# Boat Speed?



## Calyb (Feb 17, 2014)

Hi.
I have a character who sets off in a medieval-style ship from her native land and reaches her destination in 8 days time. Assuming the voyage is smooth and straight, how far realistically could she have travelled?
I honestly have no idea! 
Thanks in advance.


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## buyjupiter (Feb 17, 2014)

It depends on several factors:

-boat size
-how loaded down the boat is
-prow design
-current/wave conditions
-what the boat is made out of

And that's off the top of my head. Sailing buffs will have more accurate time span/realistic distances knowledge than I would.


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## Calyb (Feb 17, 2014)

It's a pretty large boat; a merchant vessel. I would expect it to be loaded down, since there is high demand for goods and produce from this island. Considering its owner I guess it would be made of oak wood. The sea is pretty calm throughout the whole voyage.
You have me stumped at prow though. I just imagined a big, wooden boat!


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## buyjupiter (Feb 17, 2014)

Well if you have a flatter barge-like design that will sail differently than a Viking ship. It really depends on what you mean by boat. Oh, I should also mention that most people's idea of a wooden boat as in a man of war wasn't around until way after the middle ages.


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## Calyb (Feb 17, 2014)

Ah, right. While my world is roughly on par technologically with medieval Britain I obviously do have some artistic license. I'll look into the typical ship designs of that era anyway. Thanks.


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## ThinkerX (Feb 17, 2014)

As to the 'prow' ('bow' or 'front') of the boat...a merchant ship is likely to have a fairly round prow, which means slower sailing speeds.  A raider or combat type ship (man o war) is likely to have a pointer prow, cutting through the water more easily, hence a faster speed.

Favorable and unfavorable winds also play a role.  If the vessel has to sail against the wind - especially a wide bottomed merchant type vessel, they'll have to 'tack', meaning taking a zig-zag  course which can really add to the travel time without covering much straight line distance.  In the real world, for example, rounding Cape Horn - a distance of just a few hundred miles - often took weeks or months because vessels often had to fight contrary winds and storms.  

To find the best winds and currents, maritime courses often took vessels on great looping tracks.  For example, ships rounding Africa (going east) often sailed so far into the west in the Atlantic they sighted (and called at) South America.  Likewise, when the wind died in certain area's ships could drift (making zero progress) for days or weeks at a time. 

But...and this is real crude...assuming a decent merchant type ship with a competent crew, reasonable sailing conditions (no tacking or great looping detours)...maybe a speed of 4-5 mph, or something on the order of 100 miles per day.  A ship built for speed - say a messenger cutter, pirate, or fast man o war - might manage triple that.  So...*MAYBE* 800 miles.


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## skip.knox (Feb 17, 2014)

Your question is a bit like asking how fast is wind. The answer is: sometimes fast and sometimes slow. 

Much also depends on how realistic you care to be. A real medieval merchant vessel is going to put in to every port along the way, to do a little trading there. Most vessels would anchor for the night. Sailing across open water is going to be different from sailing along the coast, not so much because of winds as because of how many stops can be made.

All that said, most medieval craft could make only a few knots even in a good wind. Let's keep the math easy and posit five knots, which is about six miles per hour. We'll assume sailing night and day with no stops. That's not very realistic because it means having fresh water on board enough for eight days, which is feasible but rather a lot. Still, we're looking to establish high end here, so 24 times 6 is 144, times 8 gives 1152 miles over eight days. 

That's too high. The wind isn't going to blow a steady five knots for eight days.

A more realistic estimate is that the pilgrim voyage from Venice to Jaffa was around three weeks, if the sailing was good. Venetian galleys took more like four weeks. I think that's about 3000 miles or so. (Google maps is crap for distances by water) So, a quarter of that figure is around 800 miles, or a hundred miles a day. I still think that's high, so I wouldn't go over it, not until you have 17thc ships.

With that as a ceiling, you can always pick a smaller distance and blame storms, the needs of merchants, or the designs of evil wizards.


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## ThinkerX (Feb 17, 2014)

Skips numbers seem to be about on a par with mine...though I was figuring deep ocean voyaging without much in the way of stops.  Either way, an eight day trip would take one something on the order of a third of the way across the Atlantic ocean.

OP might want to give some thought to the worlds geography.  Is the island coastal (aka fairly close to a continental landmass)?  Part of a mid ocean island chain?  Or something else?

Just noticed something with Skip's numbers - 3000 mile trip over a three week period means that 8 days sail (just over one week) is a third of that distance, not a quarter, which means 1000 miles a week or around 140 miles a day.  But again, like he said, that's probably high.  Though a 3000 mile distance from Venice to Jaffa also sounds high.


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## Malik (Feb 17, 2014)

If it's a medieval cog? It could take it 8 days to get across the harbor. It depends on the wind. 

A knarr or longship, which was designed to be rowed at hull speed fully crewed and laden, could make hull speed under oar and about the same in decent wind. 

Hull speed -- the maximum speed at which the boat can travel before it has to climb over its own bow wave -- can be roughly determined by taking the square root of the waterline length, in feet, and multiplying by 1.34. The answer is in knots. 

A 64-foot longship or knarr (REALLY big boat, but the math is easy) would make (1.34 X 8) . . . almost 11 knots. A knot is 1.15 mph, so maybe 12 mph? 150 miles a day in good weather; twice that if crewed 'round the clock.

11 knots, by the way, is hauling ass in a sailboat. That's closing in on old-fashioned Trans-Pacific racing sloops, the fastest averaged around 15 knots. 5-6 knots is reasonable.

A boat that is grossly overpowered and has a flat bottom can plane -- like an inner tube behind a motorboat, or a Zodiac-style raft -- and by planing, climb over its bow wave and exceed its hull speed. Most ancient boats can't; even if you had someone invent a semi-dory type hull and somehow apply that much power to it, the construction methods would be so unreliable that it would be a very short trip. The stresses on the hull are unbelievable.

A classic or traditionally-styled boat can exceed its theoretical hull speed but the power required to do so is logarithmic.


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## ThinkerX (Feb 17, 2014)

Hmmm...again Malik's figures seem roughly on a par with mine.  Typical wallowing merchant scow or cog...maybe 100 miles a day, ship built for speed (knar or cutter) maybe three times that...assuming both are crewed and sailing around the clock.


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## Graylorne (Feb 18, 2014)

I picked up this once, somewhere...

A medieval sailing ship would rarely if ever exceed its hull speed. Hull speed in knots is approximately 1.34 times the square root of the vessel's waterline length in feet. 
A vessel with a waterline length of 15 meter would have a hull speed of 9.34 knots 
A vessel with a waterline length of 30 meter would have a hull speed of 13.5 knots. 
Most of the time they would go more slowly than this, maybe 5-8 knots in "normal" conditions. 
In given conditions some ships would go faster than others depending on a number of factors including hull shape, rig, fouling, windage etc. 

(Fouling means the growth of weeds and barnacles on the hull. These greatly reduced the speed of the ship.)


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## Calyb (Feb 18, 2014)

Thanks to everyone for their advice! I've taken it all into account. In terms of distance, the estimates are a little under what I wanted, so I'll likely have to make slight changes in the duration of the journey and the geography of the world, but nothing major.


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## skip.knox (Feb 18, 2014)

There seems to be some expertise here. I suggest (but am unwilling to initiate!) an "Ask Me About Ships" thread. Should cover anything from steamships to coracles. Boats do tend to get used, however peripherally, in many fantasy stories. 

The one that always gets me is ferries. A practical question: given decent weather, how long would it take to ferry a Roman legion across the lower Danube? And how, exactly, would that ferry work?

This is close to thread hijacking, so if someone really can answer that, maybe repost and start a new thread.


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## buyjupiter (Feb 19, 2014)

skip.knox said:


> There seems to be some expertise here. I suggest (but am unwilling to initiate!) an "Ask Me About Ships" thread. Should cover anything from steamships to coracles. Boats do tend to get used, however peripherally, in many fantasy stories.
> 
> The one that always gets me is ferries. A practical question: given decent weather, how long would it take to ferry a Roman legion across the lower Danube? And how, exactly, would that ferry work?
> 
> This is close to thread hijacking, so if someone really can answer that, maybe repost and start a new thread.



A quick search pulls up this boat, which was powered by oar. I have no idea how long it would take to get a legion across, but it would take a while. I have no intention on starting another thread about Roman boats, because I don't know all that much about it and so much of my knowledge is mixed in with Stephen Saylor's novels.


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## skip.knox (Feb 20, 2014)

Good one, buyjupiter! Those were river patrol boats, not really intended to ferry, but I had no idea the archeologists had reconstructed one. Kewl.

Over here in Idaho, I've seen examples of ferries across the Snake River. 19thc. These consisted of rope and pulley or capstan, and they pulled the ferry across. But I don't think that would be feasible across the Danube. Or the Rhine, for that matter. And might not be feasible in flood season either.

In Bamberg, Germany I saw a small ferry that used a clever technique. They anchored the ferry to a tree or maybe a berm on one bank well upstream, maybe forty yards. The little boat--it could hold a dozen people or so--had big wheel in the middle and (presumably) a large rudder beneath the boat. Simply by positioning the rudder, the current forced the boat across to the other side. The anchored rope kept it from going downstream. To go back, just repeat the process. Very efficient, but again not much help in getting across the Danube.

Other ferries used poles, but that works only in shallow crossings. One could simply float across, of course, like that little Bamberger boat, but without an anchor point, it would drift well downstream. Then it would have to be pulled not merely back but actually upstream in order to cross back over and land at the original starting point. This is how I picture it being done, but it'd be awfully time consuming. Moreover, the banks on the lower Danube could be awfully marshy, which would make a tow-path near impossible.  No wonder Trajan built a bridge!


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## ThinkerX (Feb 20, 2014)

Since the thread is derailed anyhow...



> The one that always gets me is ferries. A practical question: given decent weather, how long would it take to ferry a Roman legion across the lower Danube? And how, exactly, would that ferry work?



Obvious answer would involve knowing how wide the river is, the capacity of the ferry, and how long to make a round trip.

But...

Romans being noted as excellent engineers, another approach presents itself:

Round up a bunch of smaller boats.

Solidly anchor these smaller craft at regular intervals in the stream.

Knock down some long trees, and lay them across the boats, in essence making a pontoon bridge.

It probably wouldn't last more than a week or ten days, but it might be quicker than trying to ferry 5000+ troops AND their gear across a river on a ferry with a capacity of 200 or so.


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## skip.knox (Feb 20, 2014)

@ThinkerX: your real name wouldn't be Gaius Julius Caesar, would it?


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## ThinkerX (Feb 20, 2014)

> ThinkerX: your real name wouldn't be Gaius Julius Caesar, would it?



Is that what he did?

But to me, given a force of five thousand men who treated building a fort each night after a twenty mile hike in full kit as routine, the pontoon bridge option is sort of glaringly obvious.


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## psychotick (Feb 21, 2014)

Hi,

To get back to the OP, if the boat is a sailing ship of some sort, the most critical factor in determining how far it will travel will be the wind, followed potentially by ocean currents. And there are plenty of prevailing winds in different geographic locations. There are also places where there are few winds - eg the doldroms near the equator. 

I'd work with the five knot guestimate and assume that you get that with a prevailing wind of say ten knots following. If the wind's a little faster than that you might go faster, but if it's too much faster you'll actually lose speed because the ship simply won'tbe able to use the wind properly, may rip out sails and may have to take shelter. If there's no wind your ship won't go anywhere. To add to your problems, depending on the design of your ship (proper triangular sail set up and assuming it has a keel) you can go faster if the wind is coming from your side. It's just geometry in action,but modern yachts can outsail the wind by about 1.5 times as I recall. In a headwind you can also make reasonable forward speed but you have to travel at an angle to the wind which means that while you may make five knots, your journey becomes much longer as you end up tacking port and starboard. I don't know how well older styled square sailed ships would manage this, but my guess is poorly. In order to make any headway against a head wind the angle of their tacks would have to be huge, meaning the journey of say a thousand knots might have to actually be three or four thousand in actual distance travelled.

And then just to make things interesting, rough seas will always slow things down while storms will mean that ships have to shelter in harbours until they blow over.

Cheers, Greg.


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## Kahle (Feb 21, 2014)

Just three things here, going with ship navigation-

1.Navigation out of sight of land was a huge breakthrough for sailors. I do believe this was accomplished between 1100-1300, and I believe it was the Arab traders who learned to follow the stars. Until that point however, most ships were confined to landmark navigation on shorelines. Hence why raiders would stick to a specific coastline, and why it was so easy to get lost at sea. One good storm and half a fishing fleet is washed out of sight with nothing to navigate by.

2.Deeper the seas and the bigger the ship, the longer the keel (long, narrow beam or crease at the base of the ship and morphs into the prow) is going to be. A shallow keel means its easier to bring up rivers, close to shore, dock, etc. Longer keels were restricted to deep water, and some ships had to be mored in those deeper waters and use smaller ships to resupply. Otherwise, you had to beach the whole thing.

3.This is more of a fun fact. On a sphere like earth-granted this requires knowledge that the earth is a sphere-by sailing in an arc away from the equator from point A to point B the distance is shorter than sailing in a straight line parallel to the equator. So sailing from New England to the UK, you'd want to sail North in an arc towards Greenland and then come back South closer to the destination. Not sure how the math works on this, but it works. (again, people have to accept that the world is round and have a trough understanding of geometry, I think Europe hit upon this finally in the late Renaissance)


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## skip.knox (Feb 21, 2014)

WRT 3, medieval people knew the world was round. At least the ones who were in a position to need to care. The flat earth thing is a post-medieval invention.

WRT 1, it reminded me of _Seebuecher_ (sea books). These were instruction guides for navigators in the North Sea and the Baltic. They weren't maps or charts, they were more like narratives. They gave detailed descriptions of landmarks of all sorts, including islands and soundings. It was rather like instead of a map you told your friend: from your house turn right and drive until you see the gas station with the green sign, then turn right. Etc.

Sea books were prized possessions, closely guarded, much like the portolani used by sailors in the Mediterranean. It's one thing to say to sail along the coast, but it's quite another to do so *without crashing*. Captains and navigators had the super-secret knowledge of how to do this. 

Sailing out of sight of land could be done, though. Sailors crossed the eastern Mediterranean regularly, sailing for about three days on open sea, if the winds were right. That's a far cry from sailing the Atlantic, of course, but even that was done by the Norsemen. Scary stuff.


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## wordwalker (Feb 22, 2014)

skip.knox said:


> WRT 3, medieval people knew the world was round. At least the ones who were in a position to need to care. The flat earth thing is a post-medieval invention.



Quite right. The idea that Columbus was the only one to see the earth as round was a myth Washington Irving added to history. What really happened shows a bit of what sailing like this can involve:

To the rest professional and educated world, the planet was simply too big for anyone to sail from Europe to Asia without starving on the way. Columbus thought it was smaller-- because he'd made an error in his math.

And of course the only way to find out there were continents there was to *be* that wrong, or otherwise be fool enough to make what seemed like a suicide mission. (That or to be a Viking starting all the way out in Greenland, but...)


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