Wave Heights

[hmason]

We left Stuart FL for a trip to Hilton Head Island, SC. Since I don't enjoy the AICW going through Georgia we went off shore for that portion of the route. Wave heights were predicted at 1 to 2 feet and we had a great offshore run. This was my second time going offshore and it was most enjoyable. The first time was on our GB 46.

Now some details and then my question. Our current boat is a Pacific Mariner, 65 feet in length (70' with the swim platform included), active fin stabilization, and weighing 70,000 lbs. loaded. We are planning to return to Stuart later this week. Current wave height predictions are east 5' to 7' with a period of 8 seconds. It will be a beam sea as we are headed south. Is this acceptable for a "comfortable" ride and more importantly what is the acceptable wave height for a SAFE ride. Thanks to all. Howard

 

[rslifkin]

5-7 feet at 8 seconds should be safe even on the beam in that boat. With stabilisers I wouldn't expect any major comfort problems, but if they're closer to 7 feet at 8 seconds that's getting a little steep, so you may still have a bit of unpleasant motion. 5 feet at 8 seconds should be pretty comfy.

 

[READY]

---

First this is a subjective issue when talking about comfort. With that boat I believe you are sell within the reasonable safety range, even if your stabilizers failed. I would think with the beam seas even with the stabilizers on it might not be your most comfortable trip but also not terrible either. You can adjust your course if needed to make the seas less "on the beam" but that will extend the time at sea. Be sure to secure EVERYTHING onboard as items moving about not only can be unsafe but it adds to the anxiety level.

I would expect you will be subject to some significant rolling during the trip so anyone prone to motion sickness is likely to be impacted.

Remember that the forecast is based upon an average so by definition you could see some waves higher. I'd on the beam it is hard to imagine that even great stabilizers can neutralize a 10 foot beam swell.
When possible keeping you speed up will help improve the performance of the stabilizers.

If you are not comfortable with rougher conditions you probably should stay in the ICW. Nothing worse than having captain or crew regretting that they could be enjoying a calm ICW trip vs bouncing outside.

 

[mvweebles]

As an ex-delivery skipper, I'd go with your boat and that forecast. As a "Civilian" these days, I'd wait for a better wx window, especially if my wife were aboard. Decent weather is fairly common (beware wind against the Gulfstream, and hug the shoreline headed south). In 7-ft/8-secs even on the beam with stabilizers, you'll get jostled abruptly and have a small crop of thigh-bruises from repeatedly leaning against a few pressure points.

Not unsafe, and not tremendously uncomfortable, but something I'd wait out.

Best of luck -

Peter

 

[SteveK]

Current wave height predictions are east 5' to 7' with a period of 8 seconds

what is the distance between wave crests? How can you calculate it from this info?

 

[guy with a boat]

As an ex-delivery skipper, I'd go with your boat and that forecast. As a "Civilian" these days, I'd wait for a better wx window, especially if my wife were aboard. Decent weather is fairly common (beware wind against the Gulfstream, and hug the shoreline headed south). In 7-ft/8-secs even on the beam with stabilizers, you'll get jostled abruptly and have a small crop of thigh-bruises from repeatedly leaning against a few pressure points.

Not unsafe, and not tremendously uncomfortable, but something I'd wait out.

Best of luck -

Peter

As usual, I agree with Peter’s assessment. I won’t even try to add anything because Peter says it better than I can.

 

[hmason]

Thanks to all. Peter makes excellent points. Unless predictions change I'll likely stay inside the AICW

 

[mvweebles]

The only thing I'd add is it helps to understand what's driving the sea state. Short duration wave sets are generally caused either by wind or convergence of different wave patterns ("Confused Seas"). I don't have much knowledge on how the Gulf Stream would affect the weather patterns, but do know enough to have a LOT of respect for an ocean river moving at 2-3 knots. I don't know the coast well, but if you can poke your nose out with a decent bail-out back to the AICW, might be worth a peek. Just know the conditions around inlets tend to be more tumultuous than just a mile or so away. Its possible there is a counter-current close to the coast so hugging the coastline might make sense, but someone with local knowledge would trump anything I have to say.

Enjoy - nice boat!

Peter

 

[psneeld]

This should always be kept in mind for wave forecasts....although I saw where the system is being changed.

....from National Weather Service site on significant wave heights....

"On average, about 15% of waves will equal or exceed the significant wave height. The highest 10% of waves could be 25-30% higher than the significant wave height. And on occasion (about one per hour) one can expect to see a wave nearly twice the significant wave height."

I have had to assist a friend whose 55 foot Viking MY that had a bad stabilizer and his pro skipper refused to run the boat in lesser conditions. and that was only a 13 mile run offshore. I took the boat up the NJICW so they didn't have to sit in Cape May till the seas died down. You never know when one can fail.

 

[dhays]

what is the distance between wave crests? How can you calculate it from this info?

Assuming deep water (ie the depth is greater than 1/2 the wavelength) I believe the formula is:
L=gT^2/2π
L=Wavelenght
g=Acceleration due to gravity (9.8m/s^2 or 32ft/s^2)
T=Period of waves (the inverse of the frequency)

Depending on if you like to think in meters or feet, you can simplify the result to either:
L=5.09 x T^2 for wavelengths in feet
L=1.56 x t^2 for wavelengths in meters.

So in the OPs example where he gave wave heights in feet, we can get a wavelength in feet by:
L=5.09 x 8^2 or about 326 feet.

 

[dhays]

This should always be kept in mind for wave forecasts....although I saw where the system is being changed.

....from National Weather Service site on significant wave heights....

"On average, about 15% of waves will equal or exceed the significant wave height. The highest 10% of waves could be 25-30% higher than the significant wave height. And on occasion (about one per hour) one can expect to see a wave nearly twice the significant wave height."

I have had to assist a friend whose 55 foot Viking MY that had a bad stabilizer and his pro skipper refused to run the boat in lesser conditions. and that was only a 13 mile run offshore. I took the boat up the NJICW so they didn't have to sit in Cape May till the seas died down. You never know when one can fail.

That is great information, thanks. Unless I'm crossing Juan de Fuca Strait I don't see ocean swells. Plenty of wind waves lots of wind/current chop however. Unfortunately, those can be very nasty and often depend on local knowledge. I just saw this from the NOAH website, which was new to me. New format for wave reporting.

 

[MYTraveler]

Assuming deep water (ie the depth is greater than 1/2 the wavelength) I believe the formula is:
L=gT^2/2π

So in the OPs example where he gave wave heights in feet, we can get a wavelength in feet by:
L=5.09 x 8^2 or about 326 feet.

Doesn't that still need to be divided by 2π?

 

[dhays]

Doesn't that still need to be divided by 2π?

Well, let me see.... not sure if I recall my basic algebra.
acceleration due to gravity is 32ft/sec/sec
L=gT^2/2π or
L=32/2π x T^2
L= 16/π x T^2
L= 5.09 x T^2
If T=8 then
L=5.09 x 64 or
L= 326 feet rounded up.

So I think it looks right, but I could always be wrong.

 

[hmason]

I was terrible at math but as I recall acceleration due to gravity is 16ft/sec/sec.

 

[psneeld]

Generic Internet source....

Near Earth's surface, the acceleration due to gravity, accurate to 2 significant figures, is 9.8 m/s2 (32 ft/s2).

 

[dhays]

Generic Internet source....

Near Earth's surface, the acceleration due to gravity, accurate to 2 significant figures, is 9.8 m/s2 (32 ft/s2).

I have no idea why, but that 32ft/sec/sec is an odd bit of knowledge that I have had stuck in my brain since high school physics. I never remember the metric so always have to convert it.

 

[psneeld]

I amaze friends with the strange things like 32ft/sec/sec I remember instead of useful stuff...wish I could remember the therapist phone numbers they keep giving me...

 

[DDW]

The confusion with 16 vs 32 is often because that is the distance an object will travel dropping in the first second. Because it starts at zero, ends the second at 32 ft/sec, therefore its average speed for the second is 16 ft/sec, and it will drop 16 ft.

 

[guy with a boat]

The confusion with 16 vs 32 is often because that is the distance an object will travel dropping in the first second. Because it starts at zero, ends the second at 32 ft/sec, therefore its average speed for the second is 16 ft/sec, and it will drop 16 ft.

Bingo.

 

[Benthic2]

So in the OPs example where he gave wave heights in feet, we can get a wavelength in feet by:
L=5.09 x 8^2 or about 326 feet.

So.....if the wave crests are 326 feet apart with a period of 8 seconds, I think that means the waves move at almost 30 miles per hour. Something's not adding up.
( 326ft/8 sec x 3600 sec/hour x1mile/5280 feet = 28 mph)

 

[MYTraveler]

Well, let me see.... not sure if I recall my basic algebra.
acceleration due to gravity is 32ft/sec/sec
L=gT^2/2π or
L=32/2π x T^2
L= 16/π x T^2
L= 5.09 x T^2
If T=8 then
L=5.09 x 64 or
L= 326 feet rounded up.

So I think it looks right, but I could always be wrong.

You are right. I stand corrected.

 

[SteveK]

Is it possible this higher math is calculating radio waves?

 

[mvweebles]

Looks like Surfers track this well ---- scroll down to tables halfway down page.

Wave - Essential Surfing

essentialsurfing.com

An 8-foot wave appears to travel around 20-feet/second. So 7-8 seconds apart would be around 150-feet apart.

Peter

 

[dhays]

Looks like Surfers track this well ---- scroll down to tables halfway down page.

Wave - Essential Surfing

essentialsurfing.com

An 8-foot wave appears to travel around 20-feet/second. So 7-8 seconds apart would be around 150-feet apart.

Peter

Shallow and deep water waves behave differently. Surfers are concerned with shallow water waves.
Deep water waves travel at:
V= L/T
V= wave speed
f= wave frequency
L= wavelength

However, when waves start to interact with the seafloor, the waves slow down. When this happens the wave height increases and the wavelength decreases. The period of the wave stays constant.

The speed of shallow water waves is dependent on gravity and the Depth of the water. In this case I believe it is
V (shallow)=√(gd)
g= acceleration due to gravity
d= depth of water

To be honest, this seems counter-intuitive. Maybe because I am not a surfer. This says that regardless of the speed of the ocean waves, when they get to shallow water their speed will slow down to that which is governed solely by the depth of the water. However, I was at the Oregon Coast again for a week this last month and it seams that regardless of the weather the rhythm of the waves remained constant even if the size of the waves changed

This would only be for depths of water that are <1/20 of the wavelength. So for the 326ft wavelength of the OPs example, this would be when those waves approaches a shoal of 16' or less.

However, before the depth gets that Shallow, the waves would have started to steepen and shorten when the depths reached 165'. My math can't handle what happens in that transition between deep water and shallow water waves.

 

[mvweebles]

Shallow and deep water waves behave differently. Surfers are concerned with shallow water waves.
Deep water waves travel at:
V= L/T
V= wave speed
f= wave frequency
L= wavelength

However, when waves start to interact with the seafloor, the waves slow down. When this happens the wave height increases and the wavelength decreases. The period of the wave stays constant.

The speed of shallow water waves is dependent on gravity and the Depth of the water. In this case I believe it is
V (shallow)=√(gd)
g= acceleration due to gravity
d= depth of water

To be honest, this seems counter-intuitive. Maybe because I am not a surfer. This says that regardless of the speed of the ocean waves, when they get to shallow water their speed will slow down to that which is governed solely by the depth of the water. However, I was at the Oregon Coast again for a week this last month and it seams that regardless of the weather the rhythm of the waves remained constant even if the size of the waves changed

This would only be for depths of water that are <1/20 of the wavelength. So for the 326ft wavelength of the OPs example, this would be when those waves approaches a shoal of 16' or less.

However, before the depth gets that Shallow, the waves would have started to steepen and shorten when the depths reached 165'. My math can't handle what happens in that transition between deep water and shallow water waves.

I dunno. I haven't been in 8-foot following seas in many years. 325-feet in 8-seconds works out to around 25-knots. I just don't remember them going that fast. The surfer dude article says around half that speed which, from memory, feels about right.

Are you sure the math is right or it's the correct formula for ocean waves? I can only add subjective input from a faded memory.

Thoughts?

Peter

 

[SteveK]

Current wave height predictions are east 5' to 7' with a period of 8 seconds

what is the distance between wave crests? How can you calculate it from this info?

I still do not have an answer. I just cannot understand how to calculate the distance the wave travels in 8 seconds without knowing the speed. I was expecting the 8 seconds to mean something other than time, like longitude/latitude maybe as a distance.
one second longitude equals 80 feet. (640 feet?)
one-second latitude equals 101 feet. (808 feet?)
That is crazier than 326 feet. Is it possible to be 32.6 feet?

 

[mvweebles]

Minutes/seconds for latitude describe distance and only have an oblique relationship to time. And only latitude measurements are universally applicable as longitude varies widely based on distance from equator/poles.

While this thread tangent is interesting, its not overly useful. Seas are widely reported in time between waves, not distance. "Square waves" that are equal numbers for time and height are, in my mind, the line where they are undeniably difficult. 8-feet at 15-seconds is a pleasure. 8-feet at 8-seconds sucks. Maybe the convention should have been 8-seconds at 160-feet but that's not where it is. Every synoptic chart shows seconds of time, not distance.

There have been two answers offered to the distance between 8-second waves. 326-feet using a mathematical formula. And 150-feet from a surfing magazine. 150-feet feels a bit more accurate to me but frankly, I've never thought of it too much. In all candor, more often than not there are waves on top of waves coming from several directions and it's hard to figure out what waves to time let alone what the distance between them are.

Peter

 

[luna]

I think one has to acknowledge that there is more than one kind of wave.

An open ocean swell of 6’ may well have a wavelength close to the length of an NFL playing field or 326’ and a period of 8 seconds which would mean it’s travelling at about 24 Kts.
This kind of wave is generally quite harmless in deep water as it is not very steep.

Wind waves on the other hand can be more than annoying at 6’ high with a period of 8 seconds.
I have no idea the underlying basis for the equation, which I should call a rule of thumb but 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, normally broadside, at least for me! The wavelength would then be about half an NFL field or 175’.

I don’t think the surfer info is of much use as they are typically looking for large swells that are slowing down as they feel the bottom, then steepen and break. That kind of place us Trawler Types would be smart to avoid.

 

[mvweebles]

I think one has to acknowledge that there is more than one kind of wave.

An open ocean swell of 6’ may well have a wavelength close to the length of an NFL playing field or 326’ and a period of 8 seconds which would mean it’s travelling at about 24 Kts.
This kind of wave is generally quite harmless in deep water as it is not very steep.

Wind waves on the other hand can be more than annoying at 6’ high with a period of 8 seconds.
I have no idea the underlying basis for the equation, which I should call a rule of thumb but 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, normally broadside, at least for me!

I don’t think the surfer info is of much use as they are typically looking for large swells that are slowing down as they feel the bottom, then steepen and break. That kind of place us Trawler Types would be smart to avoid.

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

 

[Insequent]

Tsunami behaviour is that waves slow down and increase in height as they enter more shallow water.

File:Propagation du tsunami en profondeur variable.gif - Wikipedia

en.wikipedia.org