[REQ] Rates of change of depth

[Steeltrap]

(This is something I posted in the general SHIV forum, but would obviously love some of you clever people to have a look at it, both to test it yourself, critique my conclusions, and then see if you can mod the relevant factors if there is indeed an issue)

NOTE: I made some slight changes as I realised I had made an error: I used the distance travelled at speed as the horizontal length, not the hypotenuse. Figures below now represent the correct values.

Hi all. In the spirit of "don't complain without offering a solution", I tender the following:

I've been interested in the rate at which the boat changes depths. Given that the amount of inclination (bow/stern) is very limited graphically, I noticed at low sppeds your sub seems to zoom up and down effortlessly without using compressed air.

I decided to do a few observations to see how long it takes to change depths at various speeds.

Setup
Sub: Gar
Speed: 3, 5 and 7kts
Method: start at periscope depth. Give order to dive to 320ft. Start timing from moment indicated depth starts changing. Level at 250ft using "A" command. Give order to go to periscope depth and commence measuring per above.

Results
@ speed: 3kts 5kts 7tks 3kts 5kts 7kts
Diving Climbing
100-150ft: 18s 19s 17s 150-100ft: 30s 30s 25s
150-200ft: 26s 23s 20s 200-150ft: 28s 25s 22s
200-250ft: 47s 39s 34s 250-200ft: 30s 27s 26s

What becomes immediately significant is that the rate of change of depth is largely unaffected by the boat's speed. There are 2 possible reasons for this:
1. the sim has simply got hard-coded rates of depth changes programmed
2. the angle of inclination gets steeper the slower your speed, but this is not reflected graphically when examining the boat using external view.

What should happen, IMO, is that there should be a direct link between forward speed and rate of depth change. WWII subs largely used forward motion and planes to achieve depth changes. It was possible to use trimming tanks and compressed air to effect depth changes, but this was usually done only in emergencies as it was noisy and depleted compressed air.

At 3 kts you travel forward 5.07 ft/s. Therefore, if you are travelling at 3 kts and diving at a 25 deg angle, your rate of change in depth is 2.14 ft/s (this is using a right-triangle where 5.07 is the hypotenuse and 25 deg the angle A).
Thus it should take you (50/2.14) = 23.4s to change your depth by 50ft.

Similarly, for an angle of 30 deg, it takes 19.8s to change your depth by 50ft.

Obviously, with your rate of change being a constant based on angle, you should change depth more quickly based on your dorward motion. In fact it is linear, so you will do so in proportion to your changed speed i.e. 7kts is 2.33 times greater than 3kts, so your rate of change will be 2.33 times greater than it is at 3kts.

Looking at the results, you can see that the time taken to dive from 100 to 150ft is 18s on average, which is close to what we'd expect for a dive angle of 30 deg. What isn't clear, however, is why the rates change for deeper dpeths, and why they are so different when decreasing your depth.

It could be the boat climbs at a more shallow angle than it dives. There is also the factor of "levelling off" so that the last 50ft of a change in depth will be slower than the others (note I set my desired depth to 320ft in the tests to try to minimise this factor on diving). The trouble is that the results do not reflect either of these reliably. Indeed, with almost constant results across a given depth band, there seems to be no difference in your rate of dpeth change based on your forward speed, and this is clearly not correct.

Conclusion
It seems there are issues with respect to the way the sim handles depth changes at present. There is no proportional difference between different speeds of the boat and resulting times taken to change depths.
One result of this is that it is artificially easy to evade DDs at lower speeds, as you can change depth just as quickly at 3kts as you can at 7, but obviously you are much quieter at 3kts.

Recommendation
Explore the following:
1. ask devs how depth changes are calculated at present.
2. work out an appropriate model for rates of change in depths based on different forward speeds.
3. ask if there is any way to change the amount of inclination represented graphically in the sim so it looks more realistic.

Final comment
It's interesting to note that SHIII seemed to model this much more accurately in that your rate of dchange of depth was very greatly affected by your speed. If you wanted to change depth quickly, you needed to increase your speed commensurately. This, of course, had implications for remaining silent during evasions.

I hope people find this useful/interesting. I further hope something might be done with it to improve the realism of the sim (I don't have the skills to mod this, but I will post it in the mod forum).

Comments welcome. If I've made significant errors (always possible!), please tell me. I've just nutted this out in the past 15 mins, so entirely possible I'm mistaken (hell, it's 20+ years since I've done any trig!).

Cheers all!

[Camaero]

Thanks for your work looking into this. I am sure its going to be very helpful.

[Steeltrap]

Originally Posted by Camaero

Thanks for your work looking into this. I am sure its going to be very helpful.

My pleasure. I've benefitted so much from the modders here that I thought the least I could do is try to gather some info for them to go on if they choose to look into it. If I knew more about the sim, I'd try to do something myself.

Cheers

[Steeltrap]

Is anyone interested in doing something with this? Seems somewhat out of kilter as is.

Cheers

[Laffertytig]

im just gettin back into SH4 for the 1st time in 10 months or so and am disapointed to find that this issue still exists.

im usin rfb which highlights realism so would i be right in sayin that theres simply no way to make depth changes a lot slower that what they are now?

[Hylander_1314]

Well Steeltrap, at least you remember your trig well enough! Add another ten years and might have been like me, cursing cause you can't remember how you did this stuff!

But really, I always thought the would rise quicker than it does especially when blowing the tanks to surface. Which should make the boat pop up to the surface like a cork.

[Laffertytig]

is steeltrap still about? i agree with every word of his 1st post if only there was a way to make this part of SH4 more realistic.

maybe one of the rfb guys can tell us a bit about it

[gimpy117]

is it possible to fix?? lots of stuff is hard coded...UBI like to do that

[Laffertytig]

its been a long time since ive played vanilla SH3 so i cant remember if this issue existed but i know that in gwx they had this spot on.

im really hopin theres a way to fix this

[CaptainHaplo]

SH4 does the same as SH3 in many ways - this included (to a point).

Your speed does contribute - but only to a certain extent. The vast majority of up and down speed comes from displacement calculations. To change the rate of rise and sink - you actually modify the displacement, ballast tanks and tank fill rate.

By tweaking these values, you can make speed become more of a factor. However, its a balancing act. Too big a ballast tank makes for a yoyo effect - too small makes things too slow. Same effect with the flooding time. You have to balance them both to get a nearly neutral effect - along with vessel displacement. Once you start there - you also start affecting other things like fuel consumption, etc. Its a complex problem that CAN be worked on.

[keltos01]

will you post a tweaking table ? shipsize/ballast/flooding time/ speed ?

keltos

[peabody]

As CaptainHaplo says it is a balancing act. When I was making the Sen Toku playable, I had trouble making it dive. (talking normal, not crash dive) and I changed the flooding times and it worked better BUT it created another problem. If I was at max depth and evading, and clicked on periscope depth it came up like a rocket and went right past periscope depth and the conning tower came out of the water. What was happening is the displacement number were wrong so it was too bouyant and had trouble submerging but didn't have any trouble surfacing .
So it is not an easy task to get mass, surface displacement, submerged displacement and flooding times all working together to get what you want.

Peabody

[Laffertytig]

it must must be possible cos the supermods for SH3 managed to fix it. would like to hear if anyone else is or have been workin on this.

[skookum]

IRL once the sub achieves neutral buoyancy, the only variables that affect rate of dive or climb are deck angle and speed. At neutral buoyancy the sub acts like an arrow, always traveling in the direction it's pointed. The dive planes adjust the deck angle and the force of the water moving past the hull actually does the work of changing the sub's depth. The faster the sub moves, the higher the rate of dive/climb for a given deck angle. Trim tanks are simply adjusted to maintain neutral buoyancy as the water pressure changes. I don't know if this can be duplicated in SH4.

[Laffertytig]

it was achieved in SH3 so as i said it must be possible. im gettin the feelin not thay many people are to bothered with it though as SH4 has been out for comin up to 2 years and very little has been done with regards to this situation.

or maybe it has been tryed to no avail, thats why im hopin that one of the rfb guys can post their thoughts. it would certainly add to SH4's realism and challenge if the subs turbo depth change could be slowed down somewhat