The magic depth for downwind runs

[headmount]

I finally got Google Earth running again and it came back with all my old lines and placemarks after over a year of being kaput.  Was stoked it resurrected because I've been thinking about what is roughly the ideal water depth for a downwind run. 

We all know we need wind speeds of over 20 to really have an exciting run but 20+mph wind, traversing the 10 miles of 600-800' deep Pailolo Channel is not the same as what happens when the depth reduces significantly to 80' and below.  Was fortunate enough to see the abrupt difference when I was on Conners' chase boat the last time he raced M2M.  As soon as he hit the point off Kamalo he began to average 9-10mph for the remaining 15 miles.  That's exciting.  So what happened?

Placed old GPS path line charts next to Google Earth on my screen and ran my curser over the path on Google Earth(lower right hand corner shows elevations).  Did this for Maliko, southside and that tremendous section from Kamalo to Kaunakakai on Molokai.  All had depths less than 80' and from experience of many who have done these areas,  the hot sections seemed to be around 30-40 feet deep.

On the shallow extreme I've seen very shallow areas (less than 10' deep) kick up a great roll. But they're dragging heavy and much slower than the just-outside-the-reef depth of 30-40'.

I would think any body of water would be worth a good look if you have these depths and of course, the wind.

[LaPerouseBay]

Excellent topic HM.

I found this map when we last discussed depth stuff about maliko.  Anyhow, I looked at the link I had provided for the big map, but it's dead.  All I have is this little picture. I'll search around for another depth chart for the whole run.

I overlayed this picture on google earth, saved it.  Now it opens with the overlay each time.



Then opened garmin's training center and 'view'ed one of my favorite runs (5-28-12).  

Epic conditons - 40mph, perfect direction, no NW swell - macking E swells.  It's a typical line for me in the summer.  Occasionally go further in if the swells are smaller.



Here's a close up of the fathom chart.  The white dot is where I got annihilated. (Garmin training center lets you mark specific points.  The speed chart will tell the exact time).



Here's a video of the thrashing.  The water color changes at the end of the video, I miss a glide, and take a breaking wave on the head.  Three stitches on my eyebrow.  Super fun day.  I made 3 videos of that  run.  Camera was crooked after the beating, so the best swells went unrecorded.

I learned a valuable lesson that day.  When I watch the video, it clearly shows the swell's changing shape from deep to shallower water.  The color changes, but mainly its the shape of the swells.  Now I pay close attention when entering this area - no daydreaming.  I always recite the same line, 'pay attention, you are not in Kansas anymore'.  Losing hull speed in a ski is a bad idea on that reef.  The depth changes from about 30 feet to 15 feet in the video.  IF the chart and overlay are accurate.    

C'mon summer!!!



  

[JonathanC]

That's really interesting. When I did the M to M in 2010 with my buddy Dave I noticed a huge difference when we got past Kamalo, the funny thing was that up until that point I had been struggling to keep up with him, but after that point the tables were turned. We stopped every hour and regrouped and sat on our boards for a minute or two as a discipline (well OK we did skip the last hour stop because we could see the harbour!).

How often do you find when paddling with buddies that the same situation occurs, struggling to keep up, then suddenly they are struggling to keep up. I'm convinced its not just a matter of tiredness but a matter of certain board / rocker lines that suit different conditions, depths etc.

Dave was on a Foote Maliko 14 and I was on an SIC 14, Dave weighs about 200 and I weigh about 145. My experience with the SIC F14 was that there was a certain threshold of wave face steepness (or something) that suddenly made the board come alive, up until that point I really had to work to get it on a wave, but after the threshold it was absolutely effortless and I could link and focus more on the water than on grinding. I think the rocker on the Foote Maliko 14 somehow makes it more 'slippery' in less than optimal downwind waves, my theory is that when I tipped over that threshold we were both in the magic zone of easy linked wave catching and the steering of the F14 gave me a little advantage over Dave on the Footie. Mind you he did beat me across the line by 40 seconds....

I've also experienced the same total reversal situation when doing a run in Melbourne with a buddy. I was on a Naish Glide 17 and Paul was on an F16 V2, when we started the run I felt like I had a concrete block hanging out the back, just couldn't get onto glides and Paul was having to actively miss glides to let me keep up - we stayed together because its an open water crossing with a shipping channel in the middle. Later on in what I'm pretty sure is a shallow section the wind came up and I just took off, again having to stop to let Paul catch up. Mind you I always found that the Glide 17 had a 'sweet spot' where it suddenly came alive, just too much boat for me to push the rest of the time!. Just checked the marine chart and what I described almost exactly corresponded with the depth. The Glide 17 felt slow in 11 to 14 metres of water and faster in 6 to 7 metres of water, can certainly imagine that an average depth change like that would have a marked effect on the wave shape. We are talking a very short fetch here compared to Maui waves.  

What I do know was that the last 10 miles from Kamalo to the harbour was one of the most amazing downwind runs I've ever experienced

[LaPerouseBay]

Here's a better closeup of the 5-28-12 run.  The dots are the 4.5 minutes of the video.



Here's a very fun summer run, 9-3-12.  Lucky not to get mowed that day.



Here's a big east wind day with big NW and big E swells running.  12-15-12.  No need to be close to the reef for massive waves this day.  Most exiting glide ever on the white dot.  115 feet deep.  Swell got tall, ski was broadsliding down a very steep face.  Spooky day.  


  

[XLR8]

This is really cool.  Thank you for sharing both the general point and the specifics of your runs.  I'm going to check out depth charts of my favorite runs.  Ive considered the depth factor before, but not so specifically and only to make adjustments on the run, so to speak, based on swell size and where breaking waves were.  Generally I try to stay just outside of the break point, thinking that is when the swells start to pitch a bit.  Those are great maps of a place I would love to visit.

[LaPerouseBay]

Here's the link I used for the charts.

http://marine.geogarage.com/routes

The blue transition line is about 25 feet deep.

I've always been terrified of beat downs.  That's why I research gps lines and video so much.  I'm also interested in speed, but safety is first.  I seldom chase faster paddlers 'inside' when the reef approaches.       
   
Maui has countless experienced, fast paddlers in outrigger.  They like to 'run the ridge' of big breaking surf on a maliko run.  Boats get smashed every year. 

What concerns me are tourists.  Wind direction plays a big part of safety on that run.  A guide is a  VERY good idea for a first run.  A good line adds a lot of fun too.

[covesurfer]

This is an interesting thread. I am going to try and overlay my Garmin track from my Maliko's on Google Earth if it will let me, using Garmin Connect. I'd love to see where I was on the chart with the depths. After my last Maliko, which made my eyes get real big, I intend to get a paper NOAA chart that covers the north shore of Maui and really have a look at the bathymetry.

I only have had the thrill of three Maliko's, each one being progressively windier with larger swell than the one before. The last one, on 1/6/13, was pretty spooky for me, partly because the conditions were pretty burley and mostly because I did not know where the heck I was in relation to the depth. Fortunately, I got to paddle with HM and PB who DO know where they are and are fantastic guides but it's better to know for yourself your approximate exposure to getting a beat down. Especially if you ever lost sight of your paddling buddies.

Interesting how close in LPB's track is on the summer run vs. the run in December. As a newbie to Maliko, my perspective is that I'd want to have a guide for 6 to 10 runs in a variety of wind and swell conditions before I'd feel like I could pick my own safe line.

[sup_surf_giant]

I'm not smart enough to enjoy this thread. 

[LaPerouseBay]

Here's the winter and summer lines on the same map. 

At Kanaha, summer distance offshore is 1/2 mile, in 12 feet of water.

Winter path is out 2 miles, in 170 feet of water.  I'm usually way out, but further this day, due to offshore wind and poor visibility.  Most paddlers are inside my winter paths.       

The blue shading is at 5 fathoms (30 feet).

[PonoBill]

There's a lot of variation possible. Waves begin to slow when they "feel" the bottom at roughly 20 times the wavelength. The formula for wavelength given period for ocean waves is wavelength (in meters) = 1.56T**  A long period wave of 15 seconds has a wavelength of 350 meters (about 1100 feet) so the wave is slowing everywhere on the chart. But it's doing so very unevenly, which is why you see wind waves out of the east  refracting sharp left into lower Kanaha.

A small difference in period makes a big difference in how the wave acts. And it's not just the depth of the immediate location, it's also what preceded it. It takes time for a wave to stack up, the front needs to go slower than the back of the wave for some time for the energy to stack.

It's worth remembering that the water doesn't really move much--it travels in an elliptical path. It's the wave energy that we ride--or that rides us.

[Pierre]

really interesting topic... wave interaction with bottom is a very complex matter.
the wave feeling the bottom at 20 times length; Ponobill; I guess it's more likely 2 times... at least practically. mariners know danger of combined high seas over long swell when this hits edge of continental shelf; practically swells of 14-16 secs when reaching depth about 500 m and less. the most terrific being- practically and really- at1 going down to 1/2 wavelength.I personally observed a huge sea becoming terrific -over 80 ft  freak waves- in N Atlantic mostly on the 300-200 m range. swell period was 16 secs w/a 10-12 s breaking sea over it ..
for shorter wind waves we like t ride it's similar at smaller scale; but very complicated; another factor being current. generally too short waves break over longer ones and generate sets; etc.. best for us being length 1.5 to 2 times board length to easy catch and get enough speed for planing. if length is below; we stick on them; if above; more difficult to catch.
I like paddling in channels and when current against wind in shallow waters waves become short and crappy. in deeper waters they get better for a planning speed if wind 20 + knots if wind less it's generally too short.... When no current waves are shorter but steeper in shallow part; good in hot conditions.  In those inner waters best downwind fun board is a 12'6". if current with waves it makes them flat in deep water...
generally with small wavelets over bigger ones; interaction with shore/ shallow waters wind force ; longer well waves coming in etc and current ...things become very complicated
observation of environment conditions with basic knowledge of  wind and fetch to wave period/speed/length relation is a success factor. after that to know if conditions will fit our board is a complicated cuisine.
all incomes in that may help us to understand a bit more.

[PonoBill]

Sorry--brain fade. I should actually look these things up instead of trusting my memory. The definition of a deep water wave is a depth greater than wavelength/2. In other words waves start feeling the bottom at half their wavelength.

Feeling the bottom means that there is enough epicyclic movement at that depth for the bottom to hinder the movement and therefore apply drag to the wave. Since the movement of the water molecules gets larger and stronger nearer the surface, the shallower the water, the stronger the drag is.

[LaPerouseBay]

A 15 second swell will begin to feel the bottom at 576 feet.

Square the number of seconds between swells and multiply by 2.56.

(15x15)2.56=576'

Pierre,  I agree 100% with your comment that wave behavior across the ocean floor is very complex.  The charts are a very basic part of the puzzle.  I only do them to help me understand maliko.   

Like you say, current has an effect on our waves, as do various period waves from various directions.   

One of our best paddlers, Mark Raaphorst, believes it takes about 50 runs to start to understand maliko.  It's quite a chess match.   

[Pierre]

 yes that's L/2 I rechecked sources...I do not have the chance to know Maliko expept pix and movies.. .. unfortunately what i know better is my crappy home estuary chop and open sea waves. i believe a place like Maliko, with little wind/current changes can be variable, and the board which fits these local conditions: best rocker for wave velocity according rider's weight  etc... should be quite challenging. in our place, downwinding in very various conditions we need boards be much more all-purpose.
just  beow 1 pix of my home crappy water plane ( abt 25 kts wind 20 ft wave length good for a 12.6, nothing to compare with maloiko including colour... ) where deformation of waves due currents and  and 2 sheets about wave / wind/ fetch interrelation....
which will never replace experience i'm sure: a great downwind run is like a good wine, every bottle has a different taste

[headmount]

Isn't our average wave period in the summer around 6-8 secs?  So with that formula equaling  92-164 feet (for 6-8 secs),for the swells to start feeling the bottom, where do they feel it enough to stand sufficiently high enough to catch?  It seems to me that waves begin breaking at around 60 degree slope but we need less than a third of that to catch with extra long boards. 

But I've seen tall swells in the deep but they're going too fast to catch.  So  depth creating enough drag seems important for the wave speed to slow sufficiently for us to catch them.  From what I saw from two of the very best open ocean SUP racers, they got little more than brief shoves out in the Pailolo where it was 600-800 feet deep.  No prolonged glides like they encountered along the relative shallows (20-80 feet) from Kamalo to Kaunakakai.

God knows what's happening out there in the winter.  It's like every bottle of wine in the store was thrown in the bucket.

Contrasting to the southside when the direction is on... a very refined glass.  It's so cool to start your run right at the inception of the wind fetch and watch swells grow beneath your feet.  Within 10 mins, you go from pearls on the water to actual glides.  Out in the middle, knee to waist high.