Speed of a sailboat in 20 MPH winds

This may be a dumb question, but maybe not. In any case, it’s fantasy, so bear that in mind.

If a sailboat is on relatively calm waters with winds blowing at 20 mph, how fast will it move at top speed?

Specifically, if, say, you’re running a game of Dungeons and Dragons, and the players have a decent 70’ merchant vessel with a middling crew, and they just gained a magic item that lets them affect wind speed locally, granting them a wind from any direction of up to 20 mph, for an hour at a time, what would a canny crew be able to do with this item to maximize their travel speed?

Impossible to give exact numbers as this will vary due to local conditions - even barnacles on the hull will have an effect, but as a quick explanation:

Max speed for most monohulls is determined by length. There’s a good chart here:

https://improvesailing.com/questions/average-sailboat-speed

Wind speed and vectors all come into play here, so it’s not a simple answer.

If you are able to control the direction of the wind, then you could get up to whatever speed your boat will allow.

You get the least speed if you are just running straight downwind. In that case, you’d top out at the wind speed.

But as you angle your sail against the wind, and use your keel to track, you can get more speed. If you have little friction you can get to many multiples of the wind speed. Look at Ice Yachting for examples of that.

So, it would be limited, as @FinsToTheLeft by the length of the hull, unless you get up to speeds that allow hydroplaning, which I would not recommend for a crew of middling ability.

Hmm…so it looks like a ~70ft hull could go about 11 knots under ideal conditions, and having an ideally-angled 20 mph wind & otherwise calm seas would be ideal. 11 knots is about 12.6 miles per hour. Pretty good speed!

It also makes me think it’d be useful in ship-to-ship combat: if you can control your local wind direction and speed, you should be able to outmaneuver your enemy, by pulling tricks they can’t pull. But that might take an expert crew to pull off…

Only if there’s zero drag from the hull moving through the water. Absent that piece of magic, I’d guess a 70-ft monohull running before a 20-mph wind might hope for 8 mph.

And this would not be the slowest point of sail - you’d certainly do less when close-hauled.

I’m thinking maybe the crew’s job would be minimal. If you can tune the wind at will, you can leave the sails alone (probably trimmed for a broad reach) while dashing around in any direction you choose

But less fun! Running close hauled with a good amount of heel, enough to get you out on the trapeze was always my happy place.

Also depends on what you mean by local.

Lotta fun can be had if you can control the wind in their sails.

True, I was talking optimal conditions, which would be neglecting any friction. Just that, intuitively, running with the wind should be the fastest, but it’s not. You can do better than windspeed, assuming no friction.

Well, yeah, but if you can control the wind, you are not going to be going into it. (Hopefully)

What is the level of tech of the boat? A 1500s galleon, an 1800s clipper, a mid 1900s J-boat and a 2000s race boat have very different performance. @FinsToTheLeft gave a fine reference upthread for modern tech recreational and racing boats.

You’d expect an oceangoing merchantman to be compromised heavily for seakeeping and for cargo capacity. Yes, speed is good in a merchantman, but not if you can’t carry a profitable amount of cargo or survive the open ocean in an era before weather forecasts & reports. There was no FedEx equivalent back in the Age of Sail.

It occurs to me the OP in effect asked the wrong question. He really wants to know “Is 20 extra knots of wind a lot of wind or a smidgen of wind? Is it tactically decisive or a needle in the haystack?”

In much of the world 20 knots is a lot more than ambient wind most of the time. And is enough to drive any competently sailed boat/ship to its hull speed. Said another way, in much of the ocean, 20 knots, and even better, 20 knots conveniently oriented, represent an overwhelming advantage over the nearby opposition. But an hour is not very long in Age of Sail combat. It could take a day to close the distance from first sighting to cannon range.

There certainly are parts of the open ocean and near-shore coastal waters where the ambient wind is usually high enough that 20 more knots is a hindrance, not a help.

An interesting question from a game-play perspective is whether a player can cast the wind spell defensively? i.e. team A is being pursued by Team B. Team B is gaining on Team A. Team A casts a 20 knot adverse wind onto B’s boat, slowing it dramatically. A escapes successfully while B is becalmed or forced to tack and eventually drops out of range.

It’s actually possible to make a vehicle that’ll travel downwind at faster than the wind speed, though it’s mechanically complicated: You need something coupled to the surface you’re on, and something coupled to the air, and the appropriate gearing in between them. Basically, you’d have a screw in the water, that’s driven by your motion through the water, and then that drives a fan in the air to blow backwards against the wind.

Yeah, that’s pretty much the info I needed. I didn’t really realize that hull speeds existed. I’m thinking that this device allows wind within about 50’ of it to be controlled, direction, and speed up to 20 mph total (so, you can calm a hurricane, but you can’t turn a gale into a hurricane). It’s not useful for offense, except inasmuch as it allows you to outmaneuver foes.

And I think that fancy maneuvers would require an expert crew, because fancy maneuvers would involve sudden changes of wind direction and speed in order to execute hairpin turns and such that would otherwise be impossible on the high seas. I don’t really know what would be possible with perfect control over wind direction and with a crew ready to take in/let out sails in order to take advantage of that control. Any ideas?

If you have total control of the wind, you just set the boat up on its fastest angle (for a merchant vessel this will be broad reach) and steer the boat using your control of the wind.
Your typical sailing merchant vessel is never going to be making fast manoeuvres. The hull is built with a full length keel, and that makes turning the boat very slow. Modern sailing boats have very different underwater design, making for a very different capability. But that is highly dependant upon modern materials.
State of the art round the world ocean racing yachts are quite something:

(Oh, and they sail solo.)
State of the art 70 foot monohull?

Not exactly a merchant vessel. Probably capable of close to 50 knots on 20 knots of wind.

It’s actually been done (on land): https://www.lockhaven.edu/~dsimanek/museum/ddwfttw.htm

Yeah, that page was where I learned it was possible. It could in principle be done on water, too, though I don’t know if it ever has been.

There are also toys with no moving parts (or rather, the entire toy is one single moving part) that will sit on the surface of a body of water and always move straight upwind. It looks like a broomstick with a buoy in the middle and a propeller on each end.

I think it would require hydrofoils rather than a hull in contact with the water.

I made a little Lego model that demonstrates how it is possible for a vehicle to move faster than the motion of the force propelling it (I think because there was a previous thread ages ago where someone was flatly declaring this impossible); with a single additional gear in this model, it can be made to come toward you when you push it away.

Very interesting. Do you have a link, or a phrase to Google?

There’s a diagram of one on this page: https://www.lockhaven.edu/~dsimanek/museum/ddwfttw.htm (titled ‘The push-me pull-you boat’)

20 miles per hour is about 17 knots of wind. A nice amount of wind for fun and efficient sailing (25-30 knots of wind would get scary for an inexperienced crew). Don’t forget about currents though! Currents are like moving sidewalks so if you are running against it then it doesn’t matter how fast you run, sometimes you aren’t going anywhere. On the other hand, running with the current and you are laughing.