Boat Speed as a Function of Wind Speed

[Baddox]

Apropos to nothing in particular but after sailing our 30 footer for a few years now it occurred to me that I can usually achieve a speed through the water of about half the apparent wind speed. Sea state can affect the numbers and wind angle can add or take away a little from the ratio but it’s proved a reliable target. Once we get above about 6 to 6.5kn through the water, the ratio starts to tail off and boat speed maxes out at about 7.8kn.

Her sails are past their prime and the bottom needs a scrub, but I’m happy with the numbers and it gives an easier target than using a set of polars.

 

[William_H]

Boat speed as related to wind speed is very variable but of course depends on point of sailing. Now when running you might get a regular relationship of boat speed to wind speed. Allowing that the faster you go the less apparent wind you get so less power to push boat.
But of course this assumes boat speed is well below hull speed. Hull speed is sometimes defined as square root of water line length in feet.(and some percentage above that like 15%) In fact that is the speed when water resistance starts to restrict the increase in speed you might hope with more power. It can be exceeded but the more you exceed that speed the more difficult it is to increase speed. (unless the boat starts to plane)
So OP theory is only applicable in low wind speeds and low hull speeds.
Now when reaching (90 degrees wind to boat) You can get more speed as you can get more power from the sails without the loss of apparent wind. But still you get this apparent barrier to higher speeds from hull speed limitations. Some specialised catamaran or planing boats can get hull speeds like double wind speed.
Now when beating to windward things are different again. You can probably get a lot of speed up to hull speed with enough wind. But beating is a question of speed to target (VMG velocity made good). Bearing away from the wind to get more speed will increase time and distance to target. So in fact you are better off accepting lower speed and closer to wind. Especially because of this hull speed effect. So here especially your water speed measurement becomes very usefull. You should aim for a water speed of around 5.5kts or so to windward which will ensure that the sacrifice of hull speed results in closer to wind. That closeness to wind shouild not sacrificed by trying to get more boat speed because more boat speed is harder to get.
Racing your boat next to another similar boat going to windward can be very interesting as you can look at pointing versus speed very easily. Of course all this theory depends on smooth water. I would never sail with a dirty bottom. But then I usually scrub in water before each race and once per week. (warm water and crazy fouling) ol'will

 

[Roberto]

A good start Have a look at what people manage to do, there was 12-15kt true wind

 

[Refueler]

Whats that magic word ???

Trim Trim Trim !!

My Conqubin 38CR (CR for Cruiser racer) achieved 6.4kts average with wind 35 - 40 deg on stbd bow .. F3 .... under #1 genoa alone. Looking forward to really letting her loose on the 'circuit' !

 

[johnalison]

It is very hard to judge boat speed and efficiency when under sail unless one has instruments or other boats to measure against. Many sailors will happily crawl along with hopelessly-trimmed sails, content that the hull is making the right kind of noises and the boat is heeling in a satisfying way. I am a bit of a fiddler with the sails, but not to a racing level, and it can make quite a difference. On my current boat on a reach it can seem as if we are doing OK as the wind increases but the boat will start to weave a bit and just tensioning the clew outhaul will make the course steady again and add half a knot that may not be noticeable to someone not actually at the helm.

VMG as mentioned is critical of course. I usually refer to VMG to wind rather than to waypoint, though my instruments give both. There is no magic formula for this and a lot depends on sea state. I usually sail to a consistent speed through the water, ideally 5.9 kn, and this can give me a VMG of about 4.5 kn. A VMG meter is very useful as a training tool but I find requires far too much concentration to be useful when cruising.

 

[ProDave]

I find boat speed often slows with wind speed.

As the wind comes on you get sailing nicely in a nice flat sea, easily achieving hull speed. but as the wind increases and the sea becomes lumpy, each time a big wave hits you the boat slows, then slowly speeds up before being slowed by the next wave. It becomes hard to maintain as fast a speed as you can easily in a calm sea.

 

[lustyd]

boat speed maxes out at about 7.8kn

Impressive that this is above your max hull speed with that waterline length. Is it a motor boat?

 

[B27]

The boats which overtake us cost on average 6.3 times as much.

That's more reliable than most numbers people quote about their boat's performance.

 

[flaming]

I find boat speed often slows with wind speed.

As the wind comes on you get sailing nicely in a nice flat sea, easily achieving hull speed. but as the wind increases and the sea becomes lumpy, each time a big wave hits you the boat slows, then slowly speeds up before being slowed by the next wave. It becomes hard to maintain as fast a speed as you can easily in a calm sea.

It's all about modes.

Assuming we're talking about upwind sailing, then you have 2 modes, a power mode and a height mode. The key to good performance on the race track is knowing how to access those modes in your boat, and then knowing when to use each one.

In simple terms power mode is generally a more twisted, more forgiving, setting in both sails that allows for easy flow attachment across the sails, which gives good acceleration. Absent of any tactical considerations we generally use this is 2 scenarios, very light winds and when there is significant waves or chop.

Height mode is with much more leech tension, especially in the main. This allows the boat to point higher, but the flow is more "fragile" and it's easy to stall the top of the sail, which then drastically slows you. In a cruising context this is not an autohelm mode... You need to actively drive the boat in this mode, and also be prepared to ease the mainsheet an inch or 2 and get the bow down to accelerate again when you inevitably over cook it a bit.

For context, my height mode in 12 knots true has targets of 7.3kts at 41 degrees. My power mode in flat water I'd expect to see 7.5 at 46. But if I'm not getting 7.3 at 41 it's an indication that the chop is not letting me, so we start easing down to 43-45 and looking for the speed. Targets at higher wind speeds actually start reducing, then at 18 knots I'm looking for 7.1 at 43.

So from your cruising context example, add a bit of twist to both sails, don't look for that last bit of height and allow the speed to build. Only once you're up to your normal speed start coming up for height.

 

[Buck Turgidson]

I actually use this as a minimum target below 12kt true wind. I should always make at least half the wind speed on all points unless really pinching. If I'm not then something isn't right. Once boat speed is at or above 6kt all bets are off as my un-heeled LWL is only 21.5ft. It lengthens considerably with heel because Kim Holman knew how to twist the racing rules!