Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 17-06-2016, 16:57   #1
Registered User

Join Date: Aug 2015
Location: Port Townsend, WA
Boat: Tashiba-31
Posts: 216
Images: 1
Math!

Ok, school was a looong time ago.

I'm looking at the NOAA website that shows sea conditions at various bouys across the globe. Neat site. I see that the bouy off the Oregon coast is showing a wave height of 5-ft and a average period of 6 seconds.

Question - what would the distance between wave crests be?
__________________

__________________
dmksails is offline   Reply With Quote
Old 17-06-2016, 18:06   #2
Registered User

Join Date: Jul 2015
Location: Plano, Texas
Posts: 68
Re: MATH!

Here is a handy little calculator from the fine folks at hyperphysics (one of my favorite web sites for such things). BTW, this is not a high school subject. I didn't run across this until I took a fluids course eons ago. I taught AP Physics a few years ago and mentioned this, but we didn't do the math because the derivation of this thing is a beast having to do with energy transfer.

Wave Motion

I vaguely remember the way the formula looked, but not the numbers. All I really remembered about it is was it had to do with the depth of the water and the wave motion looks mostly like a sin wave.

Good luck bouncing around.

Regards,
Doug.
__________________

__________________
dwoodall is offline   Reply With Quote
Old 17-06-2016, 18:34   #3
Registered User
 
Ziggy's Avatar

Join Date: Apr 2008
Location: U.S., Northeast
Boat: Contessa 32
Posts: 1,421
Images: 2
Re: MATH!

Quote:
Originally Posted by dmksails View Post
Ok, school was a looong time ago.

I'm looking at the NOAA website that shows sea conditions at various bouys across the globe. Neat site. I see that the bouy off the Oregon coast is showing a wave height of 5-ft and a average period of 6 seconds.

Question - what would the distance between wave crests be?
Assuming the buoy is in deep water, the relationship is:

wavelength = gT^2 / 2pi, where g is acceleration due to gravity (9.8 m/s^2), T is the wave period, and pi is the ratio of circumference to diameter of a circle (3.1415926....), or:

wavelength = 1.56 * T^2

So if the wave period is 6 s, the wavelength is about 56 m or 184 feet.

It gets more complicated in shallow water (depth less than wavelength / 4)
__________________
... He knows the chart is not the sea.
-- Philip Booth, Chart 1203
Ziggy is offline   Reply With Quote
Old 17-06-2016, 18:36   #4
Registered User
 
AnglaisInHull's Avatar

Join Date: May 2015
Location: Sailing Lake Ontario
Boat: Mirage 35
Posts: 508
Re: MATH!

WOW! This is what I love about this forum: you can ask an obscure question like this and somebody will have useful information.

__________________
My boat is like me. People look, and say "... pretty good shape ... for its age ..."
AnglaisInHull is offline   Reply With Quote
Old 17-06-2016, 18:54   #5
Senior Cruiser
 
StuM's Avatar

Cruisers Forum Supporter

Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43
Posts: 6,713
Re: MATH!

About 184 ft or 56 meters. And height is irrelevant.

A couple of useful conversions:
Speed of wave in knots = 3 x Period.
Length of wave is 1.56 x Period Squared in meters, x 5.12 in feet

It all comes back to the same equation as used for Hull Speed
Speed of a wave = 1.34 x Sqrt(length of wave)
and period is length/speed.
__________________
StuM is online now   Reply With Quote
Old 17-06-2016, 20:18   #6
Senior Cruiser

Cruisers Forum Supporter

Join Date: Feb 2012
Location: Carrabelle, Florida
Boat: Self-built 44' steel trawler
Posts: 873
Re: MATH!

To clarify the completely correct but slightly confusing formula above, wave length in feet equals 5.12 times wave period squared.
__________________
tkeithlu is offline   Reply With Quote
Old 18-06-2016, 13:37   #7
Registered User

Join Date: Aug 2015
Location: Port Townsend, WA
Boat: Tashiba-31
Posts: 216
Images: 1
Re: MATH!

Thanks - exactly what I was looking for!
__________________

__________________
dmksails is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Check My Math - DC Charger AC Draw - Generator Sizing Patrick_DeepPlaya Electrical: Batteries, Generators & Solar 11 30-01-2012 08:57
Math is dangerous; proof 2=1 avb3 Challenges 8 30-12-2011 11:34
boating math samson Off Topic Forum 4 20-03-2011 21:07
Pulley ratios or (I hate math) Tellie Engines and Propulsion Systems 10 29-04-2008 23:58
pulley/rpm/math question JusDreaming Plumbing Systems and Fixtures 1 28-06-2007 07:42



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 21:51.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.