Storms at Sea, game rules

The heart of the scenario is: once an hour (effectively, an hour becomes a round), a ship handle check must be made. Almost everything else flows into modifies of the ship handle check. This will allow the players to, at the end of the storm, have either not lost any time, been forced to flee before the storm, or the main ship itself goes under. Individual DMs will have to work out the implications and consequences of those results (swim to shore? endurance checks? offload dinghies and lifeboats? arrive too late for the ceremony / wedding / ambush the bad guy?).
Scaling: I ran this for 4 level 7 PCs, and gave the ship a total crew of 10. They were also traveling with an NPC wizard to whom I gave a bucket of spells which were useful in a storm, and gave the PCs control of how to deploy these resources.

I wouldn't run this with much fewer than 10 crew or the rolls get quite challenging. Depending on what you do, math for 10-20 crew seems to be the sweet spot. If in doubt, I recommend about 15 crew.

Hurricane Encounter
Hurricane lasts a minimum of 10 hours. After that, if the wind is at the lowest notch ("Severe") and is rolled to be lowered, the storm ends.

Setup
I would get a battlemap or a piece of paper and mark out a bunch of tracks, like a board game: three tracks of three levels for wind, waves and rain. Three tracks of at least eight levels for the Fatigue / Exhausted increasing characters, and three tracks of eight levels for the Fatigued / Exhausted decreasing characters.

Each Hour

Ship Status:
The ship can have
[Scudding - A term applied to a vessel when carried furiously along by a tempest.]

• Sailing on course (roughly, no time lost) (no modifier to check)
• Scudding before the wind (lose time for voyage, +5 to ship handle)
• Abreast of the waves (each check ending in abreast of the waves results in one piece of major damage, i.e. a sail blows a way or a key rope is smashed by the waves)
• 10 pieces of major damage: ship wreck

Ship Handle Check
Roll to for ship handling, fail = lose one ship status (i.e. go from sailing on course to scudding before the wind)

• Ship Handle DC 15 - maintain scudding before the wind
• Ship Handle DC 20 - maintain sailing on course
• Ship Handle DC 25 - increase status (i.e. move from abreast of the waves to scudding before the wind)
• Ship Handle DC 25 - maneuver or hold station (used if someone falls overboard)
Modifiers

• +1 for every 2 crew members 
• +4 for Bosun
• +5 for Captain
• -5 for each piece of major ship damage
• (If in rigging) DC 15 Climb/Acrobatics: +2 for a PC (if fail by 5 or more, PC falls)
• (If on deck) DC 10 Strength: +1 for a PC

Ship Zones

• Rigging
• Deck (option of being tied in place)
• Below decks (safe and able to rest and recover fatigue)

Environment
Roll for wind strength on d20:

• First 10 hours = hurricane
• Before 24 hours:
• 1-4 decrease wind strength
• 15-20 increase wind strength
• After 24 hours:
• 1-10 decrease wind strength
• Severe wind: no penalty to checks
• Windstorm: -1 to physical checks
• Hurricane: -2 to physical checks
• Roll for rain:
• 1-5 no rain
• 6-10 heavy rain -1 all checks, -4 Perception, light by half
• 11-20 blinding rain -2 all checks, -6 Perception, light to ¼
• Roll for waves:
• 1-10 swells 10’
• 11-15 swells 20’
• 16-20 swells 30’

Characters
Every hour:
Fortitude save, DC 10 or become Fatigued (-2 Str Dex) or Exhausted (-6 Str Dex)
Increases by 1 each hour
+5 for proper clothing
Fatigued reset by 8 hours rest
Crew bonus halved at 8 hours and zero at 16 hours - crew must rest 8 hours below decks, coming on deck resets the rest timer
Proper crew management is one of the player's resources
Roll for lightning:

• 18-19 lightning hits ship, major damage
• 20 lightning hits character (crew = 1-7, helmsmen 8-9, Captain 10) 4d8 dmg
• Helmsmen hit: Acrobatics / Climb DC 10 (deck) 15 (rigging) to stabilize wheel
• Fail: One piece of major damage

Roll for challenges:

• 1-2 Grab a crew member as they fall over / Acrobatics DC 15
• 5-9 (only after 10 hours) Morale sinking, crew bonus halved / Diplomacy DC 15 to negate
• 10-12 Stinging squid

24 stinging squid land on the deck, leaping out of the water to avoid a Scaly Needletooth! Run as 24 immobile minions, any contact causes poison damage For every squid still remaining at the end of five rounds, -1 to ship handle check that hour

• 13-14 Deepeye Toadfish

10 Deepeye Toadfish jump on deck
Deepeye toadfish are medium sized and have the ability to Swallow Whole medium creatures using their bite attack Can leap 15' in combat over characters Have a one time use acid spit

• 18-20 (20’) or 10-20 (30’) Struck by wave, all on deck not tied down must save / Fortitude or Acrobatics DC 15, or swept overboard

Ship Damage Table d10:

• Bowsprit
• Jib
• Forestaysail
• Fore topsail
• Foresail
• Foremast
• Main topsail
• Mainsail
• Mainmast
• Boom

Weather Spells

• Animate rope -Save a character falling overboard with a lifeline, prevent loss of a sail, +5 to ship handle check
• Gust of wind - one +5 to ship handle check
• Control winds - lower wind on wind track one notch (i.e. windstorm reduced to severe)
• Control water- lower wave height to 10’ for one hour

Type

Natural