Ship weight vs Cargo weight vs Speed

[irR4tiOn4L]

Hey everyone, just wanted to point people in the direction of an interesting source on the relationship between a ship's weight and decrease in speed when weight increases.

Ingame, this does occur (complete with sinking lower in the water, very well done guys!) and costs around 10-20% performance, though i may be wrong there. Here''s an interesting source on how a real boat fares;

It is clear that a lighter boat will be faster. An estimate that a 4% weight increase leads to a 1% speed decrease is based on some simplifying physical assumptions. They are listerd here.

1) Water’s drag force on the hull opposes the driving force of the wind. The water’s drag is equal in magnitude to the wind’s force.

2) The drag is proportional to the submerged cross sectional area of the hull, which is nearly proportional to the total weight.

3) The drag force is also proportional to the square of the boat speed.

4) The total drag force would stay the same if the speed decreased by 2% at the same time that the weight increased by 4%. This follows because a 2% decrease in boat speed corresponds to a 4% decrease in the square of the boat speed.

5) For downwind sailing, slower boat speed means an increased apparent wind speed (with reference to the boat). Combining the decreased boat speed with the increased apparent wind speed means the boat must slow by only about half as much (1%) when its weight is increased by 4%.

http://www.physicsofsailing.com/

Long story short - for our slow boats every 4% increase in weight equals a 1% reduction in speed.

This gets interesting when applied to, say, the flush deck frigate ingame;

//-------------------------------------------------------------------------
// Lydia France/Spain by Yo Ho Ho rexturing by Seb
// <SWS - Willemstad Builders' Trials Winter 09 S/N 149> (WBT1LF) Advanced Frigate
//-------------------------------------------------------------------------
makeref(refShip,ShipsTypes[n]);
n++;
refShip.Name = "lydia2";
refShip.All = "Corvette1";
refShip.Sname = "FrigateAdvLydia"
refShip.walk = "";
refShip.id = refShip.Name;
refShip.Class = 5;
refShip.Cannon = CANNON_TYPE_LONG_LBS24;
refShip.MaxCaliber = 24;
refShip.Weight = Tonnes2CWT(1200);
refShip.Capacity = 2000;

With a weight of 1200 and a cargo capacity of 2000, with a full cargo hold the ship weighs 3200cwt, or 266.6% of dry weight. Thats a 166% increase which should equal (divide by 4) approximately a 41.5% reduction in top speed!

Im not sure how well that '4 for 1' reduction holds for such a large increase in weight, or whether the numbers of 1200 dry weight and 2000 capacity are even possible/accurate.

But Imagine if that was how much speed you lost loading up a frigate ingame!


Personally, i think the numbers of 1200cwt empty and 3200 full are not realistic but more balanced for gameplay, and that ingame the actual speed loss is probably quite realistic. Im mainly posting this as a reference to anyone who is interested.

 

[Armada]

Hey everyone, just wanted to point people in the direction of an interesting source on the relationship between a ship's weight and decrease in speed when weight increases.

Ingame, this does occur (complete with sinking lower in the water, very well done guys!) and costs around 10-20% performance, though i may be wrong there.

I must say, I never even noticed this was a feature of the Build Mod!
It's impressive just how realistic a game we have here (considering how far we've come from the stock game!).

I'm not entirely sure how accurate the weight and capacity values are for our ships, but of course we should be able to research certain types of vessel to determine what those values should possibly be.

 

[irR4tiOn4L]

Hey everyone, just wanted to point people in the direction of an interesting source on the relationship between a ship's weight and decrease in speed when weight increases.

Ingame, this does occur (complete with sinking lower in the water, very well done guys!) and costs around 10-20% performance, though i may be wrong there.

I must say, I never even noticed this was a feature of the Build Mod!
It's impressive just how realistic a game we have here (considering how far we've come from the stock game!).

I'm not entirely sure how accurate the weight and capacity values are for our ships, but of course we should be able to research certain types of vessel to determine what those values should possibly be.

Indeed, this has come a long way since stock.

On how much speed you lose and realism, i think this bears into the cargo capacities of the various ships vs their empty weights, and these have been rebalanced at least somewhat with a view to gameplay. I doubt in reality a 1200 ton vessel could have taken on 2000 tons of cargo (or these must be different units or something!). So ingame, i think this works just fine.

 

[Armada]

I doubt in reality a 1200 ton vessel could have taken on 2000 tons of cargo (or these must be different units or something!).

I think there might be some confusion between units. Even going by the RN Rating System of the Napoleonic Wars, a real 1st Rate should only have half the capacity they have in game, if the units are the same.
This varies quite a lot, with some ships having almost three times what the RN system says, but some of the smaller vessels are where they should be.
But as you said, it seems to work quite well regardless.

 

[irR4tiOn4L]

Well I just looked this up and in ships_init.c the units for ship weight are stated as 'cwt' and in the cargo screen goods are stated as 'cwt' - so its the same unit.

However, I thought that was some kind of ton but it turns out its short for 'centum weight' or 'hundred weight ton'. There is a distinction between the imperial (long) hundredweight (112 pounds or 50.8 kg) and Us (short) (100 pounds or 45.3kg) hundredweight, and 20 of these make up a long and short ton respectively. Since the long ton is approximately equal to the metric ton, and the long hundredweight is probably what 'cwt' is referring to, we need to divide by 20 to get the actual weight in metric tons.

That means our 1200 cwt ship actually weighs 60 metric tons and since it carries 2000 cwt it can carry 100 metric tons. This is for the Lydia light flush deck frigate.

Anyway you look at it, the units dont quite make sense but as long as it works ingame who cares.

 

[Pieter Boelen]

Nathan Kell coded in the weight dependence at some point, if I recall.

Do the ships REALLY sink further in the water based on the cargo? I didn't even know that.
Thought that was impossible. If we can figure out how that works, maybe we can do a PotC style Flying Dutchman trick.
Would be pretty useless because the water looks weird from below, but still...

 

[irR4tiOn4L]

Do the ships REALLY sink further in the water based on the cargo? I didn't even know that.
Thought that was impossible. If we can figure out how that works, maybe we can do a PotC style Flying Dutchman trick.
Would be pretty useless because the water looks weird from below, but still...

Yes, they do! Not only that, they sink by quite a bit and their centre of gravity seems to drop so they move differently (dont 'bob' as much) and seem to plow through the waves more heavily.

Nowadays when i look at a ship from afar I look at its waterline, and when picking a ship for speed I consider its cargo capacity - for example, a corvette with 15 speed but only 1000 cargo capacity and full will probably be slower than a frigate with a speed of 13 but 1500 cargo capacity filled to 2/3rds, let alone the VOC Amsterdam with 13 speed and some 4000 cargo capacity filled to one quarter

EMPTY


FULL



Try taking two ships out to sea, one fully loaded and one empty. Go in the empty one, observe it, and then transfer all goods from the other to fully load the empty ship - you will notice it suddenly 'snaps' lower in the water, appearing far heavier!

I think ive seen one or two occasions where it didnt work right away but generally the ships do sink lower when loaded.

 

[Armada]

That's very interesting! Something extra to be aware of, now...

Slightly off-topic for a moment, having seen your screenshots of that corvette, you've reminded me that ship is lacking some ropes.
I looked into it, and as I suspected, there were 'fal' locators where ropes should be. They don't show up in PotC, being CoAS-based locators.
Anyway, I've corrected that now; see here.

 

[Hylie Pistof]

Since I always sail with full holds that would explain why ships like the barque longue are awash and look like it's sinking. I always raise its waterline, but now methinks mayhaps I've raised to too much.

 

[irR4tiOn4L]

That's very interesting! Something extra to be aware of, now...

Slightly off-topic for a moment, having seen your screenshots of that corvette, you've reminded me that ship is lacking some ropes.
I looked into it, and as I suspected, there were 'fal' locators where ropes should be. They don't show up in PotC, being CoAS-based locators.
Anyway, I've corrected that now; see here.

Thats my favourite ship, and youve just made it better! Thanks