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Old 16-02-2009, 05:29   #16
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Excellent essay, Joli!
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Old 16-02-2009, 05:33   #17
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Sailed Tornado for 15 years when we could only use jib and main. Always tacked down-wind to bring apparent wind forward. There was a fine line. If you started to bury the lee bow, you bore off, moved aft, and tried to prevent a pitchpole. If you tried to round up, AP moved further forward, boat accelerated, windward hull lifted, and 9 out of 10 times you went swimming.
Loved sailing those things.
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Old 16-02-2009, 05:42   #18
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Creating accurate polars cannot be done with GPS data especially of there is current. Of course most pick up speed on a bread reach compared to a run down wind. Since the wind is pushing the boat you want to maximum presentation of the sails to the wind so this mean wing and wing or else you are blanketing the head sail and only powering the boat with main alone. Depending on wind speed this may be enough cavas, but if you are not moving at hull speed than you would need more.

The angle downwind where you sails are trimmed for the most presentation to the wind should give you max speed, but that is likely sending further off your intended course line and you have to make that up by gybing over. If you have instrumentation which can cumpute VMG to windward or downwind you can steer to the highest VMG. But as noted below you need calibrated instruments.

Another consideration is the yawing motion, often present with auto pilot steering. This is the result of the pilot trying to maintain a course and over shooting in a puff or riding a wave and moving of "heading" which is then compensated and you head off a bit in the other direction. The net effect of this zig zagging is like a slalom down a mountain which is obviously a much longer path then straight down the fall line. This DOES add up the distance traveled and do for a long down wind sail you want to try to keep a steady heading IE not yaw about. The helm requires constant adjustment because of waves motion and wind puffs. When you sail off the wind the you encounter quartering seas which when large enough wreak havoc on holding a straight course and so this point of sail is prone to zig zagging yaw which increases the distance traveled.

Dead down wind you usually have following seas which add a bit to your speed since their force in in the direction you are steering. The problem here is that small direction changes amplify the apparent wind angle and this requires very subtle steering to keep the wind dead astern. If you have swept back spreaders you also risk backwinding the main and accidental gybes. YIKES

My autopilot does not have enoough helm to hand large seas high winds on a beam reach where it may be required to put the helm hard over. Autopilots typically make smaller corrections and don't have the ability to turn the helm from stop to stop (at least mine doesn't).

Additionally you can be sailing a reach, slide off a wave the auto pilot or helmsman may struggle to prevent the boat from rounding up or even broaching. YIKES.

I suppose this is why you see many sail with genny alone in strong winds on down wind courses.

Finally the dead down wind course had the boat flat and the apparent wind lowest and this is more comfortable than heeled over with a stronger wind.
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Old 16-02-2009, 06:11   #19
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Old 16-02-2009, 08:39   #20
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Deciding to go dead down wind depends on wether or not the polar is flat. Here is and example of a 30 foot mono. When you look at boat speed vs wind speed you need to look at the polar that corresponds. The 12 to 14 knot wind speed shows a "flat" polar. This indicates you should be sailling deep. If you look at the 20 knot wind speed the polar is now heart shaped indicating you should be sailing a hotter angle ~148 degrees.



Next when sailing downwind you are rarely sailing perfectly DDW to the mark but are sailing VMC "velocity made to the course". Hence one board or the other is favored and since the wind direction is constantly changing you must know the favored board and jibe on the lift to maintain the best VMC. All of this requires very good insturmentation with a central processor that has a sampling rate of 100 data points per second and good calibration to be of any use. Ideally you should be sailing to a true wind speed and target speed but few instruments are good enough to give you meaningfull data to allow you to do that. Unless you want to drop $30k and have your instruments constantly calibrated and maintained.

So it is not so simple to say just go dead down wind because it depends greatly on the boat, the windspeed, your ability to analyze data, and how hard you want to work.

For a Lagoon 38 I would point it dead down on the board I thought was favored, pop on the auto, and enjoy a coldy while watching the world float by.

The polar you are showing is an apparant wind angle polar. The white dots on it are the best vmg angles. Since you have chosen the correct vmg points it appears that you know this, but I think that you are confusing people to say look at the lines on my graph to see how this is flatter, then pick a point that doesn't look to be at the right place. These best vmg dots come from the other polar I described, the true wind angle polar. The true wind angle polar is where you look at where the lowest point in the graph is and that is the true wind angle to sail at. If you're using wind instruments, you have to calculate what apparent wind angle that point will be. The next step would be to plot that as a special point as has been done on the graph you show.

Here's a Soverel 33s true and apparent polar. See how on the true polar the best vmg boat indicators are at the bottom of each curve (horizontal tangent), then look at where that same boat is on the apparent. For the lowest windspeed plot the best angle is at about 140 degrees to the true wind. If you only have wind instruments you need to calculate what the apparent wind angle will be, this is calculated and plotted for you on the apparent diagram you buy from U.S. Sailing, it's all the way up at 88 degrees. So on a Soverel 33, at this wind speed, you sail at 88 degrees to the wind as shown on your apparent wind indicator, to sail at 140 degrees away from the true wind to make your best speed to a downwind destination.

Also all the diagrams shown on this post have discontinuous lines, this is because the top line is with jib, the bottom is with spinnaker. The links I posted for the cats previously are continuous and so they are using a jib on all angles of sail.

http://www.soverel33.com/Images/Arti...r_40810_TW.gif



http://www.soverel33.com/Images/Arti...r_40810_AW.gif

I'm posting the pictures from the links in a separate (next ) post since they messed up the formatting.

John
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Old 16-02-2009, 08:43   #21
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Old 16-02-2009, 10:08   #22
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Hi John,

What I've shown is a true wind polar for a Henderson 30. I picked that polar since it exhibits both heart shaped and flat characteristics common to a planning mono. The Soverel 33 (great boat by the way) is flat at almost all wind speeds.

Look at the Henderson polars for say 4 knots.
TWA is ~138, BS is ~4 the AWA is gonna be ~69 degrees.

Here is a calculator for your reference. http://www.onemetre.net/Design/AppWind/Apparent.xls Note the calculator is in f/s. You can also do vector analysis if that is more comfortable.

Look at the Soverel 33 polars for 6 knots.
TWA is ~139, BS is ~ 4.7 and the AWA as indicated in the AWA polar is 88 degrees.



As I said, if you have very good instrumentation it is best to sail with true wind numbers and targets but I can't imagine anyone reading this really cares about upwash angles, masthead accelerometers, gravity switches or a host of other instrument issues to help calculate the quickest way to the leeward mark. Why do racers sail to true wind numbers and target speeds and not apparent wind numbers? Because the apparent numbers move too quickly.

I would recomend for anyone interested in this, get polars for their boats and play around with them when out sailing. Sometimes going deep is quickest, sometimes sailing hot is quickest.

Cheers,

Joli

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Originally Posted by cal40john View Post
The polar you are showing is an apparant wind angle polar. The white dots on it are the best vmg angles. Since you have chosen the correct vmg points it appears that you know this, but I think that you are confusing people to say look at the lines on my graph to see how this is flatter, then pick a point that doesn't look to be at the right place. These best vmg dots come from the other polar I described, the true wind angle polar. The true wind angle polar is where you look at where the lowest point in the graph is and that is the true wind angle to sail at. If you're using wind instruments, you have to calculate what apparent wind angle that point will be. The next step would be to plot that as a special point as has been done on the graph you show.

Here's a Soverel 33s true and apparent polar. See how on the true polar the best vmg boat indicators are at the bottom of each curve (horizontal tangent), then look at where that same boat is on the apparent. For the lowest windspeed plot the best angle is at about 140 degrees to the true wind. If you only have wind instruments you need to calculate what the apparent wind angle will be, this is calculated and plotted for you on the apparent diagram you buy from U.S. Sailing, it's all the way up at 88 degrees. So on a Soverel 33, at this wind speed, you sail at 88 degrees to the wind as shown on your apparent wind indicator, to sail at 140 degrees away from the true wind to make your best speed to a downwind destination.

Also all the diagrams shown on this post have discontinuous lines, this is because the top line is with jib, the bottom is with spinnaker. The links I posted for the cats previously are continuous and so they are using a jib on all angles of sail.

http://www.soverel33.com/Images/Arti...r_40810_TW.gif



http://www.soverel33.com/Images/Arti...r_40810_AW.gif

I'm posting the pictures from the links in a separate (next ) post since they messed up the formatting.

John
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Old 16-02-2009, 11:04   #23
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Hi John,

What I've shown is a true wind polar for a Henderson 30. I picked that polar since it exhibits both heart shaped and flat characteristics common to a planning mono. The Soverel 33 (great boat by the way) is flat at almost all wind speeds.

Look at the Henderson polars for say 4 knots.
TWA is ~138, BS is ~4 the AWA is gonna be ~69 degrees.

Here is a calculator for your reference. http://www.onemetre.net/Design/AppWind/Apparent.xls Note the calculator is in f/s. You can also do vector analysis if that is more comfortable.

Look at the Soverel 33 polars for 6 knots.
TWA is ~139, BS is ~ 4.7 and the AWA as indicated in the AWA polar is 88 degrees.



As I said, if you have very good instrumentation it is best to sail with true wind numbers and targets but I can't imagine anyone reading this really cares about upwash angles, masthead accelerometers, gravity switches or a host of other instrument issues to help calculate the quickest way to the leeward mark. Why do racers sail to true wind numbers and target speeds and not apparent wind numbers? Because the apparent numbers move too quickly.

I would recomend for anyone interested in this, get polars for their boats and play around with them when out sailing. Sometimes going deep is quickest, sometimes sailing hot is quickest.

Cheers,

Joli
When I looked at your polar the vmg points really didn't look like they were at the bottom of the curves, holding a piece of paper up to them, it now looks like they actually are. The other thing that made me immediately think that it was an apparent graph was the tacking angles, I had in my mind a normal boat, seeing the 35 degree upwind angle is closer to what I picture for a cruisers apparent. Looking at the Henderson site I see the Henderson is more like the Soverel example I give.

Sorry, I was wrong about what kind of graph you had posted.

I've also made my own spreadsheet, took me awhile to do the trig. Why are the boatspeeds you're using so low?

John
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Old 16-02-2009, 11:18   #24
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Hi John,

No worries, I used to spend allot of time on these things, now I'm kruzer.

I referenced a low wind condition to illustrate the dramatic difference in TWA and AWA. If you look at the higher winds for the Hendo 30 the difference is not so great, as an example: TWS 25, BS 14, TWA 162. AWA 142. So at 25 knots TWS the boat has only pulled the breeze by 20 degrees.

The original question seems simple but the answer is very complex if you want it to be.

Cheers,

Joli


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When I looked at your polar the vmg points really didn't look like they were at the bottom of the curves, holding a piece of paper up to them, it now looks like they actually are. The other thing that made me immediately think that it was an apparent graph was the tacking angles, I had in my mind a normal boat, seeing the 35 degree upwind angle is closer to what I picture for a cruisers apparent. Looking at the Henderson site I see the Henderson is more like the Soverel example I give.

Sorry, I was wrong about what kind of graph you had posted.

I've also made my own spreadsheet, took me awhile to do the trig. Why are the boatspeeds you're using so low?

John
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Old 17-02-2009, 05:44   #25
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Hello all,

I'm curious to everyone's preference. Do you think your catamaran goes faster on a broad reach or on a dead run?

We have a Lagoon 380 and the shrouds prevent the boom from swinging out for a proper run (like most cats). Consequently I find we are maintaining a 140 degree broad reach with several jibes to arrive on a downwind waypoint. The alternative with our Genoa only appears to be much slower. What is your preference on your cat?

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G'day,
The simple answer is to note your speed dead down wind then your boat speed and angle to the dead down wind course (difference in compass headings) on the new heading.

Take the cosine of that angle and multiply it by the speed and compare it with the ddw speed.

Using Joli's polars at 20 knots windspeed this is 9 .5 knots dead down wind and 12 knots at 32 degrees (180-142 on the polar) off ddw. Cosine of 32 is .848, *12 is 10.17, so this will be quicker than running ddw.

It will in fact be quicker to sail at 11 knots 20 degrees off ddw. Cosine 20 is 0.939. Multiplied by 11 is 10.3 knots

HOWEVER, the waves help more ddw and gybing takes time.

In my experience, there are not many cruising boats that sail the apparent wind downwind faster than if they are just dragged down under spinnaker.

Standard cruising catamaran rigs are quite pathetic for downwind sailing as the main cannot be eased out enough. They are equally pathetic broad reaching as the headsail is blanketed by the main. Given that these are the most common sailing angles, it beggars belief that so many cruisers tolerate it.

regards,

Rob
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Old 17-02-2009, 07:05   #26
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You obviously have never captained a cruising catamaran.

Pure gibberish
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Old 17-02-2009, 07:18   #27
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You obviously have never captained a cruising catamaran.

Pure gibberish
The maths or the catamaran rig?

regards,

Rob
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Old 17-02-2009, 09:23   #28
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For an under 40' cat a broad reach is more comfortable that a run because the sail stablilizes pitch and roll better when the air flows from luff to leach rather than vice versa when sailind DDW. A chute provides less stability.
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Old 17-02-2009, 10:14   #29
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And there is the matter of accidental jibes.

Cruisers like auto and dead down wind can be fussy, IMHO.

Also, speed be damned, on a hot, still summer's day, more apparent wind is ALWAYS better for the crew. It's cooler.
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Old 19-02-2009, 21:16   #30
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I think that in strong trade winds it was better to sail dead downwind. To facilitate this I poled out the jib with a 22' carbon whisker pole, and kept the main very flat and off the shrouds using a heavy duty preventer.The reasons are as follows:
If tacking downwind you must sail faster, OK thats what we are talking about in this post, but is that always a good thing? Consider that we are talking multi hulls here. I found I could average 190 to 200 miles per day by sailing directly downwind wing on wing or using the kite if the winds slowed. This was done by keeping the speed down to 8 knots or less at night for sleeping and sailing faster during the day. Everyone was well rested with not much to do as the autopilot could handle everything that was thrown at it and no jibing required. The waves directly on the stern make for a smooth ride that the autopilot has no problem coping with even when surfing.
Now consider tacking downwind. The increased speed required just to maintain the same daily average will create more noise when trying to sleep, more wear and tear, more motion, more work. The autopilot will have a harder time with the waves on the quarter especially when surfing. You will also be assuming that the wind will not shift when working out your vmg. An added benefit of setting up your catamaran to sail directly downwind in strong conditions will be when reefs or islands lie to each side of you. So yes you can get there faster by tacking downwind but the price to pay will be the crews comfort.
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