Wave Heights

[dhays]

All you guys are very smart and I enjoy hearing your ideas and feedback. I will need to think about it some more but here are some of my early take-away ideas.

I think that waves may generally be traveling faster than we realize.

Slow speed waves are a lot more problematic than fast waves.

Wave height increases as the velocity and wavelength decreases. Wave velocity slows as the bottom gets shallow, when waves run into a contrary current, or when waves run counter to wind.

What matters to us isn't wave velocity, but period/wavelength. A Tsunami wave can travel at 500 mph in the open ocean yet have such a long wavelength and resultantly low amplitude that It would be felt. However, as the wave interacts with the bottom and starts to slow, it's energy is compacted and the amplitude and frequency increase. That is what created the problem.

Fundamentally, I think wind waves and deep ocean waves behave much the same as a theoretical model, however there is more "noise" in the system typically with wind waves.

As someone who has been caught both in Cattle Pass and off Point Wilson when wind, current, and ocean swell all had different ideas as to what direction the water should be moving. I can say with confidence that no equation can predict or describe the sea state that results.

 

[luna]

Discussion in this leg of thread-drifr has been either an 8-foot/8-second wave travels at 326-feet in 8-secs (roughly 25 kts); or it travels about 160-ft in 8-secs (12-kts). One or the other is true but not both. @luna, if I understand correctly, you're saying both can be true? I thought speed of a wave was bracketed by physical properties of height and water depth. I didn't do well in physics but it sounds like you're saying there are other properties that govern wave speed. Thoughts?

I of course agree there are different types of waves (wind waves, swell, upheave). There's probably a better way of saying it, but I also understand the water in a non-breaking wave does not move much along the X-axis but does in the Y-axis as an oscillation wave.

The surfer example is most definitely germane to anyone who transits a bar/inlet.

Peter

The OP questioned his boats abilities when subjected to 5’-7’ waves at 8 seconds. I picked 6’ as it’s kinda in the middle. You seem to be the only one talking about 8’ waves.
The calculations by dhays and my rule of thumb don’t use wave height, just period, pi and acceleration due to gravity.

So yes, I am saying both can be true as they are 2 different types of waves. Their height and period are the same but their shape, speed and wavelength all differ markedly.
Why would you expect one equation to define both?

 

[SteveK]

@mvweebles
I was taught long ago that the speed in Kts. of a wind wave is about 1.5 times the period in seconds. An 8 second wind wave is therefore moving at about 13 Kts

Peter. 8*1.5=12
Google helped me find "one knot (kn) = 1.68780985710119 feet per second (ft/sec)"
If we use your 1.5 times reported seconds (8) for speed (12) then we have
1.68780985710119 X 12 X 8 = 162.0297462817142 feet

Edited after post 36

 

[Benthic2]

We have all seen standing waves that don't move, so I think observable data indicates that waves can have a speed from 0 to X. The value of X doesn't matter, as it means that waves do not move at a constant speed, and therefore, distance between peaks can't be calculated.

 

[luna]

But isn’t that standing wave moving through the water?
I see them all the time in tidal passes where the water is moving at 8 to 15 knots yet the wave remains stationary referenced to land.

Don’t all waves move through the water?

 

[luna]

Steve

If one knot equals 1.69 feet per second then at 12 knots the wave crest would be moving 20.28 feet per second.

If the waves are 8 seconds apart in time, how far apart in distance are they?

 

[SteveK]

Steve

If one knot equals 1.69 feet per second then at 12 knots the wave crest would be moving 20.28 feet per second.

If the waves are 8 seconds apart in time, how far apart in distance are they?

Thanks, I forgot the 8 seconds, have edited post 33

 

[dhays]

But isn’t that standing wave moving through the water?
I see them all the time in tidal passes where the water is moving at 8 to 15 knots yet the wave remains stationary referenced to land.

Don’t all waves move through the water?

I would agree that all waves move relative to the water that is creating them. The standing waves sure are unique though.

 

[mvweebles]

The OP questioned his boats abilities when subjected to 5’-7’ waves at 8 seconds. I picked 6’ as it’s kinda in the middle. You seem to be the only one talking about 8’ waves.
The calculations by dhays and my rule of thumb don’t use wave height, just period, pi and acceleration due to gravity.

So yes, I am saying both can be true as they are 2 different types of waves. Their height and period are the same but their shape, speed and wavelength all differ markedly.
Why would you expect one equation to define both?

I only picked 8-footers because the calculations were already described in earlier posts.

I found a discussion from the Dept of Oceanography at University of Hawaii. Although academic, made sense to me. Best I can tell, it is not possible for deep water waves to have the same height but travel at dramatically different speeds. The 28-mph calculation given by @dhays is confirmed.

https://www.soest.hawaii.edu/oceanography/courses_html/OCN201/littlepages/waves.html#:~:text=waves%20regardless%20of%20ocean%20depth,Wind%20wave%20formation

Some tidbit quotes:

Major formation forces:

• Wind
• Atmospheric pressure
• Landslides or other earth movements
• Gravitational attraction

Deep and shallow water waves

• The wave does not move forward only the energy in it progresses
• Wind waves have the most energy in surface ocean
• Relationship between wavelength and water depth determines wave characteristics
• Deep water wave: water depth >1/2 wavelength orbits die away above bottom
• Shallow water wave: water depth <1/20 wavelength orbits are flattened at the bottom
• Transitional wave: water depth >1/20 but <1/2 wavelength wave "feels" bottom
• Gravity and seismic waves have very long wavelengths are always shallow water waves regardless of ocean depth

Deep water waves

• Velocity of wave energy through water determined by wavelength
• Longer waves move faster
• Use period as it is easier to measure than length
• Speed (m/sec) = 1.56 x wave period
• Typical 8 second trade wind wave moves at 12.4 m/sec=28 mph

Peter

 

[mvweebles]

But isn’t that standing wave moving through the water?
I see them all the time in tidal passes where the water is moving at 8 to 15 knots yet the wave remains stationary referenced to land.

Don’t all waves move through the water?

I was once in a town in Nova Scotia called Truro. At the time, their tourist attraction was a 'tidal bore.' Driven by Nova Scotia's infamous tidal range, the incoming tide against the outgoing river current would cause an incredibly slow moving 'standing wave' that was easily watched from a nearby restaurant strategically located on an oxbow. Type of thing that passed for entertainment in pre-internet days.

Leaving aside current and wind, waves do not move through water. According the University of Hawaii paper I cited above, "The wave does not move forward only the energy in it progresses." A tidal bore or some other current-induced event obviously involves moving water. But in their natural state, apparently the waves do not move. Wind waves are a different matter - clearly wind moves water which is shown to dramatic effect during hurricanes in bays.

As long as the OP stays out of the Gulf Stream, unlikely to encounter standing waves.

Peter

 

[dhays]

• Typical 8 second trade wind wave moves at 12.4 m/sec=28 mph

Peter

That is a great bit of information. Thanks. As I suspected, the waves are a lot faster than I had ever imagined.

Edit: the energy propagation of a wave is a lot faster than I had imagined. ;-)

 

[SteveK]

my research into waves has concluded that my old brain was used to reports in imperial. Metric wave height is a lesser number (noit amount) than imperial. The relationship between imperial wave height and seconds translated a brain message.
An imperial 4 foot (1.2m) wave height at 4 seconds is rough but at 10 seconds it is not. so if the seconds are more than 2.5 times the height it can be comfortable ride. 1.2 x2.5=3 (done in my head) did not compute.

 

[dhays]

my research into waves has concluded that my old brain was used to reports in imperial. Metric wave height is a lesser number (noit amount) than imperial. The relationship between imperial wave height and seconds translated a brain message.
An imperial 4 foot (1.2m) wave height at 4 seconds is rough but at 10 seconds it is not. so if the seconds are more than 2.5 times the height it can be comfortable ride. 1.2 x2.5=3 (done in my head) did not compute.

That is a good point, particularly with the coincident discussion on NOAA changing to metric measurements of wave heights. After moving to power from sail, I slowly absorbed the concept that the closer the wave height in feet got to the period in seconds the more uncomfortable the ride will be.

With measurements of wave height in meters, I will need to create a different "comfort metric". An easy shorthand may be 1:4. If the wave height to period ratio is less than 1:4 chances are that my wife won't be too mad at me. That 1:4 ratio is not comfortable in my non-stabilized boat, but it isn't overly uncomfortable unless right on the beam.

 

[Greg QS]

I have basically the same boat. 70000lbs with fins.
I would take that on the stern and power up to surf the seas.
Taking that on the beam for a period of time , even with fins, will be uncomfortable.
Even at 70000 and loa 67' I only plan crossings at a max of 4' seas. 4' can easily turn into 6-8' seas which is doable.
Whatever the sea state your in there are always bigger waves in the mix. So at 6-8 you will have some 10-12 mixed in and if it blows up, you need to be prepared for 10-12 with even larger mixed in. On the beam will be quite ugly. Of course it can always be nicer.