26m sailing performance

A forum for discussion of how to rig and tune your boat or kicker to achieve the best sailing performance.
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Phil M
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Re: 26m sailing performance

Post by Phil M »

mastreb wrote:I'm going to Catalina on the 4th of October from San Diego, and will spend four days beating into the wind. I've got an anemometer, chartplotter, and will have an auto helm on by then, along with new sails and a rig retention. I'm not the world's best sailor and I don't expect much from the wind but I will be beating into it for three or four days.

My goal is to try to derive a reasonable and instrumented set of polars for the 26M with as little human factor involved as possible.

I'll post what I'm able to do.
Time is up.
I'll hold your feet over the fire until you post. :P
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mastreb
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Re: 26m sailing performance

Post by mastreb »

I haven't posted a graph yet because I've only got data for very light winds of less than 10 knots.

It's very important to understand that efficiency numbers for both angle and speed on polars are reported in true wind, but that you actually set your sails and sail to apparent wind. The reason for this is that true wind describes the actual energy input and is therefore the convention for comparing efficiency between boats.

So when you TALK about wind, you talk in true. When you ACT on the wind, you act on apparent wind. Because people sail to apparent wind and its easier to measure, many people will argue that you should report performance in apparent wind, but apparent wind cannot be normalized across various wind speed regimes without resorting to trigonometry and apparent wind when running is extremely low, which would create a very distorted polar chart. Also apparent wind includes the effect of boat speed, which is also separately reported on a polar, and this causes distortion in the graph.

So everything I'm saying here is in TRUE wind. People who report pointing higher than 50 degrees in these boats are talking about apparent wind, and generally in higher wind speed regimes than I get here in San Diego.

I manage sheets pretty attentively, mainsheet in hand to keep heel to fifteen degrees consistently. There are no passengers aboard my boat: You are either crew or you're ballast, and if you're ballast you are required to be on the windward side of the boat. I also routinely pull up my windward rudder and manage the daggerboard depth based on point of sail:

Pointing: 100% deployed
Close reach: 75% deployed (1 foot back on the line)
Beam reach: 50% deployed (2 feet back on the line)
Broad reach: 25% deployed (1 foot down)
Running: pulled up.

This keeps unnecessary drag to an absolute minimum. I also have heavier that stock 6oz sails with full luff pads on the genoa and a fully battened loose-footed main. While they're easier to use than the stock sails, my performance numbers are very similar. I don't manage the outhaul of the main very much besides setting a good belly because the winds here aren't that variable.

Here's a simplified summary of what I've collected so far:

Firstly, below 5 knots of true wind, the boat is not moving fast enough to keep course, which requires 2 knots of speed through the water. With less than 5 knots of true wind efficiency is also terrible as the rudders and daggerboard drag dominate. This drag remains steady as winds rise, so there's an increase in efficiency as you pick up speed.

So 5 knots is the bare minimum to sail with my 150% genoa rig. I don't bother raising sails until we get to that point.

5..10 knot wind regime (150% RF Genoa 6oz)
======================================
< 40 degrees (true): Irons
< 50 degrees (true): The boat rapidly dwindles from 35% efficient down to 0 between 40 and 50 degrees of true wind. It's almost certainly best to beat to 50 degrees for VMG rather than to try to sail below 50 degrees.

50..70 degrees (pointing, close reach): 35% efficient. This means that in a 10 knot true wind, I make 3.5 knots. It's right about 1/3 windspeed. Probably a hank-on will be closer to 50% and may point down to 40, but I don't have that sail to test.

70..130 degrees (beam reach): 45% efficient. The beam reach is the MacGregor's most efficient point of sail except for running. When I weathervane, which is setting the autopilot to change course automatically to keep a specific point of sail, I set it to 70 degrees off the wind for maximum speed, unless I'm actually going somewhere, in which case I set it to 50 degrees for best VMG.

130..160 degrees (broad reach): 40% efficient: At this point, the mainsail is backed against the spreaders and that causes some efficiency loss. Also the Genoa is in the wind shadow of the main.

160..180 degrees (running): 50% efficient: When running wing-on-wing, the boat picks up to 50% efficiency as the Genoa is no longer in the wind shadow of the main. Those who report speeds higher than hull speed (6.4 knots) in a Mac are always running.

When running either main only, cut these numbers in half: The boat is no more than 25% efficient running main only. It's slightly more efficient running Genoa only, about 30% on a beam reach.

According to my fuel efficiency tests, the hull drag by speed progresses like this:

0.1gph, 2 knots
0.3gph, 3 knots
0.4gph, 4 knots
0.5gph, 5 knots
1.0gph, 6 knots
2.5gph, 7 knots
3.5gph, 12 knots
4.5gph, 14..17
5.6gph, 17 knots

While these are in units of gallons per hour, they also give us a perfect multiplier for determining what our losses are for a specific speed, magically normalized to 1 for hull speed. You can see that getting to 7 knots takes 2.5X as much power as 6 knots, and getting to 12 knots takes 3.5X as much power. This is because at 6 knots the boat begins climbing the bow wave, and that dramatically reduces efficiency.

It is for that reason that I'm skeptical of claims that the Mac, at least the 26M, could possibly plane downwind at speeds of ten knots. Considering the boat's downwind efficiency of about 50%, and the fact that 10 knots requires 3X more power to move through the water than 6 knots, you'd be looking at required wind-speeds of 10kts * 2 * 3, or 60 knots, to deliver that much force. Perhaps the downwind efficiency goes up in heavier air, but even at 100% efficiency it would take a 30 knot wind, hard, but not impossible, to believe.

Takeaways:

:: Below 6 knots of wind, it's not you, it's the lack of wind.
:: Think of the boat as 40% as fast as the wind in light airs. More than that and you're running better than typical.
:: If you're only going to run one sail, run the genoa. I don't have advice for a hank-on jib.
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Ixneigh
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Re: 26m sailing performance

Post by Ixneigh »

I think that is a very good summation, and one that jibes well with my experience, but note I do have a hank on jib, so I think my performance in stronger winds may be a bit better. My average boats speed is around 4.5 knots on days when one would actually want to be out enjoying the water, and not being too uptight about sail trim.
Ix
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