Period of a Given Ht Waves as Related to Hull Length

[Cacia]

Is there a rule of thumb for assessing the ocean conditions for a "crossing" in a given length boat and certain NOAA conditions?

How do you interpret period as being "too short" to pull your nose up before the next wave hits?

Any implications for the speed you are taking on water? ..and the rate your bilge can work effectively?

How does a newbie make these decisions?

I am seeing that five foot seas with a short period could be more treacherous than bigger seas with a slower period between.

How do you learn to "think" about this?

 

[captain]

Is there a rule of thumb for assessing the ocean conditions for a "crossing" in a given length boat and certain NOAA conditions?


I am seeing that five foot seas with a short period could be more treacherous than bigger seas with a slower period between.

True

How do you learn to "think" about this?

Avoid square waves, period and height are about the same. A long period big wave is a more comfortable ride than a short period small wave. Watch the NOAA weather data bouys in your area
http://www.ndbc.noaa.gov/maps/Hawaii.shtml

This will give you a feel for what different wave heights and periods feel like.
There is no rule of thumb other than the difference between height and period. But sometimes there are wave systems moving in different directions which confuses the issue more, say a swell with a SE set and a wind wave on top with a N set. Only the hard teacher, Mr. Experience will teach you what you can handle. That happened to me about 21 years ago when I got hit buy a late summer cold front packing 50 knot wind and 12 to 15 foot seas in a 30 foot boat. After that nothing much bothered me. I won't go out in conditions anywhere near that but I have the confidence I can handle it if the boat can.
l
http://www.ndbc.noaa.gov/waveobs.shtml
http://www.ndbc.noaa.gov/measdes.shtml

 

[Charlie99]

The short answer is that nothing beats the experience that comes from always looking at the marine forecast, and then watching how the wind waves and swells affect your boat.

http://www.seafriends.org.nz/oceano/waves.htm is a good review of wave mechanics, but unfortunately doesn't directly address your question.

One thing that may help is that most of the sea state forecasts for Hawaii include separate forecasts for swells and waves; and that the waves generated by the trades have a daily cycle that helps to ID them (of course, nearly always coming from N to E quadrant always helps too). Wind waves will have shorter periods and sharper faces for a given height than will the equivalent height far-away-generated swell.

Pay attention to how your boat performs under various conditions and I'll bet that in open water what really matters the most are the short period wind waves, and that the long period swells only affect you indirectly as the form interference patterns with the waves.

OTOH, as the swells enter shallow water, then their height will grow --- this is very localized and why some places are great surfing spots and others aren't. As swells hit shallow water, they not only grow, but can also get sharper faces. You can usually tell the difference between wind waves and swells that are hitting shallow water simply by noting the direction and period.

Confused seas -- where there are 2, 3 or more different swell and wave patterns -- can be really bad when every once in a while the different waves and swells combine to a bigger breaking wave. Another problem that isn't quite as obvious are "holes in the water" --- where different troughs combine. You don't see those until you are there. I've been airborne (or at least it seemed so even if not really true) in RIBs that have launched out over one of these troughs. The flight ain't bad, but the landing sure is rough.

 

[Charlie99]

That happened to me about 21 years ago when I got hit buy a late summer cold front packing 50 knot wind and 12 to 15 foot seas in a 30 foot boat. After that nothing much bothered me. I won't go out in conditions anywhere near that but I have the confidence I can handle it if the boat can.

It doesn't matter how big the boat is, there's a wave out there somewhere that's bigger than it.

The worst waves I remember were in the relatively shallow North Sea, while running at speed into heavy seas in a 600' missile cruiser. Looking up at waves from a bridge with eye height of 36' above sea level. Green water not quite reaching bridge level. Ripped off about 150' of life lines, dented up a 5" gun turret, bent up a pretty massive weatherbreak. 32 years ago last winter, and I still shudder thinking about it. Never saw anything like it before or after.

It was supposed to be a short little run from Bremerhaven to Oslo fjord and I wasn't even supposed to stand watch, but the other officers-of-the-deck were puking out their guts. So in spite of having been out to bars until 2 hours before departure, the Captain decided I'd stand watch, even after I told him I was still drunk. By far my biggest DUI offense.

 

[captain]

It doesn't matter how big the boat is, there's a wave out there somewhere that's bigger than it.

You ain't wrong about that.

 

[Cacia]

yea, I was reminded of a bad voyage yesterday lunching with someone I share a bond from a very bad crossing where my whole body shook and I stuffed trash bags in my pockets.

he just bought a 26 ft cat and was explaining to me why that crossing we did four years ago was so bad.

Confused seas the backside of Portlock, converging currents, 25 mph wind...it still gives me flashbacks. I guess they don't call it "Witches Brew" for nothing

Now I am noticing our boat does a wierd thing in following seas. I'll read the sites, thanks. I want to be able to say "oh 9 seconds....that is going to probably be tolerable with four ft seas, etc"

Cats handle well, but you have that two hull ride that can feel not so reassuring, like tripping.

32 years ago last winter, and I still shudder thinking about it.

I bet

Sometimes I feel mildly traumatized by my little adventure. I have a respect I did not have before, that's for sure. It has made me a little timid though. I am trying to work through that in bitesize peices.

 

[matt_unique]

Yeah basically ditto on the sea time. The marine forecast is as important as fuel, but you may arrive to find it totally different. The buoy's are great for some general information but the closest buoy may be 100 miles out to sea from where you will be cruising.

As we all know it's about the shape of the wave, usually determined by wind (to a lesser degree current). 10' swells can be a piece of cake....like riding a freight elevator. 5' cresting seas in Gale Force winds are scary.

In terms of handling the seas...you want to avoid accelerating over the crest of one wave and "surfing" into the trough of the next. This just comes from a sense of "feel" which I'm sure you have from your experience on your boat. Going over seas requires constant throttle adjustments to push to get up then pull back to slow down until you pass the next trough.

I suggest new captains slowly build their experience and comfort with their boat with controlled exposure to various conditions. Don't go out in a storm but try some time handling in rougher seas.

--Matt

 

[Chuck Tribolet]

The forecast tells you when to stay home,
the Mk I eyeball tells you when to go out.

Scariest I ever had was six foot breaking chop.
Got the whaler airborne at idle speed.
All of a sudden there's no water noise on the hull,
the prop jumps from 800 RPM to 1200,
and there's lots of exhaust noise.
Then the boat drops six feet straight down.


Chuck