Why ships accelerate much faster then decelerate?

[rgreat]

Isn't it must be the over way around?

Also, I think acceleration and deceleration must greatly depend on speed.

Edited November 25, 2020 by rgreat

[Bluestrategist]

I suppose that acceleration is brought about by the ship's propellers increasing in speed, actively driving the ship faster through the water. If you then order your ship to go more slowly, the propellers slow down but the tremendous impetus of the ship would mean that it decelerates more slowly. This could be changed if you threw the ship into full astern, as the screws would then be actively working to slow the ship.

[Durham Dave]

Yeah, apart from putting a ship into reverse (not as easy as you might think), there is nothing actively going into slowing a ship down (no water brakes), so just natural friction.

[HistoricalAccuracyMan]

Acceleration IRL is determined by how sleek/aerodynamic your hull is, engine/shaft horsepower, how many screws you have and overall weight of the ship (in game these are represented by hull form, engine choices, engine efficiency and propeller upgrades).

As for why it takes so long for ships to decelerate and/or stop is mostly down to one thing: inertia (the resistance to a change in motion, probably better described in this case as the ship's momentum). If you have a ship traveling at a high speed, it will take longer to slow down because you have the full displacement of that ship traveling forwards at whatever speed the ship was maintaining--it's the same principle for why if slam on the brakes in your car, the car might slide to a stop and you will be thrown forward as a result of a sudden change in speed/direction. However, a ship's momentum is directly effected by two things: speed and displacement. A lighter ship traveling at slower speeds won't take near as long to stop/reverse direction as a large ship traveling at the same speed would...and the same is true at high/flank speeds. A heavy ship going at high speed will take a long time to slow down (even if the crew throws the engines in full astern/reverse) where a light ship traveling at high speed will take considerably less time to stop.

Basically, it all comes down to elementary Physics: The more your speed and weight increase, the more time and distance you will need to stop

[rgreat]

On 12/2/2020 at 8:27 PM, Durham Dave said:

Yeah, apart from putting a ship into reverse (not as easy as you might think), there is nothing actively going into slowing a ship down (no water brakes), so just natural friction.

Do you know about water friction? Do you know how large that force is compared to air friction?

On "high" speeds like mere 30 knots water friction is so strong that to overpower it in large ships you need hundreds of thousands of horsepower.

Yes, inertia is a big factor, and at low speed water friction is quite low comparable to ship large inertia, but not at high speeds.

That's same as with acceleration but the other way around.

It is fast to accelerate first 20% of speed but very slow to attain the last 20%.

Same as it is very fast to lose 20% of max speed but very slow to come to full stop for the last 20%, unless you do engage reverse.

And with reverse engaged it is must be quite faster to decelerate compared to acceleration.

But that not the same in game.

Edited December 6, 2020 by rgreat

[rgreat]

Real time data on container ferry class ships.

Time	Speed, knots	Revolutions per minute
STOP -> FULL FORWARD
00.00	0	0
01.00	4.1	73
02.00	9.8	80
03.00	14.2	83
04.00	17.7	84
05.00	19.9	88
06.00	21.4	88
07.00	22.4	89
08.00	22.9	89
09.00	23.3	89
10.00	23.5	89
11.00	23.6	89
12.00	23.7	89
14.00	23.7	89
16.00	23.8	89
18.00	23.8	89
MEDIUM FORWARD -> STOP
00.00	18.0	67
01.00	15.4	0
02.00	13.2	0
03.00	11.6	0
04.00	10.2	0
05.00	9.2	0
06.00	8.4	0
08.00	7.1	0
10.00	6.1	0
15.00	4.6	0
FULL FORWARD -> FULL REVERSE
00.00	23.9	88
00.30	22.0	5
01.00	18.5	-68
01.30	14.2	-72
02.00	10.5	-72
02.30	7.1	-75
03.00	4.7	-76
03.30	1.0	-78
03.40	0	-78

Edited December 6, 2020 by rgreat

[coalminer]

4 hours ago, rgreat said:

Real time data on container ferry class ships.

Time	Speed, knots	Revolutions per minute
STOP -> FULL FORWARD
00.00	0	0
01.00	4.1	73
02.00	9.8	80
03.00	14.2	83
04.00	17.7	84
05.00	19.9	88
06.00	21.4	88
07.00	22.4	89
08.00	22.9	89
09.00	23.3	89
10.00	23.5	89
11.00	23.6	89
12.00	23.7	89
14.00	23.7	89
16.00	23.8	89
18.00	23.8	89
MEDIUM FORWARD -> STOP
00.00	18.0	67
01.00	15.4	0
02.00	13.2	0
03.00	11.6	0
04.00	10.2	0
05.00	9.2	0
06.00	8.4	0
08.00	7.1	0
10.00	6.1	0
15.00	4.6	0
FULL FORWARD -> FULL REVERSE
00.00	23.9	88
00.30	22.0	5
01.00	18.5	-68
01.30	14.2	-72
02.00	10.5	-72
02.30	7.1	-75
03.00	4.7	-76
03.30	1.0	-78
03.40	0	-78

A little curious what the general characteristics of this particular vessel that was quoted, also if this was in builder's trials as a laden vessel would likely take more time to come to a stop. extrapolating between the first 2 tables, it is likely the vessel would have taken a longer time to come to a stop if it was going at full forward vs medium forward without throwing the prop into reverse (at 15min it was still doing 4knots vs reaching "max" speed in between 14-16min).

IMO requirements has it that a vessel must crash stop within 15(20 for very large vessels) ship lengths, this gives a good estimate on what distances a large vessel requires to come to a stop (<4.5km for a 300m vessel). And crash stops (throwing engines/props into full reverse) is not conducted often or as part of standard manoeuvres as it places alot of stress on shaft seals, gearboxes(if any) and engines terribly causing premature failure.