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Old 06-05-2012, 02:59   #1
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What does swell do to boat speed

When sailing in a current it is quiet easy to make a guesstimate of what it will do to boat speed.
Sailing in a swell seems to be a little more difficult to work out.
I know that sometimes the swell can lift the boat and give me a real boost and sailing into it can really slow me down.
Is there some way of working out just how it affects boat speed before you go out or how fast the swell is actually travelling. I believe it has something to do with swell height and the time period between swells but i'm not sure.
Thanks
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Old 06-05-2012, 05:41   #2
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Re: What does swell do to boat speed

A gentle swell which is not too high doesn't usually have a big effect. You sail up one side then slide down the back, usually making back most of what you lose on the way up.

It's chop that kills your speed, especially the square waves you get from wind over tide. I had to do a 40 miles passage last Sunday into the teeth of a F8 (friend had to catch a plane) and wind over tide, and was making only 1 or 2 knots at times as the steep chop crashed over my bow, tons of water stopping us almost dead in the water.

A little off topic, but lesson learned -- if you absolutely have to make a passage like this, better to do it on an adverse, not favorable tide. You lose more speed from the sea state, than you gain from the favorable tide. This is a rule -- I always knew -- MOBO'ers follow, but it applies to us, as well.

In the event, I went inshore where the tide doesn't flow as strongly, to get a better sea state, and got up to 4, then 5, then 6 knots.
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Old 06-05-2012, 07:49   #3
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Re: What does swell do to boat speed

Take note of momentary water speed changes and average them (you can have your speed instrument do it for you unless the swell has longer periods than your instrument's averaging period). Then do the same with your gps measurements. The difference is what the swell adds or detracts. Instruments must be calibrated (water speed).

It may be difficult to separate more than one factor (wind, currents, swell).

My wild guess is that going in either direction, all other things equal, the swell will slow down the boat.

b.
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Old 06-05-2012, 09:35   #4
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Re: What does swell do to boat speed

Thank you for the replies.
I wondered if there was a way of possibly calculating it before you go out?
I know the sea does not move in swell, only energy and it is an excellent point made about chop and adverse tide although to be honest here I wasn't aware of it.
When I was googling swells the speeds that were mentioned for them seemed unbelievable.
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Old 06-05-2012, 09:38   #5
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Re: What does swell do to boat speed

Use time and distance to determine speed over a period of 30 minutes or so and you will get a sense of its effects. Unless your GPS can take an average speed over time, it is not going to be able to tell you anything from an instantaneous reading.

Keep in mind that the water in the swell does not travel with respect to the Earth, each water particle goes in a circle with respect to the Earth. There can be a small amount of wind current generated current right at the surface though. The cause of going faster or slower is the boat sensing an uphill or downhill effect, where if your boat spends more time on the downhill side of a swell, you will indeed go faster. The opposite of course is true when going into a swell, where your boat spends more time on the uphill side of a swell.

This is how surfing makes you go faster overall. Your boat is spending a lot more time going downhill than uphill. I realize uphill and downhill are not precise terms, but it gets the point across and it is the water equivalent of going up or down a hill on land.

Particle motion in a wave and in deeper water.....


Source: http://www.google.com/imgres?hl=en&s...9,r:0,s:0,i:75
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Old 06-05-2012, 09:58   #6
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Re: What does swell do to boat speed

depends on if you are bashing(running against it) or surfing.....
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Old 27-05-2012, 07:40   #7
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Re: What does swell do to boat speed

At the very least if you are at hull speed you are going 1.1 - 1.4 times the distance over flat water so unless you can surf you are going slower SOG. The choppier it is the slower you will be able to go.
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Old 27-05-2012, 09:07   #8
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Re: What does swell do to boat speed

Check out this new gizmo, featured on Panbo: Panbo: The Marine Electronics Weblog: Nortek Doppler Velocity Log, measuring Puma's leeway across the pond

It measures, via Doppler shift, all axes of motion through the water to provide a very clear chunk of data about how a boat is doing while cruising through seas.

Only a few thousand bucks for more data. Oh boy!

I'm happy when the boat takes off on a wave, and sometimes relieved when it slows down coming up the backside of the wave in front.
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Old 27-05-2012, 10:18   #9
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Re: What does swell do to boat speed

I love ocean swell.

In the Pacific Ocean on the way to French Polynesia, the swell was like riding on an elevator. On several occasions, the swell was so large that we got to see the green flash twice in a couple of seconds.

We would see the green flash at the bottom of the swell, and then we would be quickly lifted up on the swell and get to see the green flash a second time. We called it double green flashes. The only places I have seen double green flashes was in the large ocean swells of the Pacific.

I wish I was out there right now in the big swell trying to capture another double green flash at sunset.
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Old 27-05-2012, 17:29   #10
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Re: What does swell do to boat speed

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Originally Posted by maxingout View Post
I love ocean swell.

In the Pacific Ocean on the way to French Polynesia, the swell was like riding on an elevator. On several occasions, the swell was so large that we got to see the green flash twice in a couple of seconds.

We would see the green flash at the bottom of the swell, and then we would be quickly lifted up on the swell and get to see the green flash a second time. We called it double green flashes. The only places I have seen double green flashes was in the large ocean swells of the Pacific.

I wish I was out there right now in the big swell trying to capture another double green flash at sunset.
Dave,

Sorry to have to be the one to tell you, but the French, as an economy move from the new government, have cancelled the green flash at sea. They have permitted periodic appearances in Tahiti for the tourists, though, but likely never double ones.

Wish that you guys were here, too!

Cheers,

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Old 27-05-2012, 17:42   #11
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Re: What does swell do to boat speed

A day without a green flash is a sad thing.
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Old 27-05-2012, 19:21   #12
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Re: What does swell do to boat speed

Sorry for my unknowing, but what is green flash??

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Old 27-05-2012, 21:10   #13
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Re: What does swell do to boat speed

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Sorry for my unknowing, but what is green flash??

Guy
A green flash happens at sunset when the western horizon is absolutely clear without any clouds near the western horizon.

Just before the sun disappears beneath the horizon, a brilliant green flash of light appears on the horizon at the location of the sun. It is a bright emerald color, and is unmistakable. It's not hard to see if you look for it.

We thought it was rare when we started sailing across the Pacific, and many people never see it because they don't look for the green flash, or they look for it when clouds are on the horizon.

Once you make up your mind that you want to see it, everyone gets in the cockpit at sunset to see if there will be one that evening. If there are any clouds in the direction of the sun on the horizon, the flash will not occur. But if the horizon is crystal clear, you will usually see the green flash.

When we thought they were rare and hard to see, we kept a record of them in our log book. Once we figured out that there would be a green flash if the horizon was clear, we stopped writing them all down. In settled and clear weather, we often saw the green flash several nights in a row. We saw the green flash more frequently in the Pacific, but that was because it took more time to cross the Pacific than the Atlantic.

And when the swells were running big in the Pacific, we saw a double green flash 3 or 4 times during our circumnavigation.

It's kind of neat to see the last glimmer of sun turn into bright emerald green.
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Old 27-05-2012, 23:15   #14
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Re: What does swell do to boat speed

I am still waiting to be flashed at although I keep looking.
As you say I believe the French have moved it from this side of the Atlantic or maybe the clouds of austerity have obscured it.
Hopefully now the summer is getting nearer here we may get a chance to see one but I never get over the beauty of watching the sun set on an open sea.
I also think that those of you who get the chance to see several flashes in a row are just being greedy and using my allocation up.
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Old 28-05-2012, 00:01   #15
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Re: What does swell do to boat speed

Somebody has to look at those flashes, and it may as well be me.
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