Boat Speed as a function of Wind Speed

[DaveC426913]

This came up in another thread.

I thought it wasn't unreasonable to be doing 3 knots in a 4 knot wind.

This diagram - which admittedly is not for Macs - nonetheless seems to suggest one can easily have a greater boat speed than the wind speed.



In a 3 knot wind, on any pont of sale from close reach to broad reach, this boat is doing between 3 and 5 knots.
In a 6 knot wind this boat can apparently hit 8 1/2 knots.

(Out of morbid curiosity, I checked what size boat this would need to be, based on hull speed calculation. For a max boat speed of 20kts, it would have to be 230ft long!)


Has anyone done one of these for our boats?

[DaveC426913]

Here's a database of polar diagrams for different boats.
https://jieter.github.io/orc-data/site/

Here's a 7 metre boat
https://jieter.github.io/orc-data/site/#RUS/AC1334RUS64

(It mentions a spinnaker as part of gear, but I don't know if that's factored into the chart.)

[Russ]

Interesting.

I've seen Ice Boats that do excess of 70mph in a 15mph wind. Some crazy stuffs. Hence the need for helmets

[NiceAft]

Hobie Cats are know to be able to go faster than the wind.

[Be Free]

This came up in another thread.

I thought it wasn't unreasonable to be doing 3 knots in a 4 knot wind.

This diagram - which admittedly is not for Macs - nonetheless seems to suggest one can easily have a greater boat speed than the wind speed.



In a 3 knot wind, on any pont of sale from close reach to broad reach, this boat is doing between 3 and 5 knots.
In a 6 knot wind this boat can apparently hit 8 1/2 knots.

(Out of morbid curiosity, I checked what size boat this would need to be, based on hull speed calculation. For a max boat speed of 20kts, it would have to be 230ft long!)


Has anyone done one of these for our boats?

It's not unreasonable to go 3 knots in a 4 knot wind. It all depends on the boat. A MacGregor can't do it though.

It is also not impossible for a boat to go faster than the wind. My racing catamaran could do it on certain points of sail. A MacGregor can't do it though.

The polar you posted appears to be for a Volvo Ocean 60. That's a 64 foot boat with an 85' mast. There were only 32 of them built (1993-2002) and a used one will set you back between $250,000 and $500,000 (or more). They were built for round-the-world racing and 28 actually competed. Under the right conditions it could hit 30 knots.

In its day, that boat was the sailing equivalent of an F1 race car. We would be a VW bus.

Good polars are hard to make and tend to be more "ideal" than "reality" in a lot of cases. Polars supplied by the manufacturer are almost invariably optimistic (if not outright fantasy). I've been working for several years to refine the polars for my boat. I've found them to be useful when planning long trips; less useful for short ones.



These work for my boat. If you have an X with a hank-on jib, stock main, keep your heel no more than 15 degrees (and reef accordingly), cruise heavily loaded and single-handed then your numbers might possibly be similar.

[Jimmyt]

Thanks for posting your polar points! Very interesting.

One other point. Theoretical hull speed is for displacement hulls. If a boat planes, that formula doesn’t apply (much less so for foiling).

[DaveC426913]

Thanks!
Thought I'd like to see that visualized.

(That's a heckuva sweet spot you got there with a 4 knot wind at 160 degrees. )

[DaveC426913]

Hey @Be Free, can I ask you about your axes?

You've labeled the Y-axis as True Wind Angle (TWA).

Your data indicates that your fastest speed at any Wind speed is 90 degrees.
But if that's TWA then your Apparent Wind Angle will be much farther forward than that. A 4.5 : 6 right triangle makes for an angle of 37 degrees forward of a beam reach.That indicates that your fastest point of sail is actually a close reach.

Is this right?

[Be Free]

Thanks!
Thought I'd like to see that visualized.

(That's a heckuva sweet spot you got there with a 4 knot wind at 160 degrees. )

Sorry about that. The 4 and 5 knot columns were hand typed. That's not the only one I missed a decimal place in but it is the only one I did not correct.

[Be Free]

Hey @Be Free, can I ask you about your axes?

You've labeled the Y-axis as True Wind Angle (TWA).

Your data indicates that your fastest speed at any Wind speed is 90 degrees.
But if that's TWA then your Apparent Wind Angle will be much farther forward than that. A 4.5 : 6 right triangle makes for an angle of 37 degrees forward of a beam reach.That indicates that your fastest point of sail is actually a close reach.

Is this right?

Yes, apparent wind will move forward as boat speed speed increases. I think I mentioned in another thread that my fastest upwind point of sail was around 40 degrees so that works out pretty close to your calculation. That is 40 degrees AWA as measured by eye with the Windex at the top of the mast. Without the proper instruments (which I lack) TWA is difficult to determine while underway.

My polars are primarily used for trip planning in conjunction with GRIB files to predict a course that will tend to maximize VMG toward my destination. All too often they just end up being a reality check that states, "You can't get there from here" (at least not in the time frame I was aiming at). Absent very favorable winds I can seldom plan on much more than 40 miles a day.

[Be Free]

Also, these polars assume a clean bottom (I need new paint), perfect sail trim (which I can accomplish occasionally), good sails (which I don't really have), and tweaking of the amount of keel in the water (which I don't usually care enough to do unless it's going to be a very long leg).

If all you want is speed and you don't care where you end up then you can use these numbers as a guide. If you want to get somewhere in particular as quickly as possible they you will have to do a bit more math (using these numbers as a starting point) in order to determine the best course to set. The fastest speed through the water is not often the fastest way to get to a particular destination.

[DaveC426913]

Yes, apparent wind will move forward as boat speed speed increases. I think I mentioned in another thread that my fastest upwind point of sail was around 40 degrees

Interesting to know. I was always taught that beam reach was the fastest PoS.

That is 40 degrees AWA as measured by eye with the Windex at the top of the mast.

Indeed. Especially with a teardrop shaped mast. I m considering putting up a crossbar under the wind vein that tells me where true beam reach is.

Without the proper instruments (which I lack) TWA is difficult to determine while underway.

I have a device for that.

I built an analog "computer" (it looks like the love child of a protractor and a ruler). You simply set your boat speed and your AWA and it will indicate TWS and TWA. It works both ways. If you know your AWA and TWA, it will tell you your boat speed.

I would love to send you one of my prototypes and see it if it helps.

Absent very favorable winds I can seldom plan on much more than 40 miles a day.

I just moved my boat back from summer harbour to home harbour. That's 24nm and it took me 6 hours (the last 1.5 hour of which was motoring). That's a big trip for me that I only do twice a year. Been a few years since I've done anything overnight. Admiral's lost interest.

[DaveC426913]

I just found a cool feature in Excel called home > Conditional Formatting > Color Scales.

It does automatically what I was doing manually. You pick a colour for minimum, and colour for mid and a colour for max and it sets every cell colour according to its value:


(I don't know why the image upload feature is breaking suddenly. It worked earlier.)

Anyway, I've attached a sample excel sheet with a few values.


Let me tell you why I'm doing this. With your permission, I would like to print out your data sheet, laminate it and keep it on my boat.
Your is by far the most comprehensive data sheet I've seen for a Mac, and it would be very helpful for me to reference.

[Be Free]

All are welcome to the numbers. I hope they are useful but unless you are on my boat I can't vouch for them being applicable.



For those who prefer a traditional polar plot.

[Be Free]

Once correction and one clarification: the data I posted does allow heeling up to (sometimes a bit over) 20 degrees; not 15.

I put in my first reef at 15 knots on most reaches and then the second reef at 20 knots. I've only had to go up on the cabin top a couple of times to put in a second reef. It's scary, dangerous, and not recommended; doubly so in the dark. Try really hard not to have to reef or strike the main after dark.

Technically, it's usually done in reverse. I'll start out with a second reef if the weather is in the low 20's and expected to improve then shake them out as it does. If it's in the 20's and going to get worse I'm probably not going to raise any sails (or better yet, stay in port).