Water depth correlation to wake size

Question for all the physics gurus out there...

We normally ride in a river that is 15 ish feet deep in most spots. Occasionally when we travel to a lake where the water is quite deeper, using the same setup, the wake is noticeably larger. So what is the scientific answer for this? One other factor that may or may not be relevant - the river we ride is brackish as opposed to the fresh water of the lakes.

Deeper water equals a bigger wake. But usually the minimum depth to maximize a wake is 12 feet, but don't quote me on that.

But why? Not disputing - just curious.

Our lake is 4ft all over and the wake sucks. When we take the same boat on the sea the wake is double the size.

From my experience wake sufers below 10ft. I've not really seen any difference in wake above 10ft.

You can notice between 5 and 10 feet
you can also notice between 10 and 15 ft.
after 15 not much change.

We use two lakes usually, one average 10 ft deep, the other over 100 ft deep. With the same boat/people setup, the wake is almost twice the size on the deeper lake. I have no idea why. Would be cool to hear someone who knows what they are talking about explain it!

one half of our lake is like 20+ feet everywhere, the other half goes from like 30+ feet and works its way to like 6-8 ft deep towards the end... and if we're surfing especially it gets crazy small on the shallow end and you also see a ton of the black muck mud get turned up from the water off the bow turning things up. Its crazy how far down water gets blasted from the hull

okay i am no expert.......but in shallow water as the water is being displaced and the wake is forming it hits the bed of the lake ......when it does it slows down and spreads the volume of water out to form a lower(and wider) wake. If you are in deep water the wake can form properly and the displaced volume of water does not get slowed or broken on the bottom thus all the volume of the displaced water can form into its rightfull size (narrower and higher)when unimpeded.

so basically in the shallow water all the displaced water cant immediatly form into your large wave but gets spread into a lower wake because it is slowed down by the bottom.

(Message edited by absoluteboarder on August 30, 2009)

a good visual is when you fill a balloon with a given volume of water (as in the volume being displaced by the boat). Now take the ballon put on a table and put a hand on each side. Over 1 second squeeze your hands together slowly (as when the wave is slowed by hitting the bottom) youll get a shallow but wide bubble(wake). Now if you squeeze your hands quicker over that 1 second (such as the wake unimpeded in deep water)the volume of water in the baloon will form into a higher but narrower bubble(wake)

now as far as the brackish water goes......it has salt in it and thus will be more dense than freshwater......greater density equals greater bouancy.....thus your boat doesnt sink as much in brackish or salt water as it will in fresh=smaller wake. (although how much difference it makes who knows but it is a variable)

^^ Wouldn't the wake also be harder in brackish because of the greater density of the water?

We launched our old boat in salt water and you can tell their was a difference between fresh and salt. The wake was harder and big falls hurt more.

I also think that the thrust could be pushing off the bottom in shallower water and making the bow push over, the ground has no give, like water, kind of like the jets taking off of a carrior when they flip up the thrust plate on the deck behind the plane.

..yeah the wake would be harder.....same goes for when the water gets colder.

Not sure I follow the logic on size. Salt water is certainly more dense than fresh water, but only about 2.5-3.5%. Same goes for cold vs. warm fresh water. The density (if I recall right on this one) between almost freezing water vs. almost boiling is only about 4%.

Neither of these should be very noticeable - of course the ice cream headache you get in freezing water probably makes it feel worse...

As far as wave height goes, the way I understood it is that a wave goes as deep as it is high. So if you are getting a waist high wakesurfing wake, lets call it three foot (not to be confused with three hawaiian foot), then it is depressing three foot into the water.

I don't understand how any more depth would help? Now I understand that is an extreme - three feet of water is a little sketchy, but lets say for arguments sake you are in eight feet of water - plenty to do whatever you want in the boat - I just don't see how having 10ft or 15 makes a difference - the wave isn't hitting the bottom already...

I ride at a place that the depth is ~ 30ft and a place that the water is 4-7ft. The wake is bigger and firmer in deeper water...no clue why.

john i think you are misjudging the amount of depth to build a full wake......if you accept the argument that 8 feet is not enough water depth for full unimpeded (by the lake bed) wake formation....then you will have the answer.

just look at whats different in deeper water....the bottom is not as close.....therein lies the answer.

....sometimes we look for more complicated answers...when the obvious is simple and right in front of us.

just to make sure we are on the same page...we have all experienced that a wake is larger in deeper water.......so the deeper water is making a difference.

(Message edited by absoluteboarder on August 31, 2009)

The effect is called surge. It is the interaction of a passing vessel through the water and how it interacts with the bottom and sides of the channel. I have studied this extensively. An unlimited depth will give the biggest wake, but not necessarily the best shape. Also, the lateral distance to the underwater structures plays an important role as well, almost as much as the vertical depth. Of course, if both are effectively unlimited, the ride is now in the ocean with the roller and wind chop. Surge, the effect of the wake, is influenced by hull size, hull shape, displacement wake, attitude, prop, speed, depth, bottom type, channel configuration, salinity, TDS (total suspended solids), vegetation, and the rider. Other factors still apply, but to a reduced impact.

As far as I know, I am the only person to design a wakeboard lake with all of these factors taken into consideration.

Your wake is formed when your hull makes a depression or void in the surface of the water and the water fills the void created by a planing hull to create the wake. Naturally when there is more water available (deeper) to quickly fill the void, the bigger the wake.

From my experience, the difference in wake size is barely noticable once it gets deeper than 8 feet for wakeboarding, but is quite profound when you get below the 6 foot range. There might be some subtle differences between 8 and 15 feet but not much and once deeper than 15 feet other factors are going to matter more. Now if you are really slamming your boat, you might notice more differences. Different story for wakesurfing. Just can't seem to get a decent wave unless we are in at least 12-15 feet of water.

Temp would have a negligible effect on density unless we are talking about freezing, but no doubt it feels a lot harder when you crash but that is more of an effect of your body getting cold.... Still stings when you gently put you foot in an ice bucket.

I never rode our boat in fresh water before last Summer, but the wake looked huge when we took the boat to the lake, compared to the salt water where we usually ride. The fresh water didn't really seem softer to me on falls though.

I can certainly attest to the shallower the water the smaller the wake though, we ride both on the flats and in the channels around where I live and you can definitely tell just by the wake size when you get out of the channel.

(Message edited by jtnz on August 31, 2009)

I understand that folks have experience with deeper water making a difference - however I have not noticed that. I ride on Lake Travis and Austin - Travis is deep, Austin has a lot of spots we ride over 6' or less. I haven't noticed any difference, in fact I've always seemed to have better wakes in specific shallower spots of Lake Austin.

My point wasn't to debate it though - it was to figure out why. I thought the point of this discussion was to figure out the why. People have experience with salt and cold water being "harder" but that is generally all perception and not the reality of the actual density of the water. Similarly perception could be off on waves - hence the discussion to figure out why.

Sounds like Karl has done a lot of research on it though. Karl - how does depth figure into the equation? I believe a wave only extends its energy at a depth equal to its height, so it is confusing to me why a depth any more than that has an impact.

Good page here describing ocean waves in depth btw. http://www.seafriends.org.nz/oceano/waves.htm

Wake is a waveform - a pressure wave. Water does not compress, but it transmits waves very well. The waves also will reflect off of the bottom. So, a "pressure wave" similar to that of the sonic boom photos builds around your moving boat. If the bottom is very close, it will lift up the hull in the stern - avoiding the displacement of water. The wake gets smaller and the shape is like that of a more bow weighted boat. The water also has to move and displace. That takes viscosity - hence the TSS and salinity factors. The speed it moves out and back will determine a lot of wake size. The faster it moves, the bigger the wake, but a little less lip. You want a little TDS in the water for best shape. Weeds will slow down the flow of the water, making the wake smaller and "mushy." A wide spot for the boat and a shallower area for the wake will allow the wake to peak and lip properly since the wave form will push up before flattening out. Increasing speed will push the wave form further back of the boat with good shape.

The depth of the wave form is much greater than the height due to gravity and the ability of water to transmit a very low frequency much better than air. The density of the water is much greater than air. After a certain TSS, viscosity, and a few other factors, the water will operate as a non-newtonian liquid, and you have slime. You cannot make a wake in slime. Of course, if the lake is that bad, you have an algae filled swamp.

"If the bottom is very close, it will lift up the hull in the stern - avoiding the displacement of water."
That's very interesting. Good info.

On final point. If the bottom is too close with an inboard, you will experience "squat" where the lower pressure in front of the prop will draw the hull down to the bottom like a suction cup. The effect will stop only when the prop stops - usually with damage. So, beware of the riding in very shallow areas with a wake boat - 3 foot deep may damage the boat if you take off due to squat. Four feet, fine. Three or less, and you may be contacting me for a new prop.

"The speed it moves out and back will determine a lot of wake size. The faster it moves, the bigger the wake, but a little less lip"

thats what I was trying to refer to .....the bottom will slow down the speed of the water due to friction. (I read this but dont have the source)

...having said all that there seems to be no written answer on this in the internet.....just a lot of wave theory

john.....how much weight are you pushing? in theory if you have a lighter vessel then 6-8feet would be enough for full wake formation in which case you would not notice a difference in deeper water.

if you are fully slammed or wakesurfing I would challenge you to pay close attention to the wake.....it will be different in 6 feet vs 15.

The number of people who have studied this can be counted on one hand. Two are (were ?) employed the the ACOE, one consultant, and me. Nobody will pay for this research, I did it to design and build my lake.

An X-Star with 2,500 over factory ballast will get to 95% of wake height with 12' of water with a width of at least 70'. 15' will get to 97-98% of wake height, but with more ramp. That is what I designed for.

...thats interesting karl......any % of wake size in 6-8 feet or was that depth not need in your calculations......or can you extrapolate from your data.

6' turned to mush. However, with the 205V hull, the wake was decent at 6'. The X-Star was wider, longer, and had a V-hull. The 205V has a flat hull. For shallow water riders the new X-1 (205V) or the old CC 210 are the hulls of choice.

I'd say I'm running a pretty heavy rig... have a Maristar 230 VRS with the 350 engine. When fully weighted with ballast only I'm running it like this...

400 bow bag
400 ski locker (very far forward - might as well be bow)
750 fat sac in pax area, port side
400 on port pax seat all the way back
750 in each rear ski locker, port full, starboard 1/2
Total ballast in boat = 3450
Total full riding = roughly 3000, plus pax.

If any more than 2 pax, I ease back on weight - usually don't fill the 750 center just for space reasons.

The boat throws up a healthy wave. Even with only a rear ski locker and the center full, nothing in the boat, it is easily rideable with no rope on my LF 4'10. I'm 190. With everything full, it is a very powerful and forgiving wave.

I don't think it is quite as nice as some of the waves I've seen from the more modern boats, but it is close.

In any case, clearly Karl has done the research on this, and I think his answers make sense from a "why" perspective.

I'll try to pay more attention to the differences. There are plenty of 6-8' deep sections, and plenty that are 15' plus.

Karl - you said 95% at 12', did you figure out what % at say 6 or 8' of water? Would be interesting to have a metric like... "you can get a XX% larger wake going from 6' to 12' deep of water... and only a YY% larger wake from 12' to 50' of water..."

The height is still over 50%, but the quality is mush. It has no clear lip or crest. I don't have a good measurement technique other than riding it and pictures in relation to a known height. Please bear in mind that the wake height and quality is also related to bottom composition, the shape of the bottom, and the width of the waterway. So, if you compare a 15' deep lake with a flat, smooth, sand bottom with one that has a muck bottom, you will have a visible and felt difference in the wake quality. A harder bottom makes the wake stand up and have a good lip. Mush creates mush. Heavy weeds creates mush.

Now, if you really want to go in-depth, you need to discuss shoreline and beach composition in a small lake....

In any case, it is not a linear progression, is is closer to a logrithmic progression where you get a lot from 4-8' - a bit more from 8-12, and the the effect goes down from there.

Lets say that it really makes a difference to ride in an engineered lake.