Wednesday, February 9, 2011

Ship speed mechanics

There are already some cursory ship movement mechanics in various places (Labyrinth Lord and the DMG, for example), but if your campaign is conducted largely on the water you might want something more interesting. I should also say that the following rules are a work in progress. I'd appreciate feedback.

I don't want to get too crazily simulationist here. There are well known real-world formulas for all of this stuff, but they're a little complicated for use in play. I asked myself: what sorts of sailing situations do I want the rules to highlight?

  • Longer ships tend to have a higher speed.
  • Wind is a big factor.
  • Ships with higher displacement tend to be slower (like encumbrance).
  • Being able to carry a larger sail area means more speed, but only to the point where there's enough wind to fill those sails.
  • A racing yacht or warship will be faster than a cargo ship of the same length.
  • In light winds, a smaller ship may be able to out-sail a larger ship carrying lots of cargo.

This is what I came up with. A ship has three attributes: Length, Sail, and Displacement. To determine the current velocity of a ship, you take those attributes and combine them with Wind.

V = W ( S + L - D )

This looks like a lot of variables for use on the fly, but S+L-D isn't going to change much once the ship is under way, unless the crew starts throwing cargo overboard. And that could be fun.

This only works if Sail is less than or equal to Wind, so the full formula would be (feel free to correct my probably nonsensical math notation) something like V = W (S + L - D), S ∈ [0,W]. A ship flying too much sail wouldn't gain any speed, would handle poorly, and might even capsize.

I haven't yet worked-out the range of these numbers, or how they translate into feet and knots and tons and so forth. They might have a range of perhaps 0-12, so a ship with a Length of 3 would be 30' long. I'll think more about that tomorrow.

7 comments:

  1. Hi

    I just left a long comment but it got eaten. I like what you're doing here, especially with L and D modeling the lines of the boat. These comments may seem didactic: sorry about that. Please don't be offended.

    I don't understand what you're saying about wind and sail. Ultratech catamarans aside, you can't go faster than the wind: as wind strength goes up, you reduce sail or face terrible risks.

    After about 1500 European ships routinely carry more sail than they can put up in a strong breeze, so sail/wind ratio becomes a question of nerve rather than ship design as such. I can see a system where you balance speed against crit risk... like maybe in the James Bond RPG, which had a chase mechanic where players bid penalty modifiers against one another? Then you have to roll under your bid, and the proportional risk of crit fails goes up with the difficulty.

    For wind strength, I like the Beaufort Scale: a 1-12 range and easily recognized conditions at each force/number, actually based on sailors' experience.
    ...

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  2. ...corrigenda: I meant to say "wind strength would be an ideal upper limit on speed, most ships historically would go much slower." This page gives handy sail-handling instructions and ideal speeds for Napoleonic ships of the line, which didn't go much faster than 17th century warships or big cargo ships.

    I don't know what your game's period/parameters are but in Europe warships became distinguished from cargo ships in 1650. Before that the principal reason warships went faster was oarsmen, and after the speeds were still comparable, except for frigates and schooners - ships made first of all to be fast.

    Small vessels go faster in light winds but can't handle big seas: a force 5 is uncomfortable in a 10' boat, force 8 is scary in a 30 footer and desperate in a 10 footer. I'd make stability inverse to displacement, perhaps, but retain capsizing as a crit risk for oversailed big vessels, alongside masts/sails breaking, the hull opening...

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  3. ...and now I'm getting embarrassed by this essay, but one more titbit: Spanish galleons, and ships of the 16th century generally, were at a weird point, where they were too big to row but too undersailed to handle all conditions. Exploring the Philippines they found currents they couldn't sail out of: they'd get thrown off course hundreds of miles. At a push they could do 4 or 5 knots. Local reed and dugout caracoas, however, propelled by hordes of oarsmen, could do up to 15 knots and were hardly affected by currents. Depending on your period and the state of sailing/cannon technology, war galleys either rule or can't support strong enough hulls and heavy enough firepower to compete with sailing ships.

    Goitein's A Mediterranean Society is really excellent for the parameters of 12th and 15th century trade. If you're interested I could dig out my notes. [link points to vol 2: ships are in vol. 1]

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  4. Thanks for the feedback.

    you can't go faster than the wind: as wind strength goes up, you reduce sail or face terrible risks.

    That's what I'm saying. Sail needs to be equal to or less than Wind, or the ship risks capsize, a broken mast, etc. As long as Wind is an abstract without a specific unit of measurement, it doesn't matter that the Velocity number is greater.

    For wind strength, I like the Beaufort Scale: a 1-12 range and easily recognized conditions

    Yes, that's great. Thanks. Given what I said above, I may use that, but drop the specific wind speeds.

    Small vessels go faster in light winds but can't handle big seas

    That's a good point, and one for which I haven't yet accounted.

    really excellent for the parameters of 12th and 15th century trade

    I'm not interested in accurately simulating any particular historical period, or even real-world sailing, as long as the rules suggest interesting play situations. The rules should certainly allow for conditions in where large vessels are underpowered.

    One of my buddies is a big Patrick O'Brien fan, and he's more interested in simulating tactical naval combat than am I. I just want travel to be slightly more interesting than taking a train from point A to B.

    Obviously, I'm still hashing this out. I now have quite a bit more detail in a rough draft, and I'll probably post a PDF in the next few days.

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  5. Have you seen the Pirates! board/miniatures game? I played it once maybe 10 years ago but I remember it being fast, fun, and challenging: best/worst feature being that you had to prepare your orders one or two turns in advance, to model the response lag of square riggers, and mayhem ensued.

    I go back and forth on simulation. On one hand, you want the environment to be distinctive and make sense to players. On the other, fun first! My own efforts have always tended to work out heavier than I would like. I have a quixotic urge to find the perfect lightweight mechanic.

    veriword: boati. You must be doing something right...

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  6. I like where you are going with this. One goal I was wanting to achieve when I was thinking about ships a few sessions ago was a simple way for players to choose a ship.

    "This ship holds more but is slower," that kind of stuff is essential to player decisions. And if you could boil it down to 2 or three numbers like you are here, that would work well in game.

    Also, deciding whether to throw cargo overboard is a dramatic decision and if the system allows for events where players would have to make that decision and it would make a difference in sailing that would be great.

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  7. Very intriguing suggestions here. Thanks for making me think. Delta is the expert in stats in the D&D world. Ask him to comment. He was kind enough to respond to me.

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