Reading: Sailing optimization for Macs

[DaveC426913]

Just to be clear: the wind was five knots, you are going into at at 4 knots over ground (assuming gps measured) and then tacked through it never falling below 3 knots?

That is what I'm saying.
Though you are making me doubt myself.

The only thing I am not 100% certain of is the true wind speed because I have to estimate. I hove-to a few times and the apparent wind dropped to a gentle breeze, so yeah, I'd say it wasn't more than 5 knots. There is absolutely no way it was anywhere close to 10 since at 10 we get whitecaps and, with full main on, SeaSaw would round up. I only had 20 to 25 degrees of heel and it was very stable.

[Be Free]

Just to be clear: the wind was five knots, you are going into at at 4 knots over ground (assuming gps measured) and then tacked through it never falling below 3 knots?

That is what I'm saying.
Though you are making me doubt myself.

The only thing I am not 100% certain of is the true wind speed because I have to estimate. I hove-to a few times and the apparent wind dropped to a gentle breeze, so yeah, I'd say it wasn't more than 5 knots. There is absolutely no way it was anywhere close to 10 since at 10 we get whitecaps and, with full main on, SeaSaw would round up. I only had 20 to 25 degrees of heel and it was very stable.

Yeah, something is not adding up. You might get close to those speeds on a beam reach around 8 knots ( or a little more) but you should not be heeling more than five degrees or so under those conditions. If you are heeling that much and rounding up at 10 knots there is something odd going on.

[DaveC426913]

Yeah, something is not adding up. You might get close to those speeds on a beam reach around 8 knots ( or a little more) but you should not be heeling more than five degrees or so under those conditions. If you are heeling that much and rounding up at 10 knots there is something odd going on.

Yeah. Thinking back, I remember tacking very easily - nothing like with my genny. If I were heeling at 20 - 25 degrees, I'd have to be sitting sideways and switching my seating with every tack. It must have been more like 15 degrees, at which a tack can be done without moving from the helm.

Still, I'm certain about the headings and the speeds. And pretty certain about the true wind.
I took notes of all of it through my paces. (I was thinking about making a polar chart)

Speeds: GPS
Headings: compass
Sail plan: Full main, full 100% (brand spankin' new) jib

Test 1:

True wind speed (est): 5 knots from East
Port tack (close reach up to close haul):
4.0kn @ 170°
3.4kn @ 160°
2.8kn @ 150°
1.6kn @ 145°
Stb tack (close haul):
2.8kn @ 45°
3.4kn @ 40°
True Wind Angle (derived): 100°
(Two PoS, both at 3.4kn: 160° and 40°. Thus, TWA is 100°.)
Tack arc: 105°

Test 2

True wind speed (est): 5 knots from East
True Wind Angle (derived): 100°
Stb tack (close haul):
4.0kn @ 45°
3.3kn @ 55°
2.8kn @ 60°

Port tack (close haul):
3.4kn @ 135°
3.5kn @ 130°
2.6kn @ 120°
True Wind Angle (derived): 90°
This has got to be two different tests. If these were part of the same test, it would make my tack arc (120°-60°=) 60.


(These aren't the data that supports my "90°tack at 4.0kn" run. Too bad I didn't write that one down.)

[Be Free]

Don't do it on my account. Those revised numbers are at least within shouting distance of what I'd expect to see.

[DaveC426913]

Don't do it on my account. Those revised numbers are at least within shouting distance of what I'd expect to see.

Too late!

[DaveC426913]

This is another thing I'm constantly trying to come up with in my head while single-handing:


I mean, not trying to solve the trig or anything, just the basics like:
At a given speed for a given TWA, what is the TWS?

For simple solutions, like 4.0 knots speed, on a beam reach, if I know the TWA (by tacking).

It would be awesome to have a little handbook or notepad with the basic points-of sail and typical speeds.

[DaveC426913]

This messed me up yesterday when I was doing more data collection. I was on a beam reach at 300 degrees, port tack, and I figured that a tack through 180 degrees should bring me around to a starboard tack beam reach at 120 degrees (300-180). But I ended up at 90 degrees. That's a heading change of ... greater than 180 degrees.

Er. Is that right? The TWA moves forward with boat motion. So a tack from Beam Reach to Beam Reach should have an angle of...
Oof. Larger - or smaller - than 180? My poor brain...

Diagram time...

(I have simplified the diagram by flipping my real-life wind direction (which was zero degrees) to 180 degrees, so that the wind is coming from the top, as is convention. It was super confusing otherwise.)





OK, yes. Beam reach to Beam Reach is a 240°heading change.

So, my real life heading-300 port beam reach should have tacked around to a heading-120 starboard beam reach.

I got 90, which is a 210 degree heading change - which means I guess I was about 15 degrees nearer to a close haul on both tacks than I thought.

Wow. This is so non-intuitive, I had to correct this post at least 8 times! My brain needs a nap!

[Jimmyt]

You didn't mess up if you figured it out before you completed the project.

I agree with Bill. If you're rounding up in 10 knots, you need to check your mast rake, and see if you can find a reason.

You were more into gybe territory than tacking, since both headings were essentially down wind in 8 kn. A gybe would have been much more efficient than a 240 degree tack. But maybe I'm missing your point. I do understand that you are trying to get data for Polars or similar performance info. A gybe in 8 knots is a walk in the park.

Are you using your windex to get the apparent wind angle? ...and true wind angle? If so, I would have taken my data point at the first heading, then turned downwind (as detected by windex), noted the data heading and downwind heading difference; then gybed to the same offset from downwind on the other tack. Then, looked at the windex to see if it was in the expected spot.

In any event, looks like you're getting into the weeds with it. Can't wait to see the results of your efforts!

[DaveC426913]

In any event, looks like you're getting into the weeds with it. Can't wait to see the results of your efforts!

Here it is:



True Wind Speed: maybe 9 knots steady with a swelling to maybe 10 as the hour wore on.
True Wind Direction: due south, between 175 and 195.

Red data is real data. Pale pink data is a mirror reflection of real data to get a better curve, under the assumption that speeds will be the same on both tacks.


Note on the broad margins of error: collecting data really should not be done single-handed. Trimming the sails quickly and precisely for a given point-of-sail is critical to getting accurate data.


That being said, clearly, there are some bigger problems with my chart. The most obvious being that I can apparently sail within up to within 35 degrees of-the-wind. That's impossible, especially on a Mac.

My first assumption for error is that I poorly derived TWA. I tried to verify the angle of tack: the four faint red lines connect the two headings immediately before and after tacking. The fact that they are all almost exactly perpendicular to head-to-wind tells me that I derived TWA from my tack data accurately. So, mystery remains.

I notice that there is no specific tack that was actually less than 90.
In fact, my five tacks were 120°, 110°, 135, 210°, 105°.

I can post my data if anyone is excited by data analysis.

[Jimmyt]

Very nice! And very interesting. I applaud your efforts to do this solo.

I'd love to see your data. Please post it.

Compared to J30 data (25 ft lwl, PHRF around 150 compared to Mac 26M/X of 250-270 (ish)):





At first glance, your information passes the sniff test. You aren't showing the Mac to be faster than the J30...

[Be Free]

Great work. I really appreciate that you are posting actual measurements. Can you fill is in on how you are getting them?

I'm assuming (dangerous, I know) that you are using a 110 (or so) jib and a standard main, you have not stripped the boat down inside to reduce weight significantly, and you are running with your ballast full. Please correct me if that is not the case. Do you have a reasonable estimate (or measurement) of your boat's actual weight with your usual gear on board? For instance, I estimate that I'm 4500 to 4900 pounds while sailing provisioned for a week.

Your downwind numbers are close to what I'd expect but occasionally a little high. Your upwind numbers are (in my experience) around 2x what I'd expect to see in 9 knots. All of them are closer to what I'd expect to see with the wind around 15 knots.

This is a great discussion. I love numbers!

[DaveC426913]

PHRF Mac 26M/X of 250-270 (ish)):

That's news.
A racer at my club estimated a Mac's PERF closer to (and I quote) "a million".

[DaveC426913]

Great work. I really appreciate that you are posting actual measurements. Can you fill is in on how you are getting them?

Headings are by compass.
Speeds are by GPS.
TWA is by tacking then dividing headings in half.
TWS is by estimation.
Sail trim is the biggest source of error.

I'm assuming (dangerous, I know) that you are using a 110 (or so) jib and a standard main, you have not stripped the boat down inside to reduce weight significantly, and you are running with your ballast full. Please correct me if that is not the case.

All correct.

This is a brand spankin' new jib with an ancient main.

Do you have a reasonable estimate (or measurement) of your boat's actual weight with your usual gear on board? For instance, I estimate that I'm 4500 to 4900 pounds while sailing provisioned for a week.

No food provisions, but maybe 500 pounds of gear.

Your downwind numbers are close to what I'd expect but occasionally a little high.
Your upwind numbers are (in my experience) around 2x what I'd expect to see in 9 knots. All of them are closer to what I'd expect to see with the wind around 15 knots.

At 15 knots with full main I'd absolutely round up. Also, the lake would be filled with whitecaps, as it does above 10 knots.

There were times when I tacked and seemed to find myself in a little stronger wind than expected. I'm not sure I'd put it down to gusts. It was pretty steady. I'd put it down to very fast tacking.

I'm used to a 150 genny, hauling 5 yards of sheet at a time. This is a jib across the cabin-top fairleads, so a tack is, like, 1 yard of sheet. It feel almost like a self-tacking setup to me!

[Jimmyt]

PHRF Mac 26M/X of 250-270 (ish)):

That's news.
A racer at my club estimated a Mac's PERF closer to (and I quote) "a million".

Yep - heard that kind of talk before.

Probably hasn’t sailed one.

It’s not a farrier trimaran, but I’ve sailed on slower boats than my M.

My buddies that race can still enjoy a day sailing my boat (but they are constantly tweaking - looking for that last fraction of a knot). They do enjoy sailing, and boating in general though. We got to go sailing last Friday on the island packet 31. It was the first time I’d been on the water since my cancer surgery, and one of the other guys had just had spinal surgery. The captain said he was going invalid sailing (meaning he was taking his invalid buddies sailing ). Sure was good to be out on the water again.

[DaveC426913]

It’s not a farrier trimaran, but I’ve sailed on slower boats than my M.

The other day I was out doing my best in a 6 knot wind, when a Mac 25 came by to say hi. He passed me like I was standing still. Must have been doing 2 knots better than me.