How does fire spread?

[Zunee]

So I'm working on a small challenge in a campaign wherein the ship the PCs are riding on is shot at with fire crossbow bolts. That created 2 questions I couldn't find a definitive answer to.

1.) What is the chance of a bolt catching the ship on fire? It reads:

Quote:

Alchemical fire bolts do not cause any splash damage, and the fire burst isn’t enough to ignite targets (unless they are particularly flammable).

How flammable is particularly flammable in this case? What kind of math would I need for that?

2.) How does fire spread? I couldn't find anything that explained the spread of fire. Right now I'm thinking it's: 1d8 to pick spread direction at the start of "the fire's turn" for each burning square, then roll an attack on the AC of the ship. I'd love to see if there's an actual way to do it though.

If anyone has any idea how to play with fire, please help!

[Pizza Lord]

Quote:

1.) What is the chance of a bolt catching the ship on fire?

Almost zero.

Like the description for alchemical bolts and fireball spells say, they don't typically set most creatures or objects on fire unless they're flammable: paper, wicks, curtains, etc. Even normal cloth (cotton or wool) is unlikely to catch fire unless it's soaked in oil or something (not sure how silk holds up.) There might be a certain type of cloth that might go up faster.

Wood does not actually burn very easily, it tends to take a long time to burn through any sizable piece of it (which is actually why it makes good fuel for campfires, because it takes a long time to burn up). Most fire attacks on ships probably involve flinging oil or using oil-soaked weapons to douse the deck and then ignite them with a fire attack first. Otherwise, just the fire weapon probably won't burn long enough to ignite wood. The sails and rigging, or spare sailcloth and rope on the deck are more likely to go up and if not extinguished possibly spread.

Most likely, fire attacks are not just going to ignite the hull or wood of a ship, that's usually pretty tough wood and often a bit wet, so unless your attacks are hitting flammable cargo or going into portholes and igniting the sheets on bunks or the powder magazine you're likely just going to deal with a short-term burning area immediately after a hit.

Quote:

2.) How does fire spread?

I don't have rules specifically so if you don't want to just adjudicate how the scene plays out, you could probably just assume a 20% chance of a fire spreading. If 10% or less, let it spread in two (believable) direction. Honestly, that's probably way higher than realistic, but adjust to suit. You can always lower the odds and just shoot a lot more incendiary arrows or alchemist's fire to up your chances. Again, note that that's for a fire's chance of spreading once something catches fire, otherwise you're probably looking at about 5% (the equivalent of rolling a 1 on a saving throw) to ignite an object that isn't very flammable already.

Typically the fire will move in the direction the wind is blowing unless the ship is moving fast enough then likely towards the rear of the ship. That's assuming it doesn't have a viable trail of fuel to follow along, be that spilled oil or burning along a rope leading below decks. Otherwise, rolling a d8 for direction is a sound idea.

[Byakko]

There are no consistent rules for the spread of fire, as far as I know. As mentioned above, spells and effects generally don't produce persistent spreading fire unless they specifically say they do. As a GM, however, it would be perfectly reasonable to allow the fire to happen as a challenge for the PCs and as a plot device! Just be sure to emphasize that it is an unusual/exception situation and not the norm.

There are many example scenarios where the PCs must quickly act to put out the spread of a fire, often in the middle of combat. Most of these I've encountered have used fairly unrealistic/chance based rules for the spread of fire to adjacent squares, making them effectively impossible to put out. Your suggestion would turn out like this.

Small fires can quickly grow and spread when there is highly flammable material about, but may take quite some time to build up the heat necessary to spread to wetter parts of the ship or to penetrate walls. I am not an expert on the subject of spreading fire and ships, so you may wish to ask google about it for ideas.

Cinematically it might be nice to have a relatively cool fire spread along the surfaces and ropes of the ship but without damaging the general integrity of the hull. If a player douses an area then the fire should be prevented from returning there for a while. This will give urgency to act without making for a completely hopeless situation. Afterwards, you can have some skill challenges involving repairs too. :)

[Daniel Yeatman]

This is taken from AP #30, and it seems to be quite comprehensive. Sorry about the formatting, I mostly just copied it directly from the PDF.

FIRE
Fires occur when indoor or outdoor areas that contain or are constructed from flammable materials are exposed to flame and catch fire. As the blaze spreads, creatures in the nearby area are subject to the deadly e#ects of fire and smoke inhalation. GMs running a fire encounter might refer to the rules for forest fires found on page 426 of the Core Rulebook. The fires detailed here suggest more
easily combatable fires, such as those newly sparked or spreading within an urban setting.

The Spread of Fire:

Unchecked, fire tends to spread both rapidly and
unpredictably. Minor factors, such as the dryness of
the burning material, the presence of wind or breeze,
flammable finish on flooring, dry vegetation in an area,
and countless other factors can all contribute to the
spread of a fire. Once a fire has burned an area, it will
not return to that area. Likewise, once an area has been
doused with water or covered with a non-flammable
substance, such as dirt, that area is safe from further
e#ects of the blaze for the immediate future.
Every round that a fire burns, regardless of whether
characters are attempting to control it, roll 1d20 and
consult the following table to determine the activity
of the fire for that round and how many (if any) 5-foot
squares the fire spreads to. The GM chooses which
squares a fire spreads into if multiple possibilities
exist. Fire cannot spread into areas where it has already
been extinguished (unless noted otherwise), nor can
it spread into squares where flammable materials are
not present. Characters who are inside of a square
when it catches fire are subject to damage, as per
the rules for catching on fire on page 444 of the
Core Rulebook.

Progress of Spreading Fire:

1d20 Fire Reaction
1 The fire does not grow this round.
2 The fire grows 1 square to the north.
3 The fire grows 1 square to the east.
4 The fire grows 1 square to the south.
5 The fire grows 1 square to the west.
6 The fire grows 1 square in all directions.
7–8 The fire does not grow this round.
9 The fire grows 2 squares to the north.
10 The fire grows 2 squares to the east.
11 The fire grows 2 squares to the south.
12 The fire grows 2 squares to the west.
13 The fire grows 2 squares in all directions.
14–18 The fire does not grow this round.
19 The fire grows 3 squares in all directions.
20 The fire grows 4 squares in all directions.

Burning Buildings:

Buildings that catch fire are quickly engulfed and
are often a complete loss. If there are no characters or
NPCs attempting to put a fire out, a building becomes
unsalvageable in a space of time dependent on the size of
the building. Ultimately how fast a building burns is le!
up to the GM to decide and depends on a variety of factors,
but a rough guideline is as follows: small one-floor buildings
(as occupied by many commoners) are consumed in 6d8
minutes; larger homes (like town houses and the homes
of merchants) are totally consumed 4d20 minutes a!er
catching fire; and major structures (like villas, castles, or
cathedrals) are consumed in 2d4 hours. Buildings built
entirely of flammable materials burn in half the time, while
structures consisting mainly of non-flammable materials
take half again as much time to burn. While burning,
most structures begin collapsing.

Dense Smoke Inhalation:

Dense smoke, as might fill a burning building, can prove
even more dangerous than the flames that create it. In
addition to the rules for smoke inhalation presented on
page 426 of the Core Rulebook, a character in dense smoke
must make a DC 10 Fortitude save every round that she issubject to these conditions. A character may fail this save
a number of times equal to her Constitution modifier.
After failing to save for the last time, the character falls
unconscious and is subject to suffocation (see page 445 of
the Core Rulebook).

Backdrafts:

Fuel-starved flames bursting into freshly opened
chambers pose a lethal threat to fire fighters. Such
hazards typically arise from rooms no larger than 40
square feet and sealed from ample airflows. When these
rooms catch fire, they deplete the supply of oxygen in
2d6 × 5 minutes. After such a point, the fire continues
to burn, but the combustion is a slow smolder. When a
door or obstruction is opened or removed, the air from
outside the room rushes in and instantly restarts the
flames, resulting in a fiery eruption. Any characters that
are either already in the room or are within 15 feet of the
newly opened entryway take 5d6 points of fire damage
(DC 15 Reflex save for half ). The area opened to must
be oxygen-rich for a backdra! to happen, and does not
occur if one oxygen-starved room opens into another.

Firefighting Magic:

A number of spells have the potential to a#ect areas that
have caught fire and can serve to reduce the seriousness
of a blaze. While these are by no means all the spells that
might aid a fire-fighting spellcaster, these account for
the majority of the magical e#ects that can be brought to
bear against flames. Other obvious spells, such as control
water, rely on the specifics of a situation and are le! to the
GM to determine the effects.

Control Weather: In general, weather conjured by this
spell has a 40% chance to extinguish an uncovered square
full of flame every round. This does not prevent flames
from spreading, but those le! exposed are quenched with
relative swi!ness. Fires burning within a structure are
una#ected by this spell unless it has some obvious point
of entry (such as through an open roof or large window).

Gust of Wind: This powerful wind blows out 10 feet
of fire in its path. Flames blown out can be reignited by
nearby flames.

Ice Storm: An ice storm extinguishes fire in the area it
affects. The hail from the spell melts and leaves an area
soaked, preventing it from catching fire again.

Polar Ray: This spell extinguishes the fire along the
ray’s path. The ice from the spell melts due to the heat
and leaves an area soaked, preventing it from catching
fire again.

Pyrotechnics: Upon affecting a fire, a pyrotechnics spell
extinguishes up to four squares of fire. A#ecting flames
with this spell can backfire upon a caster, though,
potentially hindering those nearby with even more light
and smoke.

Summon Monster: Several summoned creatures might
possess qualities allowing them to aid in putting out
fires, whether special abilities or the power to cast any
of the spells noted here. Water elementals especially
can put out flames in any square they cross, though
burning squares count a di"cult terrain for them while
purposefully trying to extinguish flames. Even water
elementals take damage from fire, and can be destroyed
by entering a blaze.

Dousing a Fire:

Dousing a fire requires a large amount of water or other
non-flammable material, such as dirt, to be deposited on
the burning area. One e"ective strategy for extinguishing
a fire quickly is to surround the burning area with nonflammable
material. PCs doing this must make a ranged
touch attack against an AC of 10 to deliver their payload
to the intended square. The following indicates how many
5-foot squares of fire a number of the listed containers can
extinguish with successful delivery.

Waterskin: Twenty waterskins full of water extinguish
one square.

Bucket: Four buckets full of non-flammable material
extinguish one square.

Gallon Container: Twelve gallon containers of nonflammable
material extinguish one square.

Cauldron: One cauldron of non-flammable material
extinguishes one square.

Portable Hole: A portable hole filled with non-flammable
material extinguishes a 12-square-by-12-square area.

Bag of Holding: A bag of holding, type I filled with nonflammable
material extinguishes a 3-square-by-3-square
area, type II extinguishes a 5-square-by-5-square area, type
III extinguishes a 7-square-by-7-square area, and type IV
extinguishes a 10-square-by-10-square area.

[Zunee]

I really appreciate everyone's input, and thank you for the sourcing so I can go read through AP 30 really quick. I'll have to find a more believable way to start the fire than bolts, I guess, but luckily I plan 2 weeks in advance so I still have some time to work it all out. Thank you guys so much though!