brake.

Let me go into a little more detail as to why this analogy doesn't apply. Trains and cars ride on axles designed to reduce friction to a minimum. In addition, they are moving through a much less dense and compressible medium- air. The friction created by the axles is totally different than the drag and friction created by water flowing around an object. How long do you thing it would take a train to stop that was pushing against WATER?

One is a fairly simple and straightforward problem easy to compensate for. The other involves getting into fluid dynamics and is a completely different animal. First, you are pushing a hull through a liquid, not a gas. So instead of the object moving through a compressible medium, it moves through an incompressible one
( http://www.fas.harvard.edu/~scidemos/NewtonianMechanics/IncompressibilityofWater/IncompressibilityofWater.html ).
Kinetic energy is spent through the change in the fluid as it's pushed aside and around the hull. This is considered a deformation of the medium and energy is transferred from the boat to the water in the process. This is only one component taken into consideration.

Drag over a hull has several parameters, go here to read more on this:
http://www.sailinganarchy.com/YD/2003/whatadrag.htm

So you have drag acting over the entire surface of the hull, plus all exposed parts that are sticking out from the hull and all hull penetrations.

In addition, once the propellers are stopped they have a huge drag coefficient. One book equates a stationay small prop as having the equivalent drag components of a flat hand placed into the water with the palm facing the direction of movement. They are designed to grab and push the water to create propulsion. So when stationary they are a huge component of the total drag. In otherwords, they are similar to applying a brake. How much depends on the configuration of the propellers (blades, pitch, hub, etc) and their location in relation to the hull. (ironically they create more local drag when rotating, but this is outweighed by the thrust created).

So once again, comnparing a train and a boat are not compatible analogies. So I'm not going to argue that point anymore- it just doesn't apply.

[panthercules]

Hmm - reading this thread got me curious, as I've never really had a problem that I noticed bringing my boat to a stop. So, I loaded up my current career and went to all stop from 9 knots (just happened to be the speed I was going when I loaded in). As soon as I coasted down to 1 knot (IWS, if that's a term ), I made a mark on my nav map where my sub was, then waited till I was indicating a speed of 0 (and confirmed that I was really stopped by going to time compression in external camera - no perceptible movement over a space of about 15 minutes game time).

Turned out, the total distance traveled from 1 knot to 0 was only 250 yards. Now, this was in 5m/s seas in my Salmon class, and I haven't had time to test it in other boats or sea states, so maybe that's masking the problem somehow, but this confirmed my hazy recollection from previous sessions (especially doing agent or supply drop missions) that I've never really had (or at least noticed) this sort of "eternal creep" phenomenon that you're seeing.

I hope you get this sorted - I'd love to help but I have never seen any stats on sub (or boat) coasting-to-stop distances and I'm not even sure who would accumulate data like that or why, so unless you can back into it via some pretty complex formulas I'm not sure how you're gonna get reliable answers.

[Sailor Steve]

I've done a lot of research on ships over the last 15 years, and acceleration and deceleration are indeed two things that there is very little information on. Nobody in the real world seems to have ever cared how rapidly ships accelerate and decelerate, and I still can't find anything definite in writing on the subject.

[vanjast]

Maybe we can do our own 'simulated' modelling of the subject.
Of course there will be many factors left out, like salinity, water temp, etc....

But one can do a TIME-vs-SPEED chart for coasting to a stop, at various depths.
Do an excel chart and from there make a formula, and then see how it compares to, say, aircraft drag formulas.
The ship formula should not be much different from the aircraft one, as it's all about Friction and Fluid Dynamics..

Have fun..

[longam]

Does a large ship or sub come down off of plain? I can remember when I had my 17’ boat the it would slow down real quick and drop off plain then it would coast for a long time without reverse thrust.