Wave Crest Phenomenon - Page 2

9D280 · 03 June 2007, 22:30

Rouge,

anywhere between about 2-8 boat lengths. (Yes, I know that was stupidly close at that speed , and wouldn't normally run so close at 20 knts, but this phenomenon needed some investigation, and my bow was always pointing outsuide the lead boat's transom) All the other thoughts in the post make sense, as the water moving around in the wave has to go somewhere, and the circular motion that makes surfers happy all makes sense, however I'd have thought it would be a "handed" phenomenon on a wake, port mirroring starboard. The effect was slightly less dead astern as I suspect the exhaust mixed with the water would give less for the leg / prop to "grip" - also I didn't want to stay dead astern too long in case my water pump was moving more air than water..... This was only tested between 6-8 boat lengths for hopefully obvious reasons!

I got similar heel effects when I was in open wave water, however open waves behaved in a "handed" manner, as you would expect , granted not quite so much or as solidly as behind the Ribcraft). We did wonder about something to do with the water coming off the prop, but that would need to be one huge amount of sideways motion and lets face it, if your engine is moving that much water sideways, the rib woudn't be travelling forward quite so fast!

Not been out this weekend as the middle carb was in bits- but more of that anon. I'm not sure I'll be able to get out next weekend as I have the small matter of moving house

, but if I do I'll go find a CalMac ferry and see if it does the same thing. Watch this space, but don't hold your breath just yet......

brucehawsker · 04 June 2007, 10:46

I did this once for nearly 10 minutes coming back from the Needles towards Hurst. Really weird - seemed to be stationary - illusion - incredibly smooth - not much revs, nose up, trimmng made little difference. 23-30+ k on the GPS, with no change to throttle - 3/4 approx. 4k on the revs.

But, I argued, surely the wave can't have been doing 23k??

Big wind from SW, big tide coming up the Solent from the Needles, pretty foul weather. F5 or 6? The Brdge was a maelstrom so we turned back.

codprawn · 04 June 2007, 12:04

Quote:

Originally Posted by brucehawsker

I did this once for nearly 10 minutes coming back from the Needles towards Hurst. Really weird - seemed to be stationary - illusion - incredibly smooth - not much revs, nose up, trimmng made little difference. 23-30+ k on the GPS, with no change to throttle - 3/4 approx. 4k on the revs.

But, I argued, surely the wave can't have been doing 23k??

Big wind from SW, big tide coming up the Solent from the Needles, pretty foul weather. F5 or 6? The Brdge was a maelstrom so we turned back.

Yes it's quite possible. We were out in 15 - 20' waves and only doing 15kts at 5000rpm into the waves. When we turned around we were doing 25kts at 1200rpm!!! Sitting on top of the waves was great - no wind - total silence etc. Acceleration was crap though - boat was very unrepsonsive - felt like flying a plane at stall speed.........

jwalker · 04 June 2007, 16:37

Quote:

Originally Posted by 9D280

...but if I do I'll go find a CalMac ferry and see if it does the same thing....

In me canoeing days, I had a notion of catching the Calmac bow wave and getting a free ride from Skye to Uist.

nikster · 05 June 2007, 09:06

transverse wave motion = bottom of wave (low pressure) reasonable clean not much air or sound - as you climb up towards the crest (into the high pressure) huge turbulence, air and sound = drag and reduced performance - you basically stick to the wave as your properties match the properties of the wave chemistry - ie. your not helping as you are decreasing the densisty of the water by artificially adding more air with your prop.

Depending on engine size / configuration you could get yourself in a right tight spot......

The closer you are to the vessel and depending on the vessel size depends on severity of problem.

jwalker · 05 June 2007, 09:17

If you're going to Google, you need to do it properly. If I'm not mistaken, that picture is describing the mechanism for the propagation of a sound wave in air.

nikster · 05 June 2007, 23:55

hey jwalker - you are entirely correct - the wave structure is exactly the same as for sound - due to the airated mass in turbulence on the leeward side of the wave based on direction..... the front of the wave is clean - from the crest to the trough behind the wave is nothing but turbulence, rotating water and air = sound :-)

remember the whole reason your boat sticks to the leeward side is due to the wave vortices getting stronger to the point of the crest.

Think of a snowball the size of a tennis ball being the bottom of the trough, then envisage a snow ball the size of 3mtr rib being just before the crest of the wave ####

Now you have an idea of the structure and mechanism of a wave and how it wants to keep your boat pinned on the leeward size of the wave in front.

Imagine also trying to operate your boat in a lake full of thick heavy oil - as that is the power of turbulence / rotor of wave power..

Shall we discuss my mastermind subject wake vortex?......

Its all good..:-)

Feel free to shoot me down...

Biggles · 06 June 2007, 11:26

No it all seems good to me. I thought their had to be a technical explanation and that it just wasn't me imagining things.

jwalker · 06 June 2007, 12:55

Quote:

Originally Posted by nikster

...the wave structure is exactly the same as for sound .....

Nah, it's not. An ocean wave is based on rotating particle movement. It's thought to be trochoidal in shape rather than sinusoidal. The diameter of the paths of the particles is dependent on the depth of water, it tends to flatten with depth and eventually may become a back and forth movement. I'm not a diver, but I can recall watching diving film footage and seeing the weed and plants at the seabed moving softly back and forth. I think this may be the result of the flattened particle movement as the waves pass above.

Next time you're out in wavy conditions, get beam on the the sea and watch a particular wave. You'll see it form, advance and then subside. It'll rise again a bit further back of the position where it subsided. The particle circles advance slightly along with the wave. However, when a wave bottoms out as they do coming in to a beach the shape begins to change, they become unstable and they eventually become advancing water. If my memory serves, when this happens I think they draw their water from the front of the wave, lift it up and pour it out as the breaking crest. Consequently, the water in the trough in front of a breaking wave becomes quite shallow.
This is what produces the powerful rolling action in surf. The top of the wave moves inshore but the bottom of the wave moves offshore. Anything beam on to the front of the break has a good chance of being rolled over. It might actually be righted again too because the water within the wave front is upward moving.

I don't know for sure but I wouldn't be surprised if big ocean swells are bottomed out waves and advance continuously rather than subsiding as the lesser waves do. But I'm guessing here.

Quote:

Feel free to shoot me down...

nikster · 06 June 2007, 13:48

jwalker - you seem to have switched subjects....?? your now talking about waves based on wind and current... i thought this thread was about wake???? they are not the same - as wake is a forced movement not a natural movement - even with your particles swirling about - its all still SOUND.... :-)

either way - its all a giggle - who wants to come and see a ship test tank with dye colouring to learn about the structure of wake & waves then?

jwalker · 06 June 2007, 14:10

Quote:

Originally Posted by nikster

jwalker - you seem to have switched subjects....??

Yeh, I did a bit. I wondered about a wake being different but just went ahead anyway.

However, the phenomenon Biggles is describing also occurs on the backs of ocean waves especially if they have bottomed out. It can be difficult, or impossible, to climb up the back and over some waves.

I don't understand your explanation of the airation and turbulance in the wave and the snowballs.

Quote:

- its all a giggle - who wants to come and see a ship test tank with dye colouring to learn about the structure of wake & waves then?

I'd love to.

Biggles · 06 June 2007, 14:12

As JW says I have noticed this on both large waves and large wakes.

nikster · 06 June 2007, 14:30

you will notice it on the leeward side of any wave - but if its wake - your ability to power out of it will be greatly reduced.

Natural waves have much more powerful rotation from the trough to 40% of the wave height - if you could stick razor blades on the vortices - you would not end up with a fragment bigger than a ten pence piece - based on a 6 mtr boat -