From D&D Wiki
- 1 Boats
- 1.1 Hull
- 1.1.1 Waterline Length and Hull Speed
- 1.1.2 Length to Beam
- 1.1.3 Stability
- 1.1.4 Construction
- 1.2 Power Supply
- 1.3 Finishing Touches
- 1.1 Hull
https://en.wikipedia.org/wiki/Buoyancy https://en.wikipedia.org/wiki/Watercraft https://en.wikipedia.org/wiki/Boat https://en.wikipedia.org/wiki/Ship https://en.wikipedia.org/wiki/List_of_boat_types https://en.wikipedia.org/wiki/Sailboat https://en.wikipedia.org/wiki/Sailing_ship
Simply put, the boats presented in the PHB and DMG are bizarre. They make no sense in comparison to reality, fiction, or each other. The worst part is their speed. 5e boats are so obscenely slow, they are effectively a waste of time. So, I'm trying to write a set of rules which allow one to find the actual mechanical properties of a vessel based on its description.
The hull is the body of the vessel; the big tub you'll be floating along in. In ancient times, there was only one type of hull; a displacement hull. These vessels float via displacement of water with a buoyant material. It was an effective enough technology that we used it right up to the modern age, though it has some considerable restrictions. I will not cover any other style of hull in this article, as they are thoroughly anachronistic and pretty much impossible to build using classical technology.
Waterline Length and Hull Speed
The waterline length of an average displacement hull determines its maximum speed generally. Essentially, as the vessel increases in speed, it sinks into a trough of water formed by its displacement, eventually forcing it to a stop. The longer the vessel, the faster you can go before you reach that point. This is called hull speed.
Hull speed is estimated to be about 1.34 x the square root of the waterline length, and is measured in knots, which are really just nautical miles per hour.
So if a sailboat is 144 ft long on the waterline, the square root is 12, and the boat will probably not exceed 12 x 1.34, or about 16 knots. A 100 footer will do 13.4 kts and a 16 footer will do 5.36. A thirty footer should do 7.34 knots.
Now, all that calculation is a pain for a game of D&D, so I'll simplify it to the part that actually matters: speed. The following list tells you the minimum length to attain a given hull speed. The length is given in nearest rounded feet and then in nearest rounded 5ft map squares.
20kn = 223ft or or 44sq
19kn = 201ft or 40sq
18kn = 180ft or 36sq
17kn = 161ft or 32sq
16kn = 143ft or 28sq
15kn = 125ft or 25sq
14kn = 109ft or 22sq
13kn = 94ft or 19sq
12kn = 80ft or 16sq
11kn = 67ft or 13sq
10kn = 56ft or 11sq
9kn = 45ft or 9sq
8kn = 36ft or 7sq
7kn = 27ft or 5sq
6kn = 20ft or 4sq
5kn = 14ft or 3sq
4kn = 9ft or 2sq
Note that the above is the maximum possible theoretical speed for each hull length. This is not likely to be the normal travel speed, which is dependent upon weather conditions, the actual dimensions of the boat, the sail plan, and how the sailors have chosen to manage the sails. As such, no measurements are made for anything slower than 4kn, as any vessel with a maximum speed that low would be nonfunctionally small... like, "surf board with a sail" small.
Some features of the hull's design may allow it to exceed this theoretical speed, while some inefficient designs may actually reduce it. For example, in modern naval architecture, hull speed was basically forgotten with the development of planing and wave piercing hulls. In modern vessels, the only real limit is the power supply's ability to overcome whatever drag acts upon the hull, and the hull's ability to withstand the force of it all!
Length to Beam
So, now that we have hull speed, let's consider beam, which is the width of the boat measured at its widest point on the waterline. Beam plays a major role in the stability of the boat, and can also have a hydrodynamic impact.
The beam measurement itself is not as important as the ratio between it and length; the length to beam ratio, or L/B. The L/B is always given as a measurement of feet in length relative to each foot of beam it has. The L/B can range anywhere from 1/1 (a circular boat) to 30/1 (a boat so thin it looks like a needle from above). For average stability and speed, the vessel's L/B should be 3/1. At this measurement, your boat's hull speed is not altered by its L/B.
If you choose to increase or decrease the L/B from its nominal value, you will begin to see some performance changes, as you are altering the total wetted surface of the hull; the part experiencing water resistance and drag. The exact math is complicated and not really relevant to this so again we will simplify.
For every 1ft increase in L/B, add 1 knot to its hull speed. For every 1ft decrease in L/B, subtract 25% from its hull speed. (Thus, a perfectly round boat has half the expected hull speed.) As you can see, the bigger the ship gets, the more it matters; you are dealing with larger surfaces which make a bigger impact on drag. A vessel's L/B cannot reduce its width below 5ft. We will pretend that's how wide racing canoes are. I am NOT making 1cm:1ft square map grids. One person wide is all that matters in gameplay.
Now we need to consider balance. A boat has two stability variables: initial, which is its stability when upright, and secondary, its stability when tipped edgeward. L/B determines initial stability. Initial stability is the vessel's resistance to leaning due to various forces such as waves. The wider the boat, the less it rocks. As a result, it takes more force to push the boat over to the point where secondary stability kicks in, which is anything beyond 10°.
To calculate initial stability, we will invert the L/B ratio. Our thinnest possible L/B is 30/1. Subtract your L/B from that number. The result should be a single numerator. The widest possible boat, practically a bowl, has a stability of 29. The thinnest possible boat has 0 stability- it is 100% dependent on external forces to resist waves and other forces. Record your vessel's initial stability and continue on.
Draft & Secondary Stability
Now that we have length and width covered, it's time to delve into the third dimension and examine the vessel's depth- its water depth to be precise. Draft is the distance between the boat's keel (its bottom) and the waterline. In real life this changes as cargo and people present on the vessel alter its weight, but the change is rarely a life or death matter so we will pretend that does not happen! Also, a vessel's bow (front) and aft (back) may have different drafts, and the difference is known as "trim"- we will pretend that doesn't happen either!
The obvious impact of a vessel's draft is that it determines how shallow of waters it can sail in, and can impact how many interior decks there are, but it also influences hull speed and balance! On the subject of balance, in reality the positioning of mass inside the vessel alters its center of gravity, and thus its balance. Like everything else that would force us to recalculate the boat's stats every time someone moves, we will pretend that does not happen either. Instead, we will assume that the crew knows this and intentionally positions all cargo to maximize hull stability.
Now, in reality there is no such thing as a "natural" draft for a boat. Unfortunately, for an RPG we need it. So, take the total height of your hull, from main deck to keel and halve it. That is your boat's natural draft for its hull speed. Now, we can modify that a bit to alter speed and stability. If you increase the draft by 25%, setting the boat lower in the water, add 5 to its stability and decrease its hull speed by 1 knot for every 2 L/B it has. (IE, a deep draft on a boat with an L/B of 6/1 would lose 3 knots) If you decrease the draft by 25%, subtract 5 from its stability and increase its hull speed by 1 knot for every 2 L/B. Additionally, reduce the hull speed by 1 knot for each full deck, and 1 knot for every two partial decks, the vessel has.
Let's conclude stability by talking about capsizing. A capsize is when a boat flips over and ceases functioning as a boat. The initial stability of a boat resists deviation from a straight up-and-down alignment, but secondary stability is what prevents a capsize. Basically, once enough force is present to exceed initial stability and lean the boat more than 10° to one side, you're looking at a potential capsizing. At about that point all the stuff inside the boat shifts due to gravity, actually adding to the unidentified force! Secondary stability is a boat's desire to return to an upward position once put on edge like that. Basically, if you have a deep keel with a lot of weight very low in the keel, like having a lead ballast on the bottom, gravity will pull that weight down, hopefully arighting the boat.
Even if all the weight in the boat is made up of loose cargo, if all that cargo is in the keel, it won't matter if the cargo is all on one side of the boat, it's still on the "bottom" side of the center of gravity as the boat begins to roll. (In real boats you would never put your cargo in the bilge like that, but hey, this is just an example.)
Once you hit a certain angle, which varies from boat to boat, there's no stopping it- all of your stabilizing forces are neutralized and all that's left is the force pushing you over; water enters the hull through deck openings reducing buoyancy and raising the center of balance; your stabilizing weights are pushed across the center of gravity and pull you upside-down.
In-game, I calculate this as a simple check: Whenever something is trying to bowl a ship over, I do a d20 check comparing the incoming force to the ship's stability. So, as an example, let's say I have a 223ft boat travelling at 20kn (hull speed) and it rams into a boat with 34 stability (L/B of 1/1, deep keel) the ramming vessel needs to roll at least 14 to capsize the fat boat on collision, which is a 30% chance of success. Does it account for mass and momentum? NOPE! Does it need to? NOPE! Remember, that example says nothing about whether or not either ship will survive the collision, only whether or not the target boat will flip over. This calculation works (well enough at least) regardless of whether the target boat is 10ft long or 200ft long.
Now we need to mention structural limitations. You obviously cannot have boats which go beyond physics. At the moment, your boat is a floating box, we need to discern its finer details, what holds it together, how was it constructed, what are its actual shapes and design features, etc. This section will have a major impact on the appearance and practical abilities of your vessel as well.
The first and easiest limit is decks. In real life, deck refers only to the number of physically walkable floors, but for our purposes, the word "deck" refers to a man-height vertical interior space, regardless of whether it has a floor or ceiling. In this regard, decks is actually a measurement of the height of the boat's hull. Each deck is assumed to have a floor, walls, and no ceiling. (Thus a 1-deck boat has the characters walking on the sloped interior of the vessel's hull, such as with a canoe or longboat.) Each deck is assumed to be 8ft tall. You may only have a number of decks equal to the vessel's beam divided by 10 rounded down to the nearest whole number, with a minimum of 1. In a real boat, decks tend to get smaller as you go down, but for simplicity's sake, we are ignoring that.
Thus, your beam determines your maximum number of decks as follows:
5-15 ft = 1 Deck
20-25 ft = 2 Decks
30-35 ft = 3 Decks
40-45 ft = 4 Decks
50-55 ft = 5 Decks
60-65 ft = 6 Decks
70-75 ft = 7 Decks
80-85 ft = 8 Decks
90-95 ft = 9 Decks
100 ft = 10 Decks
Any part of any deck which is exposed to the sky, (Such as your main deck) may be "decked over" to form a bonus deck out of its ceiling. A bonus deck has only rails to keep people aboard, and costs only half as much as a normal deck.
Likewise, for any deck which has another deck beneath it, you may remove up to one third of its floor for a 50% discount on that floor space.
On a multi-deck vessel, you may also leave up to one third of your main deck (Your topmost full deck) open to the air at its sides, to give the impression of a partial deck. An open deck reduces HP by 25%. You can roof over or build a partial deck above the open part of an open deck; the effect is that of a long, wide opening in the side of the hull. Though never done on classical vessels, modern ferries have such an opening, which allows a broad ramp for vehicle loading and offloading.
You can also have several partial decks, called castles, above the top deck. These do not count as full decks for any sort of calculation. You can have a number of partial decks equal to half the number of full decks your vessel has, rounded down. A partial deck occupies up to one third of a deck, at the fore (Forecastle) or aft (Aftercastle) of the boat. Each partial deck counts separately, even if they occupy the same layer. So, for instance, if you have a forecastle and aftercastle, they each count together as two partial decks, even though they are at the same height. The number of partial decks you can have relative to full decks is as follows:
Main Deck = 0 Partial
2 Decks = 1 Partial
3 Decks = 1 Partial
4 Decks = 2 Partial
5 Decks = 2 Partial
6 Decks = 3 Partial
7 Decks = 3 Partial
8 Decks = 4 Partial
9 Decks = 4 Partial
10 Decks = 5 Partial
You can also build a small house-like structure on any open-sky deck floor space. Such a structure is called a deckhouse, and is assumed to have a roof you cannot normally walk on, with 8ft walls. It must be at least 10ft from the edges of the floor space. (So a boat with a 25ft beam can have a 5ft wide deckhouse, maximum.
Then there's structural integrity, its HP. Simply add together its length, beam, and total height in feet. This base number is then multiplied by two modifiers, first by its construction method, then its material strength.
- Canoe construction is an RPG invention representing boats made of fabric/leather stretched over a frame, or composed of a hollowed out tree. These vessels are small and crude, making them rather fragile. Their modifier is 0.5- half what it should be. Canoe construction vessels cannot have a waterline length greater than 50ft, nor can they have more than a main deck.
- Mortise-and-Tennon boats are the oldest types, typical of the classical greek/roman period, they were made entirely of wood held by tension caused by the wood's expansion as it absorbed water, sealing its joints to be air-tight- they have a modifier of 1.
- Lapstrake or Clinker construction has an actual frame which helps support a hull composed of overlapping panel strips, called strakes, these were typical of the viking and medieval period in europe- they have a modifier of 1.5.
- Finally, Carvel construction has an underlying skeletal frame which is made to float via external paneling, the structure is entirely dependent on the strong frame, granting it a modifier of 2.
As for material strength, you're on your own there pal. The official consensus is, "we don't know what wood works best for what, and there's so many factors there's no way to test it.". Basically, consider the wood in question, fictional or real, and start with a base modifier of 1, increase that modifier by 1 if it is a hardwood, flexible, resistant to rot, or highly buoyant, each. If a wood possesses all five properties at the same time (Nonexistent in reality) it is the perfect boat wood, with a material strength modifier of 5. Reduce the modifier by .25 each if it has any of these properties: is a particularly soft wood, is a particularly brittle wood, rots quickly, or has low enough buoyancy that it can actually sink once waterlogged. The worst boat wood then has a 0 modifier, meaning the boat can't even be made from that material- it has 0HP no matter what!
The longer the vessel is relative to its beam, the less structurally sound it will be, as the forces acting on either end of the hull have greater leverage upon its center of gravity. Reduce the hull's HP by its L/B numerator x10. This gets you an HP penalty ranging from -10 (1/1) to -300 (30/1).
If you find your vessel has 0 or less HP, you have exceeded the limits of physics, and your boat is impossible.
Bow, Prow, & Stem
https://en.wikipedia.org/wiki/Bow_(ship) https://en.wikipedia.org/wiki/Prow https://en.wikipedia.org/wiki/Beakhead https://en.wikipedia.org/wiki/Head_(watercraft) https://en.wikipedia.org/wiki/Figurehead_(object)
This is the very front tip of your boat, its actual nose. It is mainly formed by the stem; that part of the keel which extends upward out of the water to form the front of the boat. This central beam of the keel is called the stem, and its arrangement in the bow determines its appearance and functionality. The prow is the part of the bow which is above the waterline. There are basically only two styles of bow.
- A raked stem is the most common style, as the stem comes up at a curved slant, making the tip of the boat an acute angle. These were the easiest form to construct using classical techniques. A raked bow has no impact on performance, but you must cumulatively remove 5ft or 10ft of length from each deck beneath main deck. The more you remove, the more it will reduce the vessel's value.
- A plum stem comes stright up out of the water, making the tip of the boat an axe-like wedge. This is sometimes called an "axe bow" because of its appearance. These are wave-piercing bows, which literally cut through the water, rather than riding over top of it. Increase your hull speed by 25% if you have one of these. They are extremely difficult to build using classical construction methods, resulting in a dramatic increase in price.
- An inverted stem is the inverse of a raked stem. The bow comes to a point below the waterline, then dramatically curves backward, resulting in a beak-like appearance. These are treated as an extension to waterline length, (Even though we design them as a reduction of interior space) resulting in a full 50% bonus to hull speed. You must cumulatively remove 5ft of length from each deck above the lowest. These are pretty much impossible to construct using classical methods, resulting in an insane price increase.
The final step of designing your stem is to set its angle. The sharper its angle, the better it'll glide through the water, but the more you'll have to remove from your decks' floor space in order to get it. The sharpness of the bow is measured by its length from the tip back to where the hull changes angle to converge. Simply measure this length in 5ft increments and subtract the beam in 5ft increments from that number. Add half the result to your hull speed. (So, if you somehow manage to build a 30/1 L/B vessel that is only 5ft wide and therefore 150ft long, and your bow extended from stem to stern, you would get +15kts of hull speed. That boat is impossible to build, mind you.)
https://en.wikipedia.org/wiki/Aft https://en.wikipedia.org/wiki/Stem_(ship) https://en.wikipedia.org/wiki/Sternpost https://en.wikipedia.org/wiki/Transom_(nautical) https://en.wikipedia.org/wiki/Deadwood_(shipbuilding)
|Schematic of a boat's stern.|
This is the back or "tail" of the boat, its aft-most portion. The stern's only real impact is drag and vulnerability. Because it is usually the largest part of the boat, it is also usually the primary target for cannon fire. A standard classical stern is just a flat box; this was usually where the crew and captain lived, and its construction was essentially the same as a house, though with dedicated spaces for the steering machinery if the vessel had any. These sterns were unfortunately terrible in combat, as they could neither withstand incoming fire, nor could they support the stresses of delivering cannon fire. This left them a considerable weak point: a large target and a blind spot for return fire.
This was remedied just prior to the age of sail with the invention of the round or elliptical stern. In this case, the stern takes a rounded shape as the ribbing of the boat's skeleton continues in a fan of "whisker" beams. This created a raked shape what was more structurally sound, and thus capable of carrying cannons, as well as being sloped and cut-away, reducing the effectiveness of cannon fire upon it. However, it reduces interior space and leaves the rudder open to fire somewhat. To add an elliptical stern, simply add a semicircle with a radius equal to half the vessel's beam to the aft line of each deck. Cumulatively remove 5ft of length from the stern from each deck below main. Increase the vessel's HP by 10%. Enemies may choose to fire on your vessel's rudder directly. These are fairly expensive to build.
A cruiser stern is essentially a second bow, but at the back of the ship. The stern comes to a point and dramatically reduces drag, thus improving hull speed. Simply design your stern as you would a bow, and add its hull speed bonus as you would a bow. The % bonuses added by the Plum and Inverted designs do not apply; the stern is not cutting through waves, so this has no effect whatsoever. These are fairly cheap to build, but reduce the interior floor space of the vessel.
The keel is the key structural component of all boats. It is a long wooden beam which forms the bottom central ridge to which all other components are attached. The only thing your keel will affect in this game is your draft, stability, and the shape of your vessel's bilge. There are effectively only two types of keels: Flat or Round.
A flat keel means the belly of your boat is simply flat. The sides of your boat are more or less vertical and your vessel's keel looks like a floor. Our design process assumes this design, so you don't have to change anything. You will add a bilge deck beneath your lowest deck, however. The bilge is 4ft deep, so anyone planning to go in there has to crawl.
A round keel implies the walls of your hull extend upward in a sort of sloped angle like what you see in almost any boat, resulting in a central ridge on the underside of your boat. In this case, your draft gets an additional 8ft added, as your bilge is 8ft deep. This grants +1 stability. Your bilge has only 5ft wide of walkable floor space and is as long as the deck above it.
https://en.wikipedia.org/wiki/Sailing_ballast https://en.wikipedia.org/wiki/Canting_keel https://en.wikipedia.org/wiki/Centreboard https://en.wikipedia.org/wiki/Daggerboard https://en.wikipedia.org/wiki/Bilgeboard https://en.wikipedia.org/wiki/Leeboard https://en.wikipedia.org/wiki/Bruce_foil
This refers to what makes the boat travel. If a boat has no power supply, it must either be towed or ride a current, like a barge or raft. Aside from that, classical boats had only two modes of propulsion: Man Power and Sail Power.
Sail Power & Sail Plan
Sail power uses huge sheets of fabric (called sails) to catch wind and redirect its force to propel the boat. The actual structures used to do this are actually quite complex, but it essentially boils down to this: A big pole (called a spar) sticking out of your boat is used to lift a second spar (using ropes and pulleys) with a sail attached to it. The ends of the second spar and the sail are tied to different spots on the boat to hold it in a certain position to catch wind. Extra ropes are tied to all of this to keep it stable and to allow people to climb up into it to do more work. The arrangement of all of these spars, sails, and ropes, is your sail plan.
A spar is just a big pole used as a frame work to support rigging and sails.
These are the tall vertical poles which support your entire sail plan and redirect the force of the wind into your hull. They are arranged along the center line of the boat, and pass through all decks, right down to the keel. It is pretty much impossible to find a tree that is the perfect width and shape from top to bottom to make a single-piece mast. As a result, masts are composed of sections. (In real life, these sections are called masts unto themselves, which is confusing, so we will call them submasts) The ends of each submast are bound together, with the upper submast attached to the front of the lower submast. A vessel may have up to one mast per 20ft of length, up to a maximum of 5, with a minimum of 1. (Thus, a 5 mast boat must be at least 100ft long) The main mast can have 1 submast section per every 50ft of length, up to a maximum of 3. (Thus, the tallest main mast, composed of three submasts must be at least 150ft long)
Main-mast- The tallest mast on the ship, usually at or near the center.
Fore-mast- A shorter mast in front of the main mast. Not common on 2-masted ships.
Mizzen-mast- A shorter mast directly behind the main mast.
Bonaventure Mizzen-Mast- The fourth mast on a 5-masted ship. Shorter than the mizzen.
Jiggermast- The last and shortest mast on a 4- or 5-masted ship.
A boom is a lateral pole hinged on a mast which is attached to the bottom of a sail, rather than its top. These are used for triangular main sails.
Gaffs & Sailsprits
A sprit is a pole which extends from the end of the boat. They allow a sail to be tied to a point beyond the boundaries of the vessel's hull.
Bowsprit- Extends from the prow of the vessel. Like a mast, especially long bowsprits may be constructed in sections. The additional spars are called the jibboom, and the flying jibboom on top of that.
Sternsprit- Extends from the stern of the vessel.
Boomkins- A pair of smaller bowsprits, one on either side, which hold a headsail.
This covers all of the lesser stationary poles which may go into a sail plan.
Jackstaff- Not counted as a mast; all it does is sail the ship's flag, or its Jack. Not all ships have these; some fly their jack from one of their masts.
Strikers- Perpendicular poles extending vertically above or below the bowsprit, intended to alter the angle of a line tied to it to handle higher tension. If it is above the bowsprit, it is a pelican striker. If it is below the bowsprit, it is a dolphin striker.
Spreaders- Poles which extend laterally from a mast or sprit to give stabilizing lines better leverage and to keep them away from the rest of the rigging.
Measuring the "normal" travel speed of a vessel is a physics nightmare, far too complex to handle in a tabletop RPG. Among the many factors complicating matters, you have,
- The wind is not constant.
- The boat is not traveling in uniform velocity.
- There may be a mast in front of the sail, disturbing the airflow, although this may be mitigated by profiling it.
- A mast is not infinitely stiff.
- The boat profile and position influence the airflow.
- A sail is usually made of thin and deformable fabric.
- The air is viscous, causing losses by friction.
- The flow of the air varies from slow to fast and turbulent to laminar.
And that says nothing of the forces enacted on the hull by the movement of the water itself! However, for simplicity's sake, we will assume that under calm idyllic conditions, (Think a sunny carribean day) normal sailing speed is half the maximum. This assumption was made after reviewing a number of old ships logbooks and comparing their travel pace under ideal conditions with the hull speed of the boat. It is not empirical or realistic, just an observation of what seemed "normal". Also keep in mind that the vessels I looked at, though premodern, were definitely NOT medieval.
Rowed vessels have an additional step that sailing vessels do not. Instead of halving the hull speed, you have to count up the power of the rowing crew. Each person contributes to the actual speed the vessel will travel, based on their STR modifier. Essentially, treat each +1 as .1 of a knot. So, a character with a modifier of +5 will travel at a speed of 1kn every two hours. Simply add up the total STR modifiers of everyone rowing and divide the result by 10 to get your result. Remember that their travel speed can never exceed hull speed, no matter how many oarsmen there are, or how strong they are.
We can, from that assumed normal speed, then simulate the effects of varying weather conditions. At the start of an hour of travel, the ship makes a d20 conditions check. The roll is made with a crew bonus equal to the number of actively involved crewmen with the sailor background.
Natural 1 = Dead in the water. The boat must be towed or rowed.
1-5 = Poor Conditions. Travel is reduced to half normal.
6-10 = Ideal Conditions. Travel is at normal pace.
11-15 = Beneficial Conditions. Travel is 50% above normal.
16-20 = Perfect Conditions. Travel is at hull speed.
Natural 20 = Unusual circumstances push the boat 1kn faster than its theoretical maximum.
If you crunch the numbers, that means 20 active sailors guarantees maximum speed at all times. However, sailing is hard work. You can't do it all day. As such, the crew needs to break sailing into shifts. The number of shifts the crew should be divided into is based on how long the trip is. For instance, for a two hour trip, there is no reason to break up into shifts at all! The following lists off how many hours of the day are worked by a shift under a given scheme, and how long it can be maintained before incurring exhaustion.
1 Shift = 24h, Maximum of 1 day
2 Shift = 12h, Maximum of 1 week
3 Shift = 8h, Maximum of a month
4 shift = 6h, Everything greater than a month
So, the navigator determines how long a trip is likely to take, based on distance, weather forecasts, and the normal pace of the ship. Then, based on that, the captain determines how to divide the work. So, on a ship with 20 sailors, (which is a lot of people to have living together on a soggy wooden boat) as few as 5 of them could be contributing to the overall speed of the vessel on a 4 shift rotation.
Of course, on a simple vessel, such as a sloop, there is very little actual work to be done, regardless how many people there are to do it. Most of the work on such a vessel involves hoisting and trimming sails (or, in other words, pulling a rope). Each ship, then, has a minimum and maximum staffing requirement. On extremely complex rigs, such as a fully rigged galleon, the minimum staffing number is likely to be quite high, with the maximum number well exceeding actual accommodations. A vessel cannot set sail without the minimum staffing. Staff exceeding the maximum staff limit do not contribute to the conditions check.
The staffing requirements are based primarily on the sail plan of the ship. The sail plan determines a number of technical features of how the ship handles. Most of it doesn't come up outside of combat. In combat, (and some other tense situations, such as races and storms) the precise handling of the ship becomes extremely important. Among other things, the sail plan influences:
- How close to the wind the ship can sail
- How fast it travels downwind and how that speed is impacted as it turns into the wind
- How heavy the sails are, affecting how many people it takes to move them in a given time.
- How many sails there are, affecting how many groups of sailors are needed to move them.
- Some rigs have a subtle effect on normal travel speed when they reach certain dimensions.
- The axis point where the ship turns.