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Old 15-02-2009, 07:33   #1
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Faster on a Broad Reach or Run?

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?

Seth
Lagoon 380, Honeymoon
Currently in Bonaire en route to Panama
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Old 15-02-2009, 08:13   #2
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Reaching is almost always the fastest point of sail - faster than a run.

Different boats have different performance characteristics - so on some boats, the beam reach is the fastest point of sail; on others, a broad reach is faster; and others still a close reach is fastest.

See also "The Points of Sailing" (pg 296):
Chapman Piloting and Seamanship - Google Book Search
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Old 15-02-2009, 08:24   #3
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Hi GordMay, I was wondering what other cat users have found, particularly on my boat, a Lagoon 380. Any ideas there?
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Old 15-02-2009, 08:34   #4
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If your destination is dead down wind, broad reaching and jibing back and forth to get there is almost always quicker than running down wind, regardless of the boat. There may be some exceptions, such as in high winds, but for the most part broad reaching and jibing is quicker and a faster point of sail.
Brian
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Old 15-02-2009, 08:36   #5
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I'm assuming you are looking for the best vmg downwind speed? Almost all boats get to a downwind destination faster on a reach, the break even point between extra distance sailed versus increase in speed is best found by creating a polar diagram. Plot your speed versus angle to the wind on a polar graph. Bring a horizontal line to the tangent point on the curve. The one on top gives your best angle to sail to minimize time upwind, and the bottom one is the minimum time downwind. You need to do this for a variety of wind speeds. Depending on your instrumentation, you'll have to do some calculating as the polar you need to do the tangents is the true wind speed true wind angle polar, so if you are going by wind instruments alone some trig is in your future.

Or if you're lucky it's on the web Here's an Outremer polar its best vmg downwind is about 140 degrees in 20-22 kts of wind. (Scroll about half way down the page.)

Outremer 50/55 S catamaran

Here's a FP Salina 48, its best looks to be 150 degrees.

Fountaine Pajot Salina 48 catamaran


If you're just looking for best speed and don't care where you're going, find on the graph where the line is farthest from the center.

John

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Old 15-02-2009, 16:13   #6
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Have you considered flying a kite?

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Old 15-02-2009, 16:23   #7
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If your destination is dead down wind, broad reaching and jibing back and forth to get there is almost always quicker than running down wind, regardless of the boat.
Brian
Not my experience, last summer a monohull 10' longer than our monohull did the downwind jibe routine while we sailed straight as an arrow, we were both going to the same cove and started at the same place at the same time. We got there first. Every time we crossed paths it was apparent we were pulling ahead slowly. We had a lovely downwind sail in 10 or 12 knots of wind, they didn't look to be enjoying the gibes quite so much.
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Old 15-02-2009, 17:01   #8
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Not my experience, last summer a monohull 10' longer than our monohull did the downwind jibe routine while we sailed straight as an arrow, we were both going to the same cove and started at the same place at the same time. We got there first. Every time we crossed paths it was apparent we were pulling ahead slowly. We had a lovely downwind sail in 10 or 12 knots of wind, they didn't look to be enjoying the gibes quite so much.
You're comparing apples and oranges. I've been in races where a 45 footer had the same handicap as the 26 footer I was on. And that was just an Islander Excalibur 26, not a lightweight flyer. The boat you beat might have been doing their best or didn't have or know how to use a polar.

John
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Old 15-02-2009, 17:15   #9
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The answer is how much apparent wind your catamaran can generate. The more apparent wind it can generate, the higher it can head and the more boat speed it can generate. As your apparent wind increases, so will you ability to maximize your velocity made good dead downwind.

Lagoons are not exactly speed demons, so your optimal apparent wind angle will be more downwind than say a catamaran like a Tornado with a much higher sail area to displacement ratio.

Another way to look at it is the pressure differential between the two sides of the sail will be greater when they work as a foil than as a traditional looking circular parachute.

Think of an aircraft wing, it produces more lift (pressure differential) when it has a laminar flow on both sides rather than when it is stalling.

Pointing the bow dead downwind is only fastest for the slowest sailboats.
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Old 15-02-2009, 17:16   #10
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You're comparing apples and oranges. I've been in races where a 45 footer had the same handicap as the 26 footer I was on. And that was just an Islander Excalibur 26, not a lightweight flyer. The boat you beat might have been doing their best or didn't have or know how to use a polar.
Agree. Or was simply defeated by wind shifts ...
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Old 15-02-2009, 18:13   #11
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Originally Posted by amarinesurveyor View Post
If your destination is dead down wind, broad reaching and jibing back and forth to get there is almost always quicker than running down wind, regardless of the boat. There may be some exceptions, such as in high winds, but for the most part broad reaching and jibing is quicker and a faster point of sail.
Brian
This has always been my observation.
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Old 16-02-2009, 02:18   #12
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Have you considered flying a kite?

Monster second hand bullet proof kite in a squeezer sock was the cheapest sail on the last boat.

Leave the expensive main in the bag, pop the kite have the shade up under the boom and autopilot on.

Easy peasy and ran for many many miles for days on end like this with no issues.



Dave
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Old 16-02-2009, 04:29   #13
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Kite = spinnaker? Got any pictures? Thanks, John
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Old 16-02-2009, 05:09   #14
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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.
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Old 16-02-2009, 05:25   #15
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Kite = spinnaker? Got any pictures? Thanks, John
Not under kite, no

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