Cruising Catamarans and Apparent Wind

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rexd666
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Cruising Catamarans and Apparent Wind

Post by rexd666 » Tue Oct 01, 2013 10:38 pm

Ok so after spending a lot of time on the AC YouTube channel, I think I actually learnt something, but I wanted to check if it applies to any cat and not just racing machines.

So all my catamaran sailing was beach cats in the early 90's, where I was the only person in the school club on a cat (Maricat 4.1?), against Mirror's and 125's. So I would win line honours, but no one really knew how to sail a cat any different. So downwind we would swing the boom out one side and jib out the other wing on wing and lay back and relax.

But watching the AC's I am thinking "Hang on these guys never stop flying along", it actually took me a while to work out which was the downwind leg. Then they did the little overlay technology thing to explain apparent wind, light bulb went on.

So does it work on cruising cats to get a better VMG (another term I now actually understand thanks to AC education spots), or is it all too much effort and generally easier to head directly or close to directly down wind and not look for minor speed gains from apparent wind.

Steve

Finally
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Re: Cruising Catamarans and Apparent Wind

Post by Finally » Wed Oct 02, 2013 9:38 am

I'll dip my toe in the water with this one.

In short, "Yes" it does work even for most modern cruising cats. We used to call it "tacking downwind". Having said that, it really depends on a number of things including wind speed (works best in light to moderate conditions), sea state and direction, what sails you have up, the boat's weight (the heavier the boat the less of an advantage it is). But it's also more work as you will need be making more gybing manoevres compared to going directly downwind.

Think of it this way. If you are heading directly down wind, the fastest you can go is wind speed. Tacking down wind moves the apparent wind direction more forward and, depending on your boat and what sails you have up, potentially increase in speed, which increases apparent wind speed (and moves the apparent wind further forward), which increases boat speed etc. But there is a bit of trial and error involved to work out what apparent wind angle is best for the boat and the conditions you are experiencing to then work out what advantage (if any) it is making to your VMG. This is simply because you are travelling a greater distance with this technique compared to going in a straight line from A to B. You just have to get out there and see how your boat performs. I haven't investigated it but I think modern chartplotters can give you route VMG - perhaps someone else can confirm/deny my thinking on this.

One other advantage of tacking downwind is that it largely avoids the potential of gearing damaging accidental gybes though you will have to perfect your gybing technique during the intended gybing manoevres.

In the end, if you are in cruising mode, the best angle is the one that is the most comfortable for you, the crew and the boat.

Corley
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Re: Cruising Catamarans and Apparent Wind

Post by Corley » Wed Oct 02, 2013 10:11 am

I know it's going to sound like heresy but I'd argue that most cruising cats are not fast enough to worry too much about chasing apparent wind. There are always exceptions though and if you have a decent rig and sails it's worth giving it a go and checking your VMG results DDW vs tacking downwind.
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Darren
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Re: Cruising Catamarans and Apparent Wind

Post by Darren » Wed Oct 02, 2013 1:05 pm

Following on from Finally, you probably need to collect some data about your boats performance, and over time, build up a plot of boatspeed vs true direction relative to wind (a polar plot). Make one for each wind speed. Then you can work out if there's a faster option.

Did you know that it's possible to "sail" directly down wind faster than the wind? By sail, I mean using wind as your only power source? It's very contentious, as it is not intuitively obvious how it works, and involves propellers/mills rather than sails. Google for DWFTTW (down wind faster than the wind). Some very serious discussions about it.

44c
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Re: Cruising Catamarans and Apparent Wind

Post by 44c » Wed Oct 02, 2013 2:37 pm

Corley wrote:I know it's going to sound like heresy but I'd argue that most cruising cats are not fast enough to worry too much about chasing apparent wind. There are always exceptions though and if you have a decent rig and sails it's worth giving it a go and checking your VMG results DDW vs tacking downwind.
I'd agree. We often sail DDW "goosewinged". To get an angle where we'd gain anything in boatspeed, we'd need to be sailing at about 30 degrees higher (TWA).

It really does depend on where your destination lies. If it's DDW, then it's hard to beat sailing DDW to get there. As long as you're set up for it.

rexd666
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Re: Cruising Catamarans and Apparent Wind

Post by rexd666 » Wed Oct 02, 2013 9:38 pm

Thanks for all the responses so far, if anyone does some tests for their boat and wants to share numbers at a later stage feel free to update.

Smooth Cruiser
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Re: Cruising Catamarans and Apparent Wind

Post by Smooth Cruiser » Tue Oct 15, 2013 12:58 pm

I would agree with all of the above posts - in theory it will be often better but in practice it may not be. Most cruising cats are still not performance oriented enough for "heating it up" downwind to make much difference, and the speed increase you see will not justify the extra distance travelled.

To expand on the points above - if you "tack downwind" or "run angles" where you come up from DDW by around 35 degrees (gybing through 70 degrees from one course to the next) then you will travel 22% further than going direct downwind - so you would need to have a speed 22% greater than you were getting DDW. Most cruising boats won't get this at this angle - probably needing to go 50 degrees or so off DDW to achieve this gain, which then means they are travelling even further.

Try it and see though - you can watch your VMG on your instruments, or your ETA, and see if you are getting a benefit - you will be working harder though as you will be running different sails and gybing more often. But in big uneven swell you may also have no option as DDW might be too unstable or risky for unintended gybes.

My cat is quite a performance boat and with a racing crew on board and setting an appropriate fractional asymmetric spinnaker we can go significantly faster downwind by running angles - but when I go cruising I often just use the big kite, goosewinged sails or even just the jib in stronger winds and sail DDW - because that's what you do when you are cruising - you don't need the extra workload of heating the boat up and gybing all the time.

Jim
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Re: Cruising Catamarans and Apparent Wind

Post by Jim » Wed Oct 16, 2013 6:59 pm

At a few different times while sailing DDW I have thought about gybing, so I went inside and worked it all out on a chart and came to the conclusion that it just makes for a longer day. If you are already looking at a 10 or 12 hour day to reach your next destination, which will in some cases works out to be late afternoon, then to gybe and put another hour or two on your trip time, to me just doesn't add up. To do it on a short social day sail may be a bit smoother and a little bit quicker but when cruising for full days with a boat loaded with usually too much stuff on board......i'll pass.
IMO, social day out, do whatever, full cruising day, just go DDW, long ocean passage, don't know, haven't done one.
Jim.

44c
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Re: Cruising Catamarans and Apparent Wind

Post by 44c » Sun Dec 18, 2016 7:14 am

some evidence to support sailing DDW, if that's where you're going.

Recently "School's Out", an Oram 44C left Pancake creek for Bundaberg. DDW, under a symmetrical spinnaker. Leaving just behind them was "Zero", a Schionning G-Force 18 SSS- a multimillion dollar, super lightweight 60' performance cat, with huge carbon rig, carbon sails etc.

"Zero" tacked downwind using their screecher and main.

And lost ground.

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